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Aviation History
1936
1936 - 2102.PDF
July 30, 1936 Supplement to ^0@Gn7 B^ro3 FLIGHT ENGINEERING SECTION No 126 (Vo,uNT ,XI") nth Year July 30, 1936 THE AERODYNAMIC LOADS for STRESSING a WING A Simplified Method : The Quarter Chord as Reference : Particular Advantages for Twisted Wings By W. R. ANDREWS, A.F.R.Ae.S. The method of the following article is based upon the con ception of the quarter chord as reference, instead of C.P. The quarter chord remains fixed, whereas the C.P. does not, and thus provides a fixed basis on which to work. The author claims that greater accuracy is obtained by the methods out lined, especially in cases where large changes in weight and/or speed have to be investigated. The methods are particularly useful when investigating twisted wings, and Mr. Andrews, mho is, of course, on the Technical Staff of A. V. Roe and Co., Lid., estimates that such wings can be stressed in less than half the time provided the number of cases investigated is four or more, including a T.V. dive. Probably "old hands" will find some slight difficulty in forgetting their old friend the C P., but it is thought they will quickly become accustomed to—and find a new friend in—the quarter chord. THE usual method of splitting up the loads on a wing into the components of normal force, drag and torque does not readily allow the stresses for any particular case to be determined from a previously stressed one. This is because every new case involves a fresh C.P. position, at which the lift is applied, and the moment about the flexural line must be determined by integration. No simple relationship exists between flexural line and all C.P. positions, except in isolated cases. The Flexural Line 1 lie fiexural line is denned as that line along the wing at which the distributed loading must be applied so that only flexural distortion is produced. That is to say, the incidence of each section remains unaltered as the load is applied. For the case of a single concentrated load with out any distributed load superimposed, a flexural line Q oes not exist. A concentrated load together with a distributed load requires a flexural line with a discontinuity at the point of application of a single load. The torsional deflection due to a concentrated load is dependent upon the stiffness of the wing outboard of the point of application as well as that of the inboard portion. It follows that to stress such a wing rigorously must be very involved and to bring the computations within prac tical limits many approximate and simplifying assumptions must be made. In the present stressing procedure, no allowance is made for the effect of the wing-twist upon the aerodynamic loading along the span, which for a flexible wing can be very appreciable. This twisting of the wing is a cumu lative effect, since the more the twist the greater the variation of loading along the span, which always tends to increase the twist until a state of balance is obtained. In view of the uncertainty as to the method of stressing a wing for a concentrated load, it is not thought that the accuracy will be greatly diminished if the stresses due to it are determined independently and added to those due to the aerodynamic loading. The method of this article is evolved on that assumption. In any case, it is doubtful if the loss of accuracy is as great as neglecting the variation of load due to wing-twist. Although a point may be found at any section where an applied load causes no twist with respect to some other section, the front and rear spars at that section may not have the same relative slope as for the unloaded wing. In other words, the point of application of the load depends upon the choice of reference section, except for the case where IF/IR and E are constant along the span. This explanation is necessary since in some quarters the flexural line is referred to as the locus of the point at which a con centrated load is applied to give no twist between the section considered and the wing root. The latter line, is
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