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Aviation History
1929
1929 - 1128.PDF
SUPPLEMENT TOFLIGHT 40 THE AIRCRAFT ENGINEER MAY 30, 1929 one second at the minimum take-off speed Vs (relative to still air), and is assumed to have no acceleration. Its actual speed relative to the ground or deck will be (Vg — Vo), and the run while dropping the tail will be (Vg — Vo) X 1 ft., if Vs and Vn are in ft./sec. A curve of run while dropping tail against deck speed Vo can there- fore be plotted, and. as before, the run will be zero when To = the minimum take-off speed Vg. The calculation is then completed by adding the final curves of (A) and (B), and thus obtaining a curve of total run required to take off against various head winds or deck speeds. 300 10 HEAD WIND or DECK SPEED M.P H. The accompanying curves give typical results of the use of the method on a normal tractor biplane having a gross weight of 1,400 lbs. Fig. 3 shows the thrust, total drag, and accelerating force plotted against air speed in miles per hour for the machine in the " tail up " attitude, and in the same figure is the total lift at various speeds with the " tail down," giving the true air speed for take off. Fig. 4 gives the curves of section (A), i.e., run with "' tail up " plotted against true air speed for various values of head wind or deck speed. Fig. 5 gives the final curves of run with " tail up " (taken from Fig. 3 for the true value of take-of speed), run while dropping tail (as outlined in section (B)), and the total run required to take off plotted against head wind or deck speed. In conclusion, the method outlined above, though rather laborious, is very straightforward, and will yield quite good results. The work will be facilitated by the adoption of a standard system of tabulation. It can be adapted to different atmospheric conditions, and the principle can also be used to find the time and run to get off of flying-boats and seaplanes. In this case, of course, the given assumptions do not apply, the attitudes during the run being governed to a large extent by the hull or float design. These attitudes, together with the water resistance, must be taken from tank tests of the hull or float, and the method modified to suit the case. STABILITY OF SEAPLANES AFLOAT. By " C. W. P." The following notes may be found of use to the aeronauti- cal engineer when planning the disposition of floats in sea- planes or flying-boats. An accurate estimate can be made by the folloiring methods in order to find the righting couple that will be available to counteract any upsetting force likely to be met in service when the machine is at rest or travelling on the surface. c1 CF . M HG.I D Case I. Fig. 1. Twin float machines in which two equal floats are placed symmetrically about the centre line of the machine. In this case the shift of moment equals. 6 2 (I 8 6) 1 = 1 about centre of flotation of each float -f- Area of waterplane X D3 = N.L x B X D2 since I, O.F. is negligible. 2 N.L X B X D2or BM = v = (T Z = G M sin 0. and the righting lever Flying boat. Case //. J^. 2. I L. B3 V G.ZX = — GMsin A. Unstable, therefore wing tip floats must be fitted. It is now necessary to ascertain the ^e and disposition of the wing tip float. Several trials should be made, using the following i notation:— -•. ' r V I = total volume of water displaced. = moment of inertia of waterplane. = length X breadth X function/. 446d
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