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Aviation History
1939
1939 - 1736.PDF
June 1, 1939. Supplement »fflj0Sjt E AIRCRAFT ^ ENGINEER No. 160. (VO,N™e5XVn) ,4tb Vear June I, 193V ELASTIC STIFFNESSES OF A SKIN-COVERED FRAMEWORK The Effects Upon the Flexural and Torsional Stiffnesses of a Wooden Box- wing Test Specimen Due to Different Dispositions of the Grain of Thin Ply-covering By E. W. H. THOMPSON, A.F.R.Ae.S. IT is now generally observed that the attainment of higher speeds of flight has brought with it the problem of designing aircraft structures primarily to meet the re quirements of rigidity ; that for a certain speed of flight, if sufficient rigidity in flexure and torsion is obtained in a structure such as a monoplane wing, the strength of the wing, based o«i the present specified load factors, is also ensured. Whether the; problem of the rigidity of an air craft component can be made subordinate to the strength problem and be solved simultaneously during the customary stress analysis by suitably modifying the load factors for the affected part is open to question. In a monoplane wing with spars it has been usual to obtain torsional stiffness by means of a skin covering between the spars ; a box wing is thus formed capable of resisting torque by differential bending and twisting of the spars and by shear stresses in the skin. Flexural stiffness in this arrangement is mainly supplied by the spars, although the skin contributes a not negligible portion. Any addition to the torsional stiffness oi a given wing is most economically made when the available material is disposed in the skin covering. Such a disposition of material is also most effective in securing flexural stiffness. There arises then the tendency to design a wing as a shell that can resist both bending and torsion Up to the stage when the design oi a wooden wing was determined solely by load factors, that is, before the flutter problem became of major importance, conservative values for the elastic constants of aircraft timber were adopted as affording a safe basis for design. Although the 8B00H»Mx*Ail». use of safe values lor'the elastic constants oi timber is still desirable in strength estimates, the use of actual values is imperative in any consideration of the elastic stiffness of a wing for avoiding flutter; here it is of importance to know the actual deformations under load. Since, however, timber is a variable material the assignment of any par ticular value as an actual value to an elastic constant must become eventually a matter of probability of a fre quency distribution. In practice, of course, selection of timber and control tests are usual. The following experiments were earned out in order Firstly, to observe generally the behaviour oi panels ei thin skin under flexure and torsion; "g- i. Uncovered framework. C -C A4 SECTION A-A BUT FRAMEWORK COVERED WITH PLY I ONE WIN FULLY OPERATIVE) INBENDlNy THE OTHER PARTLY SO.
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