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Aviation History
1926
1926 - 0787.PDF
October 28, 192i Supplement to FLIGHT FLIGHT IGINEERI? SECTION K ENGINEERING Edited by C. M. POULSEN October 28, 1926 CONTENTS Aircraft Performance. By J. D. North, F.R.Ae.Soc Duralumin. By Leslie Aitchison, D.Met., B.Sc, F.l.C, M.I.A.E. Technical Literature 91 AIRCRAFT PERFORMANCE. Structural Policy in Design. By J. D. NORTH, F.R.Ae.S. (Cmifiviied from /. 77.) OUR CONTRIBUTORS Mr. J. D. North, who was "" missing " from the last issue of THE AIRCRAFT ENGINEER, returns to the attack this week upon problems connected with the stresses in aircraft in flight, more particularly in the wings. He calls attention to the somewhat unsatisfactory state of our present knowedge (or absence thereof) of the nature and magnitude of the various loads to which aeroplanes of different classes are subjected in flight, and points out the urgent need of experimental investigation, quoting passages from the 1922 report of the Load Factor Sub-Committee, and maintaining that the position in October, 1926, is practically the same as it was in January, 1922. since nothing appears to have been done towards obtaining the necessary experimental evidence. Fig. 16 of Mr. North's article is interesting in showing that the maximum load factor on the wing spars occurs when the centre of pressure is at approximately one-third of the chord from the leading edge, a condition which is generally chosen in experimental testing abroad. Mr. North then proceeds to examine the problem for what it is worth under conditions imposed at the present time, and point sout that the general expression for structure weight percentage does not cover the case of the wings, since some part of the forces on these is relieved by gravitational and inertia forces. He considers the case where the weight of the wings is made up of parts following the structure law and parts whose weight varies as W:>\ The curves given for the two cases are interesting and should be closely studied. Dr. Leslie Aitchison brings his very interesting and instructive series of articles on Duralumin to a close this week. and we feel certain our readers will join us in expressing to Dr. Aitchison our thanks for his valuable contribution to the general knowledge of the characteristics, peculiarities and treatment of this material which, in the view of many, is destined to become increasingly popular as a material for all- metal aircraft construction. We are aware that there are still those who are " afraid " of Duralumin, but such firms as have had most experience of it. among them being Short Brothers, have nearly always found their fears ill-founded, and we believe that Dr. Aitchison "s articles will have done much towards a better understanding of the characteristics of this material. The experimental evidence quoted in the last article may appear alarming in the light of English practice in regard to load factor ; but, it must be remembered, the true strength figure (corresponding to the French '" In dice d'essai") is higher than the specified factor or at least normally so, the amount being usually 15 per cent, under extreme centre of pressure conditions or as much as 3(1 per cent, with the centre of pressure in its most favourable position. In the first case with the normal type of biplane structure, relief is obtained by redundancy through the incidence bracing. An approximation to the influence of the redundant members may be obtained by strain energy methods up to, roundly, half the failing load. Before actual failure takes place plastic extension of members, notably bracing wires, modifies the relative rigidity of the members composing the structure, causing relief to the overstrained members. We have here some protection from the possible detrimental influence of unconsidered redundancies and bad rigging. Where, however. stresses from these causes contribute to fatigue conditions, either actual alternating loads or by shifting the mean stress figure, failures will readily occur. In the second case, it is evident that as the centre of pressure moves backwards, the load on the front spar decreases, and as it moves forward the load on the rear spar decreases. This is illustrated in Fig. ] 6. It will be observed that the maximum factor occurs with the centre of pressure roughly one-third of the chord from the leading edge, a condition frequently chosen in experimental testing, except under the English system. Figures for experimental strength must therefore be carefully examined to ensure that they are on a common basis. This influence of centre of pressure on load factor does not affect the rest of the aeroplane in the same way as it does the wings. At the time when the wings have their maximum load factor the tail load is considerably less than in high speed conditions, e.g., c.p. back and terminal nose dive; further, the scantlings of the rear portion of the fuselage are often determined by the tail skid load. The strength of the front part of the aeroplane is generally influenced by the under-carriage loads and engine installation over normal strength for flight conditions. The nature and magnitude of the various loads to which aeroplanes of different classes are subjected in flight is in 702a D 2
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