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Aviation History
1943
1943 - 1908.PDF
FLIGHT JULY 29TH, 1943 PLYWOOD AND PLASTICS —II TABLE 8 A COMPARISON OF IMPORTANT WOODS USED IN THE MANUFACTURE OF PLYWOOD Species Group 1.Beech BrchMaple High Density Woods : Group 2. Medium Density Woods :American (Black) Walnut Douglas Fir (Coast Type)Douglas Fir (Mountain Type). African Mahogany _Cuban Mahogany Mexican MahoganyNicaraguan Mahogany.. Peruvian Mahogany Group 3. Low Density Woods :Yellow Poplar Spruce—SitkaSpruce—Eastern Red and White Average Value Spec. Grav. 0.640.62 0.63 0.550.48 0.430.45 0.590.45 0.480.55 0.400.40 0.41 Per cent. Walnut 116113 115 100 87 78 82107 8287 100 7373 75 Group No. 1 2 3 • Thiekness 1.00 1.00 1.00 TABLE 9 EQUAL THICKNESS Wt 100 82 64 Bending Strength 100 82 66 Stiffness 100 83 80 Shock Resistance 100 76 45 Wt UJ./CU. ft 44 36 28 EQUAL WEIGHT 1.00 1.221.57 100 . 100100 100 122 1G4 100 152 311 100 93 71 TABLE 11 Wood Grain lengthwiseGrain crosswise 3-ply Plywood :(a) Grain outerplies lengthwise (b) „ „ crosswise5-ply Plywood: (a) Grain outerplies lengthwise(b) „ „ crosswise 7-ply Plywood :(a) Grain outerplies lengthwise (b) „ „ crosswise BendingStrength per cent 100 c • 8217 67 30 . • 6034 Stiffness per cent. 100 5 069 81 25 7333 A cornparison of the strength of wood on a thickness and weight basis. All valuesin these tables are relative. is no saw-waste and it can be handled and transported faster and cheaper. The strength properties of most im- portance can be emphasised in laminating, and likewise a combination of several species of wood can be arranged in one pre-fabricated unit. The veneer used in all forms of plywood is generally sliced off a log revolving in a lathe, and may well be regarded as a circumferential plank. Due to the fact that the stem of a tree generally has a taper and is not accurately circular, the veneers are not cut truly parallel to grain. The want of parallelism between length of grain and length of veneer can reduce the strength wherever it occurs. Recent experiments in riving off veneers that are exactly parallel to the grain have met with success, except for the fact that cutting can be wasteful, due to irregularities in the grain producing a proportion of spoiled sheets. Veneer produced in this manner is referred to as '' rotary-cut,'' and is almost exclusively employed in 'this country in the manufacture of plywood. The B.S. TABLE 10 STRENGTH PROPERTIES AIRCRAFT STRUCTURAL PLYWOOD Species Name Group II. Medium Density Woods : American Walnut ...Douglas Fir African Mahogany .. " True" Mahogany .. Sroup I. High Density Woods :Beech BirchMaple Group III. Low Density Woods: PoplarSitka Spruce Spec. 6ra». Per cent. 0.590.48 0.52 0.48 0.67 0.670.68 0.500.42 Walnut100 82 88 82 114 114115 85 71 Column Modulus to Grain Bending Parallel of Face Plies lb. sq. in. 12,6609,340 8,070 8,500 15,390 16,00015,600 8,8607,710 Per cent. Walnut 10074 64 67 121 126 123 70 61 Mod. of Elasticity Parallel Grain Face lb. sg. in. 1,7401,530 1,260 1,250 2,150 2,2602,110 1,5401,370 Plies Per cent. Walnut100 88 72 72 123 130121 89 79 Tension Parallel Grain of Face Plies lb. sq. in. 8,2506,188 5,370 6,390 13,000 13,21010,190 7,3905,650 Per cent. Walnut100 75 65 77 157 160 123 89 68 Splitting Resist-ance 100 121 No data No data 122 130 148 66 100 A comparison of strength properties of three-ply panels (Data based on tests of plywood, allplies being one species). As the number oi plies per panel is increased there is a little change in the strength relationship between species. Above values based on material (plywood) in which all plies are same thickensand same kind of wood. Specification for plywood also permits veneers to be sliced or sawn from the log, providing the samples from stock of the finished plywood can pass the test laid down. The thickness of veneers may vary from extremely thin sheets of'o.oiin. thick, but with most species of timber it is difficult to obtain rotary-cut veneers of less than o.osin. thick. Very little information is available to the designer on the characteristics of the numerous species of wood veneers. As a result the American Walnut Manufacturers' Associa- tion recently prepared and distributed a compilation a1 "'i, analysis of important data on each of the woods in corl siderable use. Abstracts from this publication are of par- ticular interest. The species of timber are divided into three groups: high, medium and low-density, and the characteristics are compared as a percentage of those for walnut. In Table 8 a comparison of the important woods used in plywood construction has been tabulated. A com- parison of the strength of wood on a thickness and weight basis for the three groups are given in Table 9. Table 10 gives a comparison of strength properties of three-ply panels, the data having been based upon plywood tests in which all plies were of the same species. It is stated that as a number of plies per panel are increased there is little change in the strength relationship between species. The data in Table 11 are especially interesting, and show the effect of the run of the grain of the outer plies on bending strength and stiffness. These values are bas^j^ on plywood in which all plies are of the same thickness and same species. The values quoted in these tables should receive consideration in the design of components, par- ticularly where the properties of stiffness and shock resist- ance are important. Tang-ential and Radial Shrinkage It should be noted that the less dense, Group 3, woods are far more stiff on a weight basis, but they have a much lower rating in shock resistance compared with other woods. By a well-thought-out combination of the several woods upon this basis, the best and most consistent results may be obtained. Before dealing with that most important aspect, moislure content, some observations on the shrinkage of wood are worth atten- tion. All woods shrink more tangentially parallel to the growth rings than radially (perpendicular to the growth rings). An ideal wood for aircraft purposes would be one in which a low percentage was combined With a low ratio between the amount of shrinkage in either direction. The optimum ratio would be 1.0 indicating that the wood shrinks an equal amount in both directions, a condition never found in any species. In the subsequent issue the question of radial arid tangential shrinkage is continued. Moisture content, its effect upon the modulus Of.elasticity and the effect of grain angle upon this latter property is also dealt with prior to finally concluding with some fundamental design data.
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