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Aviation History
1939
1939 - 0079.PDF
JANUARY 12, 1939 FLIGHT. Fig. 5. The 800-ton hydraulic moulding press in which 30ft. (or even longer) 6in. "planks" or I-section spars can be produced. ing the behaviour of materials under shear and compression, we at once meet the difficulty that the size of the structure cannot be left out. It is, in fact, only possible to make comparisons when Wagner's " Kennzahl " * (which I have, per haps clumsily, renamed '' The Structure Loading Coefficient,'' and which is numerically equal to the square root of the applied load divided by the length of the member) is the same. However, for low structure load ing coefficients it is possible to say that the figure of merit of a material under compression is given by Ei/p, where E is the value of Young's Modulus, '; is a fraction, and p is the density. You will see at once that, because >j is a frac tion, a low density is of more im portance than a high Young's Modulus in a material for low struc ture loadings. It will thus be seen that it is always worth while '' blowing out ' a compression member at low struc ture loadings, both on an atomic scale by the choice of a low- density material, and on a macro scopic scale by the use of a section with a big cross-sectional moment of inertia. We will assume that a designer * H. Wagner, Z.J.M., 1928. p. 241. is free to choose the best form of cross-section for a com pression member, and is free to choose his material. In making his choice he should bear in mind that a low density is in all cases worth a pro rata sacrifice in Young's Modulus. Suppose we begin by choosing steel with E =30 x io' lb. /in.- and ? = 7.8; we shall find that at low struc ture loadings the material fails at a low stress because even the best profile we can develop will have excessively thin walls. Therefore let us "blow out" our steel on an atomic scale so that its density falls to 2.8, and suppose the value 2.8 of E falls, pro rata, to —?. x 30 x 10" lb./in.2 = 10.8 x 10" lb./in.2 We shall now have a better material for low structure loadings because &V/P will be larger, and you will note that in fact we have chosen duralumin (0 = 2.8 and E=iox io6 lb./in.2). We can continue this process further by expanding our steel on an atomic scale until P = 1.8 with a pro rata re-1.8 duction in E to a value of —^ x 30 x io° lb./in.2 =6.9 x io* lb. /in.2 We shall then have Elektron. (p = 1.8 and E = 6.4 x io6 lb. /in.2). But, even with Elektron, the stress under compressive failure does not attain the ulti mate compressive strength of the material, though it will be a much higher fraction of the ultimate strength than in the case of steel at the same low structure loading coefficient. What is the next step? Clearly, to use beryllium or to explore the possibilities of organic materials. The carbon atom itself is not a specially light atom; packed together in the closest possible way, as in a diamond, its density is 3.52, which is about twice that of magnesium. But when combined with oxygen and hydrogen atoms it suffers a change, and the density of the resultant compounds falls to the order of unity ; nevertheless, the linkages between the atoms are strong That, in brief, is the prima facie case for organic materials in air craft structures, and since (as I hope I have been able to demonstrate) they are equally useful for high structure loadings, I think it may fairly be said that there are good reasons for believing in their utility. Fig. 7. The load on the joint at a spar root-end is brought home amusingly but forcibly by this illustration, shown on the screen by Dr. de Bruyne at his lecture ; 42 \ tons has been carried by a joint of the type shown in Fig. 8. Fig. 8. (Right) A spar root-end consisting simply of a " sandwich " of duralumin plates and synthetic material, secured with 2 B.A bolts and nuts.
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