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Aviation History
1955
1955 - 1573.PDF
688 FLIGHT JET-FLAP DEVELOPMENT Mr. L M. Davidsons Lecture before the Royal Aeronautical Society Part II INTRODUCING the second part of his paper, Mr. Davidsonsaid that before the jet-flap principle could be applied pro-perly much of the theory and practice of low-speed flight must be written anew for, aerodynamically speaking, the situationwas akin to that which prevailed in or about the year 1900. With technology as it now stood this might not, of course, take long;but for the present it would be foolish to attempt any prediction concerning die aircraft types which would emerge. Nevertheless, it remained to be shown that a practicable air-craft could be constructed around the principle. The lecturer now continued: "The analogous flap-size for atypical aircraft of, say, Comet calibre is illustrated in Fig. 21, which suggests that in cruising flight the machine could be regardedsimply as a conventional aeroplane. In low-speed flight the situa- tion is not so obvious for, although there is now a rough workingknowledge of the properties of a jet-flapped wing, little has been deduced about the likely behaviour of a whole aircraft. Onething, however, is certain—that in flight through still air the maximum CL value will be determined solely by die availablethrust and that it will be very large indeed. "For example, in landing trim this machine might display a wingloading of 55 lb/sq ft and, with fully developed by-pass engines, a jet reaction to aircraft weight ratio of about 0.55. With no con-tingency allowance and with a jet angle of some 40 deg even this quasi-conventional aircraft would be off die present experi-mental map, but a slight extrapolation suggests for it an approach speed of 35 kt at a total lift coefficient of about 13^. The low-speed problem is dius mainly that of so designing the machine that, with all due allowances for engine failure, gusts, baulking andhuman error, and so on, the ideal, still-air performance can be approached as closely as possible. That is, of course, the commonproblem of all high-lift aircraft, but with the jet flap diere exist two novel and potentially powerful possibilities. " (t) There is in principle a chance of making the aircraftunstallable. " (it) Due to the inherent lift-thrust coupling, it should bepossible to control far below the minimum-drag speed. "However, in striving for these characteristics it is found thatnot only the lift but also all its controlling increments must be generated aft of the effective mid-chord point, and this suggeststhat die conventional elevator system be abandoned. So far as the pilot is concerned there could be a perfectly conventional cock-pit layout and the rudder would follow normal practice, but die tailplane would be fixed, while the jet flap would assume thefunctions of the elevators, ailerons, flaps and air brakes. "In practice die flap control might lower the jet through some30 to 40 deg, while at the same time trimming the one-piece tail- plane to an appropriate downwash angle. On top of this, die con-trol column would provide at all times a vernier action of plus and minus a few degrees, the jets on either side of die fuselageacting together as elevators and differentially as ailerons. Finally die jet would be divided into a number of continuous spanwisesections, adjacent elements moving differentially about any com- mon datum for die purpose of generating induced drag independ-ently of die overall lift." Discussing the shrouded jet, the lecturer said that, in practice,with a pure or simple jet flap there were two quite unacceptable snags—-the responsibility for the aircraft control system must behanded at once to the engine designer, while the very survival of the craft clearly depended upon the unfailing operation of itspowerplant. These difficulties could be overcome by the intro- duction of a small hinged flap or jet shroud as illustrated inFig. 22. The term "shroud" arose because the arrangement was originally conceived only for the control of the jet and the reduc- ON October 20th, Mr. I. M. Davidson, B.Sc, A.F.R.Ae.S., of theNational Gas Turbine Establishment, read before the Royal Aero- nautical Society a paper on "Jet-flap Development." Last week wegave a slightly abridged summary of the first part of the paper, and in these pages the second half is given at almost full length, together withthe majority of the illustrations—which, incidentally, are Crown copy- right. The jet-flap principle, first demonstrated by the Ministry ofSupply at the S.B.A.C Show last month, was explained in an article in "Flight" of September 30th. tion of its entrainment drag; but, since then, the appendage hadbeen found to offer so many engineering advantages that its atrophy now seemed unthinkable. "In operation," continued die lecturer, "die jet follows dieshroud by what is commonly termed die Coanda effect and, with a litde care, there is no difficulty in turning any sort of jet dirough90 deg and more. The best shroud size will vary from one design to another, for it depends upon many diverse factors; buttypical values might lie in die range from three to ten per cent of the wing chord, with a bias towards the lower figure. "Since the lift from a small flap varies as the square root of itschordal extent, the effects of die shroud and jet will clearly not be additive. One approximate theoretical result is displayed inFig. 23, which suggests, for example, that while die separate CL values from a five per cent shroud and a jet of Cj value 1.0,may be respectively 1.8 and 5.4, they will yield in combination only 6.1. Thus, while the shroud will be all-important in cruisingflight, it can, with perhaps one exception, be neglected in first order high-lift calculations. The exception is its effect cm thecentre of total lift, and this could be important because it makes possible a reduction of the unusually large tailplane volumecoefficient. "This large tail volume may prove embarrassing, since it implies,of course, extra drag and structure weight. The difficulty arises because, as in Fig. 24, the aircraft centre of gravity will be in thevicinity of its mid-chord point, so that the distance between die aerodynamic centre of the wing and that of the whole aircraft—that is, the neutral point—must have about twice its conventional value. "In high-speed flight die equivalent of elevator control will bea movement of the shroud, so that the controlling lift increment will be generated close to the mid-chord point and the responsewill be a ballooning of the aircraft with the tailplane serving purely as a weathercock to pitch it into the resulting climb. "In low-speed flight widi a high aspect-ratio and the tailplancproperly trimmed, the balance of forces should be as in Fig. 25. Its centre of lift having moved aft, the wing will exhibit at zeroincidence a large nose-down pitching moment about die centre of gravity. This will be balanced partly by the upwash anddownwash couple on the fuselage and partly by under-trimming the tailplane, but also by the whole machine adopting automatic-ally a nose-down equilibrium attitude. With sufficient of diis third ingredient it is clear that the more lift die pilot appliesthe farther does the craft move from its stalling incidence—hence the possibility of an unstallable aircraft." Mr. Davidson went on to remark that, since die jet flappedaerofoil was so insensitive to upper-surface separation effects it might well prove a cure for many of the vices commonly associatedwith sweepback. Hence, in a first look at the three-dimensional picture, it was safe to omit plan-form in favour of aspect ratio. "Two-dimensionally," he continued, "it has been suggested diatthe aircraft should be designed to adopt in low-speed flight a small negative incidence. In practice the argument remains, butthe nose-down attitude will be struck relative to the downwardly induced mainstream rather dian to the flight path. Hence, while RESULT FOR MAXIMUM CONTINUOUS ENGINE RATING. O tOO 2OO 3OO 4OO 5OO AIRSPEED-KNOTS AT SEA LEVEL Fig. 21 (left). Analogous flap-size for a typical airliner. - I • '• • Fig. 22 (below). A practical embodiment. Fig. 23 (right). Typical theoretical result. <^WNG WITH FIXED JET SLOT CONTROLLABLE '% JET DEFLECTING SHROUD. 6 5 4 • * 3 ONE • • INCREASING; •SHROUD SIZE WITH NO JET• V * s£H SHROUD \ THE |f SUM/rt f O-Ab O-48 «M>-CMO«O O52 O-54 O"56 CHORDAL STATION-FROM LEADING EDGE
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