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Aviation History
1926
1926 - 0863.PDF
NOVEMBER 25, 1926 101 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT and G2 are of course representing applied loads of the same magnitude. Hence the load in the wire and consequently its cross- sectional area KG; and the length . •. The weight of wire o: ^r • ,,2,, , ^, This result cannot of course be considered quantitatively without a knowledge of the ratio of the gap to the bay length If in the initial case G = B and in other cases G=0-9B, 0-8B, 0-7B, 0-6B and 0-5B. the ratios of the weights of wires will be 1, 1 -005, 1 -025, 1 -063, 1 • 133 and 1 -25 respectively. The ratio of the weight of wires in changing the gap by 0-1 B taking each of the above cases in turn as the original are then 1-005, 1-02, 1-038, 1-065, 1-078. It is thus apparent that for the order of changes in gap/span ratio the reduction in wire weight will be small from 0 • 5 per cent, to 3 per cent, in different bays, according to their geometry. The resistance of the wires will be reduced in direct proportion to the weight reduction supposing the fineness ratio of the cross section of the wires to be constant. Spar.—In the case of the overhang the spar is, of course, unaffected. In the bay the load on the spar due to compres- sion component from the lift wire varies as —. G2 If a is the area of the spar then cx a <X end load c •> a a primary bending, c3 a Cf. bending due to end load.(h G2 Since end load bending effect varies both with end load and deflection an approximation may be taken as c3 H Ot — ) ' < G 2 the actual index, however, would really depend on the relative importance of the primary and secondary bending. Then a and consequently the weight of spar Gi ,G, 2 a C'2 + Cl G~ s + °* \GJ In a typical case for a single bav machine values for Cj = 0-37 c2 = 0-40 c3 = 0-23 have been found. Hence in this case Then Ci a a — and c2 a is constant.G GiW a 0-40 -j- 0-37 — »•» (ly Appropriate values for the constants for multi bay con- struction can similarly be found, and the values must be, appropriate to the structure chosen for the similarity curve. Supposing the anti-lift wire and bottom spar to be deter- mined by upside down flight conditions, the same ex2)ressions will apply. As example, let us take — =0-9. G.) Then W a 0-40 -f 0-37 X 0-9 + 0-23 X 0-81 W a 0-92 i.e., the weight of the spars in the bay is reduced 8 per cent. From previous work the weight of the wires, 2 per cent., and the weight of the struts is increased 27 per cent, for constant k. The weight of the fairing is increased 10 per cent, for con- stant h. If the fairing is I the weight of the strut, the struts are increased roughly 20 per cent. For constant ijh the struts and fairings are increased in weight about 13 per cent. The optimum arrangement is probably a mean of these, say, 17-5 per cent. For the original numerical example (changing from 0• 6 to 0-5 span2/\V) reasonable values for relative weights of external bracing, struts and spars (less overhang) would be as follows :— External bracing 7 Struts 10 Spars 20 37 By the change of gap we have approximately 7 X 0-1)8 + 10 X 117-5 + 20 x'o-92 6-80 + 11-75 + 18-4 = 37-01 We reach here the remarkable result that the change of gap of 10 per cent, has made no difference to the weight of the wings, the increased weight of the struts having just balanced out the reduced weights of the spars and wires. This case has not been adjusted for the purpose of arriving at this result, but reasonable values have been chosen based on statistical evidence of metal construction. Xo great significance need attach to the closeness of the two weights, except that it affords evidence that with reasonable wing arrangements (i.e., not too unlike the practice of good designers) the influence of change of gap is not very important. The reader may easily extend the methods here used to statistical examples in which he is interested. By analogy it may be seen that the influence of chord on the primary structure is not very important. Change of chord reduces the drag loads in the spars and the wire loads, but drag struts are increased in weight. The case is actually far more complicated than the lift structure, but the influence of drag loads on spars is only about 20 per cent, of that of the primary loads. (To be continued.) TECHNICAL LITERATURE. AERONAUTICAL RESEARCH COMMITTEE REPORTS. REPORT ON THE ACCELERATED AGEING OF " Y" ALLOY. By S. L. AKCHBUTT, F.l.C, AND J. 1). GROG AX, B.A. Work performed for the Engineering Research Board of the Department of Scientific and Industrial Research. Presented by Dr. W. Rosenhain, F.R.S., Superintendent, Metallurgy Department. National Physical Laboratory. R. & Ml NO. 1038 (M.47) (10 pages and 8 diagrams)! April, 1920. Price 9d. net. The investigation here described has been carried out to discover, if possible, a method whereby the normal age- hardening period of eight days might be shortened, in order to remove the inconvenience caused in foundry practice by the period of waiting. The progress of age-hardening has been studied on both chill cast and wrought material by means of tensile and Brinell hardness tests. The effect of tem]>eratures of 150' C. and 100° 0. in the air of an electric muffle was first tried. Hardening proceeded rapidly at 100' (!., and this temperature was therefore used in subsequent work, boiling water being chosen as the tempering medium. In the case of the wrought alloy, half an hour at 100c C. is sufficient substantially to complete the age-hardening. Slight differences only in results have been observed between material which, after quenching, has been tempered (1) immediately, and (2) after a few hours' air-ageing. Material tempered (say, at 100° C.) for a period too short to complete the ageing process, continued to age in air, finally attaining physical properties similar to those of normally air-aged material or of completely-tempered material. Tempering for longer periods up to six hours does not produce any further change in mechanical properties. With the cast alloy results have been somewhat irregular and further investigation is needed. Excellent results have, however, been obtained from chill cast bars treated at 100° C. for two hours.
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