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Aviation History
1946
1946 - 1634.PDF
196 FLIGHT AUGUST 22ND, 1946 Water Gliders An Ingenious Means for Preliminary Research at High Incidence By C. G. W. EBBUTT* THE startling progress of the last few years in allmatters concerning flight at high speed has rathertended to swamp the relatively small amount of progress at the soggy end of the speed range. I mean by this that there is a tendency everywhere to take it for granted that nothing can be done at all unless it be on the grand scale. This outlook I believe to be quite unsound, and I take the liberty of giving my reasons. As far back as 1936 I constructed a simple model which was used as a free glider completely submerged in water. The model, which is shown in Fig. 1, was trimmed to glide gently into the stall so that changes in whole mechanism of wing-dropping and the incipient spin could be seen extremely clearly. I suggest that water gliders are, for this type of work, of far greater value to demonstrators in aerodynamics at educational establishments than are small wind tunnels, since the model need not be restrained in any way and everything happens in slow-motion. The effect of wing- tip slots can be seen particularly well and, as a matter of interest, I found right at the start that breakdown or reversal of flow right on the upper surface did not necessarily indicate that the wing had stalled. This fact was not generally known at that time, but is now com- mon knowledge. The silk threads were spaced at the surface and at intervals up each pin of one-tenth of the local chord, there being twelve pins to a semi-span and four threads to each pin. The cost of finding the plan form having the 1 est stalling properties was a matter of shillings instead of a few thousand pounds. An attempt was made, also in 1936, to develop some Fig. 1. The adjustable glider chassis carrying a " c " typewing (see Fig. 2.) The series of holes seen above the wing was a vernier arrangement for finding the position of the e.g. flow within the boundary layer as well as in depth could be observed by means of silk threads attached to pins stuck into the wing. This was done for four model wings, all of 2 inches mean chord and aspect ratio 6, the section being N.A.C.A. 2200 series. The first was of constant chord and thickness, the other three having a taper ratio of 0.25^ and a thickness of 15 per cent at the centre and 9 per cent at the tip. Tapered wing (a) had a straight leading edge, (b) had a straight 50 per cent line, and (c) had a straight trailing edge. The results of observations were systematically recorded and actually turned out to be in fair agreement with wind tunnel results reported in " R. and M." bf various later dates. They also agreed wi^h the photo- graphic recordings made in flight by Mr. W. E. Gray at about the same time, if my memory is reliable. Fig. 2 is a photograph of the three tapered wings. The Reynold's Number of these tests was rather ludicrous, but since the full-scale stalling speeds concerned are low, this would appear to be of little im- portance. The really interesting thing about these water gliders was the behaviour at the stall, where the aFig. 2. • The author has been in aviation for a great num- ber of years, many ol them on the Heston Aircraft Company's technical staff. Fig. 3. The nose flap of a wind tunnelmodel of R.A.F.31 aerofoil. The slight curl at the lip of the flap was foundnecessary on the water glider. u cModels of the three wings tested. All were of 2in. mean chord and aspect ratio 6. device which would delay the stall of fairly sharp-nosed wings without incurring the heavy drag penalty of the ordinary front slot. I argued that, since the large drag increment of the slot was mostly due to the input of energy into the upper aJr- stream, and to extra skin area, the best bet was to try to avoid losing the energy in the first place. This was done by developing, on water models, a nose flap which, full-scale, would be either hinged at the wing leading edge or slid in and out of it as if it were a rigid curved shop blind. This was intended to reduce the peak velocity on the upper surface. The result was very much as ex- pected as regards stalling properties,
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