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Aviation History
1943
1943 - 1961.PDF
AUGUST 1943 FLIGHT 13* PLYWOOD AND PLASTICS —III TABLE 12 RATIO OF TANGENTIAL TO RADIAL SHRINKAGE WELL-KNOWN WOODS OF Species Bisswood... ...» Beech Birch (sweet) Birch (yellow) ... pouglasFir\frican Mahogany- Maple (hard)Spruce (Eastern) Spruce (Sitka)Poplar (yellow) ... Walnut Per cent Radial Shrinkage 6.6 5.1 6.5 7.2 • 5.0 4.1 4.9 S.8 4.S 4.0 5.2 Per cent Tangential Shrinkage 9.3 11.08.5 9.2 7.8 &.8 9.5 7.8 7.5 7.1 7.1 Ratio 1.41 2.1G1.31 1.281.56 1.421.94 2.06 1.751.75 1.36 Fig. 13. The cellulosenitrate covering on the Rotol Jablo wood blade. Based on volume when green and when oven dry.(From the U.S. Forest Products Laboratory data.) effect on the tensile strength. From the numerous articles written on the determination of strength characteristics for different moisture contents, reference to the recent %vork of Mr. G. B. Parsons, chief of stress and structural 2-1 20 1-9 1-8 17 1-6 1-5 t-4 r- l"3 in 1-2 0 iu r-0-8 W_ O 0-7 vO 0 O CJO-5 \ \ \ s\ N YELLOW POPLAR ,•• \ ~x i •<! i IN N V V -*, ^* 2-1 2-0 IB 1-8 17 1-6 1-5 1-4 1-3 t-2 1-1 1-0 06 06 07 06 n-fi \ \ • \ \ \ \ \ SITKA SPRUCE •<> 1 1 \, y 024 68 10 12.14 16 • t-a 1-3 t-2 1-1 to OP 0-7 0-5 \ \ \ I \ A\ \ \ s.v ^. - \\ \ BIRCH 1 i ! 1 *** • 18 20 22 24 -0 2 4 60.8MOISTURE CONTENT % 0 2 4 6 8 10 12 14 16 18 20 2Z 24 MOISTURE e. i 20 1-9 1-7 1-6 1-5 1-4 1-3 1-2 11 10 09 0-8 0-7 06 OS \ N — AFRICAN MAHOGANY *^ \ 1 1I •^. 0 2 4 6 CONTENTS Fig. 12. Strength-moisture content curves for aircraftveneers. • MODULUS OF ELASTICITY MODULUS OF RUPTURE AND TENSION COMPRESSION PARALLEL TO GRAIN research, Duramold Aircraft ROTOLOID Corporation, U.S.A., will be ****- COVERING found of practical value. In this are plotted the strength- moisture content curves for woods commonly used in the aircraft industry. Some examples of these resulting curves for modulus of elasticity, modulus of rupture, and com- pression parallel to the grain, have been grouped and repro- duced in Fig. 12. These graphs are plotted with 12 per cent, moisture content as unity, which appears to be more or less the standard adopted in America. To correct to 12 per cent, moisture any specimen with known moisture content, it is only necessary to divide the value obtained at the known moisture content by the value read Lom the curve corresponding to the strength characteristic in question. In moulded plywood construction in which successive veneers are laid with the grains crossing, the bonded sur- face area is large in relation to the thickness of the veneers, and therefore is better able to cater for any internal strains that may be caused by possible changes in moisture content. The problem of moisture resistance and dimensional stability is being attacked from two standpoints. The first involves the use of protective coatings on finished surfaces, and will revolve around the development of adequate painting schedules. Upon certain wood airscrews of British manufacture a cellulose nitrate cover- ing is already successfully employed in combination with compressed wood. The second method involves the impregnation of the veneers, and re- sults in superior and improved mois- ture characteristics. Almost any de- gree of resistance may be achieved by this method, but it entails an appre- ciable increase in cost, and has not for this reason been widely used. This scheme may be accomplished by thoroughly impregnating both sides of all layers of veneers by '' dipping '' in a suitable resin for a specific period, determined by the veneer thickness. Moisture Resistance and Dimensional Stability Another method which may be used, and, in fact, must be used where the veneers are Vsin. thickness or greater, is to apply the resin to the veneers under vacuum pressure, liter- ally forcing the resin into the wood. Since wood is made up of tiny cells surrounded by porous walls there can be various degrees of penetration which can be predetermined and for all practical purposes controlled. Complete waterproofness can now be obtained, and seaplane floats have already been produced of impregnated plywood material moulded to shape. The use of this material in the con- struction of flying boat hulls of the smaller type with complete absence of water soakage (a fault with wooden hulls of the past) now appears to be a practical proposition. Probably the one variable in plywood manufacture left uncontrolled is that of glue spread. Economically, it is only of secondary importance, but the quantity of glue 10 12 14 16 18 20 22 24 8 10 12 W 16 18 20 2224
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