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Aviation History
1919
1919 - 0563.PDF
MAST I, 1919 l/j?!@gd ] The dolled lines indicate values of ZtkD di<«n$ M- conitznl throughout length of Strut, but Ike Jul I lines jre Ike values adopted for practical reasons, Fig. 27. is the material. This is due to difficulties in manufac ture, which are to a certain extent against its general employment. The same applies, although in a smaller degree, to the hollow square section, of which the two halves have to be spindled out and grooved, a hardwood tongue being inserted in the groove and the whole glued together and, in most cases, wrapped with fabric. Thus, production is considerably slower than that of sections needing spindling only. As a compromise between the most efficient section and ease of manufacture, the X section, therefore, would appear to have undoubted advantages, being nearly as good as the hollow sections, and much quicker to manufacture. IV.—STREAM-LINE STRUTS. In addition to the struts of various sections employed internally in an aeroplane, there are others which are left exposed to the air, and which, there fore, have to be of stream-line section in order to reduce their head resistance. These include the struts of the undercarriage and the inter-plane wing struts. An examination of modern aeroplanes reveals con siderable variety in the proportions of the stream line sections employed, but as one can only include a limited number of sections the two most usualfy employed—having fineness ratios of 4 to 1 and 3^ to 1 respectively—have been selected for the purpose of these notes. Apart frcm the question of fineness ratio, which is largely a compromise between aerodynamical and constructional desiderata, there is the problem of the longitudinal shape of these struts, modern machines showing straight, tapered, and curved struts. We shall first examine the Strength of a Tapered Strut CASE I.—It is assumed that the strut tapers with a straight taper from centre to end, and that the dimensions at the ends are -65 times the corres ponding dimensions at the centre of the strut. Portion A to B in Fig. 27 represents this case. For any strut, the least moment of inertia I equals K x d*, and hence the section modulus Z equals 2Kd*. When, as in this case, the variation of d throughout the length of the strut is given, we can evaluate and plot curves, showing the variations in I and Z, and these are shown plotted in Fig. 27. When a strut buckles and fails, the stresses are for the main part due to the bending moments induced in the strut. Fig. 28 J shows a strut buckling. The bending moment at any point is equal to F x x. If it is / assumed that the strut buckles to a parabolic shape, the bending moment diagram is also parabolic, and such a curve is shown plctted in Fig. 27. The bending stresses are proportional to M y, and by dividing the ordinate of the bending moment curve by the corresponding M ordinate of the Z curve, the curve of =r or bending stress is obtained. For the straight taper, the -^ curve has a maximum L Fig. 28. ordinate at about the quarter points, indi cating that these are the weakest sections, and that this type of strut is inefficient. For uniform strength throughout the length of the M strut -~ should be as nearly constant as possible. 563 H
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