AIR SERVICE HANDBOOK Vol. I AVIATION SECTION SIGNAL CORPS ^^r.3^ J- AIR SERVICE HANDBOOK VOL. 1 AVIATION SECTION, SIGNAL CORPS INTENTION OF BOOk The contents of this manual have been coi.ied from various works, and the chapters which apply have been reproduced word for word It is iutend.-d thai il should be given to every pilot, and that a number should l)e available for mechanics The handbook should be sufficient for elementary training in rig- ping, engines, instruments, magnetos, meteorology and theory of flight, so iSat pupils need not read books which are not useful or which cove.- the same ground, in other words WASHINGTON ^ GOVERNMENT PRINTING OFFICE 1918 4 ^War "Depabtment, J)ocumeiit No. 769. ^ ^ \i^ War Department, Office op the Chief of Staff, If arch 26, 1918. The following Air Service handbook, A Course of Instruction for Pilots and Mechanics, prepared in the Office of the Chief Signal J Officer, is approved and published for the information and guidance ^ of the Schools of Military Aeronautics. V2 (A. G. O.-NO.062.1.) ^ By order of the Secretary of War. !^ D. W. KETCHAM, '^ ^ Colonel, General Staf. Official: " H. B. McCAIN, \ The Adjutant General. i c 483635 CONTENTS. Page. I. General rules 7 Routine in hangar 7 General organization of the workshop 9 II. Rigging 13 Rudiments of flight and stability 13 General rules 17 Planes, etc 19 Struts, wires, cables 27-34 Metal 35 Wood 36 Stresses and strains 37 Handling of airplanes 39 Keeping an airplane in good condition 40 III. SailmaMng 43 Tools and materials used 43 Patching 46 Doping 47 Markings on airplane 48 IV. Engine material 49 V. The gasoline motor 54 Detailed description 58 VI. Engine efficiency 69 VII. Propellers 74 VIII. Starting the engine 79 IX. Defects in the engine 80 X. Ignition 85 XI. Motor transport 91 XII. Instruments 95 XIII. Navigation of the air 116 XIV. Notes on flying 124 XV. Meteorology 134 XVI. Theory of flight 146 XVII. Stability of machines 157 XVIII. Nomenclature 170 Appendix 178 5 AIR SERVICE HANDBOOK. I. GENERAL RULES. THE GENERAL ROUTINE IN A HANGAR. Because the endurance and air worthiness of aircraft largely depends upon the care which is spent upon them, they should be well looked after when in the hangars. Airplanes should not be exposed to extremes of heat and cold. However well seasoned wood may be, if it is allowed to absorb moistui'e it will invariably deteriorate. Hangars therefore should be kept dry and, as far as possible, at an even temperature. Cleanliness. — An airplane can never be too clean. Rust, mud, dirt, and superfluous oil should be at once removed when the machine retui'ns to the hangar. Supports. — Fui'ther, an airplane once housed must have its weight supported in such a manner that there is no strain on the shock absorbers. The tail should also be supported so that there is not a continual strain on the fuselage. In this connection it must be remembered that supports should be so placed and in such a posi- tion that the main weight of the machine is directly over them. The best position is immediately under the points where the land- ing gear struts meet, or meet the longerons. In case it is necessary to support the wings or tail, the support should be under struts; in the case of the wings, under the interplane struts neai'est to the fuselage. Inspection. — Before an airplane proceeds on a flight and after its return all parts — such as control and aileron cables, dope, places where cables cross, the longerons, etc. — must be thoroughly exam- ined and the least sign of wear in any part must be at once corrected. It is important to watch the weai' of control wires at points where they pass over pulleys or fau-leads. For efficient inspection the machine must be cleaned, oil and grease must be removed before the cables can be properly seen. The mere fact of cleaning a machine insures that every part has at least been looked at. All engines must be thoroughly tested before flying and after any repair's or overhauling has been effected. Once a week, at least, a thorough routine examination must be made of all struts, internal 7 8 AIR SERVICE HANDBOOK. bracing wires of fuselage, etc., with a view to checking any damage or want of alignment. If an airplane makes a forced descent and has to be left in the open it is important to guard against possible contingencies, and it should be looked after as described under "Cross country flying." The planes of a machine must be cleaned dii'ectly after a flight, to prevent the oil and dirt soaking into the machine. Oil deterior- ates wood and the machine may therefore lose its factor of safety. At the end of a hundred hoiu-s' flying, the machine should be inspected by the engineer officer, who may or may not allow it to be flown farther. Any repairs necessary to a machine should be done at once, as a machine is often wanted in a hurry Tires of machines are injured by oil and grease. No oil there- fore should be allowed to collect on the floor of the hangar. Hangar floors. —Floors of hangars may be kept clean by the appli- cation of hot water and caustic soda. In this connection, it should be remembered that caustic soda rots leather boots so that wooden clogs should be used by the men engaged in this work. Sawdust must not be allowed as it accumulates dirt. Sawdust and old rags which have become oily are liable to spontaneous combustion if they are left in the sun or near the sides of a corrugated iron build- ing. Sawdust is only permissible in a tray in order to catch the waste oil from the engine. Engine stands. — Stands should be provided for engines to rest on when they are taken out of airplanes. Clothing. — Pegs should be provided for aviation clothing and hel- mets and also for the mechanician's coats, which are changed for overalls when the men come to work. No clothing should be allowed to lie about. Fire. — Owing to the inflammable nature of an aii-plane building and the large value of the articles kept in it, every precaution must be taken against fire. Fire drill should be held periodically. Buck- ets of sand and water must be kept completely filled in convenient places in each hangar. At least one fire extinguisher, such as "Pyrene," should be kept in each hangar ready for use, and all men should know how to use this apparatus. Notice board. — A bulletin board should be provided in the hangar of each section on which orders, etc., should be posted. Spares. — Only the authorized spares may be kept in the hangars. The tendency to accumulate unauthorized parts must be checked. Care must be taken that spare parts, where applicable, are kept properly oiled or greased to prevent them rusting. Every part must bear a label, showing exactly what it is and to what it belongs. AIR SERVICE HANDBOOK. 9 No mechanician should be allowed the opportunity of making a private collection of spare parts for use in effecting repaii-s. Pigeon holes should be provided for engine parts and various small stores, which should be labeled. When drawing new parts from the supply- room the mechanician should, as a general rule, and if possible, hand in at the same time the corresponding broken parts. Con- demned parts should be clearly marked and kept in a special ])lace, so that there is no possibility of their being used again by a careless workman. Spare planes should be stored in such a manner that their weight is evenly supported. One plane must not be allowed to butt into another. It has been found best to suspend planes by means of canvas slings hung from overhead. Within the loop of the slings there must be a wooden batten about 2^ inches wide, so that the leading edge of the plane is supported the whole way along. Records. — Rough records should be kept in each section in which all details of flights, overhauls, expenditm'e, fuel, and oil, etc., are entered whenever each event occurs and are of assistance in making the record an accurate history of the aii'plane and engine. Log books must in\ariably be made up to date, signed and forwarded at the same time as an engine or airplane is transferred from one unit to another. Smoking. — Smoking in the hangar is strictly prohibited. GENERAL ORC.VNIZATION OP THE WORKSHOP. Personnel. — The personnel available should be divided into three separate departments, with a master signal electrician in charge of each. A commissioned officer should exercise general supervision over each department. The three departments are: (a) Aero repair shop. (6) Dope shop, (c) Machine shop. It is desirable that the aii]jlane and dope work be conducted in a separate building from the engine and machine work. Care. — ^All mechanicians must be made to realize that the greatest care and attention to the minutest details is absolutely necessary. Bench. — Where airplane hangars or workshops are provided with benches and \dses it is convenient that the benches be fitted with lock-up drawers for the storage of tools. Benches must not be allowed to become mere shelves for the assortment of rubbish, spare parts, and discarded breakages. No article should be kept on a bench close to where some particular Work is on hand which has not a direct bearing on that work. Boxes should be provided in which metal such as brass or steel fittings 10 AIR SERVICE HANDBOOK. may be kept. At regular intervals, not less than once each week, all rubbish must be removed. Tool boxes. — Places should be allotted for the mechanics' tool boxes, and their contents should be periodically inspected. A list of the correct contents of the box should be pasted inside the lid. Care of machinery.- — Only the mechanics authorized by the engineer officer should be allowed to use the lathes, saws, etc., with which the shops are provided and to start the motors for dri\dng these machines. With electrically driven machinery care must be taken that all switches are "off" before the shops are closed at the end of the day. Lathes, etc., and their accessory parts must be kept properly oiled and greased and free from rust. All belting must be provided with suitable guards. All flat-faced surfaces in lathes should be suitably protected by wood to prevent them from being damaged by tools falling on them. All shavings, sawdust, and metal turnings must be cleared away from the machines daily. The metal turnings should be preserved for future use or sale, different metals being kept separate. Examination and dismantling of airplanes .—This work, if it is made a matter of routine, is simple and occupies a small amount of time. The following is the system which has been found most suitable: (a) In the case of serious damage or periodical overhaul the airplane must be taken at once to the shops by the men of the section to which it belongs. (6) The engineer officer then carries out his detailed examination and prepares a statement setting out in detail the general condition of the machine. (c) The airplane is then stripped and all parts labeled. (d) In all cases in which the machine has been in an accident the engine must be removed for a thorough examination and over- haul. For this purpose it must be turned over to the machine shop. (e) The parts which are not repairable are removed to the author- ized place and all small stores, such as turnbuckles, bolts, etc., which are apparently ' still serviceable are handed into the shop stock room. In the stock room they are kept separate from the other spares until they have been pronounced ser\dceable or otherwise by the officer in charge. ('/) The parts which can he repaired and made fit for service are labeled and sent to the department concerned where they will await their turn for repair. (g) The undamaged parts are handed into the stock room properly labeled. These should be taken on temporary charge as spares in AIR SERVICE HANDBOOK. 11 the workshop store, until the airplane is again ready for them. If the airplane can not be repaired the undamaged parts must be taken on permanent charge in the store account of the unit. (h) All instruments requii'ing repairs should l/e returned to the stockroom and the engineer oflicer should decide whether the instru- ments are to be returned to the makers for repair or not. (i) All parts repairable or sound should be thoroughly cleaned before being turned over to the supply officer. (j) Parts of airplanes while awaiting erection must be properly supported or stored. When the airplane is being erected all parts intended for that particular machine must be kept together so as to avoid using wrong ])arts. Engine repair wrk.- — ^A system must be established and worked on whenever an engine is taken down for repair and reassembled. Suitable stands must be provided on which to place engines. Trays divided up into compartments in which to put each part of the engine as it is removed are a necessity. It is a good thing to have each compartment numbered and the parts of each cylinder and its attachments put into the compartment coi'responding to its number in the engine. A little shelf can conveniently be con- structed on the outside of these compartments on which to lay the tools so as to prevent them getting lost or dirty. Only in cases of urgency should parts of one engine be used to complete another. If such a course is necessary the parts so used should be numbered afresh so as to correspond with the numbering of the engine in which they are used. Thus, if "number 5" piston of one engine is to be used as '"number 7" in another, it should be renumbered 'number 7." When reasseml)ling an engine the utmost care is necessary to insure that all parts are free from grit and dirt. Gasoline baths are a necessity. Every part should be thoroughly oiled before l^eing replaced. Any metal part which has been bent should on no account be straightened and replaced in the engine without the knowledge of the engineer officer. As a general rule such bent parts must not be straightened and used again. When the engine has been overhauled and tested it should bear a label showing date of test, time run, and number of revolutions obtained. It should then be put to one side and greased pending a necessity arising for its use in an ahplane. Engines must be turned daily by hand. Logbooks must be kept, made up to date, showing work done on the engine and these books must always accompany the engines. 12 AIR SERVICE HANDBOOK. The care of tools. — Protect all edges. Keep all edged tools sharp. If you dull a tool by using it sharpen before returning it to its place. In sharpening an edged tool do not blunt it. Grind an angle and keep it until the tool becomes sharp. If you round off an edged tool you ruin it. Do not use a file like a hack saw, as most files are made to cut on the forward stroke only. Keep files in a case where they do not rub against each other and keep them free fi'om oil and gi'ease. When using a file always put a handle on it. Have a file cleaner handy and use it often. For a fine finish on steel do not use a file, use an oilstone. Do not keep soldering acid near tools nor handle tools after using acid without fu'st washing your hands. Have your list of tools pasted in the top of your chest and check your tools each evening before quitting. Keep bits sharp and in a case and do not expect to drill a true hole ■with a dull bit. Do not use a steel hammer on metal parts, use a brass or rawhide hammer, or copper or brass drift. Oil your tools often in rainy or wet weather. Do not keep sulphur or salt near your tools. Always oil the steel tape before putting it away. Do not use a monkey wrench as a hammer. Do not use a screw driver as a punch, drift, or lever, and keep the sides of the point parallel. Do not tise pliers on nuts. Do not use a 24-inch monkey wrench on a quarter-inch nut. Do not use an end wrench on a nut unless it fits proi^erly. The same remark applies to all wrenches on nuts, and to screw drivers on screws. Keep yoiu' fine measiu*ing tools in a case. Do not hold work that is being heated with a torch with a pair of pliers; use regular tongs. Use only regular nippers for cutting piano wire. The ordinary side-cutting pliers are usually not strong enough for this. In using a tap always be sure that the right size hole has been drilled. In using taps and dies use plenty of lard oil on the work and be careful of backing up the tap or die. Always read instructions furnished by the manufacturer. In cutting large, heavy stock it is better to take more than one cut. Always clean taps and dies before putting them away. Do not use a Stilson wrench on nuts. Sharpen your tools over all the surface of an oilstone and not only in one place. Always keep Ihe cutting edge of your saw well protected. AIR SERVICE HANDBOOK. 1» It is useful to have a place for each tool so that they can not shift when the boxes are put on the motor transport in a hurry. Each mechanic is advised to keep a memorandum book in his possession in which he may record such notes as may be considered useful to him. II. RIGGING. RUDIMENTS OF FLIGHT AND STABILITY. In order that a pilot may take an interest in the rigging of his machine he should have a sound idea of the rudiments of flight and stability. He need not know how to design a machine, as this is a highly technical procedure which is done by the machine designers. The pilot should know how to fly any machine he is given to the best advantage -rl. J'-.'tcV 4 U> UJ^WC Fig. 1, Flight is secured by driving through the air a surface or surfaces inclined to the direction of motion. The surface meeting the air tends to drive the air downward and this causes a resultant reaction on the surface. The resultant acts approximately at right angles to the plane or, if the plane is curved, to the chord. The total reaction on the aerofoil can be considered as made up of two forces, one acting in a vertical direction and one in a horizontal. Lift. — The lifting planes by being driven through the air secure a lift, and when the speed is great enough the lift will become greater than the weight of the airplane, w^hich must then rise. Thus, the vertical part of the total reaction on the plane is called "Lift." Bear in mind that the lift is always trying to collapse the planes upward . Drift. — The resistance of the air to the passage of an airplane is known as "Drift." Thus, the horizontal part of the total reaction 14 AIR SERVICE HANDBOOK. on the plane is called "Drift." This is overcome by the "thrust" of the propeller, which thrusts the airplane (or drags it) through the air and so overcomes the drift. Bear in mind that the drift is always trying to collapse the planes backward. You will see from the above diagram that there are four forces to consider. The lift, which is opposed to the weight, and the thrust, which is opposed to the drift. The lift is useful; the drift is the re- verse of useful. The proportion of lift to drift is known as the " Lift- drift ratio." This is of paramount importance, for upon it depends the efficiency of the airplane. In rigging an airplane the greatest care must be taken to preserve the lift-drift ratio. Always keep that in mind. This means that the lifting planes of a machine must not be damaged, that the adjustments of the machine are exactly in accordance with the rigging diagram, and that all work is done neatly and carefully. Angle of incidence. — The angle of incidence is the inclination of the lifting surfaces. If the angle of incidence is increased over the angle specified in your rigging instructions, then both the lift and drift are increased also, and the drift is increased in greater pro- portion than the lift. If, however, the angle of incidence is de- creased, then the lift and drift are decreased and the lift decreases in greater proportion than does the drift. You see then that in each case the efficiency is spoiled, because the proportion of lift to drift is not so good as would otherwise be the case. Balance. — The whole weight of the airplane is balanced upon, or slightly forward of, the center of the lift. If the weight is too far forward, then the machine is nose heavy. If the weight is too far behind the center of the lift then the air- plane is tail heavy. In either case an adjustment must be made which spoils the efficiency of the machine. Stability. — By stability of the airplane is meant the tendency of the airplane to remain upon an even keel and to keep its course; that is to say, not to fly one wing down, tail down, or nose down, or to try and txurn off its course. Directional stability. — By directional stability is meant the natural tendency of the airplane to remain upon its course. If this did not exist, it would be continually trying to turn to right or to left, and the pilot would not be able to control it. For the airplane to have directional stability it is necessary for it to have, in effect, more keel surface behind its turning axis than there is in front of it. By keel surface is meant everything you can see when you look at the airplane from the side of it — the sides of the body, landing AIR SERVICE HANDBOOK. 16 gear, wires, struts, etc. Directional stability is sometimes known as "weather-cock stability." If in the case of the "weather cock" there was too much keel surface in front of its turning axis, which is the point upon which it is pivoted, it would turn around the wrong way; and this is just what would happen in the case of the airplane. Directional stability will be badly affected if there is more drift (i. e., resistance) on one side of the airplane than there is on the other. This may be caused by the following: 1. The angle of incidence of the main planes or the tail plane may be Avrong. If the angle of incidence on one side of the machine is not what it should be, that will cause a difference in the drift between the two sides of the aii^plane, with the result that it will turn off its course. 2. If the alignment of the fuselage or fin in front of the rudder or the stream-line struts is not absolutely correct, that is to say, if they are turned a little to the right or left instead of being in line with the center of the machine in the case of the fin and dead on in the direction of flight, they will act as an enormous rudder and cause the machine to turn off its covu-se. 3. If the dihedral angle is wrong that may have a bad effect. It may result in the propeller not thrusting from the center of the drift, in which case it will pull the machine a little .=iideways and out of its course. 4. If the struts and stream lines on the wires are not adjusted to he dead on in the line of flight, then they will produce additional drift on their side of the airplane, with the result that it will turn off its course. 5. Distorted surfaces may cause the airplane to be dii'ectionally bad. The planes are "cambered "; that is, curved to go through the air with the least possible drift. If perhaps owing to the leading edge spars or trailing edge getting bent, the curvature is spoiled, with the result that the amount of drift on one side of the airplane is altered, causing the macliiue to have a tendency to turn off its course. Lateral stability. — By lateral stability is meant the sideways bal- ance of the machine. The only possible thing that could make the machine fly one wing down is that there is more lift on one side than there is on the other. This may be due to — 1. The angle of incidence may be wrong. If the angle of inci- dence is too great, it will produce more lift on that aide than on the other. The result will be that the machine flies one wing down. This remark also applies to too little incidence on one wing. 16 AIR SERVICE HANDBOOK. 2. Distorted surfaces: If the planes are distorted, then their cam- ber is spoiled and the lift will not be the same on both sides of the airplane. 3. If stability is not horizontal, it will cause a twisting movement. Longitudinal stability. — By longitudinal stability is meant the fore- and-aft balance. If this is not correct, the machine will try to fly nose or tail down. This may be due to — 1. The stagger may be wrong. The top plane may have drifted back a little, and this may be due to some of the wires having elon- gated their loops or having pulled the fittings into the wood. If the top plane is not staggered forward to the correct degree, it means that the whole of the lift of the airplane is moved backward, so that it will have a tendency to lift the tail; that is, it will become nose heavy. A quarter inch error in the stagger makes a considerable difference. 2. If the angle of incidence of the main planes is too great, it will produce an excess of lift, which will tend to lift the nose of the ma- chine. If the angle is too small, the opposite happens. 3. When the machine is longitudinally out of balance the usual thing is for the rigger to rush to the tail plane, thinking that its ad- justment relative to the fuselage must be wrong. This is the least likely reason. It is much more likely to be one of the first two, or, more probable still, that the fuselage has warped upward. This gives the tail plane an incorrect angle of incidence. This is due to bad landings or to allowing the machine to rest in the hangar with its its tail on the ground, so that it always has a certain amount of weight on it and it gets no rest. 4. If the above three points are correct, there is a possibility that the tail plane itself has assumed a wrong angle of incidence. In such event, if the machine is nose heavy, the tail plane should be given a smaller angle of incidence. If the machine is tail heavy, then the tail plane must be given a large angle of incidence, but be careful not to give the tail plane too great an angle of incidence. The longitudinal stability of the airplane entirely depends on the tail plane being at a much smaller angle of incidence than the main plane, and if you cut the difference down too much the machine will become uncontrollable. Sometimes the tail plane is set on the ma- chine at the same angle of incidence as the main planes, but it ac- tually engages the air at a lesser angle, owing to the air being de- flected downward by the main planes. Propeller torque. — Owing to propeller torque, the airplane has a tendency to turn over sideways in the direction opposite to that in which the propeller revolves. In some machines this tendency is rather marked, and this is offset by increasing the angle of incidence on the side tending to fall and by decreasing the angle of incidence AIR SERVICE HANDBOOK. 17 the same amount ou the side tending to rise. In this way uioii- lift is secured ou one side of the machine than on the olher, so that the tendency to overturn is corrected. Wash in. — When the angle of incidence toward the tij) of the main plane is increased the plane is said to have wash in. Wash out.— When the angle of incidence i.* decreased it is called wash out. Sometimes wash out is given to both sides of a main plane. This decreases the drift toward the wing tips, and consequently decreases the effect of gusts upim them. It also renders the ailerons more effective. Importance oj ijood riyyiiKj.—lt is imjiossible to exaggerate the im- portance of care and accuracy in rigging. The lives of the crew, the speed and climb of the airplane, its control and general elti- ciency in flight, and its duration as a useful machine all depend upon the rigger. Consider that while the engine may fail, the pilot may still glide safely to earth; but if the airplane fails, then all is lost. The responsibility of the rigger is therefore very great, and he should strive to become a sound and reliable expert on all matters relating to his art. For an art it is, and one bound to be- come increasingly important as time passes. GENERAL RULES KOH RIGGING. There are two kinds oi machines — the tractor and the pusher. The pusher type is a type which is now dying out. The principles of rigging for both are the same. The different steps of rigging are as follows: 1. Get a blue print or rigging diagram of the machine and look at the essential measurements. 2. True up the fuselage and lix the tanks and internal fittings. 3. Put on the undercarriage (landing gear), in oi'der to insure that the machine can not fall . 4. Rig, fix, and true up the center sections of the main planes. 5. Rig the main planes separately. 6. Attach and true up the main planes. 7. Rig tail (separately, if necessary). Fix and true up. 8. Attach ])alancing surfaces and adjust controls. 9. Check all measurements and see that every pin, nut, etc., is locked. 10. Put engine in machine. 11. Look over the whole machine to see if everything is correct. Before starting work on a machine, get into overalls, because a man can not do proper work if he has to think of his clothes. See that all the necessary tools are handy. Sort and lay out the planes, 4fiG4.V- 18 2 18 AIR SERVICE HANDBOOK. struts, cables, etc., putting each more or less in its relative position (if there is suflScient room in the shed). The useful tools are as follows: 1 side-cutting pliers. 1 round -nose pliers. 1 small three-cornered file (to ease burrs on bolts and pins). 1 hammer (to be used only when absolutely necessary) and copper or brass drift. I carpenter's level. 4 plumb bobs. 1 carpenter's rule. 1 straightedge about 3 feet long. 1 long and 1 short tramel. 1 steel measuring tape. 1 ball of string. 2 turnbuckle keys. 2 pairs auto combination pliers, not for nuts or bolts. Spanners suitable to the bolts and nuts on machine. End wrenches suitable to the bolts and nuts on machine. See that split cotter pins, nuts, and bolts, etc., are handy. Truing up the fuselage. — In factories the longerons of the fuselage are clamped onto a table which has blocks on it shaped as required. In the field the rigging has to be done by measurement from the beginning. It is unusual in the field for the squadrons to have to rig a fuselage, but it may often be necessary to true it up. Before attaching any wires to the fuselage, all metal fittings should be attached in the proper places on the longerons. All struts should be fitted in their sockets in order to prevent delay in assembling. The two sides of the fuselage are trued up first, and it is usual either to make the top longeron straight or to make the whole tail symme- trical. This must be found out from the blue print. When each side has been trued up, the horizontal compression members can be placed between the two sides and the bracing and the internal cross bracing of the fuselage can be adjusted. While doing this the fuselage should be supported on two trestles; the first trestle should be toward the front and the rear trestle about two-thirds of the way toward the rear. This causes the tail to stick out unsupported and will give strains on the fuselage nearly the same as those put on the ms chine in flight . Th e bracing of a fuselage is done by means of cables, piano wire, or special tension bars. These are adjusted in different ways, as will be explained later. The engine beds are usually adjusted permanently as far as we are concerned, and it is only necessary to see that the remainder of the fuselage is trued up properly with respect to these. AIB SEEVICE HANDBOOK. 19 When the fuselage has been itself trued up, it is then necessary to put it in the flying position; that is, the engine beds must be hori- zontal and the horizontal compression members should be also horizontal. This is done by placing a straightedge and spirit level on the engine foundations, and you must be very careful to see that the bubble is exactly in the center of the level. The slightest error will be much magnified toward the tail and wing tips. Great care should be taken to block the machine up rigidly. In case it gets accidentally disturbed during the rigging of the machine, you should constantly verify the flying position by running the straightedge and spirit level over the engine foundations. Carefully test the straight- edge before using it, for, being usually made of wood, it will not long remain true. Place it lightly in a vise and in such a position that a spirit level on top shows the bubble exactly in the center. Now slowly move the level along the straightedge. The bubble should remain exactly in the center. If it does not, then the straightedge is not true and must be corrected. Both top and bottom should be true and exactly the same distance apart. Never omit testing the straightedge. In the case of the airplane fitted with engines of the Fig. 2. rotary type the "flying position" is some special position laid down in your rigging diagram and great care should be taken to secure accuracy. The easiest way of measuring the length of a wire is by means of a tramel. This is a piece of wood which carries spikes at each end — one is fixed, and the other is adjustable. If necessary, the wires and turnbuckles should now be locked and painted. Put the tanks in the machine and fix all the internal fittings. It is easy to get at the inside of the fuselage now, but when the wings are on or when it is covered with dope this will be very difficult. THE PLANES. The planes, both main, center section, and tail, and all control surfaces consist of spars and ribs covered with dope. If the siu-face ia large, they are braced internally with wires, cables, or compression members. The wings consist of two spars, the front and the rear, which are kept apart by compression ribs and kept in shape by bracing wires, which are fixed to the ends of the ribs, diagonally opposite and sometimes by diagonal ribs. The bracing thus consists of a number of rectangles, usuallv two or three in number. To true 20 AIR SERVICE HANDBOOK. up. place the front and rear spar on trestles which are the same height . Attach all metal fittings to the spars. Build the compression ribs onto the spars, and fix and tighten up the cross-bracing wires. Make certain that these wires are of the same length by means of the tramel. Look along both spars and see that they are straight. The spars are always made of ash or spruce, usually spruce. The compression ribs are usually made of solid spruce or some such ma- terial or in box form. They are sometines steel tubes. They fit into sockets, clipped around the spar. The main spars should not, as a rule, be drilled to take fittings as they are thereby weakened. On no account should a spar be pierced in a place not shown on the construction diagram. The bracing wires are attached to steel clips called wii-ing plates and are adjusted by turnbuckles. The leading edge of a plane is not meant to take any appreciable load and consists of some liglit wood rounded off in front. The L > .| r X 7 < ^/^ r- J wr I J 1 ^ ^ ~-- ^ < ^ <^ £ i ^ P > < 1 1 ^ Vtxf •sp'' 1 J.^ ^ ^^-''^^ 1 ■^"^ ■1 T-. , l> ■., iii. i Fig. 3. trailing edge has no thickness and consists usually of a thin piece of wood or wii'e stretched from rib to rib in order to support the fabric. Form ribs run from the leading to trailing edge at intervals of about 18 inches. These are only to support the fabric and are usually made of three-ply wood. Holes are bored in these ribs to lighten them. Between the leading edge and the front spar are a number of light form ribs to enable the fabric to keep its shape. In some machines these ribs are replaced by a layer of three-ply wood. A rib consists of a web with a flange on the top and bottom. The fabric is tacked onto the center of these flanges so that the tack passes down the web of the rib. Or better still, the fabric is sewn onto these ribs. It is always a bad practice to pierce wood if there are other ways of doing the job. \^Tien the wing has been trued up and all bolts, etc., locked, it may be covered with fabric, dope, and varnish, as described later. All metal fittings are now attached to the planes. The center section. — Place the center section struts in the sockets on the center section and attach the bracing wires. Lift the whole of this unit and place the bottom of the struts into their sockets on AIR SERVICE HANDBOOK. 21 the fuselage. True up this center section. The machine may or may not have stagger. If it has not, the front center section struts will be vertically over the struts in the fuselage. This may be ad- justed by dropping a string with pluml> l)Gb attached from the wing attachment of the front spar, seeing, first of all, if the pluml> bob falls immediately over that of the lower plane. If it does not. alter the incidence wares. Then hold the string in front of the center section and see if the struts are upright. If they are not, alter cross- bracing wires. If the machine has stagger, the pluml) bol) must fall a certain number of inches in front or behind the attachment for the lower plane, and this amount will be found from the blue ])rint. In some machines the struts are splayed outward from the fuselage. In any case, they will be symmetrical and the adjustments can be made by measuring the cross-bracing wires, and seeing that they are equal. Make certain that the adjustment of the center section is correct, because on this depends the whole adjustment of the machine. Fig. 4. V The icings. — Make certain that the fuselage is in the flying posi- tion. Place trestles on either side of this so that when the wings are lifted they can be rested on these trestles in approximately the flying position. The wings on each side of the machine are rigged separately. This is done by supporting the planes on the leading edges, care being taken that the leading edge is not damaged. The struts are fitted into their proper sockets and the l)racing \vires are adjusted to their approximate length so that the whole pair of planes may be lifted as a unit. Attach the mngs to the center section and fuselage. When the planes are on each side, take away the trestles and allow the strain to come onto the landing wires. True up the main planes 1)}^ adjusting these landing \vires-making certain that all the other wires are sufficiently loose so as to take no strain. The wings should be symmetrical, the corresponding wires on either side being equal. The incidence should be that shown in the blue print and the dihedral angle is shown there also. To measure the incidence. — One method of finding the angle of incidence is as follows: Take the straightedge and test it. Place one corner of the straight- edge underneath and against the center of the rear spar, holding it in line with the ribs. Put a level on the end which sticks out from the 82 AIB SERVICE HANDBOOK. plane and hold the straightedge horizontal. Measure from the straightedge to the center of the bottom of the front spar or to the lowest part of the leading edge. Make the measurement that shown in the blue print by altering the landing wiies. See that the spars are straight while this is being done. Check the measurements under each set of struts. If the machine is being adjusted after having been rigged, slacken off all wires going to the top of the strut concerned and then tighten all wires going to the bottom, or vice Fig. 5. versa. Do not attempt to secure this adjustment by merely altering the incidence wires. This latter is a very bad practice indeed, and while, owing to the airplane being of such flimsy construction, it may be possible to change the angle of incidence by adjusting merely the incidence wires, the result of such practice is to throw other wires into undue tension, which will cause the framework to become dis- torted. The dihedral angle. — One method of securing the dihedral angle, which is the upward inclination of the wings toward their tip, is as c \ / /\ \ B A E E' « 3 Fig. 6. follows, and this method will at the same time give you the angle of incidence: Throw a string over the top of both spars of the wings. Keep the string tight by attaching a weight or by attaching it to some heavy object. The strings should touch the wings at points just inside the top of the outer struts. The measurement taken from the blue print is then from the string to the top of the center section near the center section struts. This measurement should be taken near the struts and no attempt should be made to take the set measurement near the center of the center section. The wings on each side should be sym- metrical, and this is insured by making the landing wires in corre- AIK SERVICE HANDBOOK. 23 eponding bays equal. The spars should be kept straight. Sometimes the diagonal measurements are taken from the bottom of one strut to the top of another, but this is wrong on account of possible inaccu- racies due to faulty manufacture. The points between which the diagonal measurements are taken must be at fixed distances from the butt of the spars. Such distances being exactly the same on each Bide of the machine, thus: Fig. 7. It would be better still to use the center line of the fuselage instead of the butt of the spars but for the fact that t^uch a method is a trouble- some one. Another method of securing the diliedral angle and also the angle of incidence is by means of the dihedral board. The dihedral board is a light, handy thing to use but leads to many errors and should not be used unless necessary. The reasons are as follows: The dihedral board is probably not true. If you must use it, then be very careful to test it for truth beforehand . Another reason against its use is that you have to use it on the spars between the struts, and that^is just where the spars will have a little permanent set up or down which will, of course, throw out the accuracy of the adjustment. Then, again, there may be inequalities of surface on the spar due to faulty manufacture. The method of using it is as follows: If the dihedral board is used, then the bays must be carefully measured diagonally as explained above. Whichever method is UBed, be sure that after the job is done the spars are perfectly straight. 24 AIR SERVICE HANDBOOK. Stagger. — The stagger is the distance the top plane is in advance of the bottom plane when the machine is in the fl>'ing position. The set measurement is obtained as follows: The plumb lines must be dropped over the leading edges wherever struts occur and also near the fuselage. The set measurement is taken from the front of the lower leading edge to the plumb line. Remember that it makes a difference whether you measure along a horizontal line (which can be found by using a straightedge and spirit level) or along a projection of the chord. The correct line along which to measure is laid down in your rigging diagram . If you make a mistake and measure along the wrong line, this may make a difference of a quarter of an inch or more to the stagger, with the certain result that the airplane will be nose or tail heavy. If the stagger was put correctly on the center section in the first instance, it should be correct when the main planes are affixed. Now adjust the drift and antidrift wires. When the adjustment of the angles of incidence, dihedral angle, and stagger have been secured, the incidence wires and the flying wires should be tightened. A\'Tien this has been done, run over all your measurements again, as these last adjustments may possibly have thrown out your original ones. Over-all adjustments. — The following o\'er-all measurements should now be taken: The straight lines "A" and "B" must be equal. The point "C" is the center of the propeller thrust. The points 'D" and AIR SERVICE HANDBOOK. 26 " ■ E " are marked on the main spar and must in each case be the same distance from the butt of the spar. Do not attempt to make "D" and "E" merely the sockets on the outer struts, as they may not have been placed quite accurately by the manufacturers. The lines "A" and "B" must be taken from both top and bottom spars — true measurements on each side of the airi)l9,ne. Now measure the dis- tance between "F" and "G" and "H" and "G." These two measurements must be equal. "G" is the center of the fuselage or rudder post. "F" and "H" are points marked on the top and bottom rear spars, the same distance from the butt, as was done before. If these over-all measurements are not correct, then it is probably due to some of your drift or antidrift wires being too tight or too slack. It may possibly be due to the fuselage being out of true, but, of course, you should have made quite sure that the fuse- lage was true before rigging the rest of the machine. Again, it may be due to the internal bracing wires not being accurately adjusted; but, again, that should have been done before covering the plane with fabric. The tail. — The tail may be either an adjustable tail or a fixed one. The angle of incidence or the mean angle is given in the rigging diagram. If the tail is adjustable, see that the control is in the center before attaching the tail plane. To true up, see that the spars are horizontal. If they are tapered spars, see that their center lines are horizontal. The spars should be straight and the corre- sponding bracing wires on either side should be equal and should bear equal strains. Verify the position of the tail plane by stand- ing behind the machine and seeing that it is symmetrical with the center of the main planes. In some machines there is an adjustment for changing the ang e of incidence of the stabilizer. The greatest care should be taken when increasing the angle of incidence on the tail. Only a comparatively small increase in the incidence makes the machine dangerously unstable, as explained in theory of flight. If a machine is nose or tail heavy, after it has once been trued up properly, it is probably due to the fuselage becoming strained rather than a wrong angle on the tail, and the fuselage should be retrued before the tail is touched. Control surfaces. — Before attaching the control surfaces, lash the control lever and the rudder bar in the central position. The pilot depends entirely on these control surfaces for managing the plane, so that the greatest care must be exercised in properly adjusting these surfaces. When the surfaces have been attached to the planes, never let them hang down without support, as this strains the hinges. The ailerons should be rigged so that when the machine is in flight they are in a fair, true line with the surface in front and to which they are hinged. The ailerons are hinged to the main planes and 26 AIR SERVICE HANDBOOK. are then attached to the aileron balance cable or balance springs. This cable should be adjusted so that the rear edge of the aileron is 1 inch (may alter with type of machine) below the trailing edge of the plane. Connect the control cables to the ailerons. Remember that controlling surfaces must never be adjusted with a view to altering the stability of the machine. Nothing can be accomplished in that way. The only result will be that the control of the air- plane will be spoiled. If the ailerons are adjusted too high, the machine feels "floppy." If the ailerons are adjusted too low, it makes the machine unstable and tiring to fly. In both cases the machine is inefficiently rigged. The elevators, like the ailerons, should set fairly behind the tail plane when the machine is in flight. Because the controls can not be adjusted too tightly, the elevators also must hang down a little bit when the machine is at rest. They should be adjusted symmetric- ally on either side, and this should be checked by eyo as well as by measurement. The rudder is sometimes set at a small angle with regard to the cen- ter line of the machine in order to help the adjustment for torque of engine. This adjustment should be checked also by eye and meas- urement. Control cables. — The adjustment of control cables is quite an art and upon it will depend, to a large degree, the quick and easy con- trol of the airplane by the pilot. Having rigged the controlling surfaces, remove the lashing which has kept the levers in the central position. Then, sitting in the pilot seat, move the control levers smartly. Tension up the control cables so that when the levers are smartly moved there is no perceptible snatch or lag. Be careful not to tension up the cables more than is necessary to effect this. If you tighten the control cables too much, they will bind round the pulleys and the result is hard work for the pilot and it also throws dangerous stresses upon the controlling surfaces, which are some- times of rather light construction. It will also cause the cables to fray round the pulleys quicker than would otherwise be the case. Now, having tensioned the cables sufficiently to take out the snatch or lag, place the levers in their neutral position and move them backwards or forwards not more than an eighth of an inch either Bide of the neutral position. If the adjustment is correct, you should be able to see the controlling surfaces move. If they do not move, the control cables are too slack. Tail skid. — The tail skid is usually made of ash and is stream lined. Care should be taken that the tail-skid spring is at the proper ten- sion. If it is too loose the machine may jar heavily on the rudder- post, and this will strain the whole fuselage. A safety cable should be fitted through the tail-skid spring to prevent the front end stick- AIR SERVICE HANDBOOK. 27 ing into the ground in case of a bad landing. If the skid is steerable it is controlled by cables working from the rudder bar, and these controls should have springs on them to prevent sudden jerks coming on the rudder bar and surface. The landing gear. — The landing gear must be very carefully aligned as laid down in the rigging diagram. 1. Be very careful to see that the landing gear struts bed down well in their sockets. If this is not done, then after a few rough landings they will bed down farther and throw the landing gear out of alignment, with the result that the machine will not taxi straight. 2. When rigging the landing gear, the airplane must be blocked up in its flying position, and sufficiently high so that the wheels are off the ground. When in this position the axle must be horizontal. 3. Be very careful to see that shock absorbers are of equal tension and that the same length of elastic cord and the same number of turns are used in each absorber. :^y'r'^ FlC. 10. 4. Errors in the fore-and-aft adjustment of the axle may make the machine unstable when landing; and the machine may either pitch onto the propeller or break the tail at the moment of landing if the adjustment is not correct. Covering the fuselage. — When covering the fuselage, start from the front and top and work backward, so that there will be no hole near the engine to catch oil. Laco the fabric fairly tight so as to make the skin friction small. If there are any overlaps, make them so as not to catch the wind. Allowance for torque of propeller. — This may be taken up on the wings or by the rudder or both. To adjust the wings it will be necessary to increase the angle atthe tip of one plane and decrease it an equal amount on the other. That is, give the planes "wash out" or "wash in." If the propeller rotates right-handed in a tractor, it will be necessary to increase the angle on the left main plane. A tractor machine with a right-handed propeller will also require a little right rudder. In some machines it has been customary to take the allowance for torque on the ailerons, but this is a bad practice. Spars and struts. — All spars and struts must be perfectly straight. The above diagram shows a section tlirough an interplane strut. If it is to be prevented from bending, then the stress of compression 28 AIR SERVICE HANDBOOK. must be equally disposed round the center of strength. If it is not straight, there will be more compression on one side of the center of strength than on the other side, in which case the strut will be forced to take a bending stress for which it was not designed. Even if it does not break it will in effect become shorter, and thus throw out of adjustment all the wires attached to the top and bottom of it, with the result that the flight efficiency of the airplane will be y T^ .jU-^f ^>.i-5r''*r-w. Fig. 11. spoiled. Besides, an undue and dangerous stress is being thrown upon other wires. 1. Where spars are concerned, there is an exception known as the arch. For instance, in the case of the Maurice Farman, the spars of the center section plane, which have to take the weight of the nacelle, are arched upward. If this was not done, it is possible that rough landings might result in the weight of the nacelle, causing the spars * t>%.^ 4.U ^ov^xcL to bend down a little. This would produce a dangerous bending stress, but as long as the wood is arched, or at any rate kept from bending downward, it will remain in direct compression and no danger can result. 2. Struts and spars must be symmetrical; by that I mean that the cross-sectional dimension must be correct, as otherwise there will be bulging places on the outside, with the result that the stress will not be evenly disposed around the center of strength and the bending stress will be produced. 3. Struts, spars, etc., must be properly bedded into their sockets or fittings. To begin with, they must be a good pushing or gentle AIR SERVICE HANDBOOK. 29 tapping fit. They Jinist never be driven with a heavy hammer. If the sockets do not fit, it is better for them to be too large than too small. Again, spars and stmts must bed well down all over their cross-sectional area; otherwise the stress of compression will be taken on one part of the cross-sectional area, with the result that it will not be evenly disposed around the center of strength, and that will produce a bending stress. The bottom of struts or spars should be covered with some sort of paint, bedded into the socket or fitting and then withdrawn, to see if the paint has stuck all over the bottom of the fitting. 4. Do not trust to the angle of the socket being correct when the niacliine is rigged for the first time; and, as the planes are being adjusted, keep an eye on all sockets to insure that the edges do no damage the wood fibers. 5. The atmosphere is sometimes much damper at one time than another, and this causes the wood to expand and contract appreciably . Fig. 13. This would not matter but for the fact that it does not expand and contract uniformly but becomes unsymmetrical or distorted. This should be minimized by varnishing the wood well to keep out the moisture. This can be done with airplane dope, which is very good for the purpose. 6. Sometimes, for lightness, a fitting is bolted onto the end of a strut, and fabric is wound round the end to prevent the strut splitting. The funclion of interplmie struls. — These struts have to keep the planes apart and they must also keep them in their correct attitude. This is only so when the spars of the bottom plane are parallel to those of the top. The chord of the top plane must also be parallel with the chord of the bottom plane. If that is not so, then one plane will not have the same angle of incidence as the other. You may think that all you have to do is to cut all your struts of the same length, but that is not the case. Sometimes, as illustrated in the diagram, the rear spar is not as thick as the main spar. It is then necessary to make up for the lack of thickness by making the rear struts correspondingly longer. If that is not done, the top and bottom chords will not be parallel and 80 AIB SEKVICE HANDBOOK the top and bottom planes will have different angles of incidence. Also, the sockets or fittings or even spars upon which they are placed sometimes vary in thickness and this must be offset by altering the length of struts. The proper way to proceed in order to make sure that everything is right is to measure the distance between the top and bottom spars on each side of each strut, and if that distance or "gap," as it is called, is not as specified in your rigging diagram, make it correct by changing the length of your strut. "WTien measuring the gap between the top and bottom spars, always be careful to measure from the center of the spar, as it may be set at an angle and the rear of the spar may be considerably lower than its front. Wires. — The following points must be carefully observed where wire is concerned: 1. Quality: It must not be too hard or too soft. An easy practical way of learning to know the quality of wire is as follows : Take three pieces of wire all of the same gauge, and each about a foot in length; one piece should be too soft, another piece should be too hard, and the third piece of the right quality. Fix them in a vise about an inch apart and in a vertical position, and with the light from a ^vin- dow shining upon them. Burnish them, if necessary, and you will see a bar of light reflected from each wire. Now bend the wires over as far as possible; where the soft wire is concerned, it will squash out at the bend and you will see this because the bend of light will have broadened out there. In the case of the wire which is too hard, the bend of light will be broadened out very little at the turn, but if you look carefully you will see some little cracks or roughnesses on the surface. In the case of the wire of the right quality, the bend of light may have broadened out a very little at the tiu^n, but there will be no cracks or roughnesses in it at all. By making this experi- ment two or three times, you will soon learn to know good wire from bad and also learn to know strength of hand necessary to bend the right quality. 2. Wire must not be damaged; that is to say, it must be unkinked, rustless, and imscored. 3. As regards keeping vnxe in good condition, where the outside wires are concerned, they should be kept well greased or oiled, especially where bent over at the ends. This does not mean that large bits of grease must be left on the wii-es, simply that there should be a film of oil. In the case of internal bracing -n-ires, which can not be reached for the purpose of regreasing them, you will prevent them from rusting by painting them with white-lead paint. You must be very careful to see that the wire is perfectly clean and dry before painting with white-lead paint. A greasy finger mark is sufficient to stop the paint from sticking to the wire. In such a case, there will ATR SERVICE HANDBOOK. 81 be a little space between the paint and the wnre. Air can enter there and cause the wire to rust under the paint. The paint should be of a light color so as to show any signs of rust. 4. Wires and cables may be stream lined by binding onto them a V-shaped faring made of spruce. The wire and faring can be then painted to decrease the resistance. For single wires, this is not much of a gain, as a semicircle or triangle is a bad form of stream line. For the duplicated flying wires it is an advantage, as it prevents them vibrating separately and thus helps to decrease the head resistance. In any case, wires where they cro.s3 should be joined together by threading them through a little stream-line fiber washer. Tension of wires.— The tension to which you adjust the wires is of the greatest importance. All the wires on the airplane should be of the same tension, otherwise the airplane will quickly become dis- torted and fly badly. As a rule, the wires are tensioned too much. The tension should be sufficient to keep the framework rigid. Any- thing more than that spoils the factor of safety, throws various parts of the framework into undue compression, pulls the fittings into the wood, and will, in the end, distort the whole framework of the air- plane. Only experience will tell you what tension to employ and assist you in making all the wires the same tension. Learn the con- struction of various types of airplanes, the work the various parts do, and cultivate a touch for tensioning wires b)' constantly handling them. While at rest the landing wires will bear more strain than the flj'ing wii'es on account of the weight of the plane. The opposite happens when the machine is in the air. If the fl\ing \^'ii'es are trued up slackly, the whole rigging of the airplane alters directly the ma- chine gets into the air. In some cases you will find wires having no opposing wires pulling in the opposite direction. In such cases, be extremely careful not to tighten such wires beyond taking up the slack. If care is not taken, the incidence of the plane will be changed, resulting in change of both lift and drift at that part of the plane. Such a condition will cause the machine to lose its direc- tional stability and also to fly one wing down. I can not impress this matter of tension upon you too strongly. It is of the utmost importance. W^hen you have learned this and also learned to be accurate in getting the various adjustments you are on the way to becoming a good rigger. Wire loops. — Wire is often bent over at the end in the form of a loop. These loops, even when made perfectly, have a tendency to elongate, thus spoiling the adjustment of the wire. Great care should be taken to minimize this as much as possible. The rules to be observed are as follows: 32 AIR SERVICE HANDBOOK. 1. The size of the loop should be as small as possible within reason. By that I mean that it should not be so small as to create the possi- bility of the wire breaking. 2. The shape of the loop must be symmetrical. 3. The loop should have good shoulders in order to prevent the ferrule from slipping \vp. At the same time the shoulders should have no angular points. 5. The ferrule should fit the cable; if it is too large the loop will slip anyhow. 6. When the loo]) is finished it should be undamaged and should not be, as is often the case, badly scored. 7. Wires up to 12 gauge should be bent easily by hand with the aid of a round-nose plier. The bending should be done firmly and quickly. This is quite a knack with the larger sizes of wire. «-»»der SC'o-V Fig. 14, Stranded wire cables. — There are two kinds of cable used in rigging- airplanes, one of which is much harder than the other. The loops on the first are usually made by serving the cable with wire and then soldering. Loops on the latter, the softer wire, may be made by splicing. When serving the cable with wire the winding must be even, with a nice stream-line effect at the end of the winding. When solder is used care must be taken that the flux does not go beyond soldered portion of cable. Only nonacidfux should be used in solder- ing. The length of the served portion should be at least fifteen times the diameter of the cable as shown in diagram. If the cable is spliced, every strand must take its proper share of the strain. Sharp turns should be avoided. When hammering the splice, a sharp or too hard an instrument should not be used, as this is liable to injure the strand. No splice should be served with twine until it has been inspected and passed by whoever is in charge of the shop. Only the very end of the splice should be served, asthia AIR SERVICE HANDBOOK. 88 is only intended to prevent the short ends of wire from coming away from the strand. Stranded cable when overstrained nearly always breaks just above the splice. Thimbles of soft metal should always be used with stranded cable. Should a strand become broken, then the cable must l)e replaced by another. ( 'ontrol cables have a way of wearinf? oul and frayinjj; ^^ henever ihey pass over the pulley. Every time an airplane comes down from a fli2;ht the rifi;ger .'ihould carefully examine the cables wherever they pa.ss round pulleys, and if he finds a strand broken he should rejjort the fact at once. The aileron bal- ance wire on top of the top ])lane is often forgotten, since it is necessary to fetch a high pair of steps in order to examine it. Do not neglect this. Both wires and cables are liable to be damaged where they cross; to prevent this, a little block of fiber, stream-lined, is threaded on to the cables so as to prevent them touching. The wires of the interior of the machine may be wrapped with adhesive tape but as this collects moi.sture it is not a good thing to use this tape on those cables exposed to th(> air. ^ u-^^^-^M Mummimmimm Fig. lo. All the cables should be stretched before fitting and should l)e well greased where they run through pulleys or fair-leads. Fair-leads should be made of rawhide, rather than of metal, as rawhide gives less wear. When a cable is being inspected to see if it is frayed, all the old oil must be wiped off and the fingers should be gently run over the suspected place. Any broken strands will be easily felt. Sometimes the inner strands are broken, and this may l)e found out by very gently bending the cables liackward and forward. Cables may be cut by heating (juickly in a l)low torch flame. This makes the wire soft and also keeps the ends from fraying. Care must be taken not to let the heat travel far down the cable, and this can be done by holding cable in metal to conduct away the heat. Stream-line wires. — Stream -line wires are made by rolling steel rods. They are screw threaded at either end with a left and right handed thread. These rods are cut to length for each machine so that if one breaks a special rod has to be obtained in order to rei)lace it. The ends of these rods fit into special Y-shaped fittings, which are also left and right hand threaded to fit onto the ends of the rod. The rods also carry a locking nut at each end so that when the 46648—18 3 34 AIR SERVICE HANDBOOK. required adjustment is made, the wires can be fixed there. The end fittings are pinned to the wiring plates on the planes l)y means of special steel pins. There is a small hole, aljout halfway down each fitting, and to be safe the end of the wire must pass this hole. There is another kind of fitting in which the wire is screwed through a little metal rod. This latter is a l)etter method, as it allows the wii-es to vibrate without putting any side strain on them. The wires must lie in the flow of air in the same manner as do the struts. By using stream-line wires the lift of the machine is increased appreciably and the speed a little. When stream -line wires are overstrained they tend to draw out just above the screw-threaded portion, and this can easily be felt by running the fingers down the edge of the wdre and can be seen. Stream-line wires should he kept bright and slightly oiled. Tension rods. — These are rods used in the construction of many fuselages, and the same rules apply to them as to stream-line wires, except that they are round and can !)e locked in any position. ^ Turns anc^ pu// Jiyh ^<3nd Af&at, Yui. Itj. TunibucHes. — A turnbuckle is composed of a central barrel, into each end of which is screwed an eyebolt. The bolts at either end are screw threaded left and right handed. Wires are taken from the ends of the eyel)olts, and so l>y turning the barrel the wires can be adjusted to their proper tension. Eyel>olts must be a good fit in the barrel; that is to say. not slack and not very tight. There is a rule that the eyebolts must V)e screwed into the l)arrel for a distance of not less than thrice their diameter, liut it is better to screw them in a good deal more than that. If the eyebolt is screw threaded for only a short distance, the bolts should be screwed into the barrel till the last thread is flush with the end of the barrel unless otherwise stated. The turnbuckle should not be tightened so that the ends of the eye- bolts meet in the middle. If this happens, new cables must l)e fitted. Turnbuckles are chosen of a size corresponding to that of the cable used with them. The l)arrel of the turnbuckle looks solid but is really hollowed out and is much more frail than it appears. For that reason it must not Ite turned l»y soi/.ing it with pliers, as that AIR SERVICE HANDBOOK. 35 may distort it or spoil the bore. The proper method Ls to pass a piece of wire through the hole in the center and to use that as a lever. The eyebolts may be prevented from turning by holding them on the ends of another piece of wire. When the correct adjustment is obtained, the turnlmckle must l)e locked to prevent it from unscrew- ing. It is quite possible to lock the turnbuckle in such a way that it allows it to unscrew a quarter or half turn, and that will throw the wires out of the very tine adjustment necessary. The proper way is to use the locking mres in such a way as to oppose the tendency of the turnbuckle to unscrew, as is shown in the diagram. The wire used for locking a turnbuckle is hard copjjer wire or soft iron wire. Turnbuckles on internal wires must be well greased and served round with adhesive ta])e after they have been locked. On no account may the barrel of a turnbuckle be sawed off short. In case of a forced landing, it may be necessary to mend the machine from materials at one's disposal locally, and it is useful to know a little about the materials used in the construction of a machine. .MET.M.. 1. There should be no signs of rust or Haws. 2. Only bright bolts ai^d nuts should be employed. In idr])lane construction, bolts, nuts, and pins are made of special steel and those obtained locally should be looked on with susi)icion as thev are almost certain to be too weak. 3. Piano wire should not have been previously l)ent and must be free from kinks. 4. Stranded wire or cable should be regularly twisted and not frayed at any point. 5. Tubing should he perfectly straight and should not show signs of having been previously' bent and subsetiuontly straightened. Tubing is ustially welded into its sockets, but if there is much vibra- tion tubes should l>e attached to the sockets by being pinned and then soft soldered. In case an axle becomes bent, it can be mended temporarily by being straightened and by having a wood iiif-hi core put in. This should bring the machine home. 6. Threads of bolts, nuts, and screws should be clean and not worn or burred. Make certain that these are screw threadcil on the same system and that they have the same numl)er of turns to the inch. 7. Strut sockets and other metal fittings should not be l)eut out of their original shape. Such fittings should not be used if they show signs of having been bent and subsetjuently straightened. 36 AIR SERVICE HANDBOOK. In the case of aluminum sockets, care must be taken that there are no cracks, especially where the sockets have previously been subjected to severe strains. Eyeplates and eyebolts should show- no signs of wear or fracture. Wiring plates can be replaced tem- porarily by those made from mild steel. Allow plenty of metal so as to insure the plates being strong enough. This again must be only a temporary measure. The properties of iron are described under "Engine material. " 8. All metal fittings should bear the inspection mark before being used on an airplane. Any fitting which has been subjected to a strain should be inspected by a qualified officer or returned to the salvage section, and this latter applies to all airplane material. 9. No bolt, pin, or turnbuckle should be used if it has been bent. The correct wood for the various parts of an airplane must be used. The wood used must have a good clear grain, with no cross grain, knots, or shakes. Such blemishes mean that the wood is in some places weaker than in other places, and, if it has a tendency to bend, then it will go at those weak points. All wood must be properly seasoned. Struts, spars, etc., must be straight and un- damaged. When a bending stress comes on one of these members the outside fibers of the wood are doing by far the most work. If these get bruised or scored, then the strut or spar, suffers in strength much more than one might think at first sight, and, if it ever gets a tendency to bend, it is likely to go at that point. The two woods most generally used in airplane construction are ash and spruce. Spruce is the strongest wood for weight that grows. Ash is a very strong but heavy wood, but it is very good at resisting sudden shocks and will bend considerably before breaking. As a general rule, spruce is used for the main spars, struts, compression ribs, and the flanges of form ribs. Ash is used in the longerons, in the under- carriage struts, in skids, and in the three-ply webs of form ribs. It is also used for engine bearers, if metal is not employed. No spar, strut, etc. should be bored in any place where a hole was not designed. If any tiring is to be fitted to a spar, it should be clipped round it with a suitable clip. Holes in wood should be of a size that the bolts can be pushed in, or at any rate not more than gently tapped in. Bolts must not be hammered into wood, as doing this splits the wood. On the other hand, a bolt must not be slack in a hole, as it works sideways and may thus split the s])ar, not to speak of throwing out of adjustment the wires leading from the lug or AIR SERVICE HANDBOOK. 37 socket under the bolthead. As has been before stated, all wood should be well varnished so as to prevent damp creeping in and caus- ing expansion or contraction of the fibers. In case of emergency, where the proper wood is not obtainable, make certain that the wood used is sufficiently strong to carry the strain. Nature of wood under stress. — Wood for its weight takes the stress of compression best of all. For instance, a walking stick of about half a pound weight will, if kept perfectly straight. ])robably stand up to a compression stress of a ton or more before crushing, whereas if the same stick is put under a bending load it will ])robably collapse to a stre.ss of not more than 50 pounds. That is a very great difference, and since weight is of the greatest importance in an airi>lane the wood must as far as possible be kept in a state of direct compression. Splicing wood. — In the case of a fracture occurring in a solid spar or one of the box type that is wide enough (2 inches) it is often possi- ble to make a good repair by scarfing on a new length. The scarf must be long compared to the dejjth of the spar and the two pieces of wood forming it must be a good fit onto each other. After fitting the two halves of the scarf together they must be well glued and then clamped together till the glue is set. The joint is then planed up and examined to see that it is a close one and does not have a thick layer of glue between the two thicknesses of wood. The two halves of the scarf are then bolted together as an additional pre- caution, large washers being employed under the bolthead and nut so as to prevent them from cutting into the wood and crushing it when tightening the nut. A waxed whipcord lashing is then served round the joint, each turn of the cord being securely knotted to prevent it coming adrift. The cord may be glued finally as a further precaution. STRESSES AND STRAINS. In order to rig a machine intelligently it is necessary to have a correct idea of the work every wire and every part of the airplane is Fig. 17. doing. The work the part is doing is known as stress. If owing to undue stress the material becomes distorted then such distortion is known as strain. 38 AIR SERVICE HANDBOOK. Compression. — The simple stress of compression produces a crush- ing strain. As an example the interplane and fusilage struts. Tension. — The simple stress of tension results in the strain of elongation. As an example all the wires. Bending.— The compound stress of bending is composed of both tension and compression. Now we will suppose we are going to bend a piece of wood. Before being bent it will have the following appearance: You see that the top line, the bottom line, and the center line are all of the same length. Now we will bend it right round in a circle, thus The center line is still the same length as it was before being bent, but you will note that the top line being on the outside of the circle must now be longer than the center line. That can only be due to the strain of elongation. That is produced by the stress of tension. So you see that the wood be- tween the center line and the line on the outside of the circle is in tension. The greatest ten- sion is on the outside of the circle because there the elonga- tion is greatest. You will notice that the line on the inside of the circle which before being bent was the same length as the center line must now be shorter because it is nearest to the center of the circle. That can only be due to the strain of crushing. That can only be produced by a state of compression. So you see that the wood between the center line and the inside line is in compression and the greatest compression is nearest to the inside of the circle because there the crushing effect, i. e., the strain, is greatest. By this you will see that the wood near the center line is doing the least work. That is why it is possible to hollow out the center of spars and struts without unduly weakening them. In this way 25 to 33 per cent of the weight of wood in an airplane is saved. Shear. — Shear stress is such that when the material breaks under it one part slides over the other. As an example the locking pins. Some of the bolts are in a state of shear stress also because in some cases there are lugs underneath the boltheads from which wires are taken. Owing to the tension of the wire the lug is exerting a side- ways pull on the bolt and trying to break it in such a way as to make one part of it slide over the other. Torsion. — The stress of torsion. This is a twisting stress composed of compression, tension, and shear stress. As an example the pro- peller shaft and crank shaft of an engine. \ («-hS c«'>- A G- rtal t< t- \ 1 \*- CPV.LI ■»,. cl '- vj. c„ >t< h<<.»i . « ^ y G-rto.\ re»(- clr h o ilV t J^ o. y Ct 11 r< Fig . LS. AIR SERVICE HANDBOOK. 39 Washers. — Under the bolthcacl and also under the nut a washer must be placed. This should be a very large wa^sher comj)ared \vdth any other form of engineering. This is to disperse the stress over a large area of wood; otherwise the washer may be pulled into the wood and weaken it, besides jiossibly throwing out of adjustment the wires attached to the bolt or fitting. Locking. — As regards locking the bolt.^! if split pins are used be sure to see that they are used in such a way that the nut can not possibly unscrew. If a castellated nut is used the pin should fit the hole in the bolt so that it will not be sheared by the edge of the nut. If a plain nut is used washers should be put under the nut so that it reaches exactly to the bottom of the hole in the bolt or the nut may be filed a little bit to allow it to go down past the hole. If a bolt is locked by burring over the end a heavy hammer must not be used in order to try and spread the whole head of the bolt. That might damage the woodwork inside the plane. Use a small, light hammer and gentlj- tap around the edge of the bolt until it is burred over. All bolts and nuts should be locked in a positive manner. A split-ring washer does not lock a nut in this manner and should not be used without some other form of locking as a rule. IIANDUNC OF .\lin'],.AM;.S. An extraordinary amount of damage is done bj' the mishandling oif airplanes and in blocking them up from the groimd in the wrong way. The golden rule to observe is "Produce no bending stresses." 1. Remember that nearly all the wood of an airplane is designed to take stress of direct compression and it can not be safely bent. In Vilocking an airplane up from the ground the packing must be used in such a way as to come underneath the interplane struts and the fuselage struts. Soft packing should always l^e i)la( cd on the })()ints upon which the airplane rests. 2. When pulling the m.achine along the ground, ahvays, if possible, pull from the landing gear. If it is necessary to pull from ekewhere do so by grasping the interplane struts as low down as possible. 3. Never lift or put any strain on the leading and trailing edges of the planes and do not cover them with oily finger marks. 4. As regards handling ])arts of air])lanes, never lay anything covered with fal)ric on a concrete floor as any slight niovtUK-nt will cause the fabric to scrape over the concrete with resultant damage. 5. Struts, spars, etc., should never be left about the floor, as in such a position they are likely to become damaged: and I have already explained how necessary it is to protect the c)Utsi('e fibers of the wood. Remember also that wood easily }>ecome8 distorted. This particularly applies to the interplane struts. The best method 40 AIR SERVICE HANDBOOK. of storing struts is to stand them uy) in as near a vertical position as possible. 6. When lifting an airplane as might have to be done when the landing gear is In'oken, it is convenient to lift the machine by putting one's shoulders under the main spar and under an interplane strut and lifting with one's back. When lifting the tail of a machine lift under one of the fuselage struts just in front of the tail plane. The best place is usually marked by an arrow. 7. Planes kept temporarily in the hangars should be kept slung in broad bands of webbing. A 2-inch batten should be threaded through the loops of webljing and the leading edge of the plane should be placed on this to prevent its being distorted. 8. Planes packed in the wing trailer should he supported under the compression ri))s on pieces of felt. These pieces of felt are made to fit the caml)er of the plane on both top and l)ottom. 9. When a ma'^'hine is standing in the sheds the weight of the machine should be taken off the shock absorbers. This is con- veniently done by placing the landing gear on lilocks of wood. The tail should he supported on a trestle in a proximately the flying position. This takes the weight off he tail and prevents the fuselage being in a continual state of stress. KEEPING .\X .\IRPLANE IN GOOD ONDITION. Cleanliness. — The fal)ric must be kept clean and free from oil, otherwise it will rot. To take out dirt or oily patches try acetone. If that will not do try gasoline. Both^acetone and gasoline should be used with caution as they both have an effect on the dope and varnish. The best way to keep the planes clean is to use soap and warm water, but in that case be sure to use a soap having no alkali in it as otherwise it will badly affect the faln-ic. Use water sparingly or it may get insiefore the nevt is applied and before applying each coat the plane should be rubbed down well with a piece of fabric. This is to remove all roughness and imparts a good polish to the plane. On no a'.'count use anything rough in the nature of sandpaper and do not apply too much pressure so as to make the fabric slacken off. Two (■oats of varnish evenly applied completes the plane. Doping and varnishing should be done on a dry day and in a hot room, tem- perature about 70°. If the room is too cold the varnish will dry with all the brush marks showing, but if the room is kept warm the varnish dries much slower and the brush marks have time to even out. Dope and varnish should be applied with a flat, 4 inch camel 's- hair brush. If the room is damp or if the fabric is damp the dope dries in white patches instead of without color, as it ought to do. In some planes the fabric is adjusted on the bias; that is, the seams run diagonally across the plane instead of straight. This is supposed to l)e stronger and to ])revent the fabric tearing in case it were damaged. Redoping. — It is sometimes necessary to redope a plane or part of a plane. First of all, take off all the varnish with a suitable solvent. This is quite easily done and does not take long. Rub into the old dope fresh doj)e which has been thinned with acetone. This should l)e rubbed well in, so as to soften the old dope. Do not attemj)t to remove the old dope wdth acetone, because the acetone will probably dry out before the dope is properly softened, and acetone tends to make the fibers of the linen brittle. After the old dope has been softened by two coats of the thin dope two coats of ordinary dope should be applied and then two coats of varnish, as is done when doping the plane for the first time. 46 AIR SERVICE HANDBOOK. If a plaue is damaged in any way care should be taken to insure that none of the internal parts are damaged. It is sometimes neces- sary to make a comparatively large cut in a plane to insure this, although the original damage is slight. If a cut has to be made it should be made in a fore and aft direction. In repairing or patching a wing, first examine the tear and judge for yourself whether it should be sewn up or whether the damaged portion should be cut out, taking into account the condition of the fabric, the edges of the tear, and the part of the wing affected. If the damage be a clean cut running straight with the thread of the fabric the edges are drawn together with the herringbone stitch. The varnish is now carefully removed with a suitable solvent and the hole is covered with a patch square or rectangular, three-quarters of an inch larger than the tear. The edges of the patch are frayed out to make it stick better and the patch is stuck on with dope. \'arnish on a plane prevents the dope sticking. A second patch is then doped over the first. This also should be frayed, the fraying being a quarter of an inch deep. Fraying allows the dope to get a firm grip of the edges of the fabric and makes it a better stream line; a small gain, but in the aggregate worth considering. The threads of the fraying must run all parallel to each other and must be fiat with the first coat of dope. All patches should have three coats of dope and two of varnish, these being applied as described above. For sewing in a patch cut the damaged portion out in the form of a rectangle and slit the corners the length of an inch. Turn in the fabric as this gives a firm edge to which to attach the patch. Cut the patch the size of this opening and allow half an inch for turning in the edges. Make it fit the hole tightly and then sew it in with the herringbone stitch. The sewing must be perfectly regular, taking care that the stitches are all the same length and distance apart, as the strength of the patch depends on the regularity of your sewing. The patches are put on tight to pull with and match the cover of the wings, which is well strained on in the first place. The corners are often sewn very badly. When you get within a good length stitch of the corner the outside stitches must wheel round regularly, while those on the inside are much closer together, or so to say, marking time. Remember that the corner of a patch is always the weakest part. Nothing is gained by making the patch circular, because the edges must be turned in, and this in itself forms a corner. Dope this patch, put on a second three-quarters of an inch outside all stitching and then another tliree- quarters of an inch larger than this. Fray patches and dope on as described above. If you have a small patch to go on with a few AIR SERVICE HANDBOOK. 47 stitclies underneath it may l>e put on with varnish insteail oi dope, l)ut this does not apply to patches which have to be sewn in. Storing fabric. — It is essential that fabric should be kept quite dry and clean, and it should therefore be stored in a dry place. If moisttire is present when the fabric is being doped the dope will not ])enetrate })roperly and turns white. Planes when covered should be left in the doping room some time before the dope L's applied so as U^ attain the temperature of the room. Doping room.— In order to insure good results in doping it is of vital importance that a special doping room be provided and great attention [)aid to tlie maintenance of a uniform temperature, ^'aporp from dope are heavier than air, so that outlets should be provided near the floor to extract the bad air from the room. Fresh air out- lets should be provided high up in the wall opposite. Incoming air should pass over hot pipes so that the room may be maintained at a uniform temperature of 65° to 70*^. At the front the nearest approach to this will l)e a tent or hangar heated tolerably by a brazier so that doping should be done on a sunny day when the sun has had time to dry up the atmosphere. Storing of dope. — I)o])e is affected by the ultra-violet rays of the sun. so that it should be kept in light-tight vessels. Dope should not be stored for more than three months and should be used as sup- plied. Special solvents are used for loosening stuck stoppers and for washing brushes. Brushes may be kept immersed in dope and the whole covered by an inverted jar stuck to the table with dope, thus making an air-tight joint. Do])e should be stored in a room at a temperature of about 60°. Mi'lhofI of apphjing rlope. — The brush should be well dipped into the dope, but care must be taken that drops of dope are not allowed to fall on the fabric as the lirush is being carried to the point of work. The dope must I)e applied to the fabric with a smooth, backward and forward motion and air bubbles must not be formed. The first coat must be rubbed well in, particular care being taken on the parts covering the woodwork in order to make the fabric adhere to the wood. The dope must penetrate well through the fabric in order to "rivet" the coating to the fabric and prevent it peeling. In cases of doping schemes where a thin first coat is provided it will not be ne -essary to rub this coat in. lie.'ause this may cause drops to form on the inner surface of the fabric. After the final coat of dope has been applied the plane should be left as long as possible, preferably about 12 hours before the varnish is put on. In some dopes the last coat contains the pigment, so that only one coat is necessary and can 48 AIR SERVICE HANDBOOK. be put on at once. Only upper and vertical surfaces are covered with pigment. The lower surfaces are covered with transparent varnish. Identification marks should be painted on immediately after the last coat of dope and are covered with the last coat of transparent varnish. Defects. — White patches are due to moisture in the air or fal^ric or to the doping being done at too low a temperature. With dopes in which acetone substitutes are used white patches frequently occur in the first ct)at, but should disappear when a second is applied. Blisters are due to doping being done at too high a temperature. Patches refusing to dry with formation of blisters is due to faulty dressing of the faljric, proba])ly traces bf soap. • Cracks appearing in circles may be spots of new mold in the fabric. Circular cracks also appear if two coats of copal varnish are applied. Sunlight on unprotcted dope causes it to deteriorate and crack . Yellow patches appearing some time after the doping are probably due to dressing left in the faljric. Sagginess is due to moisture or sometimes to the doping l^eing done at too low a temperature. MARKINGS ON AN AIRPLANE. All airplanes should be marked as follows: One insignia should be placed on each end of the upper surface of the top planes and one on the under surface of the lower planes. The circumference of the circumscribed circle should just miss the wing flaps. The insignia consists of a five pointed star colored white with a blue circumscribed field; the center of the star is a red circle; the diameter of the circumscribed circle will he equal to the chord of the wing on which the insignia is placed. The diameter of inner circle will not extend to the inner points of the star l)y an amount equal to one twenty-fourth of the diameter of the circumscribed circle. The inner circle should be painted red and that portion of the star not covered by the inner circle will be painted white; the re- mainder of the circumscribed circle should be painted blue. The rudder should be painted in blue, white, and red vertical strips, the blue strip being nearest the rudderpost. The rudder should be marked with the machine's number in 3- inch letters on the top of the white baud. The sides of body of the airplane should be left free of all markings except such as shall be ordered to l)e carried in the field. One point of the star in each insignia will point to the front. AIB, SERVICE HANDBOOK. 49 IV. ENGINES, MATERIAL. Steel. — Steel is a form of iron containing a certain percentage of carbon and in some cases alloyed with small quantities of other metals such as nickel, chromium, vanadium, or manganese. The amount of carbon present and the treatment to which the eteel has been subjected determine its mechanical properties. The metal iron in a chemically pure state is only found in a chemical laboratory, but a good commercial wrought iron is reason- ably pure. Wrought iron contains a very low percentage of carbon (up to about 0.25 per cent). It is ductile (it can be stretched), comparatively soft and fibrous in structure. Steel contains about 0.25 (o abi retain their magnetism. Hard ca.st iron retains magnetism fairly AIE SERVICE HANDBOOK. 91 well, l)ut can not ho inagnotized to the samo extent as steel. The special magnet steels for permanent magnets iiermit of a high degree of magnetism and retain their magnetism to a remarkable extent. The armature of a magneto is composed of a xcry soft steel which is easily magnetized and demagnetized. (JoKt iro7i. — The relatively large amount of carbon ci)ntained n cast iron (2 to 5 per cent) may be divided into two parts: that con- tained in the iron as a mechanical mixture in the form of graphite and tliat chemically combined with the iron. The Indk of the carbon present is in the form of graphite held in the j)()res of the iron. In structure, therefore, cast iron resenil)les a sponge of iron with the interstices tilled with grai)hite. The })resence of grai»lute obviously weakens the metal but it confers a special ])roperty which is extremely useful. (Iraphite is a lubricant so that cast iron may be looked upon, to a certain extent, as a self-Iuliricating metal. In ])ractice it is found that cast iron surfaces run extremely well together in machinery. In practically all engines, with the exception of aero engines, the cylinders and piston rings are made of cast iron, and in internal-combustion engines it is usual to make the pistons also of cast iron, but since this metal is relatively weak, cast-irou ]nstons and cylinders must be relatively heavy so that in aero engines the pistons and fre(iuently the cj'linders are made of steel or alumi- nium alloy. ( ast-irou piston rings are, however, nearly always used. Where steel cylinders are in use, they are sometimes fitted with cast-iron liners. ( itst iron is a comparatively brittle metal, about half as strong as steel. It can not be hardened and tempered as in the case of steel, but very hard chilled castings may be obtained by using a special kind of mold. Soft or 'malleable" iron castings are ordinary castings, which have been heated for a considerable period in contact \vith an iron oxide (red hematite). Cast iron is comparatively strong in compression and weak in tension so that no meml)er of an engine which may come under tension is composed of cast iron. Copper is a soft metal of extreme ductility. It is the best con- ductor of electricity with the exception of silver only. It is there- fore used for electrical connections, magneto windings, etc. It is easily deposited or "plated" in an electrolitic bath, and in some aero engines the water jackets are formed of cojjper applied in this manner. Copper as opposed to iron becomes brittle after being heated, but may be made soft again by being '"worked." Brass, an alloy of copper with zinc, usually a])out two of copper to one of zinc (by weight). It is very easily machined and casts well. 62 AIR SERVICE HANDBOOK. It is about the same strength as cast iron and can be made hard and springy by rolling. It is used for obdurator rings, small parts of car- bureters, magnetos, etc. Bronze. — Bronzes of various composition are used as bearing bushes, tappet guides, small gear wheels, etc. Steel runs very well on bronze and the wear is not excessive. Ordinary bronzes are alloys of copper and tin, about 80 to 90 per cent of copper and 20 to 10 per cent of tin. A large percentage of tin giving a hard and more brittle metal. Ordinary bronzes also are not much stronger than brass, and phosphor bronze, containing about 2 to 4 per cent of phosphorus, is nearly as strong as steel. Phosphor bronze is also one of the best metals for bearing bushes. White metal. — Connecting rods, bearings, and crank-shaft bearings are lined with white metal (if they are not of the ball-bearing type). White metal is composed of tin, copper, and antimony in varying percentages. A typical case is: Tin, 90 per cent; antimony, 7 per cent; and copper, 3 per cent. This gives a metal of low melting point, which is fairly hard and runs well on steel. In the event of a bearing tending to seize up, the heat generated will be sufficient to melt the white metal, and if the engine is immediately shut down no further damage will result. Aluminum and aluminum, alloys. — Aluminum weighs about 160 pounds per cubic foot and cast iron about 450 pounds per cubic foot. Cast iron is therefore about three times as heavy as aluminum. Pure aluminum is not so strong as cast iron, and it is often alloyed with heavier metals with the object of increasing its strength. Crank cases, gear boxes, and various fittings of aero engines are usually made of aluminum alloy, and in some cases the pistons are made of this metal . One of the difficulties in the use of aluminum for parts that are exposed to the high temperature of burning gases is that it has a comparatively low melting point (about 1,100° F.) and becomes mechanically weak when raised to a high temperature. For this reason the heads of aluminum pistons are well supported by means of internal webs, and in some cases also by an internal pillar resting on the gudgeon pin through a slot in the top of the connecting rod's small end. These webs, etc., also help to conduct away the heat from the piston head and so further reduce the risk of collapse. Another difficulty arises from the fact that under the influence of heat aluminum expands at nearly twice the rate of iron, thus necessi- tating a large clearance between the piston and the cylinder. It is not possible to solder aluminum in a satisfactory manner. Ordinary AIR SERVICE HANDBOOK. 68 solder is quite useless, and the special solder sometimes recommended requires special treatment and generally gives very poor results. Notes on distortion. — All metals expand under the influence of heat. The amount of expansion is proportionate in any metal to the increase in temperature but differs for different metals. In the case of aero engines the Working temperature is very high, owing to the high compression used, the high speeds at which these engines run, and the absence of large masses of metal which would help to conduct away the heat. If the piston, cylinder, and Tahos, and, in the case of water-cooled engines, the Water jackets. Were made of the same metal or of metals expanding at the same rate, and if they Were all raised to the same temperature expansions would give no trouble. In practice, how- ever, not only are the parts made of different metals but they work at dift'ering temperatures with the result that uneciual expansion and subsequent distortion takes place. The hottest part of an engine is the exhaust valve, but as this is a small symmetrical part, distortion is small. The exhaust-valve seating, however, will probably be hotter on one side than the other, and in a badly designed engine the distortion will be so great as to prevent the valve seating properly wlien the engine is running on low throttle. The inlet valve is the coolest part of an engine, as it is in the path of the cold incoming mixture, and in some types of engines it is placed as close as jjossible to the exhaust valve with a view to keeping the temperature down. In some engines where the water jacket is of mild steel or copper sheet circumferential ribs or corrugations are made in the jackets in order that they may more easily follow the expansion of the cylinders. The piston head becomes very hot, as it can lose heat only by conduction through the skirt to the lower part of the cylinder wall and through the gudgeon pin to the con- necting rod. The net result of distortion is that in practice clear- ances have to be made larger than would otherwise be necessary. Fatigue of metals. — Metals Which have been subjected to repeated stresses, such as those caused by vibration, become fatigued, their internal structure changes, and they are permanently Weakened. The amount of fatigue depends upon the range of the stress or load, the number of times the material is subjected to the stress, and the rate at which the stress is applied . The prolonged application of varying stresses very much smaller than the normal breaking stress of the material will induce fatigue and eventually bring about fracture. 64 AIR SERVICE HANDBOOK. V. THE GASOLINE MOTOR. Introductory. — The object of a motor is to produce rotary motion either in itself or in a shaft. To get this motion the motor must be provided with — (a) A piston which must be free to move up and down within a cylinder. (6) A rod attached to the piston termed a connecting rod. (c) Attached to the other end of the connecting rod a crank shaft. (d) Attached to the crank shaft a flywheel or its equivalent. The action of the motor is similar to the operation performed by a man turning a grindstone. The stone corresponds to the fiy wheel of the motor, the handle to the crank shaft, the man's arm to the connecting rod, and the power exerted in turning the stone to the exploded charge. Power can not be produced without a cause. One of the most effectual methods of producing power is the expansion of gases. If a substance such as gunpowder is exploded in a cylinder with an open end (a gun for example) practically the whole effect of the explosion is felt at the muzzle; and if a bullet is placed in the gun in front of the gunpowder it is blown out wath great force. This is exactly what happens in the gasoline motor — a mixture of gasoline vapor and air is ignited within the closed end of the cylinder and the force of the explosion drives the piston in front of it. The piston in moving down the cylinder carries the connecting rod \^-ith it and the latter in its turn commimicates its motion to the crank and so to the flywheel. The flywheel once it has started rotating will carry on its motion for an appreciable time without any further application of power. Consequently it will communicate its motion to the crank and so to the piston, pushing the latt(?r uj) the cylinder again. At the same time by forcing the piston upward the burnt gases are expelled from the cylinder through a suitable port or valve and by an arrangement to be described later. By the action of the flywheel the piston will again descend, traveling along the same path as it did when the mixture was exploded, but this time the piston is dragged instead of being pushed. Immediately the dragguig motion begins the port through which the burnt or exliaust gases escape is closed and a similar port or valve leading to the mixture and inlet pipe is opened . The downward mo- tion of the piston, produces a partial vacuum at the head of the cyl- inder which results in a new charge of explosive mixture rushing into the cylinder through the port which has just been opened. Just after the piston reaches the bottom limit of its stroke this port closes. The piston is then pushed up the cylinder once more and the mixture is coTnprcssfMl. AIK SERVICE HANDBOOK. 66 It may here be noted that n-ithin certain limits the greater the compretision to which a mixture of gaHoline vapor and air is subject, the ciincker it will burn, and consecpiently the gi'eater will be the force of the explosion. When compression is at its highest, i. e., when the piston i.s on the point of reaching the top of its stroke, the mixture is ignited and the explosion occurs forcing the j)iston down. It will thus be seen that one explosion and consequently one power stroke occurs every two revolutions of the crank or four strokes of the piston. I'^or this reason the gasoline motor is described as working on the four-cycle principle. The four-stroke cycle can be summarized briefly as follows: (a) The suction stroke: The piston descends, inlet port or valve opens, and an explosive mixture of gasoline vapor and air is sucked into the cylinder. (b) The compression stroke: Just after the piston has reached the bottom of the suction stroke the inlet valve closes, piston ascends and compresses the mixture 'both inlet and exhaust valves being closed). (c) The power or working stroke: Just before the piston reaches the top of the compression stroke the explosion occurs and the piston is forced down again. (d) The exhaust stroke: Just before the bottom of the power stroke the exliaust valve opens. The piston ascends and the burnt or exhaust gases are forced out of the cylinder. Suction stroke. — The intake pipe is full of an explosive mixture of gasoline vapor and air. The intake valve is open just after the piston starts descending in the cylinder. That is when the crank is about 5° to 9° past top dead center. The piston descending draws this explosive mixture into the cylinder. As it is descending very fast it causes a partial vacuum in the cylinder which the incoming gases have not sufficient time to fill uj) till after the piston starts ascending in the cylinder. So the inlet valve is not closed till the crank has rotated to about 18° past the bottom dead center. In some fast-running engines this angle is very much bigger. The compression stroke. — As soon as the cylinder is as full of the explosive mixture as is possible and when the inlet valve is closed the piston still ascending the cylinder compresses the gases. At a variable point, normally about 2()° before the crank reaches the top dead center, the explosive mixture is ignited. The mixture takes an appreciable time to l)urn and it is ignited so that when it is com- pletely burnt the piston has finished its upward travel and is just starting to descend. This is called advancing the spark, and the amount of advance depends largely on the speed of the engine. 66 AIR SERVICE HANDBOOK. AIR SERVICE HANDBOOK. 67 The power stroke. — As soon as the piston starts descending in the cylinder the gases begin to expand and push the piston down till the crank reaches a point varying between 45° and 75° from the bottom dead center. This is the power or working stroke. The exhaust valve is now open and the gases rush out of the cylinder. This early opening is called "giving lead" to the exhaust valve, and it is found very advantageous, as it insures an effective escape of the exhaust gases and consequent absence of pressure against I g n i t i o M eo+tom Dead centre Fig. 21. the piston on its return stroke. If the lead given to the exhaust valve is insufficient the engine is liable to overheat. The exhaust stroke. — The exhaust valve remains open till the piston has passed to the bottom of the cylinder, ascended to the top and has just started to descend, that is when the crank has gone about 1° to 5° past the top dead center. The valve is closed when the piston is just past the top so as to insure that as much of the burnt gases have been cleared out of tlie cylinder as possible. There is now a very short space of time between the closing of the exhaust valve and the opening of the inlet valve. This is to make certain that the explosive mixture on entering the cylinder will not come in contact with the hot, burnt gases and so be ignited prematurely. 58 AIR SERVICE HANDBOOK. DETAILED DESCRIPTION OF THE WORKING OK THE GASOLINE MOTOR. Arrangement of valves. — The majority of motors have two valve.'* or ports for each cylinder, one to admit the explosive mixture and one to release the bmnt gases after explosion. The former is termed the inlet and the latter the exhaust valve or port. The most common arrangement for aero engines is that in which the seatings for the valves are placed in the head of the cylinder. In ordinary motor-car engines the tops of the cylinders are cast with small extensions to one side, and in each of these extensions is the circular seating on which the head of the valve rests. The valve itself consists of a mushroom- shaped head with a long, thin stem, the whole being made in one piece. The head has a beveled edge which fits closely onto the seating of the cylinder, being held down by a spring mounted on the stem. The bottom of the stem when the valve is closed and the engine is warm should be just clear of what is termed a ""push rod." The push rod itself is raised and lowered by means of a cam, and so communicates its motion to the valve. From the description of the cycle of operations it is clear that each valve must open and close once in every two revolutions of the crank. It will therefore be seen that the cams operating the valves must be worked at half the speed of the engine. This half-time speed is obtained by fixing to the crank shaft a gear wheel with, say, 16 teeth and providing the .shaft carrying the cams with a geai' wheel having 32 teeth. Then, when these two wheels are enmeshed and the engine is turning the cam shaft will be driven at half the speed of the crank shaft. Valves worked on this jjrinciple are called ■'mechanically operated valves.'' Exhaust valves are always mechanically operated. The necessity for this can be clearly seen, because at the moment it is necessary to open these valves they are being held tight .shut by the pressure of the gases in the cylinder. Inlet valves, on the other hand, are sometimes automatically oper- ated — that is to say, they are opened by the suction effect caused by the piston moving down the cylinder, the exhaust valve of course being closed. A light spring is fitted to the valve stem to bring it back onto its seating at the end of the suction stroke. The auto- matic inlet valve is not as a rule considered advantageous because it is extremely hard to balance all the springs exactly so that a differ- ent amount of mixture is sucked into each cylinder. This causes bad running. The necessity for sucking also prevents the cylinder from getting as much of the mixture as it would if the valve were opened mechanically. Owing to the very high pressure generated in the cylinder during the explosion it is very necessary that the \alves should be so AIR SEKVICE HANDBOOK. 69 designed that the pre.ssui*e due to compreHsion and explosion holds them on their seatings and so assists them to become gas-tight. For this reason valves are always designed to open inward. In some cases the inlet valve is placed close to and immediately oppo- site the exhaust valve so that the inlet gases pass over the exhaust valve and tend to keep the latter cool. N'alves are "timed" by setting them to oj)en and close when the l)iston is a certain distance down the cylinder or when the crank of the engine is at a certain angle. All valve settings must be taken with the engine turning in the ahead direction, so as to avoid any errors due to play in the various gear wheels, etc. If the engine be turned too far ahead past any particular setting, turn it back more than the amount required before starting to take the readings again. In order to obtain the direction of the revolution of an engine, turn it by hand. The inlet valve wdll open directly after the exhaust valve closes if the engine or crank shaft is being turned in the correct direction. By watching the inlet valves the order in which the cylinders fire can be determined. When taking down an engine for examination and repairs it is absolutely necessary to note most carefully the relative positions of the timing gear wheels. They should be marked unmistakably (usually done by the makers) so that they can be put back in exactly the same relative positions as those in which they were found . It has been said that the valves must fit very accurately onto their seatings. If the engine overheats, the valves are liable to warp, and this will prevent them fitting securely. In a well-designed engine this should not wcur. Sometimes little bits of carbon lodge between the valves and the valve seats. If this happens the hot gases rush across and in doing so will soon wear away the valve and the seat so that the compression in the cylinder becomes very poor. In time the guides for the valve stems become worn and the valve instead of closing squarely will close on one side before the other. This also allows ihe hot gases to rush ])ast the valve and wear it away. From time to time the valves must be "ground in." This is done by coating the bevel of the valve with valve-grinding compound, which usually consists of a paste made of fine emery powder. The valve is now pressed onto its seat and turned around by means of a screw driver or special tool. It should be turned both ways, and after every turn or two should be lifted out of its seat. This makes the bevel even, so that the valve will close properly when turned in any position. The valve, when properly ground, should make a gasoline-tight joint with the valve seat and can be tested for leakage with gasoline. 60 AIR SERVICE HANDBOOK. In many engines the push rod is done away with and the vahes are operated directly from the cam through a rocking arm. The cylinder. — The cylinders of an engine are usually made of steel. In some engines the cylinders are lined with cast iron and in others the cylinders may be made of cast iron altogether. The valve seats are either welded to the head or screwed in. There is a hole, or sometimes two, in the head of the cylinder screw threaded to fit the spark plug. In an air-cooled engine the cylinder will carry fins on the outside and in the case of a water-cooled engine the water jacket will be welded to the cylinder. In some engines the jacket is made of copper deposited electrically. Near the bottom of the cylinder will be a means of attaching it to the crank case. Most of the wear on the piston will come on one side, and after the engine has run over 100 hours this may become large enough to inter- fere with the efficiency of the engine. The piston. — The piston can be described as a hollow cylindrical plug, to the interior of which is hinged the connecting rod. This is done by means of a short circular steel bar called the gudgeon pin, which is set diametrically through the piston and secured firmly to it. It is important that the gudgeon pin be held firmly in the piston and also in the lugs which hold it to the piston. The gudgeon pin is generally known as the wrist pin. The piston is made of slightly smaller diameter than the cylinder (about 8/1,000 inch for a 4-inch cylinder) in order that it may move freely up and down the cylinder. This clearance depends on the materials of which these two parts are made. On account of this clearance it is evident that if other arrangements were not made the gases would leak past the piston, resulting in considerable loss of compression. This difficulty is surmounted by cutting one or more grooves around the outside of the piston wall into which "piston rings" are fitted. These rings are made of slightly larger diameter than the bore of the cylinder and are cut through sometimes diagon- ally and sometimes in the form of a step; thus, when the piston is in the cylinder the rings are compressed. At the same time they are constantly trying to expand to their normal diameter, with the result that they press tightly against the cylinder walls and keep the piston gas-tight, ^^^len two or more rings are employed the slits in the rings must not be vertically over each other. They must be set in different positions round the piston so as to avoid as far as possible the escape of any gases past the slits as would occur were they in line. The ends of these rings must be some distance apart when cold (about 3/100 inch for a 4-inch piston) so as to allow for expansion when the rings become hot. AI& SEILVICE HANDBOOK. 61 The pistou is made wath a large skirt so as to have a l^rge bearing surface on the cylinder walls. This also prevents the piston from tilting in the cylinder and helps to conduct away the hoat from the piston head. The bottom ring on the pistou does not help much to seal the escape of gas but it wipes the excess of oil from the cylinder walls and prevents it from getting into the combustion chamber where it would carbonize and soot up the engine The top piston ring in some rotary engines is made of L section brass and is called an obdurator ring and acts in exactly the same way as does the cup leather in a pump. The connectinfi rod. — The connecting rod is the bar which connects the piston to the crank pin. It is usually made of H section steel. The small end is fitted to take the bronze bearing of the gudgeon pin. The big end is fitted to take the big-end bearing, which consists of a cylinder of brass lined with white metal. In some engines of the n--.'- ?r.^.)l. Fig. 22. V type, only one of the pair of connecting rods bears on the crank pin. The other bears on the outside of the brass cylinder which holds the white metal. The two connecting rods thus work on the game pin. In some rotary engines there is one rod called the master rod, and this is the only one which bears on the crank pin. All the other connecting rods are hinged to flanges on the master rod by means of wrist pins. The rod being of H section must not be bent or twisted, as this will destroy its strength altogether. The crank shaft. — The crank shaft, usually a steel forging, revolves in the bearings in the crank case. In multicylinder engines there is usually a bearing between every two crank pins. These bearings may be ball bearings or made of white metal. If the propeller is carried on one end of the crank shaft the shaft carries a thrust bearing, and this bearing is usually made to take the thrust in both directions, so that the engine may be used in a pusher or tractor machine. The crank case. — The crank case, made of aluminum or, in the case of rotary engines, of steel, carries the cylinders and bearings for the 62 AIR SEEVICE HANDBOOK. crank shaft aud also carries the means of attaching the engine to the machine. The bottom of the crank case, except in rotary engines, is usually little more than a cover. It catches the surplus oil and sometimes carries the pumps which return this oil to the main oil pump. It is constructed of very thin material and (>ngines must never be allowed to rest "with their weight on the crank case. The cam shaft. — The cam shaft carries the cams which operate the valves and as the latter are machined and solid with the shaft it is only necessary to time one cam and the rest will be automatically adjusted. In some engines the cam shaft carries the propeller, in which case it is furnished with the thrust bearing. Ta1«.J.\' ' 0..-. e ,c ro. a.^ t ^^.~. »_r J LjiL-CX^..,t, :r - - :^ "- ' / ' Fig. 30. The field of force exerted by a magnet is easily demonstrated by placing a sheet of paper over the poles of the magnet and then sprinkling iron filings on the paper. The filings ^vill take up clearly defined positions around the poles. Some metals retain their magnetism permanently, while others lose it at once. An example of the former is hardened steel, and of the latter soft iron. Advantage is taken of this fact in the magneto. The magneto consists essentially of two or more horseshoe-shaped magnets placed side by side (in some magnetos there is a pair of double magnets side by side and in which one magnet is placed on top of the other). The ends of the magnets are termed "poles," i. e., north and south. Attached to the poles by screws are pieces of very soft cast iron, which are cut away into semi-circular form inside the horseshoe. It is across this 'polar space" that the magnetic lines of force are concentrated. Within the horseshoe and the semi- circular pole pieces an "armature" is made to rotate. This "arma- ture" consists of a shuttle-shaped core around which primary and secondary windings are coiled exactly in the same manner as in an induction coil. The "armature" is made up of a number of soft AIR SERVICE HANDBOOK. 87 iron plates in order that it may obtain and lose its magnetism very quickly. As the "armature" rotates in the magnetic field it is evident that there are two positions in each revolution when the coils are being cut by the largest number of lines of force. These are called the "'maximum positions," and it is at these points that the current is induced in the primary winding. A new field of force is then created, due to the current passing through the primary, and this field is further strengthened by the core of the ■'armature" becoming itself a magnet. These new lines of force cut the secondary winding and induce a current in that, adding still another 'field. " The current of the primary is then broken at the "contact breaker" and the field belonging to the primary collapses, but slowly owing to the influence of the lines of force of the secondary, the current still tending to flow in the same direction. At this point Fig. 31. the "condenser" comes into play and a sudden reversal of the direc- tion of the current in the primary occurs. So rapidly do these motions take place that the spark occurs at the plug at the same instant as the breaking of the primary circuit. It is thus seen that two sparks are obtained every revolution of the "armature" and the speed of rotation therefore has to be r^ulated to the number of cylinders in the engine. Although there are only two positions in which the maximum number of lines of force cut the "armature" windings, yet immedi- ately before and after these positions are reached there will still be enough lines of force cutting the primary to give a current. It is this fact which allows the ignition to be advanced or retarded at will by altering the moment when the current in the primary is broken. The primary current is broken mechanically by a fiber stop on the end of a bell-crank lever carrying one-half of the "contact breaker. " Rollers are fixed in the circular track passed through by the fiber stop in its revolution and as the top passes them the bell-crank lever is swung about its fulcrum, parting the two screws forming the sides of the "contact breaker." One end of the primary circuit is 88 AIR SERVICE HANDBOOK. "grounded" to the "'armature" core and the other connected to the fixed half of the "contact breaker, " which is carried on the "arma- ture" spindle. The secondary circuit is usually connected to one end of the primary so as to be "grounded." The other is connected to a slip ring, where a brush collects the secondary current produced by the rupture of the primary current and passes it on to the dis- tributor and thence to the spark plugs. The distributor is on the same principle as that described above for "accumulator" ignition. The "condenser" is connected in parallel with the two sides of the "contact breaker," i. e., the two plates are connected to the two parts between which the break in the electrical circuit occurs. D/imtijroi- To1)isrri6utof- conaenser ca^r^cr s ^ a — (^ — £SftK f\\tl EZ^SXS Secondar(^ Circui t-B/a Fig. 32. To protect the insulation of the "condenser" from being pierced by the high voltage when one of the leads to the plug is withdrawn a "safety spark gap" is provided near the brush collector on the slip ring. This acts as a sort of "safety valve, " for as soon as the voltage or "electrical pressure" rises too high a spark jumps across this spark gap, thus relieving the electrical pressure in the circuit. To stop the flow of current to the plugs in order, for instance, to stop the engine, the end of the primary winding leading to the "contact breaker" is connected by a carbon brush to a switch. By "closing" this switch this end of the primary winding is put to "earth" and the coil is thus turned into a closed circuit. Hence no "make" and "break" can occur in the primary and so no current is gener- ated in the secondary circuit. When, however, the switch is "opened" the "contact breaker" again comes into action and the AIB SERVICE HANDBOOK. 89 magneto if revolved will give a spark. A terminal is always pro- vided for this purpose on the magneto. There is another kind of magneto in general use in which the "armature" itself does not revolve. A shuttle of soft iron is revolved in between the ''armature'" and the pole pieces. This shuttle distorts the magnetic field and thus draws the lines of force across the conductors wound round the "armature." Maximum current is given when the lines of force are springing back to the normal, and this occurs four times during a revolution. Faults m ianitinn. -The faihire of electrical arrangements may be due to — 1. Battery and coil ignition (if fitted). 2. The magneto ignition. 3. The spark plugs. If dual ignition is fitted — that is, if there are t\\ o coils or two mag- netos — try each in turn. One of the two probably will be foimd correct, and the fault thus partly located. Should both fail, remove the spark plugs and clean them with gasoline, replace, and try again. When testing the two ignitions, the spark plugs can be short-cir- cuited to the cylinders by means of any steel tool having a wooden handle to hold it by, e. g, a screw driver. If a spark be observed between the end of the screw driver and the cylinder, it will show that the high-tension current is at any rate reaching this point, and the length of the spark will denote its intensity. This indicates that the ignition is producing a spark and hence the fault must be in either — A. The plug itself; or B. In the timing of the magneto or its electrical coiniections, or in the batteries (if fitted). To test for "A," faulty plugs, the plug should be removed and tested by passing a high-tension current through it in air imder about 100 poimds pressure per square inch. If a good spark passes across the plug terminals and no signs of "flashing" occur else- where in the plug, it may be assumed to be in working order. It is no use testing the plug electrically under atmospheric pressure, as the resistance of the spark gap under pressure is so much greater than imder atmospheric conditions that a Haw in the insulation which may have sufhcient resistance to prevent a short circuit occiu'ring under these conditions will break down luider th<» moderate i)ressures obtained when actually working. With rotating cylinder engines the terminal at the end of the plug is pulled outward with considerable force. In plugs in which mica is used as the insulation there is a danger of the center pole of the plug l)ecoming bent toward one side of the phig and thus short- 90 AIE SERVICE HANDBOOK. circuiting the current when the engine is rotating. Any plug in which the central pole is bent or at all loose should be at once changed or adjusted. To test for "B," timing magneto and "distributor," if the fault is elsewhere than in the plugs test the timing of the ignition as f ollow s : Turn the engine by hand slowly and note the timing of the spark in the different cylinders, observe the crank angle (and inlet and exhaust valves) at the instant when the two sides of the "contact breaker " come apart. This indicates that a spark will occur at this point in the cycle. If this is correct, the various ignition leads should then be traced most carefully to make sure that they are connected up to the right cylinders and securely fastened to their respective terminals. If no spark appears while testing the terminal of the spark plvig by short-cii'cuiting, examine all the electrical connections and see that none of them have come adrift or have been connected up to the wrong terminals. A common cause of the magneto refusing to work is a short circuit to "ground" on the switch connection. This prevents the primary current being broken by the "contact breaker" and consequently the production of a spark. The platinum points on the "make" and "break" may want adjusting or the coils, condenser, or wire leads in the "armature" may have become short- cii'cuited . When machines are kept in tents, especially where the engine is mounted behind the magneto, it often happens that the magnetos become damp and refuse to spark properly. This may be prevented by wrapping up the magneto at night, but it should be remembered that damp will often deposit inside a covering of tliis kind after the machine has been up in the cold and then lands and is put into the warm tent. Defects ivhich may occur when certain specific conditions are observed. — 1. A fouled plug: To find out which cylinder it is, slow down the engine by throttling as much as possible and then short-circuit each plug in turn to the cylinder. When one of the nonfaulty plugs is thus shorted the engine will slow down considerably, but when the foul or shorted plug is treated in this manner no difference is detected in the running of the engine. The temperature of the cylinders wall often indicate the defective plug. Replace the plug mth a new one. 2. Faulty distributor: Examine the carbon brush on the dis- tributor and see that no oil has got to it. If there are signs of grease, clean it off with gasoline before replacing it. The carbon brush may have worn a groove round the distributor and the metal strips lead- ing to the plug terminals may have got masked by the insulation. The best remedy for this is to turn up the inside of the cylinder carrying the distributor segments in a lathe. AIR SERVICE HANDBOOK. 91 3. Defective insulation on connecting wire to plus?: If the w-ire carries a high-tension current a spark will probably be seen at the point where the insulation has given way. Indications will be the same as in "1." Examine the insulations carefully and replace the wire if necessary by a new length. Should the flaw in the insula- tion be small, a repair can be made with India-rubber solution and sticky tape. With stranded wire care should be taken to see that all strands are neatly housed in the terminal. It sometimes happens that one strand escapes and is short-circuited by the vibration of the engine, which causes intermittent missing and is sometimes hard to detect. 4. Faulty condenser: Tliis ought not to occur in magnetos where a safety gap is pr()\-ided to prevent too high a voltage being gener- ated in the secondary circuit. It is usually indicated by sparking at the platinum points of the "contact breaker." Should the plat- inum points of The "contact breaker" be worn and pitted the same indications will be present so that it is as well to examine these platinum points first of all, and then if necessary to file them square and smooth, afterwards adjusting them to the correct distance apart at break— 0.4 millimeter (16/1000 inchi. 5. If all the cylinders fire weakly on the magneto circuit, examine the "contact breaker" and its adjustments. 6. No spark obtainable with the magneto circuit: See that the wire from the long-contact terminal to which the switch circuit is connected is not short-circuiting to the frame. The ground brush at the back of the rocking lever of the magneto "contact breaker" may be oily and so preventing the magneto secondary circuit from being completed . 7. No sparking at any terminal: A terminal of the distributor cir- cuit has probably come loose or the wire short-circuited. Examine both carefully. The switch contacts should also be examined in all these cases. 8. The magneto refusing to stop, producing secondary current when switched off: This is probably due to oil having got underneath the carbon brush on the short-circuiting terminal at the end of the long- con tact-^ screw of the magneto "contact breaker." Remove the cover over this latter and clean the end of the brush and the face it bears on with gasoline, then replace. XI. MOTOR TRANSPORT. Engines. — In general motor-car engines are governed by the same principles as those applicable to air engines. The general chapter on engines must therefore be read in conjunction with this chapter. In addition the following points should be noticed: 92 AIR SERVICE HANDBOOK. A. Bearings should normally be examined after 10,000 miles. They may only require to be tightened up or they may be badly worn, thus necessitating remetaling. B. New piston rings will require to be fitted at this period. C. Whenever, for any reason, an engine is taken down it is ad- visible at the same time to grind in the valves, clean the pistons and cylinder heads, and clean all oil leads and filters. When re- assembling the engine it is necessary to use new washers and packing throughout. D. As all motor cars are fitted with variable ignition, care must be taken in tuning to allow sufficient "advance" and "retard" to be given on the ignition quadrant. In cases where independent mag- neto and battery ignition are fitted, each system must be adjusted so as to spark at the same point in the cycle. Routine examination. — Periodical inspections of the car or truck must be made and the following points seen to: Every time the car is used — A. Tires correctly inflated and spare wheels in place, and tools for changing wheel or rims (jack and brace). B. Radiator and gasoline tank full. Carry spare can of gasoline and strainer. C. Sufficient lubricating oil in the pump or reservoir and that the feeds work freely. D. Batteries properly charged and coil working. E. Brakes working properly. Every day before duty — A. All the above points. B. Oil holes on steering arms, knuckles, universal joint, etc., cleaned and oiled. This should be done after the car has been cleaned. C. Grease cups on springs and shackles screwed down and properly supplied with grease. Weekly— A. Spark plugs and ignition looked over, magneto oiled and cleaned. B. Examine water joints, see pump packing does not leak, and also that the radiator is tight. C. Refill axle caps and examine clutch leather. D. Open gear box and see that there is sufficient grease. E. Changing tires about on wheels if uneven wear is noticed. F. Examine body work. Monthly — A. Grind in valves (or after 1,000 miles running). AIR SERVICE HANDBOOK. 93 Cure of grease and oil caps. -There are several parts on the ear which require regular lubrication and which are not supplied auto- matically. These parts are generally equipped either with grease cups or oil holes. Particular note should be made of oilers on the spring hangers, universal joints, steering pivots, knuckles, steering- gear box, and such like. It is also necessary periodically to intro- duce some lubricant between the lamiiiationti of the springs. Drivers of motor vehicles should make themselves thoroughly acquainted with all the grease and oil cups on their car.'<, and must systematically keep them supplied. The frequency of the applica- tion of oil or grease will depend on the amount of running. Care of clutch. — The clutch may want a little attention. If a leather clutch is fitted and the leather comb can be got at, it may be brushed over with at least one coat of castor oil, the latter being allowed to soak in. This should be done when the clutch is "fierce" owing to the leather becoming glazed and hard. It does not follow that a new clutch leather is necessary when a clutch is not giving satisfaction. Sometimes it will be found that a shoulder of about one-sixteenth inch deep has worn on the old leather. This should be carefully trimmed off with a sharp file, which mil give the leather a new life. This allows the comb to go farther home, giving a closer contact between the surfaces. Especial detail to watch is to see that the copper rivets are well below the surface of the leather. If they become flush with the leather, the result would be a nasty gripping or even difficulty in disengaging. A metal to metal clutch requires to be cleaned out occasionally with kerosene. A hole is generally provided for the purpose in the clutch casing. If the clutch takes hold with a jerk, a little thin mineral oil will put it right. Gears. — The gear box should be regularly inspected to see that there is an ample supply of lubricant, but not an excess. It is quite unnecessary to fill up the cases, as this will only result in the gear grease flooding out at the joints and bearings and making them a receptacle for mud and dust. The amount of lubricant used should be sufficient to cover the lower teeth of the gears; the rest will look after itself (see differential gears). Differential gear and chains. — The differential gear transmits the power from the speed-change gear to the rear axle of the car. Cars which are made with chain drive to both wheels have the differ- ential gear arranged on the countershaft at the ends of which the chain sprockets are fitted. Usually she differential and chain-speed gear are fitted in the same case. Chains require to be renewed occasionally and taken up as they wear. Clean with kerosene and lubricate with graphite on a brush. Links of various lengths can be added. 94 AIR SERVICE HANDBOOK. Care of brakes. — Attention to the brakes is very important. They should be adjusted as closely as is permissible, the jaws being set so as just to clear the drums but not to set up any permanent friction. A screw adjustment is provided for this purpose. Too much clear- ance lessens the responsiveness of the brakes, especially in an emergency. The rods actuating the brakes should be carefully examined from time to time for any signs of weakness. Particular attention should be paid to insure that the jointing pins have split pins properly fitted. Cleaning and washing cars. — The car ought to be washed down as soon as it comes in, without giving the mud a chance to set. On no account should dust, dirt, or mud be brushed off. It must, in the fullest sense of the term, be washed off or else the paintwork will be ruined. If a hose is available, it will be very useful in getting the mud off the under parts of the car and will save a lot of time and labor. In using the hose for the outside of the car (that is, for the wheels and body work in general) the following points should be observed : A. Care must be taken that the water does not go anywhere but where it is intended to go. It should not be splashed about in every direction. B. A strong pressure of water from the nozzle is of considerable advantage in cleaning the underparts of the car, where the mud is generally heaviest, and in cleaning the underside of the mud guards. C. When dealing with the paintwork, however, a strong pressure of water is quite likely in removing the gritty particles at the same time to force them over the paintwork and scratch it. Apply the water with little force, but in plenty. If this is done when the car comes in wet, the mud will be speedily and easily removed. If the mud has been allowed to dry, the water must be poured over it, so as to soften it first of all. Afterwards it will gradually be carried away as the water runs over it. On no account should the mud be rubbed off. Brushing or rubbing the mud off, even if it is wet, will cause scratching and deterioration of the paintwork. D. When all the dirt and mud has been soaked off, the surface can be gone over with a wet sponge, using clean water. E. Oils and grease are bad for the paintwork, and care must be taken that neither gasoline, kerosene, or lubricating oil is allowed to remain on any part of the paintwork. F. When dealing with a car which is soiled with dust, the same care must be used in attempting to rub it off, the surface should be gone over first with a full sponge and finished off as before. AIR SERVICE HANDBOOK. 96 Care of tires. — If the foUowiiif^ points are attended to the life of tires can be increased considerably: A. Cuts, even surface cuts, require \Tilcanizing. This keeps out the water. B. Tires must be kept up to pressure, 20 pounds to each inch cross section, i.e., 36 X4=80 pounds. C. If possible, keep two spare wheels, so that repairs can be carried out on one while the other is ready for duty. D. Watch wheels for alignment. If a tire shows abnormal wear, look to the axles or distance rods. E. Do not apply brakes abruptly, except in emergency. A rapid "pull up" takes a good deal of mileage off a tire. XII. INSTRUMENTS. The barometer. — The mercurial barometer is the standard instru- ment for measuring the pressure of the atmosphere. In this instru- ment the pressure of the atmosphere is compared with the pressure at the base of a column of mercury of known height. If a tube from which the air has been exhausted is placed with its open end in a small cistern of mercurj', the pressure of the atmosphere will force the mercury up the tube until the pressure at the level of the surface of mercury in the cistern, due to the column of mercury in the tube, is equal to that of the atmosphere acting downwards on the surface of the mercury in the cistern. The pressure of the atmosphere is conveniently given in terms of the length of this column of mercur^^ An actual barometer consists essentially of the exhausted tube dipping into a cistern of mercury as detailed above. Alongside the glass tube is fixed a scale over which moves a vernier. The vernier is set exactly level with the top of the mercury. As the mercury rises in the exhausted tube the level of the mercury in the cistern will fall, so that if the scale be fixed its readings will no longer give the true distance between the surface of the mercury in the tube and of that in the cistern. To eliminate this error one of two methods may be adopted. In the Fontin barometer the bottom of the cistern is made of wash leather and can be raised or lowered by means of a screw until the surface of the mercurj^ always just touches a fixed mark. In the Kew pattern barometer the length of the divisions of the scale on the tube is slightly altered so that it always reads the correct height without adjusting the mer- cury in the cisteni. Errors and their correction. — A. Temperature: The first thing to be allowed for is the tem- perature of the barometer. If the temperature rises it affects the barometer in two ways — 96 AIB SERVICE HANDBOOK. 1. The mercury expands and therefore rises in the tube. This is equivalent to an apparent increase of pressure. 2. The scale against which the height of the mercury is measured expands, causing an apparent decrease of the height of the mercury or a fall of pressure. To eliminate the effects of change of temperature the readings of the barometer are always corrected to what they would be if the whole barometer were at 32° F. This correction depends on the actual temperature at the time, the coefficient of expansion of mercury, and that of the scale. n J u. be ZcClU' F.Hid r'o.'AL- ^J C. Index errors: The scale may not indicate the true distance from the surface of the mercury in the cistern. D. Scale errors: The graduations of the scale may not be tlie right distance apart. These two errors 'C* and "W are best found by having the instrument tested against a standard barometer and corrections must be applied to allow for them. E. Imperfect vacuum: If a small ([uantity of aii' be left in the tube aboA-e the mercurv it will cause the reading to be too low. the 'Po.udr K///////////////:'^^^//////x/////.>\<.l.-al. 5i,"&.)jK fC^T Fig. :;9. a small metal box similar to that which is used in aneroid barom- eters. This box is connected to the pressure part of the pressure head. This metal box is fixed inside the main case of the in- strument, which is air-tight and connected to the suction part of the pres.-!ure head. .\s the small metal box expands or contracts it moves the pointer of the instniment backward and forward by means of a small rack and pinion. 104 AIR SERVICE HANDBOOK. The use of air-speed indicators on airplanes. — Ks far as the pilot is concerned, it does not matter what figure the air-speed indicator in the machine registers. This figure alters with height very consid- erably, and is also affected by temperature. The instruments do not register small, quick changes in speed and ai"e not useful in bomb dropping because they do not register the ground speed. Where they are exceedingly important is in night flying, flying in clouds, and when it is necessary to get the maximum climb from the machine. Every pilot, by trial, knows the speed of his airplane when flying level , 80 that at night he can tell with fair accuracy how his machine is flying. No pilot can tell by the feel of his machine when he is doing the quickest climb, but by reading his watch and aneroid he knows what his air-speed indicator ought to register, so that after the first trial he can simply keep his machine steady at this speed in order to insure that his machine is rising as swiftly as possible. When the air speed indicating the quickest climb has been found this figure is correct for every height, although the correct reading of the instrument alters with the '^'•'^ *■' ^ height. This is because the angle of the machine has to be increased con- siderably at the higher altitudes in order to get the best performance. The compass — General description.— Tire compass is constructed on the principle of suspending a magnet (or system of magnets fixed parallel to each other and referred to as the "compass needles") in such a manner that, remaining horizontal, they are free to take up the direction in which the magnetism of the magnetic pole directs them. This direction is called the "mag- netic meridian." A circular graduated card called the "compass card" is fixed to the compass needle so that one diameter of the card, the opposite extremities of which are marked north and south, respectively, is in the same line as the direction of the needles. The point marked "north " (in land and water compasses distinguished by a fleur-de-lis or other special mark) is placed over that end of the needle which always points to the northward. The extremities of that diameter which is at right angles to the north and south line are marked east and west, east being to the right hand when the observer is facing to the northward. The compass card is thus divided into four quarters of a circle, or quadrants, and the points thus obtained are called the "cardinal points." These divisions may again be sub- divided into the half and quarter cardinal points. AIR SERVICE HANDBOOK. 105 In the center of the conipa-a card a .small eemicircular cap is fitted slightly hollowed on it.s underneath side. This supports the card by resting on a sharp- pointed pivot made of very hard metal. Thus the card is suspended on an almost frictiouless point and ia free to maintain its direction when the airplane is turned. The pivot itself is fixed to the center of a bowl called the "compass bowl." This bowl is covered by a glass plate or a hole is cut in it to which the glass plate is fixed. All airplane compasses are of the liquid type; that is, the bowl is filled with water and spirits. This takes the weight of the card off the pivot, so that the compass becomes very sensitive. It also makes the needle come to i"est quickly, so that it very soon points to the north after a turn has been made. On the bowl of the compass ^vill be found a mark called the "lubber's point," and when mounting a compass in an airplane this point should be in the fore-and-aft line of the airplane and pointing directly ahead. Thus it follows that the "lubber's point" moves with every turning movement of the au-plane, and to ascertain the direction of the airplane's head the observer has only to notice what point on the compass card corresponds with the "lubber's point." This bearing is called the "compass course," or direction in which the airplane is being steered. The "compass course " may also, as an alternative, be described as the angle made by the point of the compass coinciding with the "lubber's point" and the north point. 106 AIE SESVICE HANDBOOK, Airplane compasses are usually marked at the half cardinal points and the card is graduated in degrees reading from north to 360 clockwise, i. e., east will be 90°, south 180°, and west 270°. As the cards are very small, it is usual only to mark every 20°, and the last figure "0" is left out for ease in reading. It is very hard, indeed, to steer an airplane within 5° of the required course, so that the graduations are not abnormally large. There is another way of designating compass bearings, but it is not used in the land services. This method is to describe a bearing assomany de-^ree? ea.stor wer't of north or south; that is, southwest would be described as S. 45° W., etc. Errors to which compasses are subject. — The compass is unfortu- nately affected by errors. The ones chiefly concerning aviation are — A. Variation. B. Deviation. C. Dip. D. Air bubbles in liquid. Table of compass bearings. Bearing. De- grees. Bearing. De- grees. Bearing. De- grees. Bearing. De- grees. N 11} 22J 33| 45 56} 67i 78f E 90 101} 112^ 123J 135 1461^ 1574 1682 S 180 191} 202.V 213| 225 236} 247i 2581 W W.by N.. W.-N W. . . NW.byW. NW NW.bvN. N.-NW... N.by W.. 270 N. by E . . . N.-NE.... NE.by N.. NE NE.byE.. E.-NE E.byN... E.bvS... E.-SE .... SE.by E.. SE SE.bvS.. S.-SE S.byE... S. by W. . . S.-SW.... SW.by S.. SW SW.by W. W.-SVV... W.by S... 281} 292i 303i 315 326} 337* 348i The above are called th? "points" of the compass. Each "point" is equivalent to an angle of 11^ degrees. A. Variation- know at what o'clock he should be over certain places. It should be noted whether any very high ground is to be passed over necessitating a greater height being maintained at that point. Selection of objects us guides. — The following remarks are the result of practical experience: Towns. — Towns are obviously of the greatest assistance. In case of doubt they are usually most easily identified by the railways. 118 AIR SERVICE HANDBOOK. No airplane should pass directly over a town as not only is such a practice contrary to law, but also unless flying at over 2,000 feet the effects of any large works with blast furnaces will be felt. On hazy days it should be remembered that smoke hangs over villages and sometimes gives them the appearance of a large town from some little distance away. Railways. — Railways are of very great assistance and can be used to a large extent as a guide from point to point. The conventional sign for a railway is a plain black line on the map, and no distinction is made between a line with perhaps four paii's of rails and one pair of rails. Thus it is quite easy to make a mistake if a single line branches off from the main line in perhaps a not too conspicuous place. Branch lines to quarries are often not marked on the map even though they may run a mile or more away from the main line. Tunnels, liridges, and cuttings are marked on maps and these will often be of assistance in picking up the correct line Sometimes grass is allowed to grow over the track esi^ecially if it is a light railway and this makes it practically invisil^le from a height of 3,000 feet. In snow a tarred road which has had a little traffic over it looks very like what a railway does in the ordinaiy times and it is very easy to make a mistake. In spite of the above few details which are liable to cause an error, a pilot may find it worth his while to keep to the railways and go a little farther round and this applies especially in misty or windy weather, when it is hard to keep to a compass course. In this case the general direction of the railway should be noted so that the pilot ■will not find himself followdng the wrong line. Roads. — As a general rule roads are not a particularly good guide- Many roads twist about considerably. Main roads are often less noticeable from a height than minor roads. The telegraph wires and poles (a sure sign of an important road) are also very hard to see. In the neighborhood of the fighting line the places where troops have marched during the night, even if they have gone across coun- try, look very like a permanent road, especially of the soil is chalky. There are exceptions to the general rule. Roman roads being usually a1)solutely straight can generally he picked out easily and also roads over a moor or plain where there are few others in the vicinity with which to confuse them. The Napoleonic roads in Europe, which are planted on l>oth sides with poplars and which are straight for miles, make very good landmarks. Water. — Water can be seen from a great distance and is the best guide. After much rain a pilot must take into consideration the possibilities of a fiooded .stream causing the siuTounding meadows, AIR SERVICE HANDBOOK. 119 etc., to be inundatod lo a (l('))th of jjcrhaps only a low iiifhcs, hut nevertheless havinu: an appearance of a t,'0()fl-sizc(l lake or l)road river which can not be located on the map. Rivers are very winding: and are often almost concealed by high trees on either bank. A pilot will usually waste time if he elects to follow a river as a means of getting from jioiiil to ])oint. On most maps the smallest rivers are marked very disiiiicily whicli will at first encourage a pilot to follow them. Large canals are easy to see and often go very straight. In dry weather the course of a river can be noticed at once by the difference in color l)etween the trees near the river and those farther away. 11 oor/.s.- Woods can be seen from a distance and can often be identified very easily by their shape or the .shape of cuttings, but it .-^hould be liorne in mind that it is very easy to alter the shape of the woods or even to cut it down altogether so that the location can not be seen when flying at a little height. High grouml.— Yvom a height of 2.000 feet and over country presents quite a flat appearance and contour can not be recognized. In early morning or late evening hills may cast a shadow and stand out from the surrounding country. A pilot should not fail to note any high ground with steep contours which will make landing difhcult and he should fly high at these points. The general lay of the country should be borne in mind. In many places the edges of the plain or downs are very distinct and may form a convenient mark. On a long cross-country flight the color of the country will change on account of the differences in trees or grass and this may aid the pilot in case of doubl . Forced laruinigs. Landing ground is hard to recognize as l^eing good from a greater height than LOOO feet. The nicest height at which to fly is about 8,000 feet. At this height the ground can be clearly seen, the machine is usually above the ■'])umi)y" air, and in case of engine failure can glide for some little^ distance Ix'fore the spot for landing on need be finally selected. The best time of year for flying is undoubtedly the autumn, when the crops are in. At this time a pilot should choose for preference a stubble field which from a height presents a lightish brown appear- ance. By doing this he can 1)0 (|uite certain that the surface will be smooth without ditches or mounds, whereas the ordinary grass field as often as not abounds in the latter. Dark green fields ixro usually found to be roots and as such should be avoided if better ground is available. In the winter rain may make the stubble and root fields very soft and may make the machine turn onto its nose on landing. When they are like this it is very difficult to make the machine rise off the ground. Grazing land mav be identified l)v the feeding 120 AIR SERVICE HANDBOOK. cattle. Should a pilot land in growin- a fresh diagram must be made. It is not sufficient to simjily add or subtract the error BAIV from the course BA if it were subtracted or added in tlic first iiistaiire. The machine AIR SERVICE HANDBOOK. 181 will be in the air a (lifl'erent length of time and tliis \\-ill cause a different angle of error. A practical way of finding the course is to pass over two points on the aerodrome which are known to be in the same bearing as that of the distant point. By trial make the macliine fly over these two points when flown on a constant bearing. Note tliia bearing. If the machine continues to fly on this bearing it will reach the distant point. Always when flying in a wind select a point some distance ahead over which one ought to pass. This will relieve one of the necessity of continually looking at the compass because the compass need not be checked till one has reached the selected point when another point on the required bearing should be selected. ■J.,u., Fig. 49. Time— In an airplane it is most difficult to estimate tim(>. On calm days it seems to pass quickly, but on a rough journey the minutes pass very slowly. Thus it often happens that a pilot who has not checked the time of passing some object expects to pass the next long before it is really due. On a reconnaissance the time should always be kept in mind so that one mil have sufficient gasoline in order to bring one home. Instniments. — The following instruments should be fitted in an airplane intended for cross-country work: A properly adjusted compass. A watch, fixed to the airplane by some suxtal)le mounting which ])revents excessive vibration. An aneroid mth adjustable height reading. An engine revolution indicator. However skilled a jiilot may be in detecting faulty running of his engine, after a long flight his 122 AIR SERVICE HANDBOOK. hearing- will not be so good and an indicator will assist him con- siderably. When flying over the enemies' lines it prevents a certain amount of mental worry wondering if one's engine is rotating pi'op- erly. An air-speed indicator. Tliis should always be carried and is especially necessary when a machine has to fly at night or through clouds. The indicator will show changes in the airplane speed so that the pilot can tell when he is climbing or going down. On a long flight one becomes tired and can not tell how one is flying without the aid of an instrument. An inclinometer is required for ascertaining the angle of flight wlien the earth is not vi-sible. For longitudinal angles the air-speed indicator is usually sufficient, as by noticing whether the speed is increasing or decreasing the pilot knows whether he is going down or u]). A map case should be ])rovided wliere it can easily he seen so that the map may be visible and run no risk of loss or damage. Lights: All the instruments should be suitably lighted in case the machine has to be out after dark. This is conveniently done by means of small electric bulbs and dry cells. Gasoline gauge: It is useful to have a gasoline gauge so that the amount left in the tank can be checked at once. . An observer should carry the following instruments: A watch fixed to the outside of the flying jacket around the arm or leg. A compass, carried in a similar manner. An aneroid is interesting, but not as a rule absolutely necessary. Maps, suitable for the job on hand which should always be carried in a map case or stuck to a board and varnished in order to prevent them becoming damaged . Message forms and message bags. Report forms (these are mounted on cardboard in triplicate with carbon paper between each sheet). Pad and pencils for making notes that may be necessary. RULES OF THE AIR. 1. Take off and land directly into the ivind. Not only does this prevent the machine from turning over, but it also does away with the risk of collision. The only exception should be emergency landings. 2. Before starting see that the section of the field you are going to use in making your get-away is clear and that no machines are landing or gliding into this section of the field. Locate position of all machines in the air. If other machines precede you in starting, allow them to gain a distance of at least half a mile before followinu, AIR SERVICE HANDBOOK. 123 Do not follow (liroctly in rear so that the propeller wash will In- avoided. 3. Machines wilh dead inotors have right of way over all others. 4. Machines gliding into field have right of way over those about to leave. Machine.s landing are often going at a greater speed than those leaving, so be careful nor to misjudge the start and he over- taken i)y another machine. '}. Before beginning a glide see that no machines are underneath you. Those (lying beneath you have the right of way. <). In flight before making a turn see that no machines are dantrer- ously near on your flanks. 7. Unless there is some urgent reason, never tly out of gliding distance from a possible landing ground. 5. Aircraft meeting each other. — Two aircraft meeting each other end on and thereby running the risk of collision must always steer out to the right. They should in addition to this pass at a distance of at least 100 yai-ds. 9. Aircraft overtaking each other.— .Vny aircraft overtaking another aircraft is responsible for keeping clear and must not approach within 100 yards (right or left, above or below) of the overtaken aircraft. An aii'craft is said to be overtaking until it ha.s drawn clear ahead of the overtaken aircraft. \Mien one of the aircraft is an airship the distance of 100 yards should be increased to (iOO yards. 10. Aircraft approaching each other In a cross direction.- When any aircraft are approaching each other in cross directions, then the aircraft that sees another aircraft on its right hand, forward quad- rant — from 0° (i. e., straight ahead) to 90° on the right hand — must give way, and the other aircraft must keep on its course until both are clear. 11. Aircraft flying over an airdrome are bound by the local rules of the airdrome and an aircraft landing on an airdrome \\-ith which it is unfamiliar must keep clear of other machines. 12. Night Ajdng machines should carry a green light on the right- hand wing tip and a red light on the left-hand wing tip. RULE.S OF THE RO.\D-^AII)S TO MEMORY. If on your right hand red appear, It is your duty to keep clear; For he has got the right of way; •'Look out right front," your rule by day. lUit if to left of you is seen An aviator's light of green, There's not so much for you to do; For gieen to left keeps clear of you. 124 AIR SERVICE HANDBOOK. XIV. NOTES ON FLYING. GENERAL INSTRUCTIONS FOR PILOTS. 1. Before leaving the ground examine the machine carefully your- self and then get reports from both the section chiefs. 2. Always start against the wind and, if possible, in a line clear of obstacles. 3. Leave with plenty of speed. Take a normal climb. Do not climb your machine to the limit. 4. In case the engine stops before the machine has reached a height of 600 feet, land straight ahead, even if the landing is bad. Never try and turn down, wind so as to get back again onto the airdrome. This nearly always causes a fatal accident, even with experienced pilots. 5. In flying level do not run the motor full out more than is abso- lutely necessary; always thi'ottle the engine, but not so much that the machine is in danger of losing flying speed. 6. Land into the wind. 7. Land with minimum speed. Touch tail and wheels together, if possible. 8. Always taxi slowly, and if there is any appreciable wind let a mechanic hold each wing tip so as to prevent the risk of the machine turning onto one or the other wing tip. 9. Never leave a machine tail to wind. When a machine is not being used, place it facing the wind and tie the controls to prevent them moving about in the gusts. The controls should be tied in such a manner that it is impossible for the pilot to sit down without loosen- ing them. 10. Always use safety belt. It is much safer to stick to the machine, even if a bad "crash" is foreseen. 11. In the air certain machines have the "right of way," but this does not relieve a pilot from the necessity of keeping a good lookout and from the responsibility of a collision. 12. Unless it is absolutely necessary a pilot should not start his engine without assistance. 13. The pilot is responsible that the switch is in the ' ' off " position when the propeller is being turned by hand. 14. When starting do not tiy to force the machine off the ground. 15. Do not make quick turns down, wind when close to the ground. 16. Remember that in a quick turn the rudder puts the nose of the machine up oi' down, .so that if it is found that the machine is diving put on less rudder and pull ])ack the elevator in order to complete the turn. AIR SERVICE HANDBOOK. 12S 17. To recover from a ''spiral dive" or "tail spin" put all controls in a neutral position. The elevator control may then be pushed forward gently. This converts the spin inUi a "nose dive," out of which the machine can easily be pulled. The spin is due to loss of flying speed, so that the essential thing to do is to, first of all, "gain speed." The use of the rudder or ailerons in a case like this merely increases the drift and preA'ents the machine from gathering speed. 18. Do not work the controls roughly, and this especially applies to the elevator controls when the machine is di\'ing at a speed. 19. Do not stand directly in the plane of a mo^'ing propeller. 20. Tie all loose articles into a machine so that they can not fall out, even if it is necessary to loop the machine. 21. WTien coming to a new airdrome or before landing on an unknown ground always fly around once or twice at a few hundred feet and make certain of picking out a good bit of ground for land- ing on. 22. When landing in a restricted area do not dive the machine in order to lose height; do proper S turns and land slowly. 23. Insure that all the drift is off the machine before landing. Land \\-ith rudder neutral. 24. In case the engine fails when flying against the wind it is probably better to make for a landing ground down \nnd rather than try to get into a field up wind with no height to spare. It is always easy to kill height by making a spiral or S turn. 25. \Vhen gliding, throttle down the engine as much as possible and glide at the proper angle. There is one angle, usually about one in seven, which gives the machine its best and longest glide. 26. There are four types of bad landings which it is easy to make. The first is a "Pancake" which results from allowing the machine to get into the rising position when landing. In this case there will be a perpendicular bounce and on the second bounce the landing gear may break. To prevent this, open up the engine, put the machine in a flying position, and then throttle down again and land. The second type is the "Pancake" which results from bringing the machine out of the gliding position at a point too far above the ground, when the machine \\ill drop, due to lack of speed, and may break the running gear. The third type of bad landing results from failure to bring the machine out of the glide at all, so that it touches the ground before it is straightened up. This is the most dangerous kind of bad landing. To rectify it, open up the engine after the first bounce and put the machine in the flying position, then throttle down again and land. The fourth kind of bad landing is to land 126 AIE SERVICE HANDBOOK. with drift. If, at the last moment, the rudder is put over the machine will swerve and the side strain on the landing gear may pull off the tires of the wheels or buckle them so that the machine may fall on one wing tip or turn onto its nose. 27. Always test the controls of the machine before leaWng the ground. 28. If you become lost, do not fly about aimlessly. Either land and ask your way or else make for some well-defined landmark which you know or can easily recognize. 29. If one has damaged the machine when landing away from an airdrome, communicate Avith headquarters and describe exactly what one requires to make the machine serviceable, if possible call- ing each part by its correct name. Do not use the word ' ' complete, ' ' such as "landing gear complete," but describe what you want as "wheels, axle, etc. (as required)." 30. When communicating your position to headquarters, describe yowc location exactly. Give the number of the map you are using and give your nearest large town or some such mark, so that your location can easily be found. Also give your address and telephone number. Cross-country flying. — Before starting on a cross-country flight be sure that the tanks are full of gasoline and oil. When taking over a new machine find out what is the consumption of the engine and where the filling plugs are. AVhile fljdng do not let the gasoline get below 4 gallons, so that in case the first choice of a landing ground is bad you can go up again and choose another. When flying against a head wind do not try to make the airdrome at nightfall, if there is any doubt about reaching it. It is much better to land while it is light and tie down the machine, rather than risk a landing in the darkness. Unless one is flying over the lines one should carry a small tool kit for minor repairs. Care of an airplane in the open. — In case a machine has to be left in the open, steps must be taken to prevent it blowing away and becoming damaged by rain and dew. Directly the machine lands it should be placed under the lee of a house or fence. The pilot should take into consideration the probable change in the direction of the wind. If the wind is likely to blow strongly, the machine should not be left too near large trees, because the branches are liable to be blown off, and falling on the machine will damage it. The controls of the machine should be tied to prevent them flapping about in the wind. The elevator control should be lashed back, so that the wind will tend to keep the tail on the ground. The AIR SERVICE HANDBOOK. 127 machine, of course, should face the wind. Do not turn the machine tail to wind, because it is not designed to meet the wind in this manner. If the wind is likely to blow strongly, lift the tail by l)lacing boxes or trestles under the taliskid. This will give the main planes a negative angle and the machine will tend to stay on the ground rather than to be blown away. The wheels of the machine should be scotched up to the rear, or small ditches may be dug in which to sink them. The landing gear should be fas. tened to a holdfast in front and the wings and tail should be lashed down to pickets in the ground. All modern machines have small rings on the underside of the main planes for this purpose. The tail may be lixed by fastening the rope to the tail skid. The pro- peller, engine, and all openings in the fuselage should be covered by waterproof covers, but if these are not available old sacks make quite a good i)rotection. The most suitable form of picket to use is the iron screw picket, such as is used with many kinds of tents; an ordinary piece of wood hammered into the ground is liable to draw, especially if the ground is wet and soft. Do not leave a ma- chine without a guard. When a machine has been left out all night, it sometimes happens that water collects inside the planes, the rud- der, etc. If this is so, let it out by pricking small holes in the underneath of the planes. Most machines now have small eyelets in all trailing edges to prevent this. In case the field is a difficult one to get out of, bear in mind that in the early morning the machine will not lift as well as it ought to until it becomes dry. A machine should always be left in the shade, because the sun's rays damage a machine more than wind and rain. When machines are left out in the damp, the magneto is liable to get damp inside and will refuse to work in the morning. To prevent this, wrap the magneto up in waste. This is a very frequent cause of delay in starting in the mornings. Choice of airdromes. — Landing grounds may be permanent or temporary. A temporary ground need only be good for the time of the year for which it is intended to use it, but when choosing a permanent ground the surface of the soil and its condition in rainy weather or winter should be taken into consideration. The following are a guide in the selection of landing ground: A. When there is a choice between two landing grounds, the one in the more open country should be selected. B. Roads sufficiently good for heavy motor transport should lead to the ground. A side road leading to it unused by ordinary traffic is also of great use if transi)ort can be parked on it. C. A permanent landing ground should l)e at least 500 yards by 300 yards in size. A temporary ground for a few airj)Ianes only 128 AIB, SERVICE HANDBOOK. may be as small as 200 yards by 200 yards if the approaches are open. If trees or telegraph lines border the ground a minimum of 300 yards to clear the obstacle is necessary. This distance must be in- creased if the trees exceed 50 feet in height. D. A good shape for a landing ground is an L, when the minimum length of the arm should not be less than 500 j-ards and the breadth 300 yards. An L-shaped ground is particularly useful if protection against weather is naturally provided by the position of trees or houses, as in diagram. T-shaped landing grounds are also often to be found. The length of the arms in this case should not be less than 500 yards or the breadth less than 300 yards. E. Landing grounds should be as level as possible. Although airplanes can rise from and land on sloping ground, the wind will often make such landings diffi- cult. An airplane rises and lands up wind. It is easy there- fore to rise when going down- hill, but difficult to land under the same conditions. Similarly it is easy to land uphill but difficult to rise. F. The surface of the ground should be firm and level. The best surface is short grass or stubble. If the surface is rough or in ridges the landing gear is apt to break on landing. If the surface is soft the airplane may tip up and break the propeller, or it may be impossible to rise at all . Plow and ridge and furrow are unsuitable. G. Landing grounds at the bottom of hollows should be avoided if possible, as they frequently become water-logged in wet weather. H. Landing grounds with telegraph posts and wires on their boundaries should be avoided. Improvevients. — In the field ideal landing grounds are few and far between, but much can be done to improve them. This is one of the duties of the Corps of Engineers. Landing grounds may be improved by- A. Rolling soft or rough ground. Steam rollers are best for rough and hard surfaces, but a large stone or iron roller weighted with pieces of timber is suitable for soft ground. If it has been necessary to select soft ground and no rollers are available a good deal may be done by a body of men, such as a company of Infantry, trampling down the ground. B. Rolling and trampling dry plow which is not ridge and furrow. C. Filling up and rolling drains and ditches running across the ground. 1 ■ G- tevMM Fig. 50. AIR SERVICE HANDBOOK. 129 D. Cutting down trees and higli hedges when the space lor landing is less than 300 yards. Trees must be felled so that they fall away from the landing ground. E. Filling uji large ]>ot lioles and rolling. If there is insutticieut time to do this a red or yellow flag or a .square of red or yellow cloth should be placed in the center of the pot hole. Care must be taken that any fi!led-in ground is made firm and solid. F. Marking any other dangerous places, such as ditches at the edge of the ground. G. Telegraph posts and wires and wire fences or iron railings must be marked by hanging strips of cloth or blankets on them, and the Signal Service should be asked to take down any air lines which might prove daiigerous to airplanes and to substitute ground lines. All work on the landing ground should l)egin from the center and proceed outward in order that a space for a machine to land may be provided as quickly as i)ossil)le. With well directed work very unlikely looking places can be turned into practicable landing grounds in a day. It is sometimes advantageous to build paths for landing, radiating out from a center, and to put down ashes so that these paths will not become muddy in wet weather. Working parties must not leave their tools lying about on the ground, and when they see a macliine about to descend they must at once clear the ground. Methods of marking a landing ground by day. — Two strips of cloth colored white and arranged in the form of a T should be laid out ou the airdrome. The head of the T .should face directly into the wind, thus: This gives the direction in which a pilot is to land. It is to be understood that a machine landing in the ordinary manner so that it will stop running when it reaches the head of the T will find good land- ing groiind. A machine should not run over the mark, but just to one side. The position of the T must be changed with any change of tlie wind . Jfethod of marking landing ground by night. — By night landing grounds will be marked with four flares as under: It is to be understood that a machine should touch the ground near the flares A or B and that all the ground between A and CD is good for landing and free from obstacles. It aids the pilot if a man stands at each of the flares so that he is able to judge his height when landing. Searchlights may be used to light up landing grounds. They should be placed near the flare A and point in the direction in which the machine will land. The light should be raised to about 10 feet. 46643—18 — —9 130 AIR SERVICE HANDBOOK. If it is any lower than this small tufts of grass, etc., cast a shadow which looks like a large hole and this is liable to interfere with the pilot when he lands. Any parts of the landing ground which may cause damage should be marked with red lamps. The most suitable form of flare is a bucket with half a gallon of gasoline in it. This will burn for half an hour and is visible from 8 miles off on a clear night even when the moon is half full. wlf?c/ _ ^/recrion of/^>3.chine Zs.f>c/i no Fig. 51. On bright moonlight nights flares and lamps may be dispensed with and the same signal used as for day. Aviator's clothing. — The clothing for an aviator must be light and Warm and ehovild allow him to make quick movements. The essen- tial for warmth is that the aviator should be drj^ when he goes up that is, he should not walk about in the wet grass and then go up into the air. The clothing should not be air-tight and should lie loose. Cap. — The cap should be of leather, lined with fur or chamois leather. It should fit tight round the face, so that when the pilot -n' Fjg. 52. looks outside his machine the wind will not be caught by the sides of the cap. If the pilot has a comparatively large wind screen it will cause the wind to blow continually on the back of his neck, so that the cap should come far enough down so that there is no gap between the cap and the collar. Sometimes caps are made so that a mask can be strapped on for flying in cold weather. To prevent frostbite of exposed parts in cold weather, these parts should be covered with special frostbite grease which is an article of store. Goggles.- — These should fit both the aviator and the cap so that they do not leave an exposed part which would cause the aviator to get neuralgia. The glass should be either triplex, or special "unbreak- AIR SERVICE HANDBOOK. 131 able" glass. The triplex glass has the disadvantage that it tuts out a lot of the light and is not nire to use in the early morning or even- ing, or at night. It also has the disadvantage that a comparatively light blow on the outside will strip the glass from the inside and cause damage to the eyes. To get the maximum efficiency from the goggles thoy should he specially colored. "Novio" glass is a good type of this colored glass. This glass is expensive and hard to make, and is quite different from the cheap, colored glas.'^es, wMch are as a rule worse than useless. Gloies. — Gloves should be made of leather, lined with Jaeger wool; sometimes silk gloves are worn also underneath. There is a good type of glove which has a little bag attached into which the fingers can 1)6 slipped at the times when nothing is happening. The gloves should be loose so that they slip on and off easily and do not stop the circulation. It is an advantage to have only the thumb and first finger separate, the other three fingers in a mitten without compart- ments so that they will keep warm. Leather gloves should be kept clean. When they become oily they make the hands very cold. If the glove is a fur glove oil destroys the fur. Coat and trousers or union suit. — These should be made of leather,, lined with Jaeger wool or fur. If fur is used there should be some sort of inner lining to prevent the fur from being torn. Pockets in the usual places are useless. The most useful places are diagonally across the front over the chest and at the side of each knee. These pockets can be got at when the aviator is strapped into the machine. For this kind of clotliing there must be some sort of wind-proof material on the outside and warm material on the inside. Leather is best for the outside material. A waterproof material on the out- side causes moisture to collect on the inside, which makes the aviator very cold. There should be a wind Hap over the opening of both coat and trousers. Boots. — Boots should be made of leather and lined with fleece, or at least two pairs of thick, woolen socks can be worn inside instead . They should be loose and can be light. They should be put on just before the aviator ascends. To keep them drj' while he is walking from the hangar to the machine he should wear a pair of rubber snow boots, which he kicks off as he climbs into the machine. lastras. — In cold weather the aviator should take up something which he can put in his inner coat pockets to keep him warm . The Japanese charcoal instra is suitable for this. There is also an electric heater which can be run off a small generator, which is suitable for this piu-pose. If an aviator has to start his engine himself, he should take off his coat so as not to get too hot. If he gets too hot before getting into the machine he will get verj- cold when he gets high up. 132 AIR SERVICE HANDBOOK. J.t is not a good thing to take alcohol }>efoi-e going cm a high flight. The reaction, by the time one has got into the cold, is I)ad and makes one colder than one would be in the ordinary way. There is a very great difference between summer and winter flying in the matter of temperature. In the winter months great care must t ■t. Fig. 53. ■-W be taken to prevent the pilot getting frostbitten. Antifreezing grease and antifreezing oil are used for the face and hands. Elec- trically heated union suits have so far proved the most satisfactory in the way of clothing. Electric heaters are also required for the guns. In order to mitigate stoppages and gun trouble generally, barrels and parts should be kept as dry and free from oil or grease as possible. Covers and blinds will be required for the radiators. 4 Finding the iiortli point. — -A well laid-oiit compass • ^ base is absolutely essential on every airdrome, and in fact every squadron should have one -V ^^■'■^>, for its own use. The compasses on all new • *^° C* machines will I'equire testing and ad- .■ justing prior to any cross-country /' flying. It is of the utmost impor- /' fance that the compass in each /' machine in the squadron be /' tested on the (;ompass base / at regular intervals, i.e.. at ^/ least once in every seven ^^-rlT'olt. days. Should, however, pj^, ^^^ anything happen to the machine that is likely to affect the compass (such as the litting in of a new engine) the compass must be tested on the base prior to any cross-country flight. There are many occasions on which the pilot will have to depend upon the accuracy of his compass for the safety of himself and his machine. The north point can l)e found by night or by day as follows: AIR SERVICE HANDBOOK. 13S /)'// iii(jhl.- In tiie Xortliciii lli'iuispln'ic it will he noticiMl thai all the stars re\'olve around a single t)ni', wliich is called the North Star. This star can be found easily with reference to the constel- lation called the Great Bear. The Great Bear consists of seven stars, arranged as shown above. One can imagine them to repre- sent a saucepan. "The handle of the saucejjan points to the left when the constellation is below the North Star and to the right wlien it is above. The part of the saucepan opposite the handle points directlj- toward the North Star, and the two stars which form this are called the pointers. The North Star itself makes a small circle around the North Pole. The actual pole may be found by drawang a line from the Pole Star to the second star in the handle of the sauce- pan, the one called p. The North Pole lies on this line and 2° (as measured from tlie (^arth) away from the Pole Star. The con- stellation wliich is on the opposite of tlie North Star to tlie (ireat ♦ >io-|-;. Bear is called Cassiopea. This looks like a big W and the North Star is the first bright star right above this letter. In the Southern Hemisphere the North Star is not visible, but the South Pole may be found with i-eference to the Southern Cross. The Southern Cross consists of four bright stars, which can be imag- ined to form the ends of the cross. There is a less bright star close to one of these corners. There is another constellation not very far away, which looks something like this, but it is not nearly so symmetrical and should not be mistaken for the projier cross. In order to find the South Pole draw a line through the long arm of the (TOSS, divide the part between the two stars into three. Then measure, awa>' from the longer part of this arm, a distance corre- sponding to nine n\ these divisions. This point ^\^ll be the South Pole. By (lay. Plant a stick in I lie giound, pointing api)roxiniately toward the north. Tie a ])lumb l)ob to the top of the stick and allow it to touch the ground. From this point draw a (irde on the 134 AIR SERVICE HANDBOOK. ground with any convenient radius. Note the two points where the shadow of the top of the stick touches the circle. There will be one place in the morning and one place in the afternoon. Join these two points to the center of the circle. The North Pole will be found by halving this angle. For c-hecking the comimss it should be remembered that these points are true north. The compass points on the ground should therefore be laid out after having made the suitable correction for the variation of the compass on the airdrome. For steering by the stars a knowledge of the constellations is not absolutely necessary, but it is very hard to keep a course on a single star without this knowledge. XV. METEOROLOGY. Introductory remarks. — Although in late years great advances have been made in the • knowledge of the conditions and changes taking place in the atmosphere, yet a very large number of cpiestions of great interest from an aeronautical standpoint still remain unan- swered . Further, changes taking place in the atmosphere ai-e very com- l)licated, so that the problem of forecasting weather in detail is a matter of the greatest difficulty. Importunately the wind, whi<'h is one of the most important factors to the aviator, is the most amenable to simple, physical laws. The composition of the atmosphere. — The atmosphere is composed of nitrogen, oxygen, carbonic acid gas, water vapor, dust, and cer- tain other gases in small quantities. All these are mixed together and are not joined chemically. The dust in the atmosphere furnishes solid ]iarticles, around which the water vapor condenses to form fog or rain and also gives the colors of the sky and causes twilight. Over the trenches there is a considerable amoimt of dust caused by the exploding of shells and guns. Not only does this help clouds to form, but it affects the visibility of different points and makes it hard to take clear photographs near the lines. The atmosphere probably extends 50 to 200 miles above the sur- face of the earth . That part of it which is dense enough or contains enough oxygen to support life is limited to about 30,000 feet. Ma- chines which fly normally abovc^ 15,000 feet should carry oxygen, otherwise the li(>arts of the crew of the airplane are liable to be strained. The weight of a column of air at normal temperatiu'e and at sea level is about L5 pounds per square inch, which corresponds to the weight of a column of mercury 30 inches high. The air at approxi- mately 20,000 feet is half as dense as it is at sea level. The density of the air affects the efficiency of the airplane engine considerably- AIR SERVICE HANDBOOK. 136 Atmospheric pressure. — The pressure of the air — which will be seen later to be a variable quantity and its changes to be of gi-eat use in forecasting weather — is clue to the weight of air above the place where the pressure is exerted. It will be readily seen that the longer the vertical cohimn of air, and the greater the density of the air, the greater also will be the pressure exerted at the bottom of the column. Hence at two places, one above the other, the pressure at the lower place will be equal to the pressure at the upper plus the pressure due to the weight of tlie air between the two. This differ- ence in pressure will not be constant, but will depend on the density of the air, which in turn varies with the temperature and pressure. The formula connecting the difference of pressure at two places has been given under ''Barometer." The temperature of the air generally falls off with height, but near the surface of the ground the rate of decrease is often far from con- stant, and it is not uncommon to find a warmer layer of air above a colder one. The average rate of decrease is about 1° F. for every 300 feet. Above 5,000 or 6,000 feet the rate of decrease of temperature becomes nearly constant at 1° F. for every 300 feet. At very grea heights (over 30,000 feet) the temperature ceases to fall with height and may sometimes rise again. This region, however, is at present aboA'e the height practicable for flying. If readings of the atmospheric pressure as measured by a barometer are taken at the same time at a number of diffei-ent places situated over a wide area, and are then plotted on a map against each station the readings will be found to he arranged in some order, ('ertain areas will have low pressure and others will have higli pressure. On any topographical map lines or contours are drawn showing the heights of the ground equally on the pressure map it is possible to draw similar contours showing the heights of the barometer. As all places on any one contoiir are the same height so all places on any line on the pressure map will have the same atmospheric pressure. These lines are called "Isobars." The isobars are generally drawn for each tenth of an inch of mer- cury, i. e., the difference of pressure between two places on two con- secutive isobars will be one-tenth of an inch of mercury. In regions where there is a large difference of pressure between places not far apart the isobars will necessarily be close together, just as on a map, where the slope of the ground is steep the contours will be close together. The rate at which pressure changes from place to place is known as the "Pressure gradient." When the pressure is changing rapidly the "Gradient"' is said to be "steep." 136 AIR SERVICE HANDBOOK. On weather maps there are other lines marked in red which show lines of equal temperatures. These lines are marked for differences of 20° F. and are called '"isotherms." The wind and its connection vith atmospheric pressure. — When one part of the country is under high pressure and another under low pressure, it might be supposed, at first, that aii- would be forced out of the region of high pressure toward that of the lower pressure, and that winds would be found everywhere blowing straight outward from the high pressures and straight inward toward the low pres- sures. Reference to any weather chart will, however, show that this is not what happens. The winds blow in a direction which is more nearly parallel to the isobars than at right angles to them. The explanation of this phenomenon is found in two facts: A. The earth is revolving about its axis. This causes all winds in the Northern Hemisphere to be deflected to the right of the path which they would follow if they were affected only by the " ' Pressure gradient," and tends to make them blow parallel to the isobars; similarly winds in the Southern Hemisphere are deflected to the left of this path. B. Friction between the air and the surface of the ground tends to lessen the deflection of the winds caused by the rotation of the earth. The result of these phenomena is that the winds at the surface blow round the centers of low pressure in incurving spirals in an anti- clockwise direction (in the Northern Hemisphere) and in outcurving spirals in a clockwise direction round the center of high pressure. If it were possible to eliminate sin-face friction, the velocity of the wind could l)e calculated theoretically froni the ''Pressure gradient." An imaginary wind having this theoretical velocity and direction is called the '"Gradient wind." and its velocity and direction are known as the ''(iradient velocity" and the '"Gradient direction . ' ' At a height of I .OOO to 2,000 feet al)Ove the .surface of the ground the effect of surface friction is very small and the actual wind has A'erv nearlv the "(Jradient velocity and direction." AIR SERVICE HANDBOOK. 187 Table — shows the ■"Gradient velocity" for different values of the "Pressure gradient. " The strength of the wind is generally expressed in terms of its velocity in miles per hour. For some purposes it is more convenient to use a rougher classification and to divide all winds from calm to a hurricane into 12 groups, denoting the strength of the wind ])y the numhor of the group into which it would fall. As this system is duo to Admiral Beaufort, it is known as the "•Beaufort scale." The strength of the wind may also be given in terms of the pressure exerted by it. say. on a flat plat(v This pressure varies as the square of the velocity. Table — • gives the relation between the velocity in miles per hour, the Beaufort number, and the pressure exerted on a Hat plate in pounds per square foot. "Gradient direction" is along the isol)ars with the low pre.saure on the left hand. Gusthic.'i.s of the irinrl. — It is found that the velocity of the wind does not remain constant. It is continually changing, and as a result is always rather gusty. The gustiness varies with different places and with different diiections of the wind. The difference between the average maximum velocity attained in the gusts and the average minimum velocity attained in the intermediate lull is known as the "" Fluctuation" of the wind. The ratio of the ""Fluctuation" ' to the mean velocity of the wind is called its 'Gustiness,"' and this is found to 1)e roughly constant for any one direction at any ))lace. whatever may l)e the mean A'elocity of the wind. Effect of the irind sirikiny obstacles. — If the ol)Stacles are low hills, such as are found on the plains and in rolling country, the wind may approximately follow the surface. If the obstacle is abrupt, such as the side of a house or a vertical cliff, the air striking the oljstacle will be deflected upwards and will not touch the surface of the earth again for some distance. Thus, in the lee of a tall building there is often a calm area or the wind may Ix- l)lowing in the opposite direction to the wind tip above. 138 AIR SERVICE HANDBOOK. This is especially noticeable in i)laces like (iil)raltar when the east Avind is lilowing. A iierson standing on the edge of the cliff may l)e in absolutely calm air but on pushing out his arm he will feel the wind l^lowing vertical u])ward at a considerable speed. The wind blowing over the hangars on the airdrome has caused a number of accidents because pilots forget that there is a down current which is often strong enough to prevent the machine clearing the hangars. In the old days machines which had practically no reserve of power and which could only fly level when near the ground found it very difficult to fly in windy weather. A machine on passing over a wood, for instance, might be caught in the down current on the lee side and in some cases machines had not enough power to prevent themselves hitting the ground. In the summer in sunny weather it was quite dangerous to cross a river because the down current would suck the machine practically into the water. The pilot at the present time has nothing to fear from these causes. Machines are so highly powered that they liave a much greater reserve of power than they will be called upon to use when affected by the changes of velocity in the air. At the present time if the pilot does not go merely seeking danger, wind only means that the machine will take longer when flying upwind and will take a shorter time to reach a place when flying down wind. The only thing which sto|)s war flying is low fog which prevents a pilot seeing where he lias got to and will prevent him from flnding his airdrome when he returns. If a pilot obeys the ordinary rules, such as getting off and landing directly into the wind, he has nothing to fear from such things that used to l>e called ' ' holes in the air. " ' " air pockets, ' ' etc . . and from such things as are called cyclones, aerial cataracts, etc. FORECASTING. From the foregoing it has been seen that when baxometric pres- sures are plotted on a map they are arranged according to some order. It is now necessary to consider certain typical cases of pressure distribution. A. The cyclone. — This type consists of a center of low pressure from which the pressure rises on all sides. The isobars are roughly circular about the center of low pressure. The winds blow in an anticlockwise direction round the center (clockwise in the Southern Hemisphere). The different parts of a cyclone have each their own type of weather of which the following description may be given. The temperature is always higher in front than in rear, the warm air in front having a peculiar close, mugg>' character, quite inde- AIR SERVICE HANDBOOK. 139 pendent of the actual height of the thermometer. The cold air in the rear on the contrary has a peculiarly exhilarating feeling, also quite independent of the thermometer. The force of the wind depends almost entirely on the '"Gradient." In the center it is dead calm and the steepest 'Gradients''' are usually found at some distance from center. The relative steepness of the "Gradients^' measures the intensity of cyclones. If two lines are drawn thiough the center of the cyclone, one in the direction parallel to that of its motion and another at right angles to this direction, the cyclone will be divided into four quad- rants, each of which has its pecidiar tj-pe of weather. The line at yVin<^y/S C?iancf/r,a />/K>V /"o Sr-^ Fig. 5s. right angles to the direction of motion is known as the line of the "trough." The broadest feature of the weather in an average cyclone con- sists of an area of rain near the center surrounded by a ring of cloud, but both rain and cloud extend farther to the front than to the rear, of the center. When, however, examining the nature of the cloud and rain as well as the general appearance of the sky. it is found that the cyclone is di\'ided into two well-marked halves by the line of the "trough." The front may be further divided into right or southeast, and left or northeast fronts which, though they have much in common, are sufficiently different to be classified separately. Coming now to more minute detail, in the left or northeast front when the steepest "Gradients" are somewhere south of the center, the first symptoms of the approach of a cyclone are a halo, with a gradual darkening of the sky until it becomes quite overcast with- out any appearance of the formation of true clouds; or else liglit 140 AIR SERVICE HANDBOOK. wisps or barred stripes of ciri'us moving sideways, apj^ear in the l>lue sky. and gradually soften into a uniform black sky of a strato- 'umulus type; near the center light ill-defined showers fall from a uniformly lilack sky, the wind from some point between southeast and northeast blows uneasilj^, and though the aii' is cold and cliilly there is an opjjressive feeling about it. These appearances con- tinue until the barometer commences to rise, when the character of the weather at once begins to alter. In a cyclone, when the steepest "Gradients" are somewhere to the north and east of the center, the general character of the weather is the same as above described, but much more intense. The wind rising at times to a heavy gale, and the ill-defined showers developing into violent squalls. In the right or southeast front, when the steepest "Gradients" are to some point south of the center, which are the commonest cases, the first symptoms are likewise a gradual darkening of the sky into the well-known pale or watery sky, with muggy, oppressive air; or else, as in the northeast front, msps of cirrus first appear in the blue sky which gradually becomes hea\'ier and softer until the sky is uniformly overcast with a strato-cumulus type of cloud. Near the center rain usually in the form of a drizzle sets in and the wind from some point between southeast and southwest, vary in force according to the steepness of the "Gradients," drives the cloud and rain before it. In winter snow takes the ])lace of rain and in the autumn the northeast wind brings with it little flurries of snow. The line of the "trough" marks the line of heavy showers or squalls, especially tlie portion on the southern side of the center. The general character of the west of the depression is a cool, exhilarating feeling in the air, with a high, hard sky of which the tendency is always to break into firm, detached masses of cloud. The rain which occurs near the center is usually in cold, hard, brisk showers or hard squall*?, and the general look of the weathsr presents a marked contrast to the dirty appearance of the weather wluch characterizes the whole front part of a cyclone. Farther from the center showers or squalls are replaced bj^ simply detached masses of cloud, and finally these disapjiear leaving a blue sky- The wind fi'om .some point between west and north blows gustily. The whole of the rear of a cyclone partakes of this general char- acter, but the change of weather along the north of the cyclone is not nearly so well marked as along the southern portion. The motion of cyclones is as a rule from west to east, the general direction being about west-southwest to east-northeast. Occa- sionallv thcv move north or south, but seldom from east to west. AIR SERVICE HANDBOOK. 141 They may also at times remain stationary for a (.lay or two, but this is rare. The followinj^ indications are printed on every weather map: When the wind sets in from points between south and southeast and the barometer falls steadily a storm is approaching from the west or northwest, and the center will pass near or north of the observer within 12 or 24 hours, with wind shifting to northwest by way of southwest and west. WTien the wind sets in from points between east and northeast and the barometer falls steadily a storm is approaching from the south or southwest, and its center will pass near or to the south or east of the observer ^vithin 12 or 24 hours, with wind shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be in- dicated by the rate and the amount of the fall in the barometer. B. The antiq/clone. — The distribution of pressure in an anticyclone is almost the reverse of that in a cyclone. It consists of a central area of high pressure from which the pressure gradually decreases on all sides. The "'Gradients" are generally very slight so that the winds, which blow around the center in a clockwise direction, are very light. Unlike a cyclone, no very definite description of the weather can be given; in fact, almost any tjT)e of weather may be found in an anticyclone except strong winds; heav^' rain is also in- frequent. On the whole, the weather is tine, but in ^vinter periods of dull, cloudy weather often accompany an anticyclone. On the other hand, days with cloudless skies, and keen, frosty weather in winter or hot weather in summer, frequently occur in anticyclones . Unlike cyclones, they have no general direction of motion, but move very slowly and in any direction. They frequently remain sta- tionary for several days together. In the colder months anticyclones are very frequently accompanied by fog. C. Secondary (/epnssions. — On the outside of a cyclone irregu- larities in the isobars frequently occur. These may be mere kinks in the isobars, or they may be well marked and have an independent area of low pressure. A very common form is for the isobars to have approximately the shape of a V, such cases being known as V de- pressions. The secondaries travel around the main depression in the same direction as do the winds, viz, anticlockwise. The weather in these secondaries varies according to whether they are well marked or not. ^Vhere only a small kink occurs, only cloudy skies and temporary rain may be produced, but if they are well marked and the "Gradients" are steep the winds may become very strong and the weather very bad. 142 AIR SERVICE HANDBOOK. In a secondary depression of average intensity the sequence of weather is as follows : As the secondary approaches, the weather is similar to that in the right front of a cyclone; as the secondary passes, the barometer sud- denly begins to rise and there is frequently a heavy squall, as in the "trough" of a cyclone, and the wind suddenly veers to a more northerly quarter. On the side away from the center of the main cyclone the winds are generally very strong, but between the secondary and the main depression they are light. In the rear of a secondary the weather is similar to that in the rear of a cyclone. D. The wedge. — It frequently occurs that a series of cyclones pass across the country in a continuous succession. Between two of these cyclones the isobars will be roughly V-shaped, but in this case with the highest pressure within the V. These are times of brilliantly tine weather, cloudless skies, and clear atmosphere, but as another cyclone is approaching they last only a short time. E. Line squalls. — It has been said that as the " trough "of a cyclone passes there is frequently a heavy squall. This is generally of the type known as a "Une squall." Such a squall stretches in a Line for a long distance across the country and may be as much as 500 miles in length . The squall moves in a direction approximately at right angles to its length with a velocity of between 30 and 50 miles an hour. The breadth of the squall is usually narrow, so that it does not last long — generally from half an hour to two hours. The squall gives no sign of its approach, except that if the sky is fairly cloud- less a long Une of well-marked cumulus may be seen in the distance, gradually coming nearer. As the squall reaches the observer the wind suddenly increases (or occasionally increases sUghtly and then suddenly decreases) and at the same time the direction suddenly changes to a more northerly quarter. The barometer generally shows a small, sudden rise, and the tem- perature always suddenly falls. Heavy rain and frequent hail, with sometimes thunder, set in at once. The whole squall is of a violent nature and it may do considerable damage. This phenom- enon seems to be caused by the sudden inrush of a cold current of air from some northerly quarter, which forces the warmer air in front of it to ascend. As these squalls give no warning of their ap- proach, and as they are very violent, they are of a particularly dan- gerous nature. They are to be expected when the "trough" of a depression passes, and especially in a V, or secondary. After one squall has passed, others frequently follow in the next few hours. These "line squalls" may also occur at times in conditions that AIR SERVICE HANDBOOK. 143 would be expected to give only a moderate westerly or southwesterly wind. An observer can only forecast these phenomena when he is in possession of information that a squall has passed certain points and is traveling in his direction. F. Fog. — True fog (clouds on the surface of the earth are not true fog) on land is only formed when there is little wind and when the sky is cloudless. During the day the air is warm and takes up water vapor. On a calm, cloudless evening the ground is cooled by out- ward radiation and the aix- near the ground is also cooled. This cold air being heavier flows down the sides of hills and mixes with the warm, moist air over the valleys, which is thereby cooled. The cooling so produced may be sufficient to cause some of the water vapor to condense, and fog is formed. If there is much wind the air is kept stirred up and no cold air is formed. If, on the other hand, the sky is cloudy, the ground is not cooled by radiation, so that in this case no fog is formed. G. Conditions of the atmosphere affectinq aviation. — The available knowledge of the upper air is still rather small, but some information has been obtained which is of use to aviators. If it is required to ascertain what the wind is at a height above the ground, there are several methods by which this information may be obtained. First, by sending up a small balloon which drifts along with the velocity of the wind at the height it has reached. The balloon is observed by theodolites, and the velocity and dii-ection of the wind at any height can be calculated accurately. This, however, is an elaborate method and takes considerable time. Secondly, some information may be obtained by observing the motion of the clouds. From these the direction of the wind at their level may be accurately gauged. It must, however, be remembered that the height of the clouds can not definitely be fixed without suitable instruments, and therefore the velocity is only very approxi- naate. Nevertheless, a rough idea may be obtained by noting ■whether the clouds are moving ([uickly or slowly. The lower the clouds are the faster they "appear" to move. Thirdly, an estimate of the upper w.ind may be obtained from a daily weather map by calculating the "Gradient" wind. At a height of a thousand feet or more the "Gradient" wind is found to agree very well with the winds at those heights as found by means of kites or pilot balloons. Fourthly, it is possible to estimate the upper wind from the known surface wind at the time. It is nearly always found that for the first 1,000 or 2,000 feet above the surface the velocity increases 144 AIR SERVICE HANDBOOK. dii-ectly as the "height above sea level."' Hence, if at a place 500 feet above sea level the surface 'wind were 15 miles per hour, the velocity of 500 feet above the "surface" (i. e., 1,000 feet above sea level) would be expected to be 30 miles per hour; and at 1,000 feet above the "surface" (i. e., 1,500 feet above sea level), 45 miles per hour. This regular increase in velocity goes on until the "Gradient velocity" is reached, after which the wind generally remains almost constant, but may increase or decrease. In the case of easterly winds there is very frequently a decrease at higher altitudes. The direction of the wind a few thousand feet up is slightly changed in a clockwise direction from that of the surface wind, i. e., if the surface wind were south the upper wind might be expected to be south- southwest or southwest. A table of changes in velocity and direc- tion which are the results of observation is given in the Appendix. A rough rule for the pilot is this: The wind at a flying height may be expected to be double that on the airdrome, and to have changed two points to the right of the direction in which one should leave the airdrome. It is a well-known fact that the wind is frequently stronger in the day than at night. This is nearly always the case except in rough Weather. At sea the effect is not so marked. The decrease at night, however, only takes place at the surface. At a height of from 1,000 to 2,000 feet the wind is stronger at night than in the day. The cause of the surface decrease in velocity at night seems to be the formation of a shallow layer of air on the ground, which does not take part in the general movement of the air. "^Miile the velocity of the wind increases with height, thegustiness almost invariably falls off, so that the wind is more steady above than at the surface. No definite rule can be given about the rela- tion of gustiness and height. Allied with gusts are "remous" experienced in fl\ing. These may be due to two causes: First, a horizontal gust suddenly striking the airplane and causing a temporary change in its velocity tkrough the air: this will produce a momentary change in the lift. Second- ly, there may be an ascending or descending current, which will make the airplane rise or fall. These upward or downward currents may be caused either by trees, buildings, the contour of the ground, or they may be due to rising currents of hot air. Another possible occurrence is for the airplane to pass into a mass of air mo\'ing in a different direction to that in which it had been AIR SERVICE HANDBOOK. 146 before. This alt^o causes a temporary change in the speed through the air, but this last cause is not a common occurrence. Clouds. — The water vapor in the air is chiefly supplied by the evaporation from the surface of oceans, lakes, etc. The air can only hold a certain amount of water vapor. Hot air holds more than cold, so that if the temperature of warm air which is saturated with water vapor be lowered the result is the formation of fog clouds, rain, orsnow. The formation of these is facilitated by the presence Ci'rofr-a'i/t A NifhestAfoi/ntains in /'heWortd. ' ■ ' ^ ft t» t S ^'^ /Vlinbur -C Cei/ing /orZefitielin: /^,j '\'<^ ^ . I'/f /■'" 'JftfeSa//o: r,j -"^ Fig. 59. of dust in the atmosphere around which the particles of water can form. The warm, moist aii- near the sirrface of the earth rises, meets the colder air at higher levels, which causes the formation of clouds out of which rain falls. The highest clouds are the cirrus, which occur at about 30,000 feet. They are composed of particles of snow and ice. The sun can shine through them and they appear delicate, fibrous, and hair- like. These clouds are sometimes called "Mares tails." 46643—18 10 146 AIR SERVICE HANDBOOK. Below at about 20,000 feet are the cirro-cumulus clouds, which consist of detached, white, globular masses. They form during the hottest months of the year, when the air is still, and foretell the breaking up of an anticyclone. At about 16,000 feet is a formation of cloud, sometimes spoken of as the "Mackerel sky." This is a calm-weather cloud and is often observed apparently motionless tor some time. lM0^ Below this are the clouds formed by the ascending currents of air which may be met with from close to the ground to kbout 10,000 feet. XVI. THEORY OF FLIGHT. Air has weight inertia and momentxun. It therefore obeys New- ton's laws and resists movement. It is that resistance or reaction which makes flight possible. Fig. 61. Flight is secured by driving through the air a surface inclined upward and toward the direction of motion. This surface may be either straight or curved. Chord. — The chord is, for practical purposes, taken to be a straight line from the leading edge of the surface to its trailing edge. For purposes of considering the lift of a surface this line is drawn too low. The neutral lift line, for a curved surface, is found by means of wind tunnel research and it varies with the differences n the camber of surfaces. This neutral lift line is above the ' ' chord " o£ the surface. In order to secure flight the inclination of the surface must be such that the neutral lift line makes an angle with and above the AIR SERVICE HANDBOOK. 147 line of motion. If it is coincident with the line of motion there is no lift. If it makes an angle with the direction of motion and below it then there is a pressure tending to force the surface down. Angle of incidence. — This angle is defined as the angle the chord makes with the direction of motion. This is a bad definition, as it leads to misconception and is described thus chiefly so that the incidence of a plane can be measured easily when rigging an air- plane. The angle of incidence for the purposes of considering flight is best described as the angle the neutral lift line makes with the direction of motion relative to the air. It will be no good giving a practical rigger this definition, as he would be unable to find the neutral lift line and he would probably not know the direction of motion relative to the air, whereas, he can easily put the machine nv^i Fig. 62. with the thrust horizontal and measure how high the leading edge is above the trailing edge. This is explained because there are certain machines which are described as ha\'ing a negative angle of incidence on the main plane, and one might get the idea that these machines lift, although the angle is negative. This is not so because the neutral lift line must always be above the line of motion. These remarks only apply to cambered surfaces. In the case of flat surfaces the neutral lift line coincides with the chord. Flat lifting surfaces are never used in a machine. A surface acts upon the air in the following manner: As the bottom surface meets the air it compresses it and accel- erates it downward. As a result of this definite action there is, of course, an equal and opposite reaction upward. The top surface in moving forward tends to leave air behind it, thus creating a semivacuum or rarified area over the top of the surface. Consequently, the pressure of air on the top of the surface is decreased, thus assisting the action below to lift the surface upward. The reaction increases approximately as the square of the velocity. Approximately three-fifths of the reaction is due to the decrease of 148 AIB, SERVICE HANDBOOK. density on the top of the suiiace, and only some two-fifths is due to the upward reaction secured by the action of the 1;>ottom surface upon the air. A practical point in respect to this is that in the event of the fabric covering the surface getting into l)ad c-ondition it is more likely to strip off the top than off the bottom. The value of the reaction on an inclined surface is given l)y the equation R = KSVH where li is total reaction: A' is a coefficient which varies with \arious wing curves: S is the surface of the aerofoil: T^is the velocity of the surface through the air: i the angle at which the aerofoil meets the stream of air measm-ed in. Radians and this angle in practical flight is always very small. This formula is inserted here, as it is the fundamental formula of flight. The direction of the reaction is, at efficient angles of incidence, approximately at right angles to the neutral line of the surface, and it is, in considering flight, convenient to divide it into two com- ponent parts or values thus: 1. The vertical component of the reaction, i. e., lift which is opposed to gravity, i. e., the weight of the airplane. 2. The horizontal component, i. e., drift (sometimes called re- sistance), to which is opposed the thrust of the propeller. The direction of the reaction is, of coiu-se, the resultant of the forces lift and drift. The lift is the useful part of the reaction. The drift is far from useful and must be overcome by the thrust in order to secure the necessary velocity to produce the requisite lift for flight. Drift. — The drift of the whole airplane (we have considered only the lifting surface heretofore) may be conveniently divided into three parts, as follows: Active drift, which is the drift produced by the lifting surfaces. Passive drift, which is the drift produced by all the rest of the airplane, the struts, wires, fuselage, landing gear, etc., all of which is known as the "detrimental siu-face." Skin friction, which is the drift produced by the friction of the ail- with roughness of surface. The latter is practically negligible, having regard to the smooth surface of the modern airplane, and its com})aratiA ely slow velocity com])ared with, for instance, the veloc- ity of a propeller ])lade. IJFT-DKIFT RATIO. The importance of lift to drift is known as the lift-drift ratio and is of paramount importance, for it expresses the "efficiency" of the airplane (as distinct from the engine and propeller). A knowledge of the factors governing the lift-drift ratio is, as will be seen later. AIR SERVICE HANDBOOK. 149 ail al).s(ilulc iH'ccs.sity lo aiiNoiic rcsjioriMiblf lor I lie rigt;inij of an airplane and the maintenance of it in an efficient and safe condition. These factors are as follows: Velocilij. — The greater the velocity the greater the proportion of drift to lift, and coiise(|uently the less the efficiency. Considering the lifting surfaces only, both the lift and (active) drift, being com- ponent parts of the reaction, increase in the same pro])ortion (as the square of the velocity) and the efficiency remains the same at all speeds. However, considering the airplane as a whole, we must remember the passive drift. This also increases as the square of the velocity, but there is no attendant lift. This passive drift adds itself to 1 he active drift and results in increas- ing the proportion of total drift to lift. But for the increase in passive drift the ef- ficiency of the airplane would not fall with in- creasing velocity, and it would be possible Ity doubling the thni.st to approximately double the speed or lift . This can never be done, but every effort is made to decrease the passive drift by 'stream lining," i. e., by giving all "detilinentar' parts of the airplane a form by which they will pass through the air wdth the least possil)le drift., The fuselage, struts, wires, etc., are all 'stream lined"' as much as possible. In the case of a certain well-known type of airplane the replacing of the ordinary wires by '"stream-lined"" wires added oxcv 5 miles an hour to the flight speed. ■'Head resistance" is a term often applied to passive drift, but it is apt to convey a wrong impression, as the drift is not nearly so much the result of the head or forward part of struts, wires, etc., as it is of the rarified area behind. The aVjove illustrates the flow of air around two ol)jects moving in the direction of the arrow. In the case of .\ you will note that the rarclied area behind the object is very considerable, whereas in the case of F^ the air flows around it in such a way as to meet very closely in the rear of the object, thus decreasing the rarefied area. Fi<;. ti: 160 AIR SERVICE HANDBOOK. The greatei' the rarelied area the less the density, and consequently the less the pressure of air upon the rear of the object. This means that it will require more thrust to overcome this backward pressure. The "fineness" of the stream-line shape, i. e., the proportion of length to width, is determined by the velocity — the greater the velocity the greater the fineness. The best degree of fineness for any given velocity is found by means of wind-tunnel research. The practical application of all this is, from a rigging point of view, the importance of adjusting all stream-line parts to be dead on in the line of flight. Angle of incidence. — The most efficient angle of incidence varies with the thrust at the disposal of the designer, the weight to be car- ried, and the climb-velocity ratio desired. The best angles of incidence for these varying factors are found by means of wind-tunnel research and practical trial and error. Gen- erally speaking, the greater the velocity the smaller should be the angle of incidence in order to preserve a clean stream-line shape and prevent the formation of a rarefied area and the formation of eddies. Should the angle be too great for the velocity then the rare- fied area over the top of the surface becomes of irregular shape with attendant turbulent eddies. Such eddies possess no lift value since it has taken power to produce them; they represent drift and ad- versely affect the lift-drift ratio. Also too great an angle for the velocity will result in the under side of the surface tending to com- press the air against which it is driven rather than accelerate it downward, and that will tend to produce drift rather than the upward reaction or lift. From a rigging point of view one must presume that ca ery stand- ard airplane has its lifting surface set at the most efficient angle, and the practical application of all this is in talcing the greatest jjossible care to rig the surface at the correct angle and to maintain it at such an angle. Any deviation will adversely affect the lift-drift ratio, i. e., the efliciency. f'kamber. — The lifting surfaces are cambered, i. e., curved, in order to decrease the horizontal component of the reaction, i. e., the drift. The bottom camber: If the bottom of the surface were flat every particle of air meeting it would do so with a shock, and such shock would produce a very considerable horizontal reaction or drift. By curving the surface such shock is diminished and the curve should be such as to produce a uniform i not necessarily constant) accelera- tion and compression of the air from the leading edge to the trailing edge. Any unevenness in the acceleration and compression of the air produces drift. AIR SERVICE HANDBOOK. 151 The top camber: If this was flat it would produce a rarefied area of irregular shape. The bad effect of this upon the lift-drift ratio has already been explained. The top surface is then curved to produce a rarefied area the shape of which .shall be as stream-line and free from attendant eddies as possible. Fk;. 1)4. The camber varies with the angle of incidence, the a elocity. and the thickness of the surface. Generally speaking, the greater the velocity the less the camber and angle of incidence. With infinite velocity the surface will be set at no angle of incidence (the neutral lift line coincident with the direction of motion relative to the air). XI iCKtra, Fig. tjo. and would be top and bottom of pure stream-line form, i. e., of infinite fineness. This is, of course, canying theorj' to an absurdity, as the surface would then cease to exist. The best cambers for varying velocities, angles of incidence, and thicknesse.s of surface are found bv means of wind-tunnel research. Id2 AIR SERVICE HANDBOOK. The practical application of all this is in taking the greatest care to prevent the surface from becoming distorted and thus spoiling the camber and consequently the lift-drift ratio. The advantages of a cambered plane over a flat surface are these: 1. A cambered plane continues to lift when the chord is parallel to the line of liight. 2. The total lift is much greater than that of a flat plane of equal surface. ;'. The resistance of a cambered plane in relation to its lift is much less than that of a flat surface. 4. The top and bottom cambers can be made of different shapes so that each will give the maximum lift. 5. This enables suitable spars and bracing to be placed inside the surface without loss of efficiency. Aspect ratio. — This is the ]:)roportion of span to chord. Thus, if the span is for instance 50 feet and the chord 5 feet, the surface would be said to have an aspect ratio of 10 to 1. If a flat surface is acted on by a stream of air at right angles to this surface the shape does not very miich matter. But when this surface is inclined to the du'ection of motion of the air the shape makes a great difference. For a given velocity and a given area of surface, the higher tlie aspect ratio the greater the reaction. It is obvious, I think, that the gi'eater the span, the greater the mass of undisturbed air engaged, and, as already explained, the reaction is partly the result of the mass of air engaged. The woE-d "undisturbed" is iised, for other- wise it might be argued that whatever the shape of the surface, the same mass of air would be engaged. The word "undisturbed" makes all the difference, for it must be remembered that the rear ])art of the under side of the surface engages air most of which has been deflected downward by the surface in front of it. That being so the rear part of the surface has not the same opportunity of forcing the air downward (since it is already fiowdng downward) and secur- ing therefrom an upward reaction as has the surface in front of it. It is therefore of less value for its area than the front part of the sur- face, since it does less work and secures less reaction, i. e., lift. Again the rarefied area over the Idji of I he surface is most rare toward the front of it, as owing to eddies tlic rear of such area tends to become denser. AIR SERVICE HANDBOOK. 158 Thus you see that the Iruiii pari ol tlie .surlacc is ilie iu„.st valuable troni the point of view of securing an upward reaction from tlie air- and so b>- increasing the proi)ortion of front, or span, to chord we increase the amount of reac-tion for a given velocitv and area of surface. That means a better proportion of reaction to weight of surlace, tiiough the designer nuist not forget the drift of struts and \nres necessary to brace up a surface of high aspect ratio. Xot onl> that, but proi-ided the chord is not decreased to an extent making it impossible to secure the best camber owing to the thick- ness of tlie surface, the higher the aspect ratio the better the lift-drift ratio. The reason of this is rather obscure, ll is sometimes advanced that it is owing to the -spill " of air from under the u-ing tips; mth a high aspect ratio the chord is less than would otherwise be the case. Less c-liord results in smaller wing tips and conse- .J.I-- quently less -spill." This, however, appears to be a rather inade- quate reason for the high aspect ratio producing the high lift-drift ratio. Other reasons are also ad vanced , but thev are of such a con- tentious nature that it is hardly well to go into* them here They are ot interest to designers, but not to the same extent to the prac- tical pilot and rigger. • Stagger. — This is the advancement of the top surface relative to the bottom surface and is not of course applicable to a single surface i e a monoplane. In the case of a biplane having no stagger, there will be '-mterfereuce" and consequent loss of efficiency unless the -ap between the top and bottom surfaces is equal to not less than about one and a half times the chord. If less than that the air enga-ed by the bottom of the top surface will have a tendencv to be drawn into the rarefied area over the top of the bottom surface, with the result that the surfaces will not secure as good a reaction as would other%vise be the case. 154 AIR SERVICE HANDBOOK. It is uot practicable to have a gap of much more than distance equal to the chord owing to the drift produced by the great length of struts and wires such a large gap would necessitate. By "stag- gering" the top surface forward, however, it is removed from the action of the lower surface and engages undisturbed air, with the result that the efficiency can in this way be increased by about 5 per cent. Theoretically, the top plane should be "staggered" forward for a distance equal to about 30 per cent of the chord, the exact distance depending upon the velocity and angle of incidence; but this is not always possible to arrange in designing an airplane f — owing to difficulties of bal- H K ance, desired position, and view of pilot, observer, etc. Horizontal equivalent. — The vertical component of the reaction, i. e., lift varies as the horizontal equivalent (H. E.) of the surface, but the drift remains the same. Then it follows that if the H. E. grows less the ratio of lift to drift must do the same. The above are front views of three surfaces of equal area. The top view has it full H. E. and therefore, from the point of \aew from which we are at the moment considering efficiency, it has its best lift-drift ratio. The two lower views possess the same sm*face as that of the one above, but one is inclined upward from its center and the other is straight but tilted. For these reasons their H. E.'s are, as illustrated, less than in the rase of the first view. That means less vertical lift and the drift remaining the same (for there is the same amount of surface in each) the lift-drift ratio falls. The margin of power. — ^This is the power a\-ailable above that necessary to maintain horizontal flight. The margin of lift. — This is the height an airplane can gain in a given time starting from a given altitude. As an example, thus — 1,000 feet the first minute and starting fi-om an altitude of 500 feet above mean sea level. The margin of lift decreases with altitude owing to the decrease in the density of the air which adversely affects the engine. Pro- vided the engine maintains its impulse with altitude, then, if we AIR SERVICE HANDBOOK. 155 ignore the problem ol the jiroj^eller, the margin of lift would not disappear. Moreover, greater velocity for a given power would be secured at a greater altitude, owing to the decreased density of air to be overcome. At the present time machines are being designed to be most efficient in air of decreased density and attention is being paid to keeping up the power of the engine at a height by means of "blowers" and "supercharges" which increase the charge sucked into a cylin- der at a high altitude. The minimum angle of incidence is the smallest angle at which, for a given power, surface (including detrimental surface) and weight, horizontal flight can be maintained. The maa-imuvi angle of incidence is the greatest angle at which for a given power, surface (including detrimental surface) and weights horizontal flight can be maintained. The optimum angle of incidence is the angle at which the lift-drift ratio is highest. In modern airplanes it is that angle of incidence possessed by the surface of the main ])lane when the axis of the propeller is horizontal. The best climbing angle is appro.ximately halfway between the maximum and optimum angles. All present day aii7)lanes are a compromise between climb and horizontal velocity. Exficntials for maximum climb. — 1. Low velocity, in order to secure the best lift-drift ratio. 2. Having a low velocity, a large surface will be necessary in order to engage the necessary mass of air to secure the requisite lift. 3. Since (a) such a climbing machine will move along an upward sloping path, and (6) will climb with its propeller thrust horizontal, then a large angle of the main plane relative to the direction of thrust will be necessary in order to secure the requisite angle relative to the direction of motion. 4. The velocity being low then it follows that for that reason also the angle of incidence should be comparatively large. 5. Since such an airplane would be of low velocity and therefore possesses a large angle of incidence, a large camber would be necessary. The propeller thrust should be always horizontal because the most efficient flying machine (having regard to climb and velocity) has so far been found to be an arrangement of an inclined surface driven by a horizontal thrust — the surface lifting the weight and the thrust overcoming the drift. 156 AIR SERVICE HANDBOOK. This is in practice a far more efficient arrangement than the hellicopter, i. e., the propeller revolving about a vertical axis and producing a thrust opposed to gravity. If when climbing the propeller thrust is at such an angle as to tend to haul the airplane upward, then it is in a measure acting as a hellicopter and that means inefficiency. The reason of a hellicopter being inefficient in practice is due to the fact that, owing to mechanical diffi- culties, it is impossible to construct within a reasonable weight a o/1'recfion o/ motion 9 tf '*'♦« ■yo'-t^ontsi a /s/~o/3&r- fncHnjrt art /^ //^fi O'r-ac n ori of friofioft Fig. 6S. propeUer of the requisite dimensions. That being so it would be necessary in order to absorb the i>ower of the engine, to revolve the comparatively small surface propeller at an immensely greater velocity than that of the airplane surface. As already explained, the lift-drift ratio falls with velocity on account of the increase in passive drift. This applies to a blade of a propeller, which is nothing but a revolving surface set at an angle of incidence, and which it is impossible to construct without a good deal of detrimental surface near the fuselage. 'ssf)^'S/s /'Ofv S/'rectfon. of ^otzon . JYOrizonfci/ Fig. 69. Essentiah for maximum velocity. — The following are the -essentials for au airplane of maximum A'clocity for its ])ower. and possessing niereh- enough lift to get ot'f the ground, but no margin of lift : 1. Comparatively high velocity. 2. A comparatively small siu'face because, l)eing of greater velo(it>' than the maximum climber, a greater mass of air will be engaged for a given surface and time, and therefore* a smaller surface will be sufficient to secure the requisite lift. AIR SERVICE HANDBOOK. 167 '.\. A small angle relative to the projjeller thrust, sijue the latter coincides with the direction of motion. 4. A comparativeh small angle of incidence b>' reason of the high velocity. 5. A comparatively small (•aml)er follows as the result of the small angle of incidence. It is mechanicall}- impossible to construct an airplane of reason- able weight of which it would be possible to ^'ar\■ the above opposing essentials. Therefore, all airplanes are designed as a compromise between climb and velocity. As a rule airplanes are designed to have at low altitude a slight margin of lift when the propeller thrust is horizontal. By this means when the altitude is reached where the margin of lift dis- appears (on account of loss of engine power), and which is, conse- quentlj-, the altitude where it is just possible to maintain horizontal flight, the airplane is flying with its thrust horizontal and with maximum efhciency (as distinct from engine and propeller effi- ciency). The margin of lift at low altitude and when the thrust is horizontal should then be such that the higher altitude at w'hich the margin of lift is lost is that altitude at which most of the aii'- planes' horizontal-flight work is done. That insures maximum velocity when most required. Unfortunately, when airplanes designed for fighting are concerned the altitude where most of the work is done is that at which both maximum velocity and maximum margin of lift for power are required. At present designers are unable to effect this. XVII. STABILITY. Stabil'Uy is a condition whereby an object disturbed has a natural tendency to return to its first and normal position. For example, a ■weight suspended by a cord. Instability is a (condition whereby an object disturbed has a natural tendency to move as far as possible away from its first position with no tendency to return. For example, a stick balanced vertically upon the finger. Natural instability is a condition whereby an object disturbed has no tendency to move farther than it is displaced by the force of the disturbance, and has no tendency to return to its first position. In order that an airplane may be reasonably controllable, it is necessary for it to possess some degree of stability longitudinally, laterally, and directionally. Longitudinal stability in an airplane is its stability about au axis transverse to the direction of normal horizontal flight, and without which it would pitch and to.ss. 158 AIR SERVICE HANDBOOK. Lateral stability is its stability al)Out its loiigitiuliiial axis and without which it would roll sideways. Directional stability is its stability about its vertical axis, and without which it would have no tendency to keep its course. For such directional stability to exist there must be "in effect" more "keel surface" behind the vertical axis than there is in front of it. By "keel surface" is meant everything which can be seen when looking at an airplane from the side — the sides of the body, landing gear, struts, wires, etc. The words "in effect" are used because the actual area of the "keel surface" in front of the vertical axis may be greater than the actual surface behind this axis; but such surface will be much nearer to the axis so that it has not nearly so much leverage as the surface behind. The above illustration represents an airplane (directionally stable) flying along a course B. A gust striking it as indicated acts ' I>/rect/c>n of "motion s^ae to >r>omofitt/m tbrusr "^ /Vsrr-Caroportion of "keel surface" behind the turning axis and throws it into a new course. The machine, however, does not travel along this new coiu-se, owing to its momentum in the direction B. It travels as long as such momentum lasts in a direction which is the resultant of the two forces — thrust and momen- tum. But the center line of the airplane is pointing in the direc- tion of the new course; therefore its attitude relative to the direction of motion is more or less sideways, and it consequently receives an air pressure in the direction C. Such pressure acting along the "keel siu'face" presses the tail back toward the first position, in which the airplane is upon its course B. This tendency to turn is continually taking place during flight, but in a well-designed airplane such stabilizing movements are, for the most time, so slight as to be imperceptible to the pilot. If an airplane was not stabilized in this way it would not only be continually trying to leave its course, but it would also possess a dangerous tendency to "nose away" from the direction of the side gusts. In such case the gust shown in the above illustration would turn the airplane around the opposite way a very considerable dis- AIR SERVICE HANDBOOK. 169 tanr-p; and tlio ri^rlit wing being on the outside of llic turn woukl travel with greater Avlocity tlian the left wnng. Increased veloeity means increased lift; so that, the riglit ^\^^g lifting, the airplane would t\nn over sideways very quickly. Longitudhud atnbility. — Flat surfaces are longitudinally stable, owing to the fact that with decreasing angles of incidence the center line of pressure (C. P.) moves forward. The r. P. is a line taken across the surface, transverse to the direction of motion, and about which all the air forces may be said to balance, or through which they may be said to act. ■^:i\ Fi<;. 71. Imagine A to be a flat surface, attitude vertical, traveling through the air in the direction of motion M. Its C. P. is then obviously along the exact center line of the surface as illustrated. In B, C. and D, the sui'faces are shown with angles of incidence decreasing to nothing, and you will note that the C. P. moves forward with the decreasing angle. Tlie reason the C. P. of an inclined surface is forward of the center of the surface is because the front of the surface does most of the work. Now, should some gust or eddy tend to make the surface decrease the angle, i.e., dive, then the C. P. moves forward and pushes the Fig. 72. front of the surface ii|). Sliould the surface tend to assume too large an angle, then the reverse happens— the C. P. moves back and pushes the rear of the surface up. Flat surfaces are then theoretically stable longitudinally. They are not, however, used on account of their poor lift-drift ratio. As already explained, cambered surfaces are used, and these are longitudinally unstable at those angles of incidence reducing a reasonable lift-drift ratio, i. e., at angles below about 12°. A is a cambered surface, attitude approximately vertical, moving through the air in the direction M. The C. P. coincides as before with the transverse center line of the surface. With decreasing 16f) AIR SERVICE HANDBOOK. angles clown to angles of about 30° the C. P. moves forward as in the case of flat surfaces; but the angles above 30° do no interest us, since they produce a very low ratio of lift to drift. Below angles of about 30° (see C) the dipping front part of the surface assumes a negative angle of incidence resulting in the down- ward air pressure D and the more the angle of incidence is de- creased, the greater such negative angles and its resultant pressure D. Since the C. P. is the resultant of all the air forces, its position is naturally affected by D, which causes it to move backward. Now should some gust or eddy tend to make the surface decrease its angle of incidence, i. e., dive, then the C. P. moving backward and push- ing up the rear of the surface, causes it to dive more. Should the surface tend to assume too large an angle then the reverse happens; the pressure D decreases with the result that the C. P. moves forward T^i/ surf nyazr? SZ//y^ctce G./t-/7aay/7 /'/'^ec/ ta aero/>/srrs <3ts^r7fe sny/e . Fig. 73. and pushes up the front of the surface thus increasing the angle still farther, the final result being a "tail slide." It is therefore necessary to find a means of stabilizing the naturally unstable cambered surface. This is usually secured by means of a stabilizing surface fixed some distance in the rear of the main sur- face, and it is a necessary condition that the neutral lift lines of the two surfaces, when projected to meet each other, make a dihedral angle. In other words, the rear stabilizing surface must have a lesser angle of incidence than the main surface — certainly not more than one-third of that main surface. This is known as the longi- tudinal dihedral. The tail plane is sometimes mounted upon the airplane at the same angle as the main surface, but in such cases, it attacks air which has received a downward direction from the main surface, thus: The angle at which the tail surface attacks the air (the angle of incidence) is therefore less than the angle of incidence of the main surface. AIR SERVICE HANDBOOK. 161 First, imagine the airplane traveling in the direction of motion, which coincides with the direction of the thrust T. The weight is of course balanced about a 0. P., the resultant of the C. P. of the main surface and the C. P. of the stabilizing surface. For the sake of illustration the stabilizing surface has been given an angle of in- cidence and therefore has a lift and C. P. In practice the stabilizer is often set at no angle of incidence. In such case the proposition Fl«. 74. remains the same, but it is perhaps a little easier to illustrate it as above. Now, we will suppose that a gust or eddy throws the machine into the lower position. It no longer travels in the direction of T, since the momentum in the old direction pulls it off that course. M is now the resultant of the thrust and the momentum, and you will note that this results in a decrease in the angle the neutral lift line makes with the direction of motion, i. e., decrease in the angle oi incidence and therefor a decrease in the lift. '^aZSZHZTrrrr, Ji^^ij-, c^nc^^^ -^n^'e of ^ nccUe nee Momentum ^jt^rmc^'on of r/jrci^n Yu\. We will suppose that this decrease is 2°. Such decrease applies to both main surface and stabilizer, since both are fixed originally to the airplane. The main surface, which had (say) 12° angle, has now only 10°, i. e., a loss of one-sixth. The stabilizer, which had (say) 4° angle, has now only 2°, i. e., a loss of one-half. The latter has therefore lost a greater proportion of its angle of incidence and consequently its lift than has the main surface. It 46643—18 11 162 AIR SERVICE HANDBOOK. must then fall relative to the main surface. The tail falling, the airplane then assumes its first position though at a slightly less altitude. Should a gust throw the nose of the airjjlane up then the reverse happens and the airplane will assume its first position, though at a slightly greater altitude. Do not fall into the widespread error that the angle of incidence varies as the angle of the airplane to the horizontal. It varies "with" such angle, but not "as" anything approaching it. Re- member that the stabilizing effect of the longitudinal dihedral lasts only as long as there is momentum in the direction of the first course. These stabilizing movements are taking ^ ~ ~ " ^ place all the time, even though impercepti- ^ V ble to the pilot. The gyroscopic action of a rotary engine will affect the longitudinal stability when an airplane is turned to the right or left. When a right-hand rotary engine is fitted in a tractor machine the nose of the air- plane will rise when it is turned to the left and will fall when it is turned to the right. In modern airplanes this tendency is not sufficiently important to bother about ex- cept in the matter of spiral descents. Lateral stability is far more difficult for the designer to secure than is longitudinal or directional stability. Some degree of lateral stability may be secured by means of the lateral dihedral, i. e., the upward inclination of the surface toward its wing tips, thus: Imagine the top " V " to be the front view of a surface flying away fi'om you. The horizontal equivalent (H . E .) of the left wing is the same as that of the right wing. Therefore, the lift of one wing is equal to the lift of the other, and the weight being situated always in the center is balanced. If some movement of the air causes the surface to tilt sideways then you will note that the H. E. of the left wing increases and the H. E. of the right wing decreases. The left wing, having the greatest lift, rises; and the surface assumes its first and normal position. Unfortimately, however, the righting effect is not proportional to the difference between the right and left H. E.'s. In the case of A the resultant direction of the reaction of both wings is opposed to the direction of gravity or weight. The two Fig. 76, AIR SEBVICE HANDBOOK. 163 forces, K. H. and gravity, are then evenly balanced and the surface is in a state of equilibrium. In the case of B you will note thai ihe resultant reaction is not directly opposed to gra^'ity. Tliis results in the appearance of M, a side pressure, and so the resultant direction of motion of the airplane is no longer directly forward, but is along a line the resultant of the thrust and M. In other words, it is while flying forward at the same time mo^^ng sideways in the direction of M. In moving sideways the "keel surface" receives, of course, a pres- sure from the air equal and opposite to M. Since such surface is greatest in effect toward the tail then the latter must be pushed sideways. That causes the airplane to turn; and the highest wing being on the outside of the turn it has a greater velocity than the lower wing. That produces greater lift and tends to tilt the airplane 3^ 1 -' - - ^ 'T<7 \ k ^ 1 1 1 1 :<,-l_^ji>i 1 1 .| Jr Fig. 77. still more. Such tilting tendency is, however, opposed by the dif- ference of the H. E.'s of the two wings. It then follows that for the lateral dihedral angle to be effective such angle must be large enough to produce when the airplane tilts a dif- ference in the H. E.'s of the two wings, which difference must be sufficient not only to oppose the tilting tendency due to the airplane turning, but sufRcient also to force the airplane back to its original position of equilibrium. The above should make it clear that the lateral dihedral is not quite so effective as would appear at first sight. Some designers, in- deed, prefer not to use it since its effect is not very great and since it must be paid for in loss of H . E. and consequent loss of lift, thus de- creasing the lift-drift ratio, i. e., the efficiency. Also it is sometimes advanced that the lateral dihedral increases the "spill" of air from the wing tips and that this adversely affects the lift-drift ratio. The disposition of the "keel surface" affects the lateral stability. It should be, in effect, equally di\ided bj^ the longitudinal axis of the airplane. If there is an excess of "keel surface" above or below 164 AIR SERVICE HANDBOOK. such axis, then a side gust striking it Avill tend to turn the airplane over sideways. The position of the center of gravity affects lateral stability. If too low it produces a pendulum effect and causes the airplane to roll sideways. If too high it acts as a stick balanced vertically would act. If disturbed it tends to travel to a position as far as possible from its original position. It would then tend, when moved, to turn the airplane over sideways and into an upside-down position. From the point of \dew of lateral stability the best position for the center of graxaty is one a little below the center of drift. This pro- duces a little lateral stability without any marked pendulum effect. Propeller torque affects lateral stability. An airplane tends to turn over sideways in the opposite direction to that in which the propeller revolves. This tendency is offset by increasing the angle of incidence (and consequently the lift) of the side tending to fall; and it is always J^eutral arr^/e 0/ Incidence advisable, if practical considerations allow it, to also decrease the angle upon the other side an equal amount. In that way it is not necessary to depart so far from the normal angle of incidence at which the lift-drift ratio is highest. Wash in is the term applied to the increased angle. Wash out is the term applied to the decreased angle. Both lateral and directional stability may be improved by washing out the angle of incidence on both sides of the surface. The decreased angle decreases the drift and therefore the effect of gusts upon the wing tips, which is just where they have the most effect upon the airplane, owing to the distance from the turning axis. The wash out also renders the ailerons more effective, as, in order to operate them it is not then necessary to give them such a large angle of incidence as would otherwise be required. The less the angle of incidence of the ailerons the better their lift- drift ratio, i. e., their efficiency. For the same amount of move- ment therefore the ailerons are more efficient when attached to the surface with washed out angle of incidence. AIR SERVICE HANDBOOK. 165 The advantafi;eti of wash in inu.st oi' course l^e paid lor with some loss of lift, as the lift decreases with the decreased anglp. Banking. — An airplane turned off its course to right or left does not at once proceed along its new course. Its momentum in the direction of its first course causes it to travel along a line the resultant of such momentum and the thrust. In other words, it more or less skids sideways and away from the center of the turn. Its lifting surfaces do not then meet the air in their correct attitude, and the lift may fall to such an extent as to become less than the weight, in which case the airplane must fall. This bad effect i.s minimized by "banking," i. e., tilting the airplane sideways. The bottom of the lifting surface is in that way opposed to the aii- through which it is moving in the direction of the momentum and receives an opposite ail- pressure. The rarified air over the top of the surface is rendered still more rare and thLs of course a.ssists the air pressure in opposing the momentum. The velocity of the "skid" or sideways movement is then only such as is necessary to secure an air pressure equal and opposite to the centrifugal force of the turn. The sharper the turn, the greater the effect of the centrifugal force, and therefore the steeper should be the ''bank." The position of the center of gravity effects banking. A low C. G. will tend to swing outward from the center of the turn, and will cause the airplane to bank — perhaps too much, in which case the pilot must remedy matters by operating the ailerons. A high C. G. also tends to swing outward from the center of the turn. It will tend to make the airplane bank the wrong wa}-, and such effect must be remedied by means of the ailerons. The pleasantest machine from a banking point of view is one in which the 0. G. is a little below the center of drift. It tends to bank the airplane the right way for the turn and the pilot can, if neces- sary, perfect the bank by means of the ailerons. The disposition of the "keel surface" affects banking. It should be, in effect, evenly divided by the longitudinal axis. An excess of "keel surface" above the longitudinal surface will when banking receive an air pressure, causing the airplane to bank perhaps too much. An excess of "keel surface" below the axis has the reverse effect. Side Hiipping. — This usually occurs as a result of overbanking. It is always the result of the airplane tilting sideways and thus decreas- ing the horizontal equivalent and therefore the lift of the surface. An excessive bank or sideways tilt results in the H. E. and there- fore the lift becoming less than the weight, when of c()urs(\ the air- plane must fall, i. e., side slip. 166 AIR SERVICE HANDBOOK. When making a very sharp turn it is necessary to hsuik very steeply indeed. If at the same time the longitudinal axis of the airplane remains approximately horizontal then there must be a fall and the direction of motion will be the resultant of the thrust and the fall, as illustrated in sketch A. The lifting siu'faces and the controlling surfaces are not then meeting the air in the correct atti- tude, with the result that in addition to falling the airplane will probably become quite unmanageable. The pilot, however, prevents such a state of affairs from happening by "nosing down," i. e., by operating the rudder to turn the nose of the airplane downward and toward the direction of motion a F IG. 79. illustrated in sketch B. This results in the higher wing which is on the outside of the turn traveling with greater velocity, and there- fore securing a greater reaction than the lower wing, thus tending to tilt the airplane over still more. The airplane may be now almost upside down, but. its attitude relative to the direction of motion is correct and the controlling surfaces are all of them working efficiently. The recovery of a normal attitude relative to the earth is then made as illustrated in sketch C by gently pulling back the elevator control. The pilot must then learn to know just the angle of bank at which the margin of lift is lost, and, it a sharp t,urn necessitates banking beyond that angle, he must ' ' nose down . " In this matter of banking AIR SERVICE HANDBOOK. 167 and '"nosing down" and indeed regarding stalnlity and control gen- erally, the golden rule for all but verj- experienced pilots should be: "Keep the airplane in such an attitude that the air pressure is always directly in the pilot's face." The airjilane is then always engaging the air as designed to do so, and both lifting and controlling surfaces are acting efficiently. Spinning. — A spin is due primarily to the loss of flying speed. It is quite different from a quick turn of small radius. The usual form of spin is for the machine to come down at an angle of aljout 00° ^vith the tail turning rapidly, and the angle may become steeper and steeper. When this happens the attitude of the lifting .surfaces to the direction of motion is roo great, and there is a greater pressure trying to collapse the wings than there ought to l)e. 0^^dng to the small radius of such a spiral the mass of the airplane may gain a rotarj^ momentum greater in effect than the air pressure of the "'keel surface" or controlling surfaces opposed to it: when once such a condition occurs it is difhcult to see what can be done by the pilot to remedy it. In this connection e^ery pil(jt of an airplane fitted with a rotary engine should bear in mind the gyroscopic effect of such engines. In the ca.se of such an engine fitted to a tractor machine its effect is to depress the nose if a right-hand turn i.s made. The sharper the turn the greater such effect. An effect which may render the airplane uiunanageal)le if the spiral is one of vexy small radius and the engine is revohing with sufficient speed to produce a material gjn'oscopic effect. .Such gjroscopic effect will, however, assist the pilot to navigate a small spiial if he turns his machijie the opposite way. The a.ssistance will only be slight Ijecau.so (he engine should of course be throttled down for a spiral descent. Nearly all machines can l)e made to sjiin more or le.ss. Some are harder than others. In order to get into a spin pull back the control until the machine is almost stalled, then kick the rudder one way or the other and the machine will spin. To get out of a spin tluoltle back the engine, put all controls in neutral and then slightly i)ush forward the elevator control. All controls must be put in neutral to give the machine a chance of regaining flying speed. Any control which is in action increases the drift of the aii'plane and prevents this. As machines are not specially designed to take the stresses of a spin, machines should not be spun without the sanction of the de- signers and testers. Gliding descent uithout propeller thrust. — All airplanes are or should be designed to assume their correct gliding angle when the power 168 AIR SERVICE HANDBOOK. and thrust is cut off. This relieves the ])ilut of work, worry, and danger -should he find himself in a fog or cloud. The pilot, although he may not realize it, maintains the correct attitude of the airjjlane by observing its position relative to the horizon. Flying into a fog or cloud, the horizon is lost to view, and he must rely upon his instruments: (1) The compass for direction: (2) an inclinometer, mounted transA^ersely to the longitudinal axis, for lateral staliility; and (3) an inclinometer mounted parallel to the longitudinal axis, or the air speed indicator which will indicate a nose-down position by increase in air speed, and a tail-down position by a decrease. The pilot is then under the necessity of watching three instru- ments and manipulating his three controls to keep the instrument indicating longitudinal, lateral, and dii'ectional stability. That is a feat beyond the capacity of the crdinarj- man. If, however, by the simple movement of throttling down the power and the thrust Fig. 80. he can be relieved of looking after thclungitiidinal stability he then has only two instruments to watch . Airplanes are therfore designed, or should be, so that the center of gravity is slightly forward of the center of lift. The airplane is then, as a glider, nose heavy, and the distance the C. G. is placed in advance of the C. L. should be such as to insure a gliding angle producing a velocity the same as the normal fljdng speed. In order that this nose-heavy tendency should not exist when the thrust is working and descent not required the center of thrust is placed a little below the center of drift and resistance, and thus tends to pull up the nose of the airplane. The distance the center of thrust is placed below the center of drift should be such as to produce a force equal and ojjposite to that due to the C. G. being forward of the ('. L. AIR SERVICE HANDBOOK. 169 Looping and upside-down Jlying. — If a loop is desired it is best to throttle down the engine at a point A when the top of the loop is reached. The C. G. being forward of the C. P. causes the airplane to nose down and assists the jiilot in making a reasonably small loop along the course C and in securing a quick recovery. If the engine is not thi'ottled down then the airplane may be expected to follow the course D which results in a longer nose dive than in the case of the course C. When pulling the machine out of the nose dive a steady and gentle movement of the elevator is necessary. A jerky movement may change the direction of motion so suddenly as to produce dangerous -y^- ""^-^ Fig. si. air stresses upon tlu" surfaces, in which case there is a possibility to collapse. If an upside-down flight is desired, the engine may or may not be throttled down at point A. If not throttled down, then the elevator must bo operated to secure a com-se approximately in the direction P>. If it is throttled down, then the course must be one of a steeper angle than 15 or there will be danger of stalling. To start a loop it is necessary with some machines to push the nose down in order to gather speed. Some machines will go straight over from the horizontal flying position. In any case the elevator should be pulled back gradually until the machine has got very nearly to the position A, when it should be pulled back as far as it will go so 170 AIR SERVICE HANDBOOK. as to bring the machine over top of the loop. As the machine goes over, the rudder must be put over; that is, in a machine which flies with left rudder, the rudder must be put over to the right as the machine goes over the top, otherwise the loop will not be clean. If the elevator control is pulled too roughly the machine will stall before it goes over the top and will not complete the loop. In machines which will be used for looping and nose diving, and also in high-powered weight-carrying machines, the greatest care must be taken about the tension of the incidence and flying wires. Remember that a machine is designed to take certain stresses when flying. If the bracing wires are tightened to such an extent that they "sing" when vibrated it means that the spars, etc., have a considerable initial strain for which the machine is not designed- All bracing wires should therefore never be tight but should only have the slackness taken out of them and nothing more. XVIII. NOMENCLATURE. Aerofoil. A thin wing-like structure, flat or curved, designed to obtain reaction upon its surfaces from the air through which it moves. Aileron. A movable auxiliary surface, used for the control of rolling motion, i. e.. rotation about the fore and aft axis. Aircraft. Any form of craft designed for the navigation of the air. Airplane. A form of aircraft heavier than air, which has wing suifaces for sustentation. with stabilizing surfaces, rudders for steering, and power plant for propulsion through the air. The landing gear may be suited for either land or water use. Pusher, a type of airplane with the propeller or propellers in the rear of the wings. Tractor, a type of airplane with the propeller or propellers in front of the -wings. Air-speed meter. An instrument designed to measure the velocity of an aircraft with reference to the air through which it is moving. Altimeter. An instrument mounted on an aircraft to continuously indicate its height above the surface of the earth. Anemometer. An instrument for measuring the velocity of the wind or air currents with reference to the earth or some fixed body. AIR SERVICE HANDBOOK. 171 Angle. Of attack, the antrle between the direction of the relative \vind and the chord of an aerofoil, or the fore and aft axis of a body. Critical, the angle of attack at which the lift is maximum. Gliding, the angle the flight path makes ^vith the horizontal when flying in still air under the influence of gravity alone. Aspect ratio. The ratio of spread to chord of an aerofoil. Axes of an aircraft. Three fixed lines of referejice; usually centroidal and mutually rectangular. Longitudinal axis, usually parallel to the axis of the pro- peller, is the principal longitudinal axis in the plane of symmetry. Sometimes called ''fore and aft axis." Vertical axis, the axis perpendicular to the above in the plane of symmetry. Trans\erse or lateral axis is the third axis perpendicular to the other two. Sometimes called "athwartship axis." In mathematical discussions the tirst of these axes is called the X axis, the second Z axis, and the third the Y axis. Ballonet. A small balloon within the interior of a balloon or dirigible for the purpose of controlling the ascent or descent, and for main- taining pressure on the outer envelope to prevent deforma- tion. The ballonet is kept inflated with air at the required pressure, under the control of a blower and valves. Balloon. A form of aircraft comprising a gas bag and a oar. whose susteu- tation depends on the buoyance of the contained gas. which is lighter than air. Captive, a balloon restrained from free flight by means of a cable attaching it to the earth. Kite, an elongated form of captive balloon fitted with tail appendages to keep it headed into the wind and deriving increased lift due to its axis being inclined to the wind. Bank. To incline an airplane laterally, i. e.. to rotate it about the fore and aft axis. Right bank is to incline the airplane with the right wing down. Barograph . An instrument used to record variations in barometric pressure. In aeronautics the charts on which the records are made are prepared to indicate altitudes directly instead of barometric pressure. 172 AIR SERVICE HANDBOOK. Biplane. A form of airplane in which the main supporting surface is divided into parts, one above the other. Body of an airplane. A structure usually inclosed, which contains in a stream-line housing the power plant, fuel, passengers, etc. Oabre. A flying attitude in which the angle of attack is greater than normal; tail down; down by the stern — tail low. Camber. The convexity or rise of a curve of an aerofoil from its chord, usually expressed as the ratio of the maximum departure of the curve from the chord as a fraction thereof. Top camber refers to the top siu^ace of an aerofoil, and bottom camber to the bottom surface. Capacity. Lifting, the maximimi load of an aircraft. Carrying, excess of the lifting capacity over the dead load of an aircraft, which latter includes structure, power plant, and essential accessories. Center. The point in which a set of effects is assumed to l)e accumulated producing the same effect as if all were concentrated at this point. Of buoyancy, the center of gravity of the fluid displaced by the floating body. Of pressure of an aerofoil, the point on the chord of an ele- ment of an aerofoil, prolonged if necessary, through which at any instant the line of action of the resultant air force passes. Of pressure of a body, the point on the axis of a bgdy, pro- longed if necessary, through which at any instant the line of action of the resultant air force passes. Chord . Of an aerofoil section, a right line tangent to the under curve of the aerofoil section at the Front and rear. Length, the length of the chord is the length of an aerofoil sec- tion projected on tlio chord, extended if Tiecessary. Controls . A general term ap])lying to tiie means provided for operating the devices used to control speed, direction of flight, and attitude of an aircraft. Dirigible. A form of balloon the outer envelope of which is of elongated form, provided with a ])ropelliiig system, car, rudders, and stabilizing surfaces. AIR SERVICE HANDBOOK. 178 Dope. A general term applied to the materials used ia treating the cloth surface of airplane members to increased strength, pro- duce tautness, and act as a filler to maintain air tightness; usually of the cellulose type. Drag. The total resistance of motion through the air of an aircraft, i. e., the sum of the drift and head resistance. Drift. The component of the resultant wind pressiu'e on an aerofoil or wing surface parallel to the air stream attacking the surface. Elevator. A hinged surface for controlling the longitudinal attitude of an aircraft, i. e., its rotation about the athwartship axis. Engine, right or left hand. The distinction between a right-hand and a left-hand engine depends on the rotation of the output shaft, whether this shaft rotates in the same direction as the crank or not. A right- hand engine is one in which when viewed from the output shaft end, the shaft is seen to rotate anticlockwdse. Entering edge. The foremost part of an aerofoil. Fins. Small planes on the aircraft to promote stability; for example vertical tail fins, etc. Flight path; The path of the center of gravity of an aircraft with reference to the air. Float: That portion of the landing gear of an aircraft which provides buoyancy when it is resting on the siu'face of the water. Fuselage. (See body.) Gap: The distance between the projections on the vertical axis of the entering edges of an upper and lower wing of a biplane. Glide: To fly without power. Head resistance: The total resistance to motion through the air of all parts of an aircraft not a part of the main lifting surface. Sometimes termed ' * parasite resistance . ' ' Helicopter: A form of aircraft whose support in air is derived from the ver- tical thrust of large propellers. 174 AIR SERVICE HANDBOOK. Inclinometei' : An instrument for measuring the angle made by any axis of an aircraft with the horizontal. Keelplane area: The total effective area of an aircraft which acts to prevent skidding or side slipping. Landing gear: The understructure of an aircraft designed to carry the load when resting on, or running on, the surface of the land or water. Leeding edge. (See entering edge.) Leeway : The angular deviation from a coursse over the earth, due to cross currents of wind. Lift: The component of the force due to the air pressvue of an aerofoil, resolved perpendicular to the flight part in a vertical plane. Longeron : A fore-and-aft member of the framing of an airplane body, or of the floats, usually continuous across a number of points of support. Metacenter : The point of intersection of a vertical line through the center of gravity of the fluid displaced by a floating body when it is tipped through a small angle from its position of equilibrium and the inclined line which was vertical through the center of gra\'ity of the body when in equilibrium. There is in general a different metacenter for each type of displacement of the floating body. Monoplane: A form of airplane whose main supportiilg surface is disposed as a single wing on each side of the body. Nacelle. (See body.) Nose dive: A dangerously steep descent, head on. Ornithopter: A form of aircraft deriving its .support and propelling force from flapping wings. Pilot tube: A tube with an end open square to the fluid stream, used as a detector of an impact pressure. More usually associated with a concentric tube surrounding it, having perforations normal to the axis for indicating static pressure. The velocity of the fluid can be determined from the difference between the impact pressure and the static pressure. This instrument is often used to determine the velocity of an aircraft through the air. AIR SERVICE HANDBOOK. 176 I'ropeller: Disk area of, tlie total area oi' the 3 1158 01262 0331 :? > > -n^- 1 DTT^ on the last UC SOUTHERN REGIONAL LIBRARY FACILITY AA 001 168 405 ''^^ Z^-€^