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Aviation History
1936
1936 - 0764.PDF
March 26, 1936 Supplement to ^j@CW FLIGHT ENGINEERING SECTION No. 122 (Vo'S™e|C") 11th Year March 26, 1936 THE STRESSING of MONOPTANE WINGS By HAROLD N. HORNE A Rapid Graphical Method of Treating a Two-spar Wing With Stiff Covering and Parallel or Non-parallel Spars THE author of this article has had considerable experience in stressing monoplane wings with stiff covering, having been a Technical Officer at the R.A.E. where, in 1927, he investigated the original Fokker monoplane bought by the Air Ministry ; since then he has been for seven years a senior on the structural strength side of the technical department of a well-known aircraft firm. When a monoplane wing with stiff covering is being designed for use in the R.A.F. it has to be investigated by the Roxbee Cox method stated in A.R.C Report and Memoranda No. 1617. The original method is somewhat laborious, and for rapid working a simplified adaptation is an advantage in the design stage. In The Aircraft Engineer of December 26, 1935, and January 23, 1936, was published an article on the same subject by Mr. B. B. Walker. However, Mr. Home's treatment is different from that of Mr. Walker, and whereas the article already published dealt with two- spar wings with parallel spars, the present article explains a graphical way of treating R. & M. 1617, covering all kinds of two-spar wings whether the spars are parallel or not. In a subsequent article Mr. Home will describe an extension of his method to cover the case of the multi- spar wing, with parallel and non-parallel spars.—ED. Ihe demand for high-speed aircraft has resulted in much rreiition being given to the monoplane, and this is the type snee 1 appears t0 ^ rapidly on the increase. When the of J WaS S the re£ion of IO° miles per hour the question sneed f S ness was relatively unimportant, but as the the wi n tInCreased> the stiffness, especially in torsion of for m^ st["cture. has become more and more important, control Wmg is Stiff' troubIe due t0 loss of aileron may occur & tUm &t high speed' or PerhaPs WU1S flutter, Several ways of dealing with this problem have been tried ; for example, the monospar, the diagonal bracing wing structure with pyramid bracing to take the torque, the wing structure with a large tubular spar, etc., but the most general way has been the use of some form of covering between the spars, which replaces the fabric and is capable of resisting the torsion on the wing by acting together with the spars, and so forming a unit capable of resisting torsion. Tests on a two-spar wing have shown that the torsional stiff ness can be doubled by adopting this kind of construction. When a wing can resist torsion purely by differential bending of the spars, as is the case of a wing with two spars and fabric covering, the estimation of the stresses in the spars is a simple procedure, but when the wing has a covering which, together with the spars, is capable of resisting the torsion on the wing the allocation of the loads between the members is much more complex. In this case the torsion is taken parti)' by differential bending of the spars and partly by shear in the covering. A complete method of dealing with the problem is given in R. & M. 1617 (Ref. 1) and also in a paper read before the Royal Aeronautical Society by Dr. Roxbee Cox, and published in The Journal of the Royal Aeronautical Society, February, 1934 (Ref- 2)- These, whilst being very complete, are not in a form easy to apply when a wing is in the design stage, and the present article is written to show how the problem can be dealt with and the loads in the structure obtained quickly and easily. In the above-mentioned papers as in the method to be explained, it is assumed that the contribution of the skin to rlexural strength is negligible. This is the case if the covering is corrugated, and perhaps less so if the covering is of plj-wood attached to the spars, but to include the effect of this would be to make the solution too long and difficult, except as a problem for research ; so it is seen that extreme accuracy is not warranted by the nature of the assumptions. It will be seen later that the method to be explained gives results which, although obtained quickly and easily, are accurate enough for practical purposes.
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