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Aviation History
1943
1943 - 1960.PDF
I5O FLIGHT AUGUST 5TH, 1943 Plywood and Plastics—III Basic Design Factors and Stress Data for Plywood Structures : The Simple Plywood Fuselage : Moulded Plywood Design By W. NICHOLS, A.R.Ae.S. X X /// AMERICAN WALNUT X ''// MEXICAN MAHOGANY f Af PICAN MAHOGANY T '•'/. DOUGLAS FIR '" '/. YELLOW POPLAR #? : SITKA SPBUCE %Ji^ , HAPD MAPLE ; BEECH 0 5 IO THE previous article concluded with an approach tothe subject of tangential and radial shrinkage ofwood. It was stated that all woods shrink more • tangentially (parallel to the growth rings) than radially (perpendicular to the growth rings). It was pointed out that the ideal wood for aircraft purposes would be one in which a low percentage of shrinkage was combined with a low ratio between the amount of shrinkage in either direction, the optimum ratio being 1.0, a condition never found in practice. The ratio for various species is shown in Fig. 10. From Table 12 it will be seen this.ratio varies from 1.28 in yellow birch to 2.16 in beech. In the manufacture of veneers these shrinkage values may be empha- sised, depending on the method of cutting. With rotary-cut stock, the broad surface is all tangential wood, and this shrinks a great deal more laterally than in thickness' In quarter-sliced stock only the radial face is in evidence, and the whole piece is more stable. In flat- sliced stock the predominating flat surface is radial, and this is also more uniform than rotary-cut veneer. It is this fact which accounts for the greater amount of "buckle" in rotary veneers. Moisture Content Obviously panels made of rotary-stock woods having a high ratio, as indicated in Table 12, are much more apt to develop internal stresses due to shrinking than are panels made of quartered or sliced stock, particularly on the thin panels commonly used for aircraft purposes. One of the first and most important characteristics to be understood in connection with wood veneer is the moisture content, a wrong value of which can make an otherwise perfect piece of wood worthless. Wood will always absorb or give out moisture until it is in ' equilibrium with the humidity of the surrounding atmo- sphere. At the same time there will be a corresponding change of dimensions. Wood, as already explained., is made up of a substance, mostly cellulose and lignin, ex- panded by virtue of its cellular structure to a specific gravity usually below that of water. Change of dimensions will occur, due to the absorp- tion of water in this sub-" stance, and not in the pores of the wood, which, inci- dentally, does not cause appreciable swelling. Dense woods, therefore, will swell more than light porous woods, due to the greater amount of wood substance in a given volume of dense wood X Fig. 10. Ratio of tangential to radialshrinkage of various woods. than light wood, a point to be remembered in air design. As wood becomes drier, the majority of its strength characteristics are rapidly increased, thus the obvious con- clusion is to use exceptionally dry woods and to utilise a protective coating on the surface to maintain this moisture content as low as possible. Experience has shown, how- ever, that the stress or the proportional-limit increases at a greater rate than does the modulus of rupture or the modulus of elasticity. The material appears to lose ductility in a dried-out condition, and there is a limited plastic flow range before failure takes place. A starved- glued johjt. is liable to occur in gluing if-*?«f moisture content is too low. Als^ " if the moisture content of one ph- is greater than that of another there will be a tendency for the panel to buckle or warp, because the content of each ply will tend to balance under any humidity conditions of the air. It has been determined that a moisture content of 7 per cent, to 9 per cent, js the most efficient in affording ET good bond between the veneers, which are usually kept stored at this content by holding the relative humidity of normal temperatures within a range of approximately 30 per cent. to 40 per cent, at 60 deg. F., as shown on the chart, Fig. 11. In moulded plywood con- struction, for example, if the moisture content of the ven is fairly high, a longer period under the application of heat would be needed during the moulding process. This would result in boiling out of moisture from the cellular structure of the wood, with a consequent low ductility and an exceed- ingly brittle material. Tensile strength apparently is not affected. It has been shown, in tests made on European woe -3s by Gerngross at the Technical University of Berlin, that the effect of heat- ing to 284 deg. F. for varying periods of time had little X i, 4. 15 20 2-5 -_...FOR COUCH CARPENTER'S WORK APPRECIABLE SHRINKAGE COMMENCES AT ABOUT THIS POINT SUITABLE MOISTURE CONTENT fOR PRESSURE TRCATMEN7 (^"SOLUTION! £f|ISTANCE CONTENT xnfiMVQ'///////////////"/////RANGE Of MOISTURE IN THOROUGHLY AIR-SEASONED TIMBER /////////////• V////////////////////////////A MAX FOR HIGHCLASS CARPENTRY AIRCRAFT. MOTOR VEHICLES PATTERNS eTCCtlMBER) VENEEB AMERICAN STANDARD (AVERAGE) VENEER FOB AIRCRAFT ETC. (BRITISH STANDARD 7%T0 9%) OVEN DRIED WOOD 10 JO 3O 4O SO BO 7O 60 <)O HUMIDITY OF THE AIR PEO CENT (AT 6O°F) CORRESPONDING TO THE MOISTURE CONTENT OF TIMBER Fig. ii. Range of moisture content, seasoned timber.
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