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Aviation History
1931
1931 - 0289.PDF
ch 27, 1931 Supplement to FLIGHT ENGINEERINGSECTION Edited by C. M. POULSEN March 27, 1931 CONTENTS PAGE A Graphical Method of Stressing Aeroplane Spars. By D. Williams. B.Sc, A.M.I.Mech.E 17 Technical Features of the Air Mail, By Frank Radcliffe, B.Sc., A.M.I.A.E., A.R.Ae.S 21 Technical Literature ... ... ... ... ... ... ... ... 23 A GRAPHICAL METHOD OF STRESSING AEROPLANE SPARS. ByD. WILLIAMS, B.SC., A.M.I.Mech.E. [Some time ago Mr. H. B. Howard of the Air Ministry developed a graphical method of stressing aeroplane spars, the theory of the method being described in R. <£• M. No, 1233. As there presented, the method ivas hardly suitable for use in the design office, and Mr. Williams, who is one of the Technical Officers at the Royal Aircraft Establishment at Farnborough, has now given a new presentation of Mr. Howard's method, intended partkularly'io be readily applied in the design, office. The first instalment of Mr. Williams' article appears below.—ED.] P = end load in main bay. E = Young's modulus for the material of the spar. 1 = moment of inertia of the spar section, ji* = P/EI w = sjMir loading in 1b. per inch run (-j re when in upward direction). 5 = true shear. i = slope of spar (?'u = slope at B). « = length of a sub-bay. 'AB = length of main bay AB. y. = \ia radians = 57-3 [x« degrees. [3 = defined below. 6 = „ „ M = true bending moment (+ tie when concave side of beam faces upward). m = (M — w/[i2) and is merely a convenient quantity for diagram construction. Before considering the case of the continuous beam in which, of course, the, bending moments at the supports are, unknown, the application of the graphical method to the simple case of a single beam supported at its ends and sub- p - AAAAAAAAAA/j\A/j\AA/f\/j\ A VERY useful method of stressing aeroplane spars was recently developed by Mr. H. B. Howard, B.A., B.Sc. The theury of the method and examples of its use have been ginii by Mr. Howard in E. & M. No. 1233. The purpose of tie tollowing description is to enable anyone in the design °ffii • to apply the method without necessarily mastering the .heory. T-e method makes it possible to deal expeditiously withe nd oaded spars in which any number of discontinuities, r of loading or moments of inertia of spar section, . In such cases it is much superior to any analytical met ad, as it entirely does away with the necessity fors °fr lg a large number of simultaneous equations. •^ s following nomenclature will be used :— «i in bay of spar = length of spar between two consecu- tive points of support. S' j-bay of spar = length of spar between two discon- tinuities in the same main bav. eith jected to known end bending moments and a uniformly distributed load will be explained. Suppose that for a beam AB pin-jointed at A and B, the values of P. I, I, MA, MB and w are given. (Note that when MA, MB and w are in the directions shown, thej- are -f- ve). First evaluate [i, wj\j?, and a (= 57-3!^°) and proceed with the following construction (see Fig. (a) on next page):— Draw a horizontal line OB and make the angle AOB = a. Produce AO to A1 and BO to B1. The space included within the L/ AOB, i.e., the sector AOB, is termed the + ve sector, while A1 OB1 is the — ve sector. The circular arcs used in the construction are confined within the limits of these two sectors, those portions of the ares extending into the regions AOB1 and BOA1 not being drawn. With centre 0 and radius wj\j? to a convenient scale draw an arc ktk2 in the negative sector A'OB1, the rule being that if w is -j- ve (as in the present example) the arc is drawn in the — ve sector ; if negative, in the positive sector. Measure off k2 WB in the direction BlB 272a
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