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Aviation History
1919
1919 - 0279.PDF
FMSRUAKY 27, 1919 JLJ»L^^I and bottom wings because for the top wing the compression is the same all along the spar inside B. In the two and three bay types the points of support are different for top and bottom wings as the end loads pile up, of course, in the top spars, and there are no end loads (save those due to drift and stagger) in the bottom spars. The points of support for the top wing spars are so arranged that the end loads in a biplane of gap — about .8 chord, and added bending moments due to these end loads, will, when added to the fixing moments, give the same unital compressive stress in the uniform section spar at each point. I have assumed that all spars are pin- jointed at their inner ends, as I consider that to be the soundest practice ; " fixed joints "in a composite wood and metal structure are generally questionable, especially when de- tachability comes in, and one does not know how to consider fairly in calculations a half-hearted type of " fixed " joint. It is noteworthy that in types of two or more bays the maximum bending moment between any two points of support is never as great as at the points of support. This means that a uniform section spar will be unnecessarily heavy between supports, and it would probably pay to make the spars to strength required, between points of support, and stiffen them up locally for some little distance on each side of supports. Plate V. We now come to the consideration of the 'tween wing struts. Shown here are the proportions of what I shall call a "standard strut." The proportions of the cross section are probably about as good from a weight and head resistance point of view as any other. The strut is supposed to be of solid spruce of 32 lbs. per cubic foot density, with an ulti mate compressive strength of 5,000 lbs. per square inch, and a value of modulus of elasticity of 1,500,000 lbs. per square inch. The amount of taper has been decided from figures given in the paper by Messrs. Barling and Webb published in the Aeronautical Journal of October, 1918. For convenience in calculations I give for this strut form the values of area, moment of inertia and radius of gyration of cross section in terms of its maximum breadth b. I give in equation (1) the weight in pounds, W, of the strut in terms of its length in inches, L, and its maximum breadth of central cross section in inches, b, and in equation (2), the value for the crippling load in pounds, P, in terms of L and b. From equations (1) and (2) is obtained equation (3), v' ich gives directly, for purposes of weight estimates, the value of W in terms of L and P. In equation (4) is given an approximate value for the head resistance in pounds of the strut in terms of V, L and W. where V is the speed in feet per second. The value is obtained from experimental figures given by the N.P.L. Plate VI. Here are values for weight and head resistance of tension wires. I have assumed that one can always use a wire of exactly the right cross sectional area. The form of *«.T!r!-» MoweNTorAggfteg*tfWoj«vg.*BouT ec = -3i3uw»Tsa ZO 2» SO -75 LOAQ SR*PING yr WHMG TIF*. I-OO CHQgo ,*" 1 2 *i-»l =q 1 ———— - , 0 >V ***«»!»-. 'B UJ 1 M£»*^. -5 ml" . - R-UJI I 2-*l l-fet *frW) = -iquil* «£«*,)*-IB UJ1* 'cCOMOMtC* PgQROWTiQWS *w -T5- _l__ v>.l 2.«J. 1- qel «H * -i7uji ECONOMIC PKOfORTIOMS H3* S-**l- fr *twffee B*rTTOP wing 5P*« ryg-safciol •' -«. * 9«i u»l* a-ooi —i- rLATrrlg 279
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