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Aviation History
1914
1914 - 0222.PDF
(AEHJ THE AERODYNAMIC AND OF By Professor HERBERT FEBRUARY 28, 1914. ON WINGS, EFFECTS OF GUSTS OSCILLATING WINGS. CHATLEY, B.Sc. (Engrg.J, M.I.C.E.I., A.F.Ae.S. the sand pillar, but the upper end seems to spread •IT is the object of this paper to call attention to a factor in dynamic flight which has received very little attention, and yet may prove to be of the greatest importance. Observations of wind pressure on exposed structures (<f.<f., The Forth Bridge) indicate that on small surfaces higher pressures are experienced than on large ones, but that, on the other hand, in a steady current, large surfaces tend to have a higher unit pressure than small ones. The latter fact is certainly d«e todimensional causes. The effects of dimension have received considerable attention at the hands of the Advisory Committee for Aeronautics, and the manner in which the coefficient of resistance increases with dimension, and the relation of the dimensions to the velocity will be found in the Blue Books issued by the Committee. It remains, there fore, to account for the first fact. There can, of course.be no doubt that it arises from the unsteadiness of the wind, and the complexity of its structure. It would seem probable that the dimensions of the aerial vortices and waves which are termed " gusts " are comparatively small (elsewhere I have pointed out the serious effect this has on the mathematical com putation of aeroplane stability) in one direction if not in both. In this connection I would remark on the vertical vortices known as "sand-pillars" (or, at sea, " waterspouts "). These, on a small scale, are very frequently seen in North China. The visible portion, i.«., that within which the velocity is sufficient to keep the dust in motion, varies in diameter from two or three inches upwards. I have not seen them more than a foot or two in diameter. The height is twenty or thirty times the diameter, the structure Incoming indefinite at the top. In a perfect fluid a vortex must terminate on a surface of separation if at all ; this is true of the lower end of out in spirals, in a manner similar to that which Lanchester conceives to occur at the trailing tips of pterygoid aerofoils. These pillars are generated in the " dead-water " on the leeward side of an obstacle with a vertical edge, and finally pass out into the stream and travel with it. I have seen them travel a mile or more at a comparatively low speed. Doubtless similar horizontal vortices frequently tend to be formed, but lack stability, on account of the absence of a vertical surface of separation. Have they ever been seen to travel up or down the face of a cliff? Without reference to the question of flight, it would be of great utility to know what effect gusti- ness of a prescribed amount has on wind pressure, but beyond this the abnormally high pressures which occur, due to gustiness, suggest that a means is provided in this direction for obtaining higher loading on a dynamic flying machine at a given t-peed than is possible with the present purely gliding forms. Some temerity is needed to refer to the ornithopter after its continuous failure, but the aerodynamic success of the Nicholson and Pichou wheels is, perhaps, some indication that progress may occur in that direction. The main thesis of this present paper is that:— (1) an oscillating wing may sustain a heavier loading than a gliding wing, the mean velocity being the same, and (2) if the virtual mass of air dragged with the wing during the downstroke can be feathered off at the bottom of the stroke, the upstroke being made " in the wind," there will be a further and possibly very appreciable increase in the loading possible. Professor Maurice Fitzgerald was the first to call attention to the latter fact, in a paper communicated to the Royal Society by Lord Rayleigh in 1909 (" Proc. Roy. Soc," A, Vol. 83, 1909.) He shows that the loading per h.p. due to this cause alone, varies as the ratio of the virtual mass to the actual macs of the machine multiplied by the frequency of " flapping." 222
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