GIFT OF U. S. Department of Agriculture, Forest Service FOREST PRODUCTS LABORATORY In cooperation with the University of Wisconsin MADISON, WISCONSIN WOOD IN AIRCRAFT CONSTRUCTION Approved Copy Filed I* ?f ~ MAU-i LIBRARY-AGRICULTURE DEM". AIRCRAFT DESIGN DATA. NOTE NO. 12. WOOD IN AIRCRAFT CONSTRUCTION. [Prepared by the Forest Products Laboratory, Forest Service, TJ. S. Department of Agriculture.] CONTENTS. Page. Mechanical and physical properties Variability of the strength of wood Wood nonhomogeneous Variation of strength with locality of growth Variation of strength with position in tree Variation of strength with rate of growth Variation of strength with amount of summmvood Variation of strength with specific gravity Variation of strength with moisture content Defects affecting strength -Xl/^cUIH.H-H^^l'l^JiSl'ii , ** Diagonal and spiral grain 11 Knots 21 Pitch pockets 21 Compression failures and "cross breaks" 22 Braslmess 22 Decay 22 Internal or initial stresses in wood 22 Wood fibers under stress in the tree 22 Internal stresses produced during drying 25 Initial stresses produced in assembling 25 Working stresses for wood in aircraft construction 25 Nature of loading " 27 Tensile strength 27 Torsional strength 28 Shrinkage 28 Suitability of various American woods for aircraft 34 Construction ." Conifers Hardwoods Storage and kiln drying 40 Rules for piling lumber Kiln drying of wood Advantages of kiln drying The elimination of moisture from wood Three essential qualities of a dry kiln Defects due to improper drying Case hardening and honeycombing Collapse 40 41 41 41 42 42 42 44 Brashness 45 Methods of testing conditions during drying 45 Preliminary tests 46 Current tests 46 Final tests 49 Specifications for kiln drying lor aircraft stock. 50 Treatment of wood after removal from kiln 55 Changes of moisture in wood with humidity of air 50 Veneer and plywood 57 Veneer 57 Plywood 59 Properties of wood parallel and perpendicular to the grain . 5q lo 8T> ftoq i , fiyat oiuo'Hje oil) v List of species and reference numbers for figure 1. HARDWOODS. Species. Locality. Reference No. Species. Locality. Reference No. Alder red Washington 30 Hickory Continued . Ash: Pignut Pennsylvania 160 Biltmore Tennessee 91 Do West Virginia 161 Black Michigan 60 Shagbark Mississippi 140 Do . ... Wisconsin 70 Do Ohio 159 Blue Kentucky !)() Do Pennsylvania 143 Green Louisiana 93 Do West Virginia 153 Do . ... Missouri 100 Water M ississippi Pumpkin do 79 Holly, American Tennessee Q7 White Arkansas 106 Hornbeam do 149 Do New York 128 Laurel, mountain do 145 Do West Virginia 83 Locust: Aspen Wisconsin 23 Black. do 158 Largetooth ..do 20 Honey Indiana 169 Basswood . . . Pennsylvania 12 Madrona .... California 101 Do Wisconsin 5 Do Oregon 128a Beech Indiana 110 Magnolia . I ouisiana fil> Do Pennsylvania 98 Maple : Birclr Oregon Washington 58 Paper Wisconsin 73 Red Pennsylvania 69 Sweet Pennsylvania 129 Do Wisconsin 92 Yellow ..do 107 Silver do 56 Do Wisconsin 103 Sugar Indiana ' 104 Buckeye yellow Tennessee 9 Do Pennsylvania 108 Buckthorn, cascara . . . . Oregon 84a Do Wisconsin 124 Butternut Tennessee 27 Oak: Do Wisconsin 21 Bur do 125 Chinquapin, western . . . Oregon 48b California black .... California 80 Cherry- Canyon live do 163 Black Pennsylvania 72 Chestnut Tennessee 121 Wild red Tennessee 24 Cow Louisiana 133 Chestnut . ... M aryland 46 Laurel . do 116 Do Tennessee 40 Post Arkansas 130 Cotton wood, black ... . Washington 6 ' Do Louisiana 137 Cucumber tree Tennessee 59 Red Arkansas 119 Dogwood : Do Indiana 118 Flowering do 151 Do Louisiana 117 Western Oregon 125a Do Tennessee 97 Elder, pale . do 69a Highland Spanish . . Louisiana 94 Elm: Lowland Spanish . . . . do 142 Cork Wisconsin, Marathon 126 Swamp white Indiana 150 County. Tanbark California 115 Do Wisconsin, Rusk Water ... Louisiana 111 County. White Arkansas 132 Slippery Indiana 102 Do Indiana 138 Do Wisconsin 74 Do Louisiana Richland 136 White Pennsylvania 55 Parish Do Wisconsin 53 Do Louisiana, Winn Parish 131 Greenheart 165 Willow Louisiana 109 Gum: Yellow Arkansas 122 Black Tennessee 68 Do Wisconsin 105 Blue (Eucalyptus) California 147 Osage orange Indiana 164 Cotton . . Louisiana . . . 76 Poplar yellow (tulip Tennessee 35 Red Missouri 54 tree) Hackberry Indiana 90 Rhododendron, great. do 85 Do Wisconsin 78 Sassafras do 51 Haw, pear do 146 Serviceberry do 156 Hickory : Silverbell tree do 49 Big shellbark Mississippi 135 Sourwood do 89 Do Ohio 154 Sumac, staghorn Wisconsin 61 Butternut do 139 Sycamore Indiana 63 Mockernut Mississippi 144 Do Tennessee . ... 65 Do Pennsylvania . . . 159 Umbrella Eraser do 45 Do West Virginia 155 Willow Nutmeg Mississippi 112 Black Wisconsin 11 Pignut do 148 Western black Oregon . ... 43a Do Ohio 157 Witch hazel Tennessee 114 AIECEAFT DESIGN DATA. Note 12. List of species and reference numbers for figure 1 Continued. CONIFERS. , - - Species. Locality. Reference No. Species. Locality. Reference No. Cedar: Incense California 26 Pine Continued. Lodgepole Montana, Granite 41a Western red Montana . ... 2 County. Do Washington 10 Do Montana, Jefferson 40a ; WJiite Wisconsin 1 County. t 'v press, bald Louisiana 62 Do Wyoming 34 Douglas fir California 45a Lon ir leaf Florida ... 123 Do Oregon 67a Do Ix)uisiana,Lake Charles. 113 Do Washington, Chelialis 46a Do Louisiana, Tangipahoa 96 Do County. Washington, Lewis 75 Do Parish. Mississippi 95 County. Norway Wisconsin 57 Do Washington and Ore- 67 Pitch Tennessee . . . 71 gon. Pond Florida 86 Do... Wyoming 84 Shortleaf Arkansas 77 Fir: Sugar California 22 A Ipine Colorado 4 Table Mountain Tennessee 82 Amabilis Oregon 39 Western white Montana 42 Do Washington 18 \Vestern yellow Arizona 19 Balsam Wisconsin 14 Do California 37 Grand Montana. . 36 Do Colorado 41 Npble : Oregon 16 Do Montana 32 White California 17 White Wisconsin 25 Hemlock: Redwood California, Albion 28 Black Montana 47 Do California Korbel 13 Eastern Tennessee 52 Spruce' Do Wisconsin 15 ETigelmann g Wjestern Washington 50 Do Colorado San Miguel 3 Larch,! western Montana 84 D6 - Washington 64 Red 44 Pine: i Do Tennessee 29 Ciiban Florida 127 White 7 Jack Wisconsin 43 Do Wisconsin 38 Jeffrey California 33 Tamarack do 81 LdbloUy Florida 88 Yew, western 134 Lqdgepole , Colorado 31 ! Do Montana Gallatin 35a : 1 _ 1 County. . n 62 6ft 8 . Note 12. AIRCRAFT DESIGN DATA. 7 The minimum strength values which may be expected of a particular lot of lumber can be raised a good deal by eliminating a relatively small portion of the lighter material. This lightweight material can, as a rule, be detected by visual inspection. In order to train the visual inspection and to pass judgment on questionable individual pieces, frequent specific gravity determinations are necessary. 8000 7000 6000 5000 -j 4000 I 3000 2000 1000 w R MAXIf SIR SPECI BASED SPECK A SRE HITE ASH ELATION OF 1UM CRUSHING ENGTH TO FIC GRAVITY D / (ENS TESTED IN EN CONDITION / i ,? g / 1 / z . ' > J "; / : , ? c t. / - 2 14 i ! : i> 2j v y f ^ g $ / k -; L # f- . \ > 1 \ ', / - f / $?> / / /\ / f / /' < 5.000 >%. 10 ft 30 JS 40 4S' SO SS 60 65 MOISTURE PERCENT, or DRV WEIOH1. 70 75 00 86, 90 OVtRSO S9 Fig. 5. Effect of case-hardening upon the form of the moisture-strength curve in bending tests. The upper curve is from case-hardened specimens, the lower curve from uniformly dired specimens. from defects, as in airplane construction, proper drying with careful control of temperature and humidity increases the strength of material very greatly. In whatever way wood is dried, upon its being resoaked and brought back to the original green or wet condition it is found to be weaker than it was originally. So when it is said that wood has been injured in the drying process it must be taken to mean that it is weaker than it should have been after drying and while still in a dried condition. When a stick of timber dries out below the fiber-saturation point (that is, when it has lost all its free moisture and the moisture begins to leave the cell walls), the timber begins to shrink and change in its mechanical properties. Also numerous stresses are set up within the timber. Under severe or improper drying conditions the stresses may be great enough to practically rum the material for purposes where strength is important. Improper drying con- ditions, however, do not of necessity mean fast drying conditions. When properly dried, the timber gains in its fiber stress at elastic limit, its modulus of rupture, maximum crushing strength, etc. It bends farther at the elastic limit when dry than when green, but does not bend so far at the maximum load. After having been bent to the maximum load dry timber breaks more suddenly than green timber of the same species that is, dry timber is more brash than green, although it withstands greater stresses and is stiffer. Note 12. AIRCRAFT DESIGN DATA. 11 DEFECTS AFFECTING STRENGTH. DIAGONAL AND SPIRAL GRAIN. Diagonal grain is produced when the saw cut is not made parallel to the direction of the fibers. It can usually be avoided by careful sawing unless it is caused by crooks in the log. Spiral grain, on the other hand, results from a spiral arrangement of the wood fibers in the tree. Fig. 6. Spiral grain in Sitka spruce. If a log is spiral grained, it is impossible to secure straight-grained material, except in small pieces, from the spiral-grained part. The effect of spiral grain is illustrated in figure 6, which shows three views of a piece of Sitka spruce. The center part of a log may be straight grained and the outer part spiral grained or vice versa. 12 AIRCRAFT DESIGN DATA. Note 12. Figures 7 to 14, inclusive, show the weakening effect of spiral or diagonal grain upon various strength properties of Sitka spruce and Douglas fir. The data are based upon about 1,400 static bending tests, made upon clear specimens, third point loading, 45-inch span. Similar impact bending tests have shown similar weakening with increasing slope of grain. 7OOO /00% O/-/ess OFGRA/N o Avenage vaJu.c.s Probab/e m/n/~musn Fig. 7. The effect of spiral and diagonal grain on the fiber stress at the elastic limit; Sitka spruce. - -ii AIRCRAFT DESIGN DATA. 13 /oooo 9OOO /.JO SLOPE Of /'/O /.O mtn/mum Fig. 8. The effect of spiral and diagonal grain on the modulus of rupture; Sitka spruce. 14 AIRCRAFT DESIGN DATA. Note 12. 2000 /<3OO /OO% o Menace va/ues ? X77//7//7? Fig. 9. The effect of spiral and diagonal grain on the modulus of elasticity; Sitka spruce. Note 12. AIRCRAFT DESIGN DATA. 15 <$ 3 8 * Nl tt ^* t /.'4o or /ess wa* yes 7a7, ^ ea,'? \ ^ 3 80% 60% 20% /:3o /:2o \ L L_L Fig. 10. The effect of spiral and diagonal grain on the work to maximum load; Sitka spruce, \ jr 16 AIRCRAFT DESIGN DATA. Note 12. /oooo 9OOO &OOO 7000 6000 . \ > S000 4000 \ 3O00 2000 /ooo /*/: /77// 7/rnu/r? era X \ ^ \ % /:4-0 /:30 or /ess SLOPE Or r GS?/t//V /oo% (50% 60% 0% ':o Fig. 11. The eflect of spiral and diagonal grain on the fiber stress at the elastic limit; boughs ..*. Note 12. AIRCRAFT DESIGN DATA. 17 S2.OOO . W L&EMD O or/ess /77/7S77U/7? Fig. 12. The effect of spiral and diagonal grain on the modulus of rupture; Douglas fir. 98257 19 No. 12 2 18 AIECEAFT DESIGN DATA. Note 12. 2200 2OOO o/- /ess /:30 /.20 SLOPE 607* 40% 20% 0% ^ t/a/ues 'r?//77u/77 i/a/uesr Fig. 13. The effect of spiral and diagonal grain on the modulus of elasticity; Douglas fir. >n lo witoboin ml) no aims IfinogBib briH Iuiiq to } Note 12. AIECEAFT DESIGN DATA. 19 The tests were made upon seasoned material, but since the moisture content of the indi- vidual specimens varied somewhat, it was necessary to reduce such properties as are materially affected by changes in moisture content to a uniform basis before comparisons could be made. Therefore, the values for fiber stress at the elastic limit, modulus of rupture, and modulus of elasticity have been reduced to 11 per cent by means of an empirical exponential formula. The work to the maximum load values were not reduced to a uniform moisture basis, since the correction would have been very small, and no greater accuracy would have been insured. Fig. 14. The effect of spiral and diagonal grain on the work to maximum load; Douglas fir. In addition to the curve for average values based on test data, a curve for probable mini- muni values (broken line) was calculated and plotted. A third curve was also drawn showing the probability of individual values falling below the probable minimum value for straight- grained material. This probability is expressed in per cent and, as is to be expected, increases greatly as the slope of the grain becomes steeper. The rate of falling off in strength increases abruptly at a slope between 1 in 20 and 1 in 15, and therefore this slope may be considered to be the critical one. It is to be noted, however, that even at slopes at 1 in 20 there is a decided weakening. 20 AIRCRAFT DESIGN DATA. Note 12. As a result of these tests it is recommended that for purposes of design the following values for moduli of rupture for spruce at 15 per cent moisture and different slopes of spiral or diagonal grain be strictly adhered to: From straight to 1 in 25 7,900 pounds per square inch. From 1 in 25 to 1 in 20 7,000 pounds per square inch. From 1 in 20 to 1 in 15 5,500 pounds per square inch. The effect of spiral grain upon the maximum crushing strength is much smaller than upon the modulus of rupture. The following stresses for different slopes of grain may be used with safety for compression members: From straight to 1 in 25.. -! I II 4,300 pounds per square inch. From 1 in 25 to 1 in 20 4,200 pounds per square inch. From 1 in 20 to 1 in 15 3,800 pounds per square inch. When the annual rings run diagonally across the end of a piece the true slope of diagonal grain can be obtained as shown by figure 15a. Slope of diagonal grain. D/ L -- Slope of spiral grain. Fig. 15. The measurement of the slope of diagonal and spiral grain. The direction of spiral grain is indicated on a tangential (flat sawn) face by the direction of the resin ducts. These ducts, however, are often difficult to see. Drops of ink placed on tangential faces and allowed to spread are sometimes used to test for spiral grain. The ink will tend to follow the angle of the grain. The direction of spiral grain is, howeVer, not given correctly by resin ducts or by spreading of ink unless these tests be applied to a truly tangen- tial face. In figure 15, for instance, resin ducts or spreading of ink would be practically parallel to the edges whether the material was spiral grained or not. In such cases spiral grain can be detected only by splitting on a radial line (Fig. 156) or by raising small splinters and observing if they have a tendency to tear deeper and deeper. Note 12. AIRCEAFT DESIGN DATA. 21 KNOTS. The effect of knots depends upon their location with respect to the stresses to which the piece will be subjected, as well as upon their size and character. None but sound knots, firmly attached, should be permitted. Obviously, knots of any considerable size can not be allowed in any airplane parts because the parts themselves are comparatively small in cross section. Since the weakening effect of knots results from their disturbance of normal arrange- ment of fibers, their seriousness can best be decided from a consideration of the grain. PITCH POCKETS. Tests recently completed on 112 solid Douglas fir wing beams, made especially to study the effect of pitch pockets upon the strength of beams indicate that this effect may have been overrated in previous specifications. The tests were made over a 72-inch span under third- point loading. The following conclusions from these tests are presented in the form of speci- fications, and are intended to be applied to spruce and fir wing beams: (a) In portions of the length where a slope of grain of 1 in 25 is the maximum allowed, pitch pockets 1| inches in length and not to exceed one-eighth of an inch in width or depth may be allowed in any portion of the section except the outer quarters of the flange. No pitch pockets to be allowed in outer quarters of flange. (6) Where a slope of spiral grain of 1 in 20 is allowed pitch pockets 2 inches in length and not to exceed one-fourth inch in width or depth may occur any place in the section except in the outer quarters of the flange. No pitch pockets to be allowed in outer quarters of flange. (c) Where a slope of grain of 1 to 15 is allowed pitch pockets 1J inches in length and one- fourth inch in width or depth may occur in the outer quarters of the flange, and pitch pockets 3 inches in length and one-fourth inch in width or depth may occur in any other portion of the section. (d) Pitch pockets occurring in the web may not be closer together than 20 inches. If they are in the same annual ring, they may not be closer together than 40 inches. In other portions of the section these distances may be 10 inches and 20 inches, respectively. Combining this specification with a knot and spiral-grain specification, the following table has been prepared; it is the intention that this table be used in drafting parts specifications for spruce and fir wing beams: TABLE 1. Size and quantity of defects allowable witli different slopes of grain. Allowable slop* in grain not exceeding Knots. Pitch pockets. Maximum diameter permitted. Minimum distance l>otween any two. Maximum length per- mitted. Maximum width or depth per- mitted. 1 inch in 25 Inches. i A. 4 Inches. 10 12 20 Inches. H 2 3 Inches. i } 1 inch in 20 1 inch in 15 Supplementing the table are the following clauses: 1. All knots must be sound. 2. No defects must fall or cause irregular grain greater in slope than that allowable for cross grain in the outer quarter of the upper or lower flange; except that where a slope of 1 in 15 is allowed, pitch pockets 1J inches long ana one-fourth inch wide or deep may be permitted. 3. Pitch pockets occurring in the web may not be closer together than 20 inches. If they are in the same annual ring, they may not be closer together than 40 inches. In other portions of the section these distances may be 10 inches aud 20 inches, respectively. 4. The equivalent of the diameters specified may be allowed in a number of smaller knots, provided that they are not close together. 22 AIRCEAFT DESIGN DATA. Note 12. COMPRESSION FAILURES AND " CROSS BREAKS." All material containing compression failures and "cross breaks" should be eliminated from airplane parts where strength is of importance. The cause of certain "cross breaks" near the center of large logs such as are quite frequently found in mahogany is not known. Compression failures, which are, in fact, of the same nature as "cross breaks," are known frequently to be due to injury by storm in the standing trees, to carelessness in felling trees across logs, to unloading from a car across a single skid, or to injury during manufacture. While some compression failures are so pronounced as to be unmistakable, others are difficult to detect. They appear as wrinkles across the face of the piece. Compression fail- ures not readily apparent to the eye may seriously reduce the bending strength of wood and its shock-resisting ability, complete failure occurring suddenly along the plane of injury. Figure 16 shows four samples of African mahogany containing compression failures which occurred during growth. These samples were later tested in static bending, and in all cases the compression failures developed during te'st followed those originally occurring in the samples. This is illustrated in figure 17. BRASHNESS. The term "brash," frequently used interchangeably with the term "brittle," when used to describe wood or failures in wood, indicates a lack of toughness. Brash wood, when tested in bending, breaks with a short, sharp fracture instead of developing a splintering failure and absorbs a comparatively small amount of work between the elastic limit and final failure. In impact tests brash wood fails completely under a comparatively small hammer drop. DECAY. The first effect of decay is to reduce the shock-resisting ability of the wood. This may take place to a serious extent before the decay has sufficiently developed to affect the strength under static load or to become evident on visual inspection. Unfortunately there is no method of detecting slight decay in wood except with a compound microscope. AH stains and dis- colorations should be regarded with suspicion and carefully examined. It must be remembered that decay often spreads beyond the discoloration it causes and that pieces adjacent to dis- colored areas may already be infected. On the other hand, not all stains and discolorations are caused by decay of the wood. The blue sapstain of some hardwoods and of many coniferous woods, including spruce, and the brown stain of sugar pine are not caused by decay-producing organisms and do not weaken the wood. INTERNAL OR INITIAL STRESSES IN WOOD. WOOD FIBERS UNDER STRESS IN THE TREE. Wood products are quite similar to metal castings as regards internal stresses. It is probable that wood fibers are continually under stress of some kind. The fact that freshly cut logs of some species split through the center (this frequently happens as the result of heavy shocks or jars and without the use of a wedge) is evidence of some tensile stresses in the outer portion of the tree and compression in the inner portion. These stresses are independent of the stresses due to the weight of the tree and pressure against it. Note 12. AIRCRAFT DESIGN DATA. 23 Fig. 16. Compression failure occurring during growth. African mahogany 24 AIRCRAFT DESIGN DATA.| Note 12. Fig. 17. Influence of compression failure occurring during growth on failures in static bending. African mahogany. Note 12. AIECRAFT DESIGN DATA. 25 INTERNAL STRESSES PRODUCED DURING DRYING. The natural stresses may be partially or wholly relieved by sawing the tree into lumber, but other stresses are likely to be introduced by subsequent seasoning. Checking, honey- combing, warping, twisting, etc., are manifestations of the internal stresses which are produced in the drying of wood or whenever any change of moisture content takes place. Presumably such stresses are due to unequal distribution of moisture and consequent unequal shrinkage combined with more or less inherent lack of homogeneity. Air drying for a number of years, which is practiced in some woodworking industries, has for its object the equalization of moisture and the relief of stresses induced in the early part of the drying. Careful and correct kiln drying followed by a period of seasoning under proper and controlled atmospheric conditions should produce results at least equal and probably superior to those obtained by long periods of air drying. Relieving these internal stresses is important because they amount to an actual weakening of the material. If the fibers of a piece of wood are under stress when the piece is free, they are just that much less capable of resisting stresses of the same kind produced by exterior forces or loads applied to the piece. INITIAL STRESSES PRODUCED IN ASSEMBLING. When a member of any structure is stressed in assembling the structure and before any load is placed on it, it is said to be under initial stress. If the initial stress is of the same char- acter as the stress for which the member is designed, it constitutes a weakening, for when the structure is loaded the safe working stress of the member will be reached just that much sooner. If this initial stress is opposite in character to that for which the member is designed, it amounts to a strengthening of the member, for when the structure is loaded the initial stress must be overcome before the member takes any of the stress for which it is designed. Many of the curved parts of an airplane frame could be simply sprung to place on assembly. Were this done, they would be subjected to initial stress and usually of the same sign to which the member would later be subjected. In order to avoid initial stress, such parts are steam bent before assembly. It is desirable, of course, that this bending be so done as not to injure the material and to leave little tendency to spring back from the curves to which it is bent. In order that the material may be made sufficiently plastic to accomplish this result, it is essential that the steaming and bending be carried out while the wood is at a relatively high moisture content. If it is attempted on kiln-dry or thoroughly air-dry material, there is the tendency to spring back after the clamps are removed. Bending of such stock can not be compared to a considerable part of the bending done in other woodworking industries, where the strength of the wood is very greatly damaged by the bending process but without destroying its use- fulness for the purpose for which it is intended. Some of the unexpected failures of bent parts in airplanes have doubtless been due to the initial stresses set up in the member during the bending. WORKING STRESSES FOR WOOD IN AIRCRAFT CONSTRUCTION. Table 2 gives strength values at 15 per cent moisture (which is probably close to the maxi- mum moisture content of wood in a humid atmosphere) for use in airplane design, as well as the minimum specific gravity and average density which should be allowed. It is suggested that the working stresses for design be obtained by applying factors to the values for static load conditions as given in this table. 26 AIRCRAFT DESIGN DATA. Note 12. 1C * Tji < .r-l^ CO< O O O O ' ' t~ CO 00 O i p OOOCO CM' co ic *" co co c S t~ CO OO t-- CO 1 00 Tfri CN OS J> OS CO O OO OOOOOO OO 1C COOS OCNiCt-iCiC (N't 1 CO r-l t-- CO O OS CO CO i I OS OS 5; .O Q,c?O ** CD CD CD ' O O O < i OS CM CO - CO l> ^ CO I w'eis.S M J2 ? "- S .4. a Q +3 ftft&slsg' ulilfl '" B ) Oi CO 1C i I 1C OS i 1C rfi CO CO -* Tj< Tt< CD o ^lM ~ S T)< TJH CO- CO t-' (N t> O5 rH g u r-lrH i-l 1-1 .-I I-H r-l Oi-HCOOS l>(Mi-l COoio'lN C^5CO (M r-l i-l iH I-H f~lC rHCNOOSrHil os'ic lot^ic'cdcdco oooooo iO OO O O O O t^ l> i-H ^ CM !> 'S " 3 ^ , t>iCCNCOiCCOOiC^ S S't O? CD CD CD CD CD I I CO "t 1 GO i rt< co 10 t^ t^ OS O rrl i I t- OO OS oo"i>ri>roo' N co"-^"i>r oooooo o oooooo o (M 00 CO i I i I Oi iH i i, . II O5 O5 CO Tt< C5 (M t^ t> OS O OO t- OS OO OS r- ( "*! 1C OS OS CD b- 1C O I>CO CO 1C OO OO CO rH iw o^ihtmifi To 28 AIRCRAFT DESIGN DATA. Note 12. TABLE 3. Strength of various woods in tension parallel to grain. [From tests of small clear specimens of green timber.] Species. Number tests averaged. Number trees repre- sented. Moisture content. Specific gravity. Tension par- allel to grain average. Probable variation of individual from average. Mahogany African 20 27 50 59 63 48 10 7 4 42 5 13 5 7 4 18 9 10 10 10 2 2 9 3 5 Per cent. 49.7 50.1 47.1 49.9 35.0 24.1 23.0 50.0 39 to 98 31.0 41 to 86 34.5 40 to 155 c 0. 457 c.492 .550 c.645 .399 c .530 c.477 . 369 .390 .401 .351 .500 .400 Lbs. per sg. in. 15, 110 16, 400 14, 900 14, 012 11, 730 16,200 13. 300 7,972 7,716 9, 760 9,580 9, 8SO !), GOO Lbs. per sq. in. 2, 075 2,400 Mahogany Central American Oak northern white a . 2,900 1,210 1,735 2, 050 1,400 1, 570 Cedar Port Orford Douglas fir (1) Douglas fir (2) Fir white Hemlock western Pine white 1, 405 Redwood ' 1,170 a Not identified as to species. i> Araucaria from Chile, South America. Specific gravity based on oven-dry weight and volume. (1) Specimens from the 8 feet immediately above stump, rom same trees. Other specific gravities based on oven-dry weight and volume as tested. (2) Specimens from the fifth 8-foot bolt above stump and higher. (1) and (2) TORSIONAL STRENGTH. Resistance to torsion is important in connection with control surface spars. The following fragmentary data are based on only 30 tests in all, 15 of each species: TABLE 4. Torsional strength q/ commercial white ash and Sitka spruce. Properties. White ash. Sitka spruce. Number of testa 15 15.8 .62 1, 753 2, 371 88, 500 8.8 24.0 15 15.7 .39 1, 090 1, 654 72, 300 4.4 19.7 Moisture per cent of oven-dry weight Specific gravity (based on oven-dry weight and oven-dry volume) .. Shearing strength at elastic limit pounds per square inch Shearing strength at maximum load pounds per square inch Shearing modulus of elasticity pounds per square inch Work to elastic limit inch-pounds per cubic inch . . Work to first failure, inch-pounds per cubic inch (1) (1) For the spruce and ash tested the first failure oscurred at maximum load in all cases. SHRINKAGE. Ordinarily when a piece of green lumber is dried no change in dimensions takes place until the fiber saturation point is reached. The wood then begins to shrink in cross-sectional area until no further moisture can be extracted from the cell walls. It also shrinks longitudinally, but in most cases the amount of longitudinal shrinkage is so small as to be negligible. The shrinkage in cross-sectional area in drying from the green to the oven-dried condition varies with different woods, ranging from as much as 22 per cent (based on the original area before drying begins) to as little as 6 per cent. When dry wood absorbs moisture it continues to swell until the fiber saturation point is reached. Figures 18, 19, and 20 illustrate the progress of shrinkage and swelling between zero moisture con tent' and the fiber saturation point. Note 12. AIECEAFT DESIGN DATA. 29 The shrinkage of wood, like its strength, is very closely related to its specific gravity. This illustrated by figure 21. On this curve, "Per cent shrinkage in volume" is the total shrinkage from fiber saturation to dryness. It will be noted that*shrinkage, in general, increases with specific gravity. This relation in individual specimens of a single species (white ash) is shown in figure 22. Radial shrinkage, or the shrinkage in width of quarter sawn boards, averages about three- fifths as great, as tangential shrinkage, or the shrinkage in width of flat sawn boards. /# /z /o ^L P < # I, fr i: * i- 6 * o 2 O Fig. 18. Relation between swelling and moisture. Each point is the average of from five to eleven specimens. Black dots indicate specimens that were kiln-dried and then allowed to reabsorb moisture. The fiber- saturation point is at c. fc- 6 3 SO 5 M /6 J9 2O ZZ Z4 36 Z8 30 3Z 34 36 33 4O . iab .Kama lo *oi):>>8 eecr) ^dJ bn Jnalnoa arfl noUaloH .OS 30 AIRCRAFT DESIGN DATA. Note 12. IIS tit III no __ ==< / ^ -^ V i r* ? s tot i- a 107 k 'OS E 105 104 MS tot 101 no DRYING CU RVEx X T / ^ I 2 / / /x REAISOMPTION CURVE 7 / / / / / / / // I W K 10 ZS SO JS 40 41 sr-H * MOISTURE PERCENT 19. Relation between the moisture content and the cross section of small, clear pieces of western hemlock. K torn g" br ABSORPTION POHV / 3S 40 MOSTURC PERCCMT Fig. 20. Relation between the moisture content and the cross section of small, clear specimens of western larch. Note 12. AIRCRAFT DESIGN DATA. 31 "" 7 / li / '3 J / PI ' j i lie / / i a / IJ? ' 'o ! X, jj / I 101 up 130 ,it ifi 18 t I/ 60 / 1^ 5. J. 5S ' 0*D ,11 I4 1 o r S PER CENT SHRINKAGE IN VOLUME lo / . g J8 1} | isb ^ i 1.0 .'j M 4 1 "o 1 ' J, 16 30c 'V / " ^ I 85 106 "' .0-. > 7, 9, fS d JJ 17 n? 7. e t g ,, / if oi 86 Z t" .7 ff ft j | ^* 'o 4< / U Ou, # 158 / *' 9 ,11 10 64 1 fr / ?2 / , 25 4-A ' / to / i / / 1 / / / I / i/ ( HRINKAGE - 28G / / >' / SPECIFIC GRAVITY ^ Fig. 21. Relation between shrinkage in volume and specific gravity of various American woods. .>I-Mif?f AIRCRAFT DESIGN DATA. Note 12. List of species and reference numbers for -figure 21. HARDWOODS. Species. Locality. Reference No. Species. Locality. Reference No. Alder, red Washington 30 91 60 70 99 93 100 79 106 128 83 23 20 12 5 110 98 73 129 107 103 9 84a 27 21 46b 72 24 46 40 6 59 151 125a 69a 126 102 74 55 53 165 68 147 76 54 90 78 146 135 154 139 144 159 155 112 148 157 Hickory Continued . Pignut Pennsylvania 160 161 140 152 143 153 141 87 149 145 158 162 101 J^.Sa 66 58 69 92 56 104 108 124 125 80 163 121 133 116 130 137 119 118 117 97 94 142 150 115 111 132 138 136 131 109 122 105 164 35 85 51 156 49 89 61 63 65 45 11 -43a 114 Ash: Biltmore Do West Virginia Black Michigan Shagbark Mississippi Do \Viscoiisin Do Ohio Blue Xentucky Do Pennsylvania Green Louisiana Do West Virginia Do Missouri Water Mississippi Pumpkin do Holly, American Tennessee White Hornbeam do Do New York Laurel, mountain do Do West Virginia Locust: Black do Aspen Wisconsin Largetooth do Honey Indiana Basswood Pennsylvania Madrona California Do Beech .... Wisconsin Indiana .... Do Oregon .... Magnolia Louisiana Do Pennsylvania Maple: Oregon . . Washington Birch: Paper Wisconsin Red Pennsylvania Sweet Pennsylvania Do Wisconsin . . Yellow do Silver do Do Wisconsin Sugar Indiana . Buckeye, yellow Tennessee ... Do Pennsylvania Buckthorn, cascara .... Butternut Oregon Do Wisconsin Tennessee Oak: Bur ... .do Do Wisconsin Chinquapin, western . . . Cherry: Black Oregon California black Canyon live California Pennsylvania do Chestnut Tennessee Wild red Tennessee Cow Louisiana Chestnut Maryland Laurel > .. ..do . . Do Tennessee Post Arkansas Cotton wood, black Washington Do Louisiana Cucumber tree Tennessee Red Arkansas Dogwood: Flowering do Do Indiana .... Do Louisiana Western Oregon Do Tennessee Elder, pale do Highland Spanish . . Lowland Spanish. . . Louisiana Elm: Cork Wisconsin, Marathon County. Wisconsin, Rusk County. Indiana do Swamp white Indiana Do Tanbark California Water Louisiana Slippery... White Arkansas ... . Do Indiana Do Wisconsin Do Louisiana, Richland Parish. Louisiana, Winn Parish. Louisiana White Pennsylvania Do Do Wisconsin Greenheart Willow Gum: Black Tennessee Yellow Arkansas Do Blue (Eucalyptus) . Cotton California Osage orange Indiana Louisiana Poplar, yellow (tulip tree). Rhododendron great Red Missouri do Hackberry Indiana Do Wisconsin Sassafras do Haw, pear do Serviceberry do Hickory: Big shellbark Mississippi Silverbell tree do Sourwood do Do Ohio Sumac staghorn Butternut do Sycamore Mockernut Mississippi Do TGGHGSSGG Do Pennsylvania Umbrella, Fraser do Do West Virginia Willow: Black Nutmeg Mississippi Pignut do.. Do Ohio Witch hazel Note 12. AIRCRAFT DESIGN DATA. 33 List of species and reference numbers for figure 21 Continued. CONIFERS. Species. Locality. Reference No. Species. Locality. Reference No. Cedar: Incense California 26 2 10 1 62 45a 67a 46a 75 67 48 4 39 18 14 36 16 17 47 52 15 50 84 64 127 43 33 88 ' 31 35a Pine Continued. Lod ^coole Montana, Granite County. Montana, Jefferson County. Wyoming 41a 40a 34 123 113 96 95 57 71 86 77 22 82 42 19 37 41 32 25 28 13 8 3 44 29 7 38 81 134 Western red Montana Do Do Washington White Wisconsin Do Cypress, bald Louisiana Douglas fir California Longleaf Florida Do... Oregon Do Louisiana, Lake Charles. Louisiana, Tangipahoa Parish. Mississippi Do Washington, Chehalis County. Washington, Lewis County. Washington and Ore- gon. Wyoming Do Do Do Do Norway Wisconsin Pitch Tennessee Do... Pond Florida Shortleaf Arkansas Fir: Alpine Colorado Sugar California Table Mountain Western white Tennessee Amabilis Oregon Montana Do Washington Western yellow .... Do Arizona Balsam Wisconsin California Grand Montana Do Colorado Noble Oregon Do Montana White California White \Visconsin Hemlock: Black Montana Redwood California, Albion Do California Korbel Eastern Tennessee Spruce: Engelmann Colorado,GrandCounty. Colorado, San Miguel County. New Hampshire Do Wisconsin Western Washington Do Larch, western Montana Red Do Washington Pine: Cuban Florida Do Tennessee White New Hampshire Jack Wisconsin Do Wisconsin Jeffrey California Tamarack do Loblolly Florida , Yew western Washington Lodgepole Colorado Do Montana, Gallatin County. 98257 19 No. 12 3 e. SVO-YTIVAfla 34 AIRCRAFT DESIGN DATA. Note 12. SUITABILITY OF VARIOUS AMERICAN WOODS FOR AIRCRAFT CONSTRUCTION. The difficulty of securing ample supplies of the woods heretofore considered as the standards for aircraft construction has made it necessary to consider the substitution of other species. It must must be realized that aircraft can, if necessary, be made from practically any species of wood which will furnish material in the required sizes, and progress in laminating and splicing has done much to increase the utilization of smaller sized material. It must also be borne in mind that the differences in suitability are slight for a number of species and that high-grade stock of a species considered to be inferior may actually be better than lower grade stock of PERCENT OF SHRINKAGE IN VOLUME > O N on -o c c 3 \ \ / 9 H' / p / WHITE ASH HILATION OF SHRINKAGE TO SPECIFIC GRAVITY PIRCEUT SMMIMKAai IS TOTAL men, 0*1111 TO OVIN DRY u / r Di U / 6 /I X) / C f *-d S / D n A O / o c / 3 / X 1 u / / f / / / / f / / / / y f 2 / / / y / y ,/ / / .1 .2 .3 .4 .5 .6 .7 .8 3 1.0 SPECIFIC GRAVITY-OVEN DRY ASID OH OVIH DRY VOLUME Figure 22. the species considered superior. In other words, it may be preferable to change species and keep the grade up rather than to lower the grade and use the same species. In order to give a general idea of the relative properties of the more common American species of timber, with respect to their use in aircraft, a short statement concerning each has been prepared. In those cases in which the species might possibly be considered as a substitute for spruce its properties are compared with those of spruce. CONIFEROUS SPECIES. Incense cedar. This species is somewhat lighter than spruce, but lacks considerably in stiffness and does not possess the toughness of spruce. It might be substituted for spruce for parts which are not highly stressed. Note 12. AIRCEAFT DESIGN DATA. 35 Port Orjord cedar. Port Orford cedar is somewhat heavier than Sitka spruce and equals or exceeds it in all its strength properties. Recent data upon this species indicate that it is not as strong as originally supposed, but still show it to be equal to spruce, although of slightly greater weight. Western red cedar. Western red cedar is lighter than spruce and below it in all its strength properties. It is more difficult to dry, but could probably be used with success in many parts where spruce is now used, but could not be used in parts which are highly stressed. White cedar. White cedar is very low in all its strength properties. It is a comparatively small tree and could hardly be considered as a possibility for use for the larger members. Bald cypress. Bald cypress is slightly heavier than spruce. Its average figures show it somewhat superior to spruce when used in the same sizes. The great variability in the wood of this species has, however, prevented its recommendation for aircraft construction. Cypress is very wet in its green condition and is considered much more difficult to dry and glue than many other species. Yellow cypress. Data on this species are not very complete. The indications are that it is too low in stiffness to be a satisfactory substitute for spruce. Douglas fir from the Pacific coast. Douglas fir from the Pacific coast is considerably heavier than spruce and all its strength properties are equal to or exceed those of spruce. It is quite probable that the bulk of good wing-beam stock will come from second-cut logs and that the weight and corresponding strength values will run slightly lower than the average of the species. Douglas fir is considerably harder to dry than spruce and more inclined to shakes and to check during manufacture and to develop these defects in service. It is inclined to break in long splinters and to shatter when hit. The use of Douglas fir in the manufacture of wing beams requires considerably more care than is necessary with spruce, but it should give excellent results (from the strength standpoint) when substituted for spruce in the same sizes. Douglas fir, Rocky Mountain type. The Rocky Mountain type of Douglas fir is much smaller than the coast type, is quite knotty and somewhat brash, and probably would not be satisfactory as a substitute for spruce. Alpine fir. The Alpine fir so far tested was very low in weight and in all its strength prop- erties. This material was from small knotty trees and should not be used except to resist low stresses. It is quite possible that the wood in more extensive stands of comparatively large Alpine fir will be heavier and stronger than that already tested. Amabilis fir. The amabilis fir so far tested was slightly heavier than spruce and in most of its strength properties it was practically the equal of spruce. Sufficient data are not at hand to determine how this material will kiln dry nor to determine its working properties. If it can be kiln dried and worked satisfactorily, indications are that it will be a fairly satis- factory substitute for spruce in spruce sizes in wing beams, struts, and other highly stressed parts. Balsam fir. Balsam fir is somewhat lighter than spruce and considerably lower in all its strength properties. It does not give promise of being satisfactory in airplane construction. Grand fir, nolle fir, and white fir. The grand fir so far as tested was slightly heavier than spruce, while the noble and white fir were slightly lighter. In strength properties these species compare very favorably with spruce except in the case of the shock-resisting ability of white fir, which is a little low. This, however, may be accidental. The statement made concerning amabilis fir will apply to these species also. Black hemlock. Black hemlock is quite a little heavier than spruce and lacking in stiffness. Eastern hemlock. On a basis of strength properties alone eastern hemlock appears to be a substitute for spruce, but the lumber is shaky and liable to heart rot, has numerous knots, and develops shakes and checks in service. It need not, therefore, be considered. 36 AIRCRAFT DESIGN DATA. Note 12. Western hemlock. Western hemlock is heavier than spruce, but not quite so heavy as Douglas fir. It is low in shock-resisting ability, but on a basis of strength alone it might serve as a substitute for spruce in spruce sizes. No data are available concerning proper kiln-drying methods and the possibility of manufacturing conditions which would cause this species to be rejected. Western larch. Butts of the western larch tree are very heavy. The material is shaky and is hard to dry. It would not seem feasible to use this species for aircraft in view of the supply of more suitable species. Cuban pine. Cuban pine is entirely too heavy to be considered. Jack pine. The jack pine so far tested was 9 per cent heavier than spruce and was lacking in stiffness. Jeffrey pine. Jeffrey pine is especially lacking in stiffness. Loblolly pine. Loblolly pine is quite heavy. It is very variable in its properties and need not now be considered. Lodgepole pine. Lodgepole pine is somewhat low in its shock-resisting ability and slightly low in stiffness. If extensive stands of large trees can be located, there is a possibility that it might be found practicable to use some of this species. Longleai pine. This material is considered too heavy for use in airplanes without redesign. Norway pine. Indications are that Norway pine can be used as a substitute for spruce in spruce sizes. More data are needed as to kiln drying and the difficulties which may be met in manufacture. Pitch and pond pine. Pitch and pond pine are both heavy, and it is not likely that they would ever be needed in aircraft work. Shortleaf pine. The lighter material from the shortleaf pine could be used for aircraft construction, but probably would not be as satisfactory as Douglas fir, since weight for weight it shows a lower modulus of rupture and stiffness. Sugar pine. Sugar pine is quite low in shock-resisting ability and stiffness and is quite variable. It probably would not, therefore, be a suitable substitute for spruce. Table mountain pine. Table mountain pine has about the properties of shortleaf pine. It probably would not produce clear material satisfactory for aircraft stock. Western white pine. Western white pine is slightly heavier than spruce and shows up well in all its strength properties except hardness. It is more difficult to dry than the eastern white pine, but probably could be substituted for spruce in spruce sizes. Western yellow pine. Strength data show the western yellow pine to be lacking in shock- resisting ability and stiffness. It is also quite variable. It is not considered a good substitute for spruce. Eastern white pine. Tests to date show eastern white pine somewhat below spruce in hard- ness and rather low in shock-resisting ability. It, however, runs quite uniform in its strength properties, is very easily kiln dried without damage, works well, stays in place well, and is rec- ommended for aircraft construction as a substitute for spruce in spruce sizes. Redwood. The data available on redwood are not comparable to those on other species and are too erratic to form a very definite judgment of the species. The indications are that the material is quite variable in its properties and likely to be very brash. Engelmann spruce. Engelmann spruce is quite light and low in all its strength properties. Tamarack. Tamarack is too heavy to be substituted for spruce. It probably would not furnish clear material. Yew. This wood is very heavy. The tree is small and crooked. Note 12. AIRCEAFT DESIGN DATA. 37 HARDWOODS. Red alder. Data on this species are very meager, but it is probably not available in sizes sufficiently large to make it of importance. Biltmore ash. Biltmore ash should be considered along with white ash and may be used for longerons and other work where strength, stiffness, and ability to steam bend are of impor- tance. Black ash. Black ash is very low in stiffness. It is an exceedingly tough species. It is one of the best native species for steam bending. It can not be used, however, where strength and stiffness are of great importance, as in places where white ash is used. Blue, green, and white ash. These species are known commercially as white ash and are very desirable for use in longerons and other places where steam bending, great strength, and stiffness are required. Oregon ash. Oregon ash appears i,o be about equal to the eastern white ash, although the data on this species are somewhat meager. Pumpkin ash. Pumpkin ash as a species is somewhat lighter than the white ashes. It is considerably less stiff than the white ash. Commercially the term is made to include the weak, soft material from all the other species of ash. Commercial white ash. Commercial white ash includes the Biltmore, blue, green, and white ash already mentioned. Aspen. Aspen is quite soft and lacking in stiffness. Basswood. Basswood is light in weight and low in practically all its strength properties. It is one of the best species to receive nails without splitting and is used extensively for webs, veneer cores, and similar work. Beech. Beech is quite heavy and has about the strength properties of sweet and yellow birch and hard or sugar maple. It might be used to some extent in propellers but not exten- sively in other aircraft parts. Paper birch. Paper birch is rather low in its stiffness and high in weight. Sweet and yellow birch. Sweet and yellow birch are quite heavy, hard, and stiff. They have a uniform texture and take a fine finish. On account of their hardness and resistance to wear they can be used to face other woods to protect them against abrasion. Yellow buckeye.- Yellow buckeye is low in its weight and all its strength properties. Cascara buckthorn. Cascara buckthorn is a small tree and need not be considered. Butternut. Butternut is lacking in stiffness and probably need not be considered. Western chinquapin. Western chinquapin is a small tree and need not be considered. Black cherry. Black cherry is a very desirable propeller wood. Wild cherry. Wild cherry is a small tree and lacking in stiffness. Chestnut. Chestnut is somewhat heavier than spruce and is quite deficient in stiffness. Cottonwood. The cottonwood so far tested was slightly heavier than spruce. It is soft, low in its strength as a beam or post, and lacks stiffness. It is very tough, however, does not split hi nailing, and bends well. Cottonwood can not well be substituted for spruce in wing beams and long struts but can be used in minor parts. Black cottonwood. Black cottonwood is low in weight and all its strength properties. Cucumber tree. The wood of the cucumber tree is somewhat heavier than spruce and shows up well in all its strength properties. It is one of the few hardwoods which gives promise of being a good substitute for spruce in wing beams and struts. Flowering and western dogwood. The dogwood trees are too small to be considered. Elder, pale. Elder is too small a tree to be considered. AIRCRAFT DESIGN DATA. Note 12. Elm, cork (rock elm). Cork elm is slightly heavier than ash. It is low in stiffness and very resistant to shocks. It steam bends well and if properly dried can be used for longerons as a substitute for ash. Considerably more care is necessary in the drying of elm in order to have it remain in shape as it twists and warps badly when not held firmly. Slippery elm. Slippery elm is somewhat lighter than cork elm, but when of equal density may be used as cork elm. White elm. Very dense pieces of white elm have the requisite density and strength to be used along with cork elm. Most of the white elm, however, is quite light. It is lacking in stiffness, but steam bends well. It could probably be used to excellent advantage in the bent work at the ends of the wings, rudders, elevators, etc. Considerable care would be necessary in order to hold this material in place while drying, as it warps badly. Black gum. Black gum is considerably heavier than spruce and not nearly so stiff. It probably will be but little used in aircraft. Blue gum (eucalyptus}. Eucalyptus grown in this country is quite heavy. It has large internal stresses, swells and shrinks excessively, twists badly in drying, and is very difficult to dry. Under present conditions it probably should not be used in aircraft. Cotton gum (Tupelo). This species is considerably heavier than spruce, but not nearly so stiff. At present it probably should not be considered for aircraft. Bed gum. Red gum is considerably heavier than spruce and superior to it in strength properties. On account of its locked grain and its tendency to twist, warp, and check it prob- ably should not be used in place of spruce. There is some prospect, however, that carefully quarter-sawed material of this species can be used in propellers. Hackberry. The denser pieces of hackberry might be substituted for ash in longerons. Pear haw. Pear haw is a very small tree and of no importance in this connection. True hickories, including shellbark, mockernut, pignut, and shagbark. These species are heavier than ash and are very tough and strong. They could be substituted for ash in longerons, but would probably not give quite as good service for the same weight. Pecan hickories, including butternut, nutmeg, pecan, water. These hickories are consider- ably inferior to the true hickories, especially in their ability to resist shock, and probably would not make satisfactory substitutes for ash. American holly. This species is lacking in stiffness and probably is of no importance in airplane construction. Hornbeam, California laurel, mountain laurel, black locust, honey locust, madrona. The laurels, locusts, and madrona are all heavy woods and probably have little use in aircraft construction. Magnolia. Magnolia has approximately the same properties as cucumber wood, to which it is closely related, and could probably be used as a substitute for spruce in wing beams and longerons. Oregon maple. Oregon maple has about the same properties as silver maple. It is a little more stiff and not quite so resistant to shock. There is probably little use for either of these species in aircraft. Red maple. Red maple is somewhat heavier, stiffer, and stronger than silver maple. Red maple might possibly be used in propeller work, but would give much softer propellers than sugar maple. Sugar maple. Sugar maple is quite heavy, hard, and stiff. It could be used with birch in propeller manufacture. It has very uniform texture and takes a fine finish. On account of its hardness and resistance to wear it is very often used to face other woods to protect them against abrasion. Note 12. AIRCRAFT DESIGN DATA. 39 Silver maple. Silver maple is the lightest and softest of all the maples. It is much too soft to be considered as a substitute for sugar maple and lacks the stiffness to make it a satis- factory substitute for spruce. The oaks. The oaks need not be considered as substitutes for spruce, but they play an important part in the manufacture of propellers. The oaks are all quite heavy and hard. The oaks, even when a single botanical species is considered, are extremely variable hi their strength properties. The differences in the average strength properties of the various eastern oaks are not great, and greater differences might readily be found among different logs of any one species. The white oaks, as a rule, shrink and swell more slowly with changes in the weather than do the red oaks. The radial shrinkage of the oaks is about one-half the tangential shrinkage. This accounts for the much greater value of quarter-sawed oak over plain-sawed oak for pro- peller construction. The southern-grown oaks are much more difficult to dry than are the northern oaks. Experiments are being made in the drying of both northern and southern red and white oaks. The northern white oaks when quarter-sawed and carefully dried make very satisfactory propellers. It is possible that quarter-sawed northern red oak will also make fairly satisfactory propellers but with this disadvantage: It is more subject to defects in the living tree, decays more readily, and changes more rapidly with changes in weather conditions. To be satisfactory in this work the southern oaks will require exceeding care in drying, as they are very difficult to dry without checking, honeycombing, and casehardening. Osage orange, persimmon. Osage orange and persimmon have other very important uses and are probably of no importance in aircraft construction. Yellow poplar. Yellow poplar is but little heavier than spruce, and while rather low in shock-resisting ability has good working qualities, retains its shape well, is comparatively free from checks, shakes, and such defects. It would probably be a fairly satisfactory substitute for spruce in wing beams and struts. It offers no manufacturing difficulties. Rhododendron, sassafras, service berry, silverbell, sourwood, sumac. These species probably have no place in aircraft construction. Sugarberry. This species is closely related to the hackberry and the denser pieces might be substituted for ash in longeron construction. Sycamore. The trees are very shaky and probably would not furnish material suitable for aircraft. Eraser umbrella. This species is closely related to the cucumber and magnolia previously discussed and has similar properties. The clear stock obtained might be used as a substitute for spruce. Willow, black and western black, witch hazel. Willow and hazel probably are of no use in aircraft construction. Walnut, black. Black walnut has many very important uses and need not be considered as a substitute for spruce. This species probably makes the best propellers of any of the native species. It is somewhat difficult to dry, but stays in place unusually well and is hard enough to resist wear. SYNOPSIS OF COMMENTS AS TO SUBSTITUTES FOR SPRUCE. The following species range in weight from that of spruce to 25 per cent heavier than spruce. The data available indicate strongly that these species can be substituted for spruce in highly stressed parts using the spruce design: Port Orford cedar, coast type Douglas fir, eastern and western white pine, yellow poplar, cucumber tree and magnolia. The following species give promise of furnishing substitutes for spruce, but more experiments are needed in order to over- come known difficulties before these species can be recommended: Bald cypress, amabilis fir, 40 AIRCRAFT DESIGN DATA. Note 12 - grand fir, noble fir, white fir, lodgepole pine, Norway pine, and redwood. The following spe- cies are lighter than spruce, but could be used in parts where the stresses are relatively 1 Incense cedar, western red cedar, and Alpine fir. As conditions change other species will doubtless come into consideration as subs for spruce. STORAGE AND KILN DRYING OF LUMBER. The proper piling of lumber and timber for air seasoning or as temporary storage previous to kiln drying is extremely important. Green or partiaUy dry stock is subject to various forms of deterioration, such as staining, decay, severe checking, and (especially in hardwoods) insect attack. During warm, humid weather staining may take place in a few days and decay may weaken the wood in a few months. Proper piling of such stock will tend to reduce the deterioration to a minimum. All lum- ber or timber which is to be stored any length of time should be piled on solid foundations with stickers between each two courses, and should have some protection from the sun and rain. Whenever possible, the stock should be piled in a shed with open sides. If this is not practicable, each pile should be covered so as to keep out rain and snow. Green hardwoods, especially oak, frequently check severely at the ends. This can be avoided to a large extent by coating the ends with linseed-oil paint. Stock should be cut up into as small sizes as is practicable before kiln drying. Large pieces usually check severely because the outer portion dries and shrinks considerably faster than the inner core, which always dries slowly. Timbers which contain the pith and which are to be cut into smaller sizes later should at least be cut through the pith once, or, better, be quartered before being stored away. This will avoid the large checks which are commonly produced in the seasoning of timbers containing the pith by reason of the tangential shrinkage being greater than the radial shrinkage. RULES FOR PILING LUMBER. 1. The foundations should be strong, solid, and durable, preferably concrete piers with inverted rails or I beams for skids. If this is impracticable, creosoted or naturally durable wooden timbers should be used. 2. Each foundation should be level. 3. The foundations should not be over 4 feet apart for lumber, but may be farther apart for larger timbers. For woods which warp easily or for stock less than 1 inch in thickness foundations should not be over 3 feet apart. 4. If the piles are in the open, they should have a slope from front to rear of 1 inch for every foot in length. 5. The foundations should be sufficiently high to allow the free circulation of air under- neath the piles, and weeds or other obstructions to circulation should be removed. 6. Boards of equal length should be piled together with no free unsupported ends. 7. A space of about three-fourths of an inch should be left between boards of each layer and from 1 to 2 inches between timbers of each layer. 8. The stickers should be of uniform thickness, preferably seven-eighths of an inch for 1-inch lumber and 1 inches for thicker stock. 9. Stickers should be placed immediately over the foundation beams and kept in vertical alignment throughout the piles. Their length should be slightly in excess of the width of the pile. 10. The front and rear stickers should be flush with or protrude slightly beyond the ends of the boards. Note 12. AIRCRAFT DESIGN DATA. KILN DRYING OF WOOD. ADVANTAGES OF KILN DRYING. The chief objects of kiln-drying airplane stock are (a) to eliminate most of the moisture in green or partly dried stock more quickly than can be done in air drying and (&) to reduce the moisture content of the wood below that attained in ordinary air drying, so that no more drying, with consequent checking, warping, and opening up of seams will occur after the wood is in place. Other advantages incident to kiln drying are that a smoother surface can be obtained on kiln-dried stock and that glues will hold better. . THE ELIMINATION OF MOISTURE FROM WOOD. Green lumber may contain from about one-third to two and one-half times its oven-dry weight of water. Expressed in percentage, this is from 33 J to 250 per cent moisture based on the oven-dry weight. The moisture content of green lumber varies with the species, the position in the tree, whether heartwood or sapwood, the locality in which the tree grew, and the drying which has taken place since the tree was cut. As a rule sapwood contains more moisture than heartwood, although in some species, especially in butt logs, the heartwood contains as much moisture as the sapwood. Thoroughly air-dried lumber may contain from about 10 to 20 per cent moisture for inch stock and more for thicker material. Much of the moisture in green wood is contained in the cell cavities (like honey in a comb), and the rest is absorbed by the cell walls. When wood is drying the moisture first leaves the cell cavities and travels along the cell walls to the surface, where it is evaporated. When the cell cavities are empty but the cell walls are still saturated a critical point is reached, known as the fiber-saturation point. Wood does not shrink or increase in strength while seasoning until it has dried below the fiber-saturation point, which usually ranges between 25 and 30 per cent moisture, but may be less or more, and in spruce usually is between 30 and 35 per cent. This has an important bearing on the drying operation, since no casehardening, checking, or warping can occur so long as the moisture content is above the fiber-saturation point in all parts of the stick. In practice the stock should be dried to a moisture content slightly less than it will ulti- mately have when in use. This may be as low as 6 per cent for interior work and not so low for wood to be exposed to weather. Two steps are necessary in the drying of lumber (a) the evaporation of moisture from the surface, and (6) the passage of moisture from the interior to the surface. Heat hastens both these processes. For quick drying as high a temperature should be maintained in the kiln as the wood will endure without injury. Some woods (especially coniferous woods) will endure higher temperatures than others. The general specifications for kiln-drying airplane stock which follow give the temperatures at which a kiln should be operated to prevent injury to lumber to be used for airplanes. The lumber in a kiln is heated and evaporation is caused by means of hot air passing through the piles. To insure proper drying throughout the piles a thorough circulation of air is neces- sary. The lumber must be properly piled and the kiln constructed so as to make the necessary circulation possible. Dry hot air will evaporate the moisture from the surface more rapidly than it can pass from the interior to the surface, thus producing uneven drying, with consequent damaging results. To prevent excessive evaporation and at the same time keep the lumber heated through, the air circulating through the piles must not be too dry; that is, it must have a certain humidity. The specifications give the proper humidities at which to operate the kiln for drying airplane , i stock. 42 AIRCRAFT DESIGN DATA. Note 12. THREE ESSENTIAL QUALITIES OF A DRY KILN. The merits of any method of drying airplane woods depend upon the extent to which it affects the mechanical properties of the stock and upon the uniformity of the drying. In order that complete retention of properties and uniform drying may be guaranteed, it is essential that the circulation, temperature, and humidity of the air be adequately controlled. In this connection circulation does not-mean the passage of air through flues, ducts, or chimneys, but through the piles of lumber, and the terms temperature and humidity control apply to the air within the piles of lumber in the kiln. Control of air circulation involves rate or speed and uniformity. A uniform passage of air through all portions of the piles of lumber is the most essential quality in a kiln. If the cir- culation can be made both uniform and rapid, all portions of the pile will dry quickly and at the same rate. Furthermore, uniform and rapid circulation of air are necessary before the control of temperature and humidity within the piles of lumber is possible. When unsaturated air at any given temperature enters a pile of lumber containing moisture, it exchanges heat for moisture, is cooled, and rapidly approaches saturation. With green wood and a sluggish circulation, the cooling is very rapid. The rate of cooling decreases as the lumber dries, and if the circulation is increased the loss of heat in passing through the pile is less. So if the air moves rapidly through certain parts of the piles and slowly through others, the differ- ent parts of the piles will be at different temperatures. The temperature of the air within the lumber can not be maintained at any given value unless the circulation of air is uniform at all points in the pile. Even though the air moves at uniform speed from one side of a pile of lumber to the other, if the speed is too slow the air loses its heat and approaches saturation rapidly. In general a wide variation in the temperature of the lumber in different parts of the kiln is proof of very uneven or slow circulation. Inadequate circulation and temperature control render the control of humidity and uniform drying impossible. Humidity is of prime importance, because the rate of drying and the prevention of checking and casehardening are directly dependent thereon. It is generally true that the surface of the wood should not dry more rapidly than the moisture transfuses from the center to the surface. The rate of evaporation must be controlled, and this can be done by means of the relative humidity. Stopping the circulation to obtain a high humidity or increasing the circu- lation by opening ventilators to reduce the humidity is not good practice. Humidity should be raised, if necessary, to check evaporation without reducing the circulation. DEFECTS DUE TO IMPROPER DRYING. Casehardening and honeycombing. When the surface of a piece of lumber is dried more rapidly than the moisture can pass to it from the ulterior, unequal moisture conditions exist in the lumber. The moisture in the outer layers falls below the fiber saturation point. The outer layers then tend to shrink but are held from shrinking by the more moist interior, which has not yet started to shrink; so the surface either checks or dries in a stretched condition, usually both. Later, as the interior dries it also tends to shrink normally, but in turn is held by the outside, which has become "set" or " casehardened." Consequently, the interior dries under tension, which draws the outer layers together, closing up all checks and producing compression. Casehardened lumber, when resawed, will invariably cup toward the inside if the interior if the lumber is dry (fig. 23). If the tension in the interior of the wood is severe enough, it may produce radial checks which do not extend to the surface. Wood with such checks is said to be honeycombed or hollow-horned (fig. 24). Casehardening and honey- combing can practically be prevented by regulating the humidity so that the evaporation from the surface does not take place too rapidly. Fig. 23. Sections of casehardened western larch boards. Nos. 1 and 2 are original sections; Nos. 3 to 8 are resawed sections showing cupping; No. 9 is one-side surfaced. THE SAME, WELL KfLN DRIED IN HUMIDITY REGULATED don ~__^^^^_^^^_^___^^_. ' -. -Jloe Fig. 24. Oak stock honeycombed by air drying and improper kiln drying. Also similar stock properly dried. 43 44 AIRCRAFT DESIGN DATA. Note 12. If wood becomes casehardened in kiln drying, it may be brought back to normal con- dition by steaming, provided that checks and cracks have not developed. Steaming softens the outer fibers and relieves the stresses caused by the contraction of the outer shell. Care must be taken not to steam wood which has checked or honeycombed from casehardening enought to part the fibers and weaken the piece. Steaming will close up the cracks but will Fig. 25. End view of 1-inch boards of western red cedar dried with and without collapse. not restore the strength of the piece. It will be much harder to detect cracks and checks due to casehardening if they have been closed up again by steaming. ND L H E J o( n A ^ ^ 4 ^ ti t J Y j / u Q -K _ \ p f 1 ) 1 y P ) A j ^ \, 4 & * J j \ S i ) ^ PI b 11 ^ : 1 \ 8 f (f* 4 r r~ x- ' 4 C ^ & & - J .*. -' 11 ^ i .* * PER CENT RELATIVE HUMIDlTy ,' 10 30 60 70 80 100 Fig. 28. Composite curve of moisture content of fine woods at different humidities and ordinary room temperature. Sliced veneer is usually manufactured only from the finer woods. On account of the fact that it is possible to produce quartered veneer on slicing machines, and the waste on account of saw kerf is absent, this method of manufacture is preferred where pattern is important and the value of the wood is great. The length parallel to the grain of sliced veneer is limited by the length of the knife. Sawed veneer can be produced in almost any reasonable length and from any kind of stock. The material produced may be either quartered or slash. In general, sawed veneer will not be specified for aircraft uses, to the exclusion of rotary stock, except where it is necessary to have extra long lengths or quartered stock or for some other reason it is impossible to secure the stock by rotary cutting. It may happen, for instance, -that the stock from which the veneer is to be cut can not be handled to advantage in a rotary lathe on account of its shape. 58 AIRCRAFT DESIGN DATA. Note 12. 'S 3 I I 5^ 3 I a -s 'f. ^c CO <> t-~ CO CO i I iO 1C Oi _ i I^OiOCOlOC^lCir- (TtiT*lf iTJtrW OOOi-lub ^ac^it^coeoiXJt^OiOco (M i I lO O CO OS CO CO -f iffl CO iO5(MiOu5i loO oo oi OOlOOOO--< i ; Is ||=. -* T3 C rt 3*8 "E 60 o-S . 0|(3 o X o X X ** 2 . B * & % o y i II III 111 H IP a b,s5 111 9 Si J IP II H ~N*ll|tt!tf I ? 1 .2 a < ICS?OOSeOrHOasC- "if i I OS (N CO eorHco^QQO^iflos-^osioiooo^coco J>-t--.3OSOCOOI>OO?DOOOSt^Ooo xfOCOOSOOCOCOOC^CM C^-'ftNCO COrHOlCOOl rHU5COOCOeNJ-^OOOSCNIOOOSOOOCOCOCOrHrHTf't--5DCOrHiCD4 rH C4 rH i I t>COOOS t-T ecf i-T c oo 01 o < (MO rHCO !>'.'!! i !! i o o S ^ o< * (T, fe iJCj ' - ---rfS-ii.yB'ffJjr . .rfJiSli ['g a a a ais a |)^^'| '^1! ^i ^ ^ a | |4 H O 60 flg a o a gl o ill 03.4 S C. Note 12. AIRCRAFT DESIGN DATA. 67 The results of strength tests on plywood of various common veneer species are given in table 10. Except for birch all tests are on only one shipment of the species, so that the results will in all probability be changed somewhat by the addition of future test data. The mahogany results are on thin plywood ranging in thickness from ^ inch to -fa inch, while the sizes of the plywood for all other species ranged from -^ inch to -f inch. In most cases it was found that the column-bending modulus of thin plywood was slightly less than the column-bending modulus of the thick plywood. TABLE 11. Comparison of strength of 3, 5, and 7 ply yellow birch plywood, all plies of same thick- ness in any one panel. Column-bending modulus, in pounds per square inch. Tension, in pounds per square inch. Average splitting resistance Number of plies. Average specific gravity.* Average per cent moisture. Number of tests. Parallel.t Perpendicu- Tar.f Parallel.t Perpendicu- lar.t compared to 3-ply birch, for the same ply- wood thick- ness, in per cent of 3-ply. 3 0.67 8.5 195 16, 000 3,200 13, 200 7,700 100 5 .67 6.6 25 14, 700 6,800 13, 100 8,600 129 7 .70 7.1 25 14,300 7,900 12, 900 9,300 191 * Specific gravity, based on oven-dry weight and volume at test. t Parallel and perpendicular refer to direction of grain of faces relative to direction of application of force. Table 11 shows the decrease in the unit strength of plywood in the direction of the grain of the faces when the number of plies is increased, and the increase in the unit strength of ply- wood perpendicular to the grain of the faces when the number of plies is increased. TABLE 12. Comparison of strength of three-ply wood having a core of high density with similar plywood having a core of low density of the same thickness; each ply -fa inch thick. Number of tests very limited. Results tabulated will probably be changed by further tests. Species. Num- ber of tests. Ply- wood thick- ness Per cent mois- ture at test. Specific gravity, based on oven- dry weight and volume at test. Column-bending modulus in pounds per square inch. Tension in pounds per square inch. load in pounds per square inch, 5 by 12 inch specimen test- ed as a column. Face. Core. Face. Parallel.* Perpen- dicular.* Parallel.* Perpen- dicular.* Paral- lel.* Perpen- dicu- lar.* Birch Birch Birch 30 10 33 5 20 5 5 Inches. 0.15 .14 .15 .15 .14 .15 .14 9.4 8.2 6.9 7.0 9.5 8.3 6.5 0.68 .61 .69 .62 .55 .44 .51 14,200 15,200 16, 100 17, 700 9,550 7,200 10, 100 3,170 1,600 3,210 2,600 2,060 1,400 11, 900 12, 900 9,910 12, 000 8,410 4,900 6.200 7,290 3,800 6,540 3,700 4,720 3,000 4,500 258 250 265 247 193 115 149 21 12 45 15 35 11 17 Do Sugar maple Do Basswood ...do Sugar maple Basswood. Red gum Sugar maple ...do Red gum... do Red gum... Do Basswood Do Yellow poplar.. ...do * Directions refer to direction of application of the force relative to the grain of the faces. Table 12 shows that the strength values of plywood parallel to the grain of the faces are practically the same for three-ply wood having a core of dense wood as for plywood having a core of light wood. The strength values across the grain of the faces are, however, very much 68 AIRCRAFT DESIGN DATA. Note 12. less for the plywood with core of low density. In other words, the strength values of three-ply wood parallel to the grain of the faces are almost entirely determined by the strength values of the face material, and the strength values across the grain of the faces are very largely deter- mined by the strength values of the core species. Table 13 gives a number of factors that are of value in selecting the thickness and species of the plies for a three-ply panel. TABLE 13. Thickness jactors jor veneer. Giving: (1) Veneer thickness for the same total bending strength as birch; (2) veneer thickness for the same weight as birch. Species. D. Average specific gravity of species * based on oven-dry weight and air-dry volume. Specific gravity of giued ply- wood as tested. Per cent moisture of plywood as tested. S. Per cent unit bend- ing strength compared with birch.t Thickness factor for the same total bend- ing strength as birch, VIoo IT K,,. Thickness factor for the same weight as birch, 0.63 D ' Ash black 0.50 49 9 1 52 1 39 1 26 Ash, commercial white .58 .60 10 2 72 1. 18 1 09 Basswood .38 .42 9 2 48 1 44 1 66 Beech .63 .67 8.6 94 1.03 1.00 Birch, yellow .63 .67 8.5 100 1 00 1.00 Cedar, Spanish a.34 .41 13 3 43 1 52 1 85 Cherry & .51 56 9 1 80 1 12 1 24 Chestnut .44 .43 11.7 34 1.72 1.43 Cottonwood .43 .46 8 8 56 1.34 1.47 Cypress, bald .44 .47 10.3 53 1.37 1.43 Elm cork .66 62 9 4 78 1 13 95 Elm, white .51 .52 8 9 58 1 31 1.24 Fir, Douglas .52 .49 54 8.7 10 6 60 56 1.29 1 34 1.24 1 21 Gum' cotton 52 50 10 3 48 1 44 1 21 Gum, red . .49 54 8 7 1 25 1 29 TT T 't Hackberry , 54 54 10 2 55 1 35 1 17 Hemlock, western . . 42 47 9 7 60 1 29 1 50 Magnolia .51 58 9 9 67 1 22 1.24 Mahogany, African 'stic a . 46 52 12 7 56 1 34 1 37 Mahogany, Philippine d o 57 53 10 7 68 1 21 1 10 Mahogany, true a 49 48 11 4 57 1 32 1 29 Maple, soft e 48 57 8 9 74 1 16 1 31 Maple, sugar 62 68 8 100 1 00 1 02 Oak, commercial red 63 59 9 3 59 1 30 1 00 Oak, commercial white .69 .64 9 5 69 1 20 .91 Pine, white .39 43 10 2 52 1 38 1 61 Poplar, yellow 41 50 9 4 58 1 31 1 54 Redwood a 36 41 11 2 49 1 43 1 75 Sycamore .50 56 9 2 71 1 09 1.26 Spruce, Sitka 38 43 8 4 50 1 41 1 66 Walnut, black 57 59 9 1 83 1 10 1 10 * Taken from Bulletin 556 of the U. S. Department of Agriculture. ' Average of the column-bending moduli parallel and perpendicular to grain compared to birch. a Based on subsequent tests. fr Probably black cherry. Coast type Douglas fir. d Probably tanguile. Probably silver maple. The thickness factor (Ks) is used to obtain the thickness of a ply of any species having the same total bending strength as a given ply of birch. It is arrived at as follows: The strength of any structural member is determined either by the direct load it can sustain or the bending moment it can resist without failure. In plywood the latter factor is the better criterion of strength. If we denote the maximum bending moment of a strip of Note 12. AIRCRAFT DESIGN DATA. three-ply wood 1 inch wide and of thickness d v by M t and the stress at failure by S t (column- Ci j n bending modulus), then M t = --- Similarly, the strength of another strip of a different species will be denoted by M 2 , its stress at failure S 2 , and thickness d 2 . By a proper selection of thickness d 2 the second strip may be made to withstand the same maximum bending moment, so that M 2 = M x or S 2 d 2 2 = S^ 2 . VS o* Taking d t as the unit of thickness of a birch ply- Oj wood strip and expressing the maximum stresses in percentage of birch, we have d 2 = -v a ' or, in general, K s = -J-^-> where K s is the thickness of the plywood, whose column-bending modulus corresponds to S and whose total bending strength, given by the bending moment, is the same as that of birch plywood of thickness unity. The same reasoning also applies to single plies, so that K s may be used to get the thickness of a single ply, which will give the same total bending strength as a birch ply of thickness unity. For example, for yellow poplar K s = 1.46, and a ply of this species, 1.46X^ = 0.091 inch, is equivalent in strength in bending to a birch ply yg- mcn thick. By way of explanation it must be understood that unit bending strength refers to a maxi- mum stress such as the modulus of rupture, or the column-bending modulus, while total bending strength refers to the load or bending moment a beam can sustain or the bending moment a column can sustain. It should be kept in mind that these factors will doubtless be modified, somewhat by further LGS vS. The thickness factor (K w ) is used to obtain the thickness of a ply of any species equal in weight to a ply of yellow birch of given thickness. It is obtained by simply dividing the density of birch by the density of the species for which the thickness is desired. The density data used in computing K w are the same as that given in United States Department of Agriculture Bulletin 556, "Mechanical Properties of Woods Grown in the United States." The weight of the glue in the plywood is neglected. For yellow poplar, for example, the thickness of a ply equal in weight to a y^-inch ply of birch is 1.54 X y^ 0.096 inches. The column-bending tests, upon which the data in table 10 are based, were all made on specimens of the same lengths, and it was felt desirable to determine what effect, if any, the change in length of the column might have upon the maximum unit load, the slenderness ratio remaining constant. Special panels of three-ply birch, all plies of the same thickness in each panel, were made up from veneer of the following thicknesses: ^, ^, ^r, yV, rV> s> an d test columns varying in length from 20 inches to 6 inches were cut from them and tested. The conclusion drawn from these tests is that for a given slenderness ratio the length of the column has little, if any, effect on the maximum unit load which a three-ply birch column will sustain. It is assumed that the same conclusion will apply to panels of other species. Table 9, to which reference has already been made, presents data by which it is possible to calculate the strength in tension of plywood composed of various kinds of veneer. Column (d) of this table is identical with the corresponding column in table 10. Column (e) is to be used in calculating the strength in tension of plywood made up of different species. The method of calculation is based upon the fact that the tensile strength of wood in a direction perpendicular to the grain is very small in comparison with that parallel to the grain and 70 AIRCRAFT DESIGN DATA. Note 12. may, therefore, for purposes of approximation, be neglected. To obtain the tensile strength in any direction, simply add together the tensile strength, parallel to the grain, of the indi- vidual plies the grain of which lies parallel to the direction in which the strength is desired. The sample computation will make this entirely clear. The shearing strength of plywood is of importance in connection with the design of box beams having plywood cheek pieces and for similar construction. Several series of tests are under way to determine the shearing strength of plywood of various thicknesses when unsup- ported for various distances. While these tests are not as yet completed, it is evident that it will not be possible to use a shearing strength in calculating these members much greater than that of solid wood of the same species. There is much more residual strength in ply- wood after the first failure than in solid wood, and for this reason a somewhat higher working stress would be justified. Until more data are available the shear allowed in plywood should not be over 25 per cent greater than that allowed in solid wood of the same species. This assumes that in the cheeks of horizontal beams the face plies will be vertical, a condition dictated by experience to be best practice. RIVETED JOINTS IN PLYWOOD. The matter of joints in plywood is of the greatest importance in connection with the construction of various types of built-up structures such as fuselages, boat hulls, pontoons, ')([) imibi: -< m jd s - 4 3" ... \ Mrsy/iv . 3' \ & Off// ? f O O O O O O O < ) C ) O ) O O O O O Q O C ) ( ) O , s \ Fig. 30. (a) Test specimen for single-rivet tests, (b) Test specimen for multiple-rivet tests. and beams and girders. Several series of tests have been made to determine the efficiency of various types of joint for different kinds of loading. The first series of tests was made upon riveted joints designed for tension and compres- sion. The tests were all made in tension; both solid and hollow rivets were used. Two types of test were run; most of the tests were made on specimens only wide enough to accommo- date one rivet (fig. 30a), and later enough wide specimens were tested (fig. 30b) to verify the assumption that the data on the narrow specimens could be applied without correction to wider ones. In general, most of the tests were made on butt joints, with straps on each side. In some cases the straps were of plywood and in others of galvanized sheet metal about 0.02 inches thick. The nomenclature used will become clear upon examination of figure 30. Note 12. AIRCRAFT DESIGN DATA. 71 The first tests were made upon red gum plywood composed of three plies of -^ material, riveted with solid copper rivets through sheet-metal cover plates. The grain of the face plies was perpendicular to the seam. Figure 31 shows the strength of the joint with varying mar- gins and spacing. It is apparent that the best conditions are obtained with a 1-inch margin and a one-half inch spacing. 6OO "';>>.' / / . a + . / // "3f. ~2 * ' '& '* Fig. 31. Single-riveted butt joints in plywood. Relations among strength, margin, and spacing: Red gum ply- wood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover plates; grain of faces perpendicular to seam; moisture, 7.4 per cent. AIRCRAFT DESIGN DATA. Note 12. Figure 32 shows the variation of strength when using a constant spacing of one-half inch and margins varying from one-quarter inch to 2 inches. This figure shows very clearly that no appreciable additional strength can be obtained by increasing the margin above 1 inch. 600 O.2S O.SO /.OO /A/ /MCHES /.7S 2.0O Fig. 32. Single-riveted butt joints in plywood. Relation between strength and margin: Spacing 1/2 inch; red gum plywood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover plates; grain on faces perpendicular to seam; moisture, 7.4 per cent. In fact, it was found that in case the grain of the face plies was parallel to the seam, the margin could be reduced to three-quarters inch without sacrificing an appreciable amount of strength. Similar tests made on three-ply birch, each ply one-sixteenth inch, gave similar results, as shown in figures 33 and 34. With a margin of 1^ inches, the maximum strength was secured with a spacing of one-half inch. Note 12. AIRCRAFT DESIGN DATA. 73 TOO 6OO k 300 I /00 # O.2S O. SP/JC//VG >v % O.7S /.oo Fig. 33. Single-riveted butt joints in plywood. Relation between strength and spacing: Margin, 1 1/2 inches; birch plywood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover plates; moisture, 6.6 per cent. 74 AIRCRAFT DESIGN DATA. Note 12. Fig. 3 O.Z5 a SO O.7S /.OO /.2S /.SO /.7S X/V //VC//S 4. Single-riveted butt joints in plywood. Relation between strength and margin; spacing, 1/2 inch; birch plywood, plies 1/16 by 1/16 by 1/16 inch; solid copper rivets, 0.15 inch diameter; sheet- metal cover plates; moisture, 6.6 per cent. f ,ni$iBM :snfcwjqa im* msirtroa nwied iBtem-Jooria ,-THmi;il> rfani 3I.O',8J9vh wqqo-) biloa ;ri)ni ;>t ! iaiid Note 12. AIRCRAFT DESIGN DATA. 75 The margin could have been reduced to 1 inch or even less without a great falling off in efficiency. Figure 35 indicates that a spacing of one-half inch is the best with thinner birch (each ply -fa inch). ST/?N6Tff /N POUAfaS Pft /MCH OF JO/A/r 1 8 8 t 8 8 1 / *~~ > ^ S b / 1 t s 1 1 1 r ) [4 V d ' ? fOil ' x^^ t ^< ^ ^ s / / < ^*s ) ^ \ ^^wl I / / s ^\ ^ ,' / ^. ^ / p // J' I t ( /// /MCHS Fig. 35. Multiple-riveted butt joints in plywood; relation between strength and spacing; test Joint, 5 to 5 1/2 Inches wide; margin, 1 inch; birch plywood, plies 1/20 by 1/20 by 1/20 Inch; solid copper rivets, 0.15 inch diameter; sheet-metal cover plates; moisture, 5.6 per cent. ni noiiai/boi 76 AIRCRAFT DESIGN DATA. Note 12. Figures 36 and 37 show the strength of joints made in three-ply birch (each ply one-twen- tieth of an inch) with five-eighths-inch hollow aluminum rivets and plywood cover plates. A spacing of \\ inches gave the best efficiency with a margin of 2 inches. It is possible that 700 O.S-0 /.OO 2.00 3.00 /.so //v Fig. 36. Single -rive ted butt joints in plywood; relation between strength and margin; spacing, 1.25 inches; birch plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 5/8 inch outside diameter; plywood cover plates; moisture, 5.6 per cent. greater strength could have been secured in the case of the specimens with the grain of the faces perpendicular to the seam had a greater margin than 2 inches been used. In the case of the specimens with the grain of the faces parallel to the seam a margin of \\ inches could have been used without any great reduction in strength. Note 12. AIRCRAFT DESIGN DATA. 77 7OO /.SO 2.00 O.SO S-OO boow{iti naJamflib ^blnJuo rfani \l Fig. 37. Single-riveted butt joints in plywood; relation between strength and spacing; margin, 2 inches; birch plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 5/8 inch outside diameter; plywood cover plates; moisture, 5.6 per cent. 78 AIRCRAFT DESIGN DATA. Note 12. The results of tests upon three-ply birch (each ply one-twentieth inch) with plywood cover plates and one-half inch and three-eighths inch hollow aluminum rivets, respectively, are plotted in figures 38 and 39. These tests were made with margins of 2 inches. However, smaller margins could no doubt have been used without appreciable loss in strength. 600 /.SO ^-j^ C^ V . Fig. 38. Single-riveted butt joints in plywood; relation between strength and spacing; margin, 2 inches; birch plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 1/2 inch outside diameter; plywood cover plates; moisture, 5.6 per cent. _ Note 12. AIRCRAFT DESIGN DATA. 79 700 SPA&W&7N Wtf5 /.3& Fig. 39. Single-riveted butt joints in plywood; relation between strength and spacing; margin, 2 inches; birch plywood, plies 1/20 by 1/20 by 1/20 inch; hollow aluminum rivets, 3/8 inch outside diameter; plywood cover plates; moisture, 5.6 per cent. 80 AIRCRAFT DESIGN DATA. Note 12. When the most efficient spacing and margin are used, there is practically no difference in strength for the different sizes of rivets investigated. However, the smaller rivets require a smaller spacing and therefore more labor in manufacture. On the other hand, the margin required is less than in the case of the larger rivets, and this may in some cases be a decided advantage. Cover plates may be of metal or plywood, as preferred. If of metal, aluminum sheet about three-sixty-fourths inch or one-sixteenth inch thick is recommended for the thicknesses of plywood investigated. The efficiency of the joints was determined by testing a number of samples of the ply- wood, both parallel and perpendicular to the face plies, and it was determined that under the best conditions the efficiency of the joints with the face plies perpendicular to the seam was about 30 per cent, while with the face plies parallel to the seam the maximum efficiency was a little over 50 per cent. While riveted joints may be satisfactory under certain circumstances, they can not be used where an efficiency much over 50 per cent is required. In these cases it is necessary to use glued joints, of which there are several different types. I s ! I S/sn/3/e Scarfs/a//?/- ticyona/ Scarf Jo/rtf //nf>Je uff~/o//7/- D/ayona/ fiutf-Jbtfit Satv-Joerfih SuffJoM Fig. 40. Joints in the face veneer of three-ply wood. JOINTS IN INDIVIDUAL PLIES. Joints in individual plies may be made in a variety of ways. Figure 40 shows several possible methods for joining pieces of veneer. A considerable number of strength tests upon several of these joints have been made. The simple scarf joint has been tested for a long range of slopes of scarf. The diagonal scarf joint, as well as the diogonal butt joint, have been tested for various slopes of the diagonal. The saw-tooth butt joint has been tested for various angles of the saw tooth. In balancing up the various factors of strength, ease of manufacture, and efficiency it was decided that the simple scarf joint is the most desirable of the group. The simple butt joint should not be used where strength is important. The edge joint is satisfactory if carefully made. The slope of the scarf in the simple scarf joint should be within the range of from 1 to 20 to 1 to 30. In comparison with the use of rivets, joints in individual plies are probably more practical. They have an advantage, too, in that the joints in the plies of a given panel may be staggered, so that any defect that may occur in any particular joint only partially weakens the entire panel. The time and labor involved in the preparation of this type of joint, while probably less than the time and labor involved in the preparation of riveted joints, is greater than that in preparing the scarf joint extending through the entire thickness of the panel. ) Note 12. AIRCRAFT DESIGN DATA. JOINTS EXTENDING THROUGH THE ENTIRE THICKNESS OF PLYWOOD. Many tests hare been made upon scarf joints extending through the entire thickness of a panel. Such joints were prepared by various manufacturers using different glues, different combinations of veneer thicknesses and species, and various slopes of scarf. Two types of scarf joints extending through the entire plywood thickness have been tested and are here described as the straight scarf joint and the Albatros scarf joint. The two types are shown in figure 41. The tests indicate quite conclusively that the straight scarf joint is the superior joint of the two. An examination of the Albatros joint will show that the face ply of the one panel does not meet the face ply of the second panel or only partially meets it. In place of being glued to wood that has the grain running in the same direction, the face ply of one panel is glued to the core of the second panel, in which the grain runs at right angles to the grain in the face. Joints in which the grain of the two pieces joined is at right angles are not as strong as joints in which the grain of the two pieces is parallel. / Sr? 2O -fa / //? 3O /7// / r> x< /J/bafros ocarf Fig. 41. Joints in plywood extending through the entire thickness. Tension tests on the straight scarf joint show that an efficiency of over 90 per cent may be obtained with this type of joint for a slope of scarf as low as 1 in 10. On account of the variations hi the effectiveness of the gluing by different manufacturers, it is recommended that a slope of scarf greater than this be used. A slope in the neighborhood of 1 in 25, with a range of from 1 in 20 to 1 in 30, is recommended. Severe weakening of scarf joints is often due to sanding of the face plies at the joint. Obser- vations on joints of this kind that were sanded showed that at times more than hah* of the face ply is ground away. Inasmuch as the strength of a panel lies almost entirely in the face plies (in case of three-ply panels parallel to the direction of the grain of the faces), it is obvious that a reduction in the thickness of the face plies will materially affect the strength of a panel. Con- sequently it is recommended that if the scarf joint is sanded at all that it be only lightly sanded by hand, so as not to decrease the thickness of the face veneer. Figure 42 shows the method used for cutting the scarf and for gluing the two pieces of plywood together. The board above the panel should be relatively massive and flat so as to distribute the pressure from the screws. Two or three layers of blotting paper furnish sufficient padding to accommodate irregularities in the surface. 98257 19 No. 12 6 - G . gniiaoJ ni flonil rlriw vMs-iova! beiaqnioo ^eifT .gaomiguoJ m wol fn* 82 AIRCRAFT DESIGN DATA. Note 12 THIN PLYWOOD. In an effort to develop a substitute for linen for wing covering which could be used on present types of wing framework, several different kinds of thin plywood have been developed. Among these are plywoods composed of three plies of wood, each ply as thin as one one-hundred- and-tenth inch, plywoods with veneer faces and fabric cores, plywoods with veneer faces and metal wire core, plywoods with veneer core and cloth faces, and several other types. A method was developed which made it commercially possible to glue up very thin plywood without undue loss, although the losses in making thin plywood are naturally much greater than in mak- ing comparatively thick plywood on account of the fragile nature of the thin sheets and their tendency to warp and twist when glue is applied to them. It was not found possible to produce a plywood having all the requisite properties which was as light as doped linen. The genera} conclusions drawn from the investigation follow: 1. Spanish cedar, mahogany, birch, sugar maple, red gum, yellow poplar, black walnut, and basswood may be cut into veneer sufficiently thin for consideration in plywood air-plane wing covering as substitutes for linen. METHOD OF CL/rr/MG SC/lffF METHOD OfP/?5S/MG GLUED JO/NT Fig. 42. Method of making plywood joints extending through entire thickness. 2. These species may be glued satisfactorily by the method of introducing the glue between the plies by means of tissue paper previously coated with glue. 3. It does not seem that plywood sheets of the same weight per square foot as doped linen can be prepared on a practical scale. 4. Covering made either of veneer or of a combination of veneer with fabric, such as linen, cotton, wire screening, or kraft paper, in order to be both practical from the point of view of manufacture and satisfactory in mechanical properties as shown by test, weighs from two to three times as much as doped linen. 5. Plywood that might be considered practical from the point of view of manufacture possesses from two to three times the tensile strength of doped linen. 6. The thinnest ply-wood that can be manufactured at present with any degree of facility (3 plies of one one-hundred-and-tenth inch Spanish cedar) lacks toughness and tearing strength. 7. In general the tearing strength of a practical thin plywood covering is considerably higher than that of doped linen, while its resistance to blows as indicated by the toughness test is lower. 8. In order to obtain the requisite degree of toughness, it is necessary to introduce a cloth fabric into the construction. Grade A cotton now in use in airplane construction is satisfactory for this purpose. 9. Combinations of veneer with kraft paper developed satisfactory tensile strength, but are low in toughness. They compared favorably with linen in tearing resistance. Note 12. AIRCRAFT DESIGN DATA. 10. Combinations of veneer with light wire screening, thus far tested, are heavy and unsatis- factory from the point of view of tensile strength per unit weight. Their toughness and tearing resistance are not superior to cloth when used in combination with veneer. 11. Thin plywood or a combination of veneer with cloth is more rigid than linen. 12. Thin plywood unprotected by a finish changes moisture content rapidly and shrinks or expands with a change in atmospheric humidity to the extent of either showing an appre- ciable loosening or assuming a drum-head tightness when fastened along the edges. A finish of three coats of spar varnish very largely eliminates rapid change in moisture content. WOVEN PLYWOOD. f Tests have been conducted upon plywood made up with basket-weave faces and corru- gated core. The faces are woven out of splints of spruce veneer 1-^ inches wide and 0.017 inch thick, while the core is made of spruce If inches wide and 0.018 inch thick. The total thickness over all is almost 0.2 inch. The following conclusion is drawn from the tests : The high rigidity at low loads, the high tearing strength, stability under varying humidities, and comparatively high toughness indicate that the woven plywood tested may be a very desirable material for construction in airplanes. Data concerning glues for ply-wood will be found in the text under the general heading "Glues." The following specification for waterproof plywood is based upon the strength tests just described and upon the glue tests presented farther on. SPECIFICATION FOR WATER-RESISTANT VENEER PANELS *OR PLYWOOD. r c GENERAL. ' 1. General specifications for inspection of material, issued by the Bureau of Construction and Repair, in effect at date of opening of bids, shall form part of these specifications. 2. This specification covers the requirements for veneer panels for use in aircraft where a water-resistant ply-wood is specified. MATERIALS. 3. The following species of wood may be used in plywood construction: ^mlKioqab ^a$> ISKJ Basswood. Mahogany (true and African). Walnut. Beech. Maple (hard and soft.) Western hemlock. Birch. Redwood. White elm. Cherry. Spanish cedar. White pine. Fir (grand, noble, or silver). Spruce. Yellow poplar. 4. Other species of wood shall not be used without the written approval of the Bureau of Construction and Repair. 5. Veneer. The veneer must be sound, clear, smooth, well-manufactured stock, of uniform thickness and free from injurious defects. Sap streaks and sound pin knots will not be con- sidered defects. Discoloration will be allowed. 6. The veneer may be rotary cut, sliced, or sawed. 7. Thickness. Unless otherwise specified, no single ply of veneer shall be thicker than ^ inch. In three-ply stock the thickness of the core must be between 40 and 75 per cent of the total thickness of the plywood, except for panels one-sixteenth inch or less in thickness. 8. Glue and cement. Any glue or cement may be used which will meet the tests specified in paragraphs 20 and 21. 84 AIRCRAFT DESIGN DATA. Note 12. MANUFACTURE. 9. Grain. The grain in each ply shall run at right angles to the grain in the adjacent plies unless otherwise stated in the order. 10. Manufacture. The plywood must have a core of soft or low-density wood and faces of hard or high-density wood unless otherwise specifically stated in the order. The core may be made of several plies, in which case the grain of the adjacent plies must be perpendicular. The plies must be securely glued together, after which the plywood must remain flat and free from blisters, wrinkles, lapping, checks, and other defects. Plywood manufactured with cold glue must remain in the press or retaining clamps not less than three hours. 11. Joints. Plywood 10 inches wide or less shall have faces made of one-piece stock. In order to conserve the narrow widths of veneer, accurately made edge joints will be allowed in the faces and cores of wider stock, but the number of joints permitted in any ply shall not exceed the width of the panel, in inches, divided by eight. Edge joints are joints running parallel to the grain of the plies joined. All plywood built of jointed stock must be so con- structed that all joints are staggered at least 1 inch. 12. In panels over 8 feet long scarf joints will be permitted; the smaller angle of the scarf shall have a slope of less than 1 in 25. Scarf joints in adjacent plies must be staggered. Scarf joints are joints in which the seam runs across the ply at right angles to the grain. 13. Butt joints will not be permitted. 14. In case the core or crossbanding is taped at joints only unsized perforated cloth tape or open-mesh unsized cloth tape applied with waterproof glue or cement shall be used. 15. Moisture content. --The finished plywood shall be dried to a moisture content of 9 to 1 1 per cent, with a tolerance of plus or minus 2 per cent, before it is shipped from the manu- facturer's plant. The equalization of moisture shall be effected by kiln drying, followed by conditioning. 16. Kiln drying. The panels must be piled and placed in dry kilns as soon as possible after being released from the press. The method of piling must be approved by the Bureau of Construction and Repair. After the stacking is completed the panels shall be properly weighted to prevent warping during the drying process. The best results in the kiln are obtained with a temperature of from 95 to 115 F. and a humidity ranging from 50 to 60 per cent, depending upon the thickness of plywood and number of plies. The circulation must be maintained at all times. 17. Conditioning. All panels must be conditioned before fabrication or shipment. The conditioning shall be done indoors under temperature and humidity conditions existing in the factory for a period of not less than 24 hours for three-ply panels one-eighth inch thick and proportionately longer for thicker stock. The piling and weighting shall be the same as specified for dry-kiln stacks. 18. Cutting. Cutting for length and width shall be full and true. The veneer shall be cut to the thickness desired in the finished plywood and any overallowance on this thickness for the sanding operation is very undesirable. 19. Finish. In all cases the tape must be removed from the faces of the panel, and, unless otherwise specified in the order, the plywood shall be lightly sanded to a smooth finish free from defects. TESTS. 20. Submission of samples for test. The manufacturer shall submit to the Bureau of Construction and Repair for test 20 samples, each 1 foot square, of the plywood which he proposes to furnish to airplane manufacturers. Note 12. AIRCRAFT DESIGN DATA. 85 21. Boiling or soaking test. The waterproof quality of the glue shall be tested either by boiling in water for a period of eight hours or by soaking in water at room temperature for a period of 10 days. After boiling or soaking the samples shall be dried at a temperature not exceeding 150 F. to a 10 per cent moisture content. The plies must not separate when the sample panels are subjected to this test. 22. Shear test. The strength of the glue shall be tested in five test specimens cut from a sample panel. The form of the test specimen is shown in figure 43. The ends of the speci- men shall be gripped in the jaws of a tension- testing machine and the load applied at a speed of less than one-half inch per minute. The glued surface must not fail at a load of less than 150 pounds per square inch. 23. Approved list. Manufacturers whose plywood does not comply with these specifi- cations will not be considered in awarding of contracts. The list of manufacturers whose product has satisfactorily passed the tests outlined in paragraphs 20 and 21 may be procured from the Bureau of Construction and Repair, Navy Department, Washington, D. C. - JL *- >* ' i I 1 1 1 r i i i li - . I Plywood GJue S/jeor TesT Fig. 43. Plywood glue shear test specimen. INSPECTION. iijl mo'il wiqu ob^m alonisq ii4 ni feoul^ ebifi lo ;' ni ', 25. Unless otherwise stated, all veneer and plywood shall be inspected at the plywood manufacturer's plant. 26. The inspector shall make the tests specified in paragraphs 21 and 22 on at least one sample panel from each press for each eight-hours' run. 27. In case the plywood fails to meet the soaking and shear tests it shall be rejected. If the glue fails to meet one of these tests but passes the other, the test in which it fails must be repeated on not less than twice the original number of specimens selected taken from two or more panels. If the glue fails to pass the second test, the plywood represented by the samples must be rejected. 28. In case of consistent failure or lack of uniformity in product, the manufacturer wiU be required to submit a detailed written statement giving the following information : (a) The composition of the glue and the correct practice in mixing it. (&) The maximum time between mixing and applying the glue. (c) The exact procedure in applying the glue and in pressing and curing the plywoot and such other details as the Inspection Department may direct. The inspector shall see that thereafter this schedule is observed. AIRCRAFT DESIGN DATA. Note 12. 29. The inspector shall have free access to all parts of the plants where the plywood is being manufactured and shall be afforded every reasonable facility for inspecting the materials used, the methods of manufacture, and the finished plywood. PACKING AND SHIPPING. 30. Plywood which has passed inspection shall be packed in crates which will protect all edges and surfaces from injury during shipment. ORDERING. 31. To facilitate the execution of contracts the order will state any special requirements which this material must meet. The order shall state the number of pieces, the width across the grain in inches, the length with the grain in inches, the thickness of the plywood and the individual plies, the number of plies, and the species of wood to be used for faces (to be marked " Faces"), for core (to be marked "Core"), and for cross-banding (to be marked "Crossband"). Sizes given shall be finished sizes and shall conform to commercial sizes when practicable. The order shall also bear the specification number. GLUES AND GLUING. There are a number of distinct kinds of glue commonly used in aircraft manufacture. The more important of these are as follows: 1. Hide and bone glues. 2. Liquid glues. 3. Marine glues. 4. Blood albumen glues. 5. Casein glues. In addition to these there are many kinds of glue and cement used in the arts which are not well adapted to aircraft uses and which, consequently, need not be mentioned here. HIDE AND BONE GLUES. In general only the better grades of these glues are used in aircraft, and these are made from hides and are known simply as hide glues. Occasionally nonwater-resistant plywood panels made up with bone glue are used in unimportant parts of aircraft. The principal uses of hide glues in aircraft have been in laminated and spliced construction of various kinds, prin- cipally in propeller manufacture. Hide glue is still the standard propeller glue, though it has been replaced to an important extent in other laminated work. In order to secure a very good grade of glue for propeller and similar work, suitable methods of testing were developed and certain specifications prepared. The Bureau of Aircraft Pro- duction regularly inspects lots of glue at the request of manufacturers, and glue passing the required tests is sealed and certified. It is then made available for purchase by aircraft manu- facturers, who are thus assured of uniform glue of proper quality. The methods of test devel- oped and used are given in detail in the following statement. The shearing test forms the basis for the certification of casein glue also. TESTING OF HIDE GLUE. Chemical analysis has been found practically useless as a means of testing glues because of the lack of knowledge of their chemical composition. Physical tests must, therefore, be relied upon. A considerable number of physical tests have been devised, some of which are important for one class of work and some for another. For judging the suitability of glue for high-grade joint work the tests considered most important are strength, adhesiveness, vis- cosity, jelly strength, odor, keeping qualities, grease, foam, and reaction to litmus. In the subsequent discussion of these tests their application to joint glue will be especially kept in mind. | , X i . . Note 12. AIRCRAFT DESIGN DATA. 87 Strength tests are made by gluing together two or more pieces of wood and noting the pressure or pull required to break them apart. Many different methods of making the test specimens and breaking them have been devised. These depend to a certain extent upon the character of work expected of the glue and the nature of the testing apparatus available. The simplest and most convenient strength test is to glue two blocks together, as shown in figures 44 and 45b, and shear them apart in a timber-testing machine (see fig. 45 a and c). It will MfTHOO Of PttP/trt/MQ Fig. 44. Method of preparing specimens for glue-strength tests. usually be found that there is considerable difference in the values obtained for the individual specimens. The amount of difference, however, can be kept at a minimum by using care to see that the specimens are selected, prepared, and tested under as nearly the same conditions as possible. In making strength tests the selection of the wood is a very important factor. The species selected should be the one upon which it is proposed to use the glue or one fully as strong. Care should be taken also that the wood is above the average strength of the species, in order that there may be less opportunity for the wood to fail before the glue. If the wood is too weak, the full strength of the glue is not determined. AIRCRAFT DESIGN DATA. Note 12. ot & hji . Note 12. AIRCRAFT DESIGN DATA. No block should fail below 2,200 pounds per square inch, and the average shearing strength for a propeller glue should be at least 2,400 pounds per square inch. The viscosity of a glue is determined by allowing a specified amount at a given temperature to flow through an orifice. The time required is a measure of the viscosity. The time required for water to flow through is taken as the standard. In general it is found that a glue with high viscosity is stronger than one with a low viscostiy and will absorb more water, although there are exceptions. Hide glues, as a rule, have higher viscosities than bone glues. A number of different shaped viscosimeters have been devised. In the glue manufacturer's laboratory, where many tests must be made each day, an instrument must be used which will give results quickly. This can be done with a pipette cut off at one end or with a straight glass tube contracted at one end. These instruments are not always arranged so the tempera- ture of the glue within them can be controlled, and for a number of other reasons they are not entirely accurate. Better control of temperature and greater accuracy can be had with the Engler viscosimeter. This is more complicated and more expensive than the glass tubes and also slower to operate, but it has the advantage, in addition to greater accuracy, of being an instrument which is in general use for testing many kinds of materials. The values obtained by its use are readily understood by laboratory men and can be readily checked. The instru- ment can be purchased standardized and ready for use. The term "jelly strength'' refers to the firmness or strength of the jelly formed by a glue solution of specified strength upon cooling. Strong glues usually have high jelly strength. There is no standard instrument for determining jelly strength and no standard unit for expessing it. In some laboratories the pressure required to break the surface of the jelly is measured. In others the depth to which a weight of special shape will sink is observed. Sometimes the jelly is cast in a conical shape, and the weight required to press the point of the cone a certain distance is taken. More common, however, is the finger test, in which the relative strength of two or more jellies is compared by pressing the jelly witn the fingers. In making this test with any apparatus it is important that the conditions be very carefully controlled in order that comparative results may be obtained. The temperature of the jelly when tested is par- ticularly important, as the relative strength of a number of jellies is not the same at different temperatures. In other words, the jelly strength of the different glues is not affected to the same extent by changes in temperature. The ideal condition is to cool and test the jellies in a room constantly maintained at the proper temperature. This is seldom practicable, how- ever, and the jellies must be cooled in a refrigerator and tested in a warmer room. When this is done it is important that the test be made as quickly as possible after removing the jelly from the refrigerator, so that the temperature will be practically the same as it was in the refrigerator. The strength of the glue solution must always be the same once a standard is adopted. For high-strength glues weaker solutions can be used than for low-strength glues. The odor of a glue is determined by smelling a hot solution and gives some indication of its source or its condition. Glue which has an offensive odor is not considered of the highest grade. The bad odor may be due to the fact that partly decomposed stock was used or that the glue itself is decaying. For high-grade work it is usually specified that the glue be sweet; that is. it must not have an offensive odor. The odor of different glues varies considerably, and it is difficult or impossible to express the different "shades." It is usually not difficult, however, to determine whether or not the odor is clean, or, as it is commonly called, sweet. The temperature and strength of solution are not usually specified. The keeping quality of a glue is determined by allowing the jelly left from the jelly-strength test to stand in the laboratory at room temperature for a number of days. The odor and con- 90 AIRCRAFT DESIGN DATA. Note 12. dition of the glue are noted at intervals. Glues with good keeping qualities will stand several days without developing an offensive odor or showing any appearance of decomposition. For joint work a small amount of grease in glue is not a serious objection. Too much grease, however, is objectionable, as grease has no adhesive properties. The grease can be determined by chemical means, if desired, but this is not necessary unless the exact amount of grease must be determined. The common method of testing for grease is to mix a little dye with the glue solution and paint it upon a piece of unsized white paper. If grease is present, the painted streak will have a mottled or spotted appearance. If there is no grease present, the streak will have a uniform appearance. Glue which foams badly is objectionable because air bubbles are apt to get into the joint and thus reduce the area over which the glue is in contact with both faces. Foamy glue is especially undesirable for use in gluing machines, as in them the glue is agitated much more than when it is used by hand, and the danger of incorporating air bubbles is greater. The amount of foam is tested by beating the glue solution for a specified time with an egg beater or similar instrument and then noting the height to which the foam rises and the quickness with which it siibsides. Different laboratories do not make the test in exactly the same way, but in any laboratory after a method is once adopted it should be strictly adhered to thereafter. It is common to determine the foam on the solution used in the viscosity test. .oul" Py its reaction to litmus a glue shows whether it is acid, alkaline, or neutral. The test is made by dipping strips of red and blue litmus paper in the glue solution remaining after the viscosity test or some other test and noting the color change. An acid glue turns blue litmus red, an alkaline glue turns red litmus blue, and a neutral glue will not change the color of either red or blue litmus. A glue containing a slight amount of acid is slightly preferable to one which is neutral or alkaline, because it is not quite so favorable a medium for the growth of the organ- isms which cause the decay of glue. From the above description of the various glue tests it is apparent that, for the most part, they give comparative rather than absolute results. It is rather difficult to compare the results of tests made by one laboratory with those of another, as the strength of solution, temperature, and manipulation are often different. For this reason the most satisfactory method of pur- chasing glues is to specify that they must be equal to a standard sample which is furnished the bidder to test in any way he sees fit. The bidder should also be informed as to the methods the purchaser proposes to use in testing a glue submitted to him as equal to the standard sample. *i((j iiorfcW ./HOOT 101 >n PRECAUTIONS IN USING HIDE GLUE. moll yJi'H wU iMiivoir In using hide glue there are a number of precautions that must be observed to obtain satis- factory results. If improperly used, a very high-grade glue may give poor joints. It is impor- tant, first, to find out the right proportion of glue and water to get the best results. This is largely a matter of experience, but it can also be determined by strength tests. When the right proportions are decided upon, they should be strictly adhered to thereafter, and the glue and water should be weighed out when making up a new batch of glue rather than measured or guessed at. Clean cold water should be put on the glue, which should be allowed to stand in a cool place until it is thoroughly water soaked and softened. This may take only an hour or it may take all night, depending upon the size of the glue particles. When the glue is soft, it should be melted over a water bath and the temperature not allowed to go higher than about 150 F. High temperatures and long-continued heating reduce the strength of the glue solu- tion and are to be avoided. The glue pot should be kept covered as much as possible in order to prevent the formation of a skin or scum over the surface of the glue. Note 12. AIRCRAFT DESIGN DATA. 91 The room in which the glue is used should be as warm as possible without causing too much discomfort to the workmen, and it should be free from drafts. In a cold, drafty room the glue cools too quickly and is apt to set before the joint has been put into the' clamps. This results in weak joints. It is also considered good practice to warm the wood before applying the glue. Wood should never be glued when it is cold, and of course only thoroughly seasoned wood should be used. Since high-strength animal glues set so quickly on cooling, they should be applied and the joints clamped as quickly as consistent with good workmanship. In clamping glued joints the pressure should be evenly distributed over the joint, so that the faces will be in contact at all points. The amount of pressure which will give the best results is a question which has never been definitely settled. One experimenter found that a pressure of about 30 pounds per square inch gave better results on end joints than higher or lower pressures. Apparently no tests have yet been made to show the best pressure to use on edge or flat grain joints. In gluing veneers it is necessary to use high pressure in order to flatten out the irregularities of the laminations. Pressures as high as 150 or 200 pounds per square inch are sometimes used. Strict cleanliness of glue pots and apparatus and of the floors and tables of the glue room should be observed. Old glue soon becomes foul and affords a breeding place for the bacteria which decompose glue. The fresh glue is therefore in constant danger of becoming contami- nated. Glue pots should be washed after every day's run in hot weather and two or three times a week in cooler weather. Only enough glue for a day's run should be mixed at a time, so that mixed glue will not have to be held over from one day to another. If these sanitary precau- tions are not observed, poor joints are apt to be the result. LIQUID GLUES. Liquid glues, frequently known as fish glues, have been used to quite an extent for the smaller work such as gluing cap strips, tape, blocks, moldings, etc. They are being replaced gradually by casein glues, which have the advantage of water resistance. In general liquid glues are not as strong as certified hide glue, although the shearing strength of several which have been tested has been as high as 2,400 pounds per square inch. MARINE GLUES. These glues are used mainly to apply muslin between the inner and outer skins of floats and flying boat hulls. They are required to be of a sticky, viscous nature and relatively non- drying and elastic. They are usually composed of pine tar, rosin, manila resin, and alcohol. On account of their nondrying nature, these glues have comparatively low strength. They are readily soluble in gasoline, and it is necessary, therefore, to make provision to prevent gasoline from getting into the bilge water. In general, marine glues are not used to make joints in wood construction where high strength is required. BLOOD ALBUMEN GLUES. These glues, which are made from blood albumen secured from packing houses, are the strongest and most water resistant of all so-called ''waterproof glues" in common use to-day. In general, it is necessary to use heat (about 225 F.) to set them, and consequently their use- fulness is limited largely to plywood and similar thin material, although it is possible to glue thicker material in cases where the proper heat can be applied successfully. Practically all plywood glued with blood glues is glued between steam-heated plates, which furnish a con- venient source of heat. on I hTvrnijfife ?. :-}ib9Tni tttt \d bedailqOTCHXNi od Y- 8 " 1 Sffixira eMT . <.[ edi