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Aviation History
1919
1919 - 0347.PDF
MARCH 13, 1919 JLEL^^J Naturally the larger the naps the greater would be their inertia, the greater would be the balanced tension in the control wires and the greater therefore the friction in the moving parts of the control system. So even if we assume that our model data is correct and the deductions made from them sound, it is probable that in practice it would prove better to use somewhat smaller flaps than are indicated as the best from this investigation. But it certainly seems correct to use narrow flaps and to make these flaps sufficiently long so as not to have to work them through too large a range. I should have liked to look into the case of a biplane with flaps on the top wing only, as this system has some structural advantages. But unfortunately the only model data I have been able to obtain deals only with the case of pulling down the flap on the top wing of a biplane, and rather condemns the system for inefficiency on the results. To investigate the rolling moment we should need also the results with flap pulled up. I am inclined to believe that sufficiently long flaps on the top wing only of a biplane, pulled up and down of course, should give quite efficient lateral control, particularly if the top wing be larger than the bottom and if the machine be of smallish span. I now come to the final subject of which I have attempted a simple analysis. It is the comparison of different types of biplane structure. Plate X.—Here are three biplanes with differing types of wing structure :—(A) is a single-bay type (often called " Scout " type, because it is the form of wing structure generally used on single-seated fighting machines which do not scout), with " mean " aspect ratio of 5 for top and bottom wings ; (B) is a two-bay type, " mean " aspect ratio 7 ; (C) is a three-bay type, " mean " aspect ratio 9. The comic rakes for all the tween wing struts have been perpetrated in order to obtain what the previous investiga tion gave as " economical " position of points of support for the spars of both top and bottom wings. The wing areas for these three types have been arrived at by assuming that the total weight is the same for all three, and that the stalling speed near the ground is to be the same for all three, and using the maximum " absolute " lift co efficient values proper to the different aspect ratios from the " full-size " aerofoil curves given previously, Plate I. The maximum " absolute " lift coefficients are .535 for biplane (A), .543 for biplane (B) and .550 for biplane (C). Assuming total weight of each machine to be 2,600 lbs., (A) with its 375 sq. ft. has a wing loading of 6.94 lbs. per sq. ft. ; (B) with its 369 sq. ft. a loading of 7.05, and (C) with its 364 sq. ft. a loading of 7.15. For all three therefore the stalling speed near the ground is about 50 m.p.h. Approximate calculations for the weight of the wing structure for each type were then made, using the following methods :—Total load for stress taken as 2,200 lbs., C.P. taken at .28 for stress on front spars. Spars, struts and tension wires, section and end shape of aerofoils, all taken as of " standard " forms previously described. Weights of top front spars were obtained by finding bending moment and end load just inside point B ; the bending moment was multiplied by 1.2 for (A), by 1.15 for (B), and 1.10 for (C) to allow for increment due to end load. The necessary breadth, and thence cross sectional area, of " standard " I-section front spar was found which would give a maximum fibre stress of 800 lbs. per sq. in. when subjected to this total bending moment and end load, and it was assumed that this section was constant throughout the spar length, and that the timber was of 32 lbs. per cubic foot density. From the previously given curves of spar weights the weight of rear 4 spar was taken as — times that of front spar. For the bottom front spars, the weights were obtained by finding the necessary breadth, and hence cross sectional area, of " Standard " I section spar which would give a maxi mum fibre stress of 800 lbs. per sq. in. when subjected simply to bending moment at outer support—i.e., G. for (A), H. for (B.) and J. for (C,) and assuming this section constant through out spar length, and density of wood 32 lb. per cub. ft. For bottom rear spars, the weights were taken as — times those of front. Weights of ribs were taken as proportional to square of chord length, and it was assumed that the weight of a rib of 6 ft. chord length would be J lb. and that the rib spacing would be 12 in. in all cases. Weight of covering was taken as 2 ozs. per sq. ft. of wing surface, meaning 1 oz. per sq. ft. of covering. Total weight of leading edge, trailing edge and spacing battens was taken as the same per foot run in all cases, and as of .28 lb. weight per foot run. Weight of nosing ribs was taken as varying directly as chord of wing, and it was assumed that the number of nosing ribs was twice that of the ordinary ribs and that 36 nosing ribs for a wing of 6 ft. chord would weigh 1 lb. Weight of compression members was taken as -4 lb. per ft. < fcC —-4 €1 •mag , wag, or i MB——JW In. ^ >«o*'»arccT»jio-'i. 1. '^T >w*->T»atonOMwm --1B7 W»N'MreCTR»T.o-;r TYPICAL BIPLANE WING STKTUCTUFJES. PLATEX PLATE X.—Some points in aeroplane design 347
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