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Aviation History
1955
1955 - 1539.PDF
654 FLIGHT, 21 October 1955 JET-FLAP DEVELOPMENT N.G.T.E. Aerodynamicisfs Lecture before the Royal Aeronautical Society. Part I DEMONSTRATED by the Ministry of Supply at thisyear's S.B.A.C. Show, and described in Flight ofSeptember 30th, the jet flap was the subject of an R.Ae.S. lecture given by Mr. I. M. Davidson, B.Sc., A.F.R.Ae.S. (of the National Gas Turbine Establishment) in London yesterday, October 20th. We give here a condensed report of the first pan of Mr. Davidson's paper, concerned with basic principles and illustrated by Crown Copyright diagrams. Pan 2 of the paper, Some Aeronautical Considera- tions, will be reponed in next week's issue. "In human and avian flight near the ground there are a greatmany differences," Mr. Davidson stated in his introduction, "but of these only two appear vital—that the bird sports a completemastery over the stall white, with its relatively low wing loading, it can fly much more slowly. In all species flight is naturally ameans to an end and for that reason practical aviation is, and always will be, tied to wing loadings of about a hundred timesthose of the birds. Thus, for a bird-like performance to be pos- sible, one obvious necessity is a means for the generation andcontrol of very high lift coefficients—higher perhaps than anything which has previously been considered possible. The other is,naturally, the removal of the danger of the stall. "In recent years, a possible practical solution of just those dif-ficulties has apparently emerged. This is the jet flap, which depends upon nothing more than the aerodynamic resynthesis ofthe lifting and propulsive means, the entire propulsive jet being ejected in the form of a thin, full span sheet from the trailing edgeof the wing. Although novel, the underlying physical principle is by no means new, for it is commonly found as a side effect inexperiments on boundary layer control by blowing over the top surface of a trailing edge flap. Indeed, in that guise it enjoysworld wide recognition, the American term being supercirculation and the French, hypersustentation. "The first published information was that contained in a paperwritten by Hagedorn and Ruden at Hanover in 1938. In France the discovery was made independently, the first trace being anunpublished paper written in 1942 by Valenci, Parigi and Borgel at Marseilles. The first relevant French publication was byPoisson-Quinton in 1948 while, as recently as December 1954, Jousserandot has described the results of extensive experimentsin which the active principle was all but isolated. "In the U.S.A. the effect was demonstrated as early as 1931 by STATIC HOLE POSITIONS . mourn THUS^ JET TOTAL PRESSURE TUBE Fig. I. Some details of the models. . •••'"• Fig. 2 (below). The basic principle. Fig. 3 (centre, below). The analogous flap size. 1. Fig. 4. Measured lift at zero incidence. the early results of Bamber, but there, as elsewhere, the emphasiswas on the use of the jet for boundary layer control. The British trail is again a long one for, at the Royal Aircraft Establishment, thesubject was broached by Lyon, Barnes and Adamson in 1941, and again by Mettam in 1951. "However, in all the early works the vital principle seems tohave been considered mainly as a scientific curiosity, and the crucial step was not taken until November 1952. That step wasthe observation by Constant that, the propulsive jet of a modern aircraft being a very powerful physical entity, it should be onehundred per cent combined with the wing in flight near the ground." Turning to the basic principles of the jet flap, the lecturer con-sidered first the lift on a two-dimensional wing. "Basically," he submitted, "the gas stream from a jet flap acts just like a largeFowler flap. It baulks the lower mainstream, forces it over the wing and so, following Bernoulli, aerodynamic lift is generated.This argument is simple enough, but it implies that the jet path is already known; that there is in fact a worthwhile downwardpenetration of the mainstream. Among other things, the effect must clearly depend upon the speed of the aircraft and the settingof throttles so that there is, as with the ornithopter, some inexor- able combination of the lift and the thrust. To discover the laws ofthat combination it is best to consider the mechanism in two- dimensional flow." Mr. Davidson next described the N.G.T.E. demonstrationequipment used. The bulk of the early work, he said, was done on an elliptical aerofoil having three separate trailing edges, withrespective jet angles of 31.4, 58.1 and 90.0 deg (see Fig. 1). In the basic aerodynamic mechanism (Fig. 2), it was essentialto distinguish between the engine thrust or total jet reaction (J) and the reaction thrust of (J cos 8) on the aerofoil, the lecturer con-tinued. "Since, with or without mixing, the jet must be turned to flow parallel to the flight path and since it carries everywhere amomentum flux equal to J, each of its elements must exert a centrifugal force as shown. In turn, this force is balanced by adifference of static pressure across the jet such that the total lift on its centre line is equal to J sin 6, the original downwardmomentum flux. In the theory use is made of the idea of an analogous structural flap (shown dotted) and, from the size whichthis can assume, it will be seen that very high lift coefficients must in principle be possible (Fig. 3). "In practice it is best not to divide the lift into an aerodynamicor pressure component and a jet or reaction component, but rather to think in terms of the whole . . . From the diagram (Fig. 4) itwill be seen that even the pressure or aerodynamic lift component is greater than could be generated by any other practical mechan-ism, while, beside the total, the idea of using only pure jet lift looks decidedly uneconomic. This last point is perhaps better illus-trated by Fig. 5 in which the lift gain (G) is a direct measure of the total lift which can be generated per pound of available jetreaction. Thus, for many practical purposes the jet flap can be regarded simply as a jet lift amplifier of which the gain liesgenerally between three and thirty. "In the present theory an empirical factor is involved in theestimation of the lift at zero incidence, so it would not be fair to THE SOLID FLAP V CENTRIFUGAL FORCE. "O ONE 2 3 4 JET COEFFICIENT - Cj ONE 2 3 4 JET COEFFICIENT -Cj
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