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Aviation History
1938
1938 - 2221.PDF
AUGUST 4, 1938. FLIGHT. 103 SITTING on the SLIPSTREAM Effective Lift Coefficient of Five Obtained With the Crouch - Bolas Engine Arrangement AMAZING progress has been made with high-lift devices of different sorts during the last few years. Slots, slotted flaps, split flaps, rider planes and so forth raised the maximum lift coefficient from the 0.6 or so (in the old-fashioned British "absolute" units) of the thin, moderately cambered biplane sections in com mon use before the revival of the monoplane fashion to 1.5 or more when applied to the modern cantilever wing. Obviously the increased unit lift can be used either to reduce landing speed for a given wing loading or for keeping the land ing speed the same and increasing the wing loading, thereby obtaining an improvement in maximum and cruising speeds. The tendency has, rather naturally, been towards the latter course. Not only so, but the ability to steepen the gliding path by the use of flaps has made it possible to employ even greater landing speeds and trust to the new technique and greater skill of the pilot to make landings safe at the greater speeds. Research is still being carried out in all manufacturing coun tries on devices for increasing unit lift still further, and some improvement may be possible. When it is made available, however, it will probably be utilised for still higher perform ance by a further increase in wing loading. Two Englishmen have been working along quietly on a dif ferent line of attack for several years, their preliminary work, as it happens, having been carried out in America, although one of them has now returned to this country. Mr. R. J. Goodman-Crouch and Mr. Harold Bolas "emigrated" to America several years ago and there established a small com pany for experimenting with a scheme for obtaining increased lift by making use of the slipstream from the airscrews. It had, of course, been known long before that that the portion of a wing situated in the slipstream gives greater lift by virtue of the greater relative velocity of the air. But no one, appar ently, had realised that by judicious "wangling " the degree of extra lift might be made to amount to a good deal. Messrs. Bolas and Crouch did suspect this fact and set to work on finding out exactly how much there is in it. Surprising Results Their results, from wind tunnel tests and from actual flying tests carried out with a somewhat quaint experimental biplane, indicated that rather surprising "lifts" can be obtained. For the full slipstream effect to be obtained it is, of course, neces sary that the airscrews should "cover" the entire wing. If they cover half the wing area, very roughly half the effect is obtained, and so forth. Fortunately for the two inventors the modern tendency in military as in civil aircraft is towards twin-engined and multi-engined machines, and it has been found that to obtain a fairly large percentage of the theoreti cally possible advantages it is not necessary to go to extiemes in the matter of airscrew diameters. Perhaps it will be recollected that a couple of years ago or so the annual report of the Aeronautical Research Com mittee referred to the desirability of research into the mutual effects of a large number of airscrews on the leading edge of a w, ng. The idea was, we believe, connected with large flvhig boats of 200 tons gross weight in which, with existing power plants, the total horse-power would of necessity have to be provided by a multiplicity of engine units. We are not aware if that research was carried out, or if it still remains a pious hope. Be that as it may, Crouch and Bolas have found that by covering the greater part of a wing with slipstream, and by tilting the airscrew axis downwards, a very great C-D \ i^^^^ff-^^fcW,^ *V> C-B DPS- The design of the Crouch-Bolas Speed-Ranger indicates that little departure from conventionality need be incurred to ob tain the full effect of the tilted slipstream arrangement. increase in lift is obtainable at low forward speeds. Moreover, the angle of airscrew tilt needed to give somewhere near opti mum results is not great enough materially to affect the air screw efficiency at cruising speed. At low forward speeds there is, of course, a considerable degree of slip, so that when the aircraft as a whole is travelling slowly the speed of the slipstream across the wing is high, and the wing obtains the high lift which results from high relative air velocity at large angles of incidence. That an aeroplane incorporating the Crouch-Bolas ideas need not be unsightly, nor even very unorthodox in appearance, is. indicated by the three-view general arrangement drawings of the C.-B. Speed-Ranger, a design which was evolved some time ago. In this the inboard engines develop 420 h.p. each, while the outboard engines produce 145 h.p. each. The speed range is estimated to be from 245 m.p.h. to 34 m.p.h., or slightly over 7 to 1. The tapering of the power units has certain advantages, apart from its use in the increase of lift. The distributed load along the wing spars reduces the bending moments, while the fact that the lower-powered engines are outside reduces the yawing moment on the machine in the unlikely event of a stop page of an outboard engine. Incidentally, the machine is designed to fly on one inboard engine alone. Mention has already been made of the fact that one of the essential features of the Crouch-Bolas scheme is that the air screw shaft should make a negative angle with the wing chord On the experimental biplane provision was made for varying this angle in order to determine the optimum arrangement. It was found that the angle need not be very large and that in practice there is no need for such elaborations as tilting engine mountings. The wing itself is slightly more efficient when placed in an airstream tilted upwards a little, the effect being rather similar to that found in a glider soaring in an up-current. This gain in wing efficiency is just about suffi- i^:^?)a^val.yk^ji^<#<^i L.^v.^±^^u^c.^i^;^;„, C-B / *» C-B f*?' ^ Fig. 1. Fig. 2. Fig. 3. Fig. 4. Stages in the take-off of the Crouch-Bolas biplane and an orthodox machine. Sketches prepared from a film.
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