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Aviation History
1953
1953 - 0992.PDF
146 FLIGHT SUPERSONIC-AIRCRAFT DESIGN A Hawker Contributibn to the A.F.I.T.A. Congress IN our issues of July 3rd and 17th, summaries were given of three British papers presented at the International Aero nautical Congress organized by FAssociation Francaise des Ingenieurs et Techniciens de l'Aeronautique (A.F.I.T.A.) on the occasion of the Paris Aero Show. Three further papers were read by British representatives. Of these, New Materials and Methods for Aircraft Construction, by H. J. Pollard, Wh.Ex., F.R.Ae.S., was originally given before the Royal Aeronautical Society in February last and reported in Flight of March 13th, and Integral Construction—its Application to Aircraft Design and its Effect on Production Methods, by E. D. Keen, B.Sc. (Eng.), F.R.Ae.S., was also given (as a section lecture) before the Society in February, and was reprinted in the Society's Journal of April, 1953. We give here in abridged form the final paper, Some Problems in the Design of High-Speed Aircraft and Theoretical Methods of Solution, by Mr. B. A. Hunn, senior mathematician in the design department of Hawker Aircraft, Ltd. The final part of Mr. Hunn's paper, The Development of a Design, is given in full. By reason of the increasing rapidity with which resources of power were becoming available, Mr. Hunn began, man had advanced at an increasing rate into the realms of high speed hitherto unknown. In the aircraft industry, new frontiers were approached long before previous advances had been consolidated, and so, unfortunately, many problems of high-speed flight had necessarily to be tackled long before they were sufficiemly understood. It was the lecturer's intention to discuss some typical problems, to indicate theoretical methods of attack, and to evolve a design on the basis of the emerging integrated theoretical knowledge. In any industry, progress was based upon experience, which was in fact synonymous with theory. With increasing complication of the product, adequate interpretation of new effects became more difficult, while the implication of misapplied experience was economic disaster coupled, in the aircraft industry, with possible loss of life. A systematic approach involving the scientific collection of all relevant data on past designs, and the use of mathematical method in the development of theory on the basis of such data, was the ideal to be aimed at. The general principles of design were then outlined by the speaker. Giving an example of their application, he postulated a wing planform design requiring straight leading and trailing edges, streamwise tips, and a given profile. There then existed four freedoms left to the designer; the span, aspect ratio, taper ratio and leading edge sweep- back, which together were sufficient to define uniquely the planform. An aerodynamicist might be able to give functional relationships be tween the various aerodynamic derivatives, such as the lift-curve slope and aerodynamic centre, and these four quantities. If, therefore, three of these derivatives were required to have certain values, in order to obtain a given performance, the four freedoms reduced to one, and an indefinite number of designs was possible. If four derivatives were specified, the planform would be unique. The task of the theorist was to determine the quantitative dependence of the machine's performance upon the set of independent properties inherent in the design. This was not easy, for performance depended upon almost every such property, such as weight, centre of gravity position, moments of inertia, taper ratio of wings, fuselage geometry, structural stiffness and many more. It was essential that the number of these qualities should be minimized consistent with a unique des cription, and that the design specification should demand only the irreducible minimum of independent performance qualities. Turning to some of the problems of high-speed flight and their methods of solution, the lecturer dealt in turn with drag, the influence of structural distortion on the aerodynamic efficiency of the main surfaces, chordwise pressure-distribution changes in the transonic region, flutter, control reversal, viscous heating effects at supersonic speeds, and high local stresses at material junctions due to large ambient temperature changes. After discussing each problem and its solution in detail, summaries of the respective main points were given. From considerations of drag at supersonic speeds, and in particular wave drag, Mr. Hunn claimed, desirable features were a small frontal area, a small thickness/ chord ratio, a small weight /span ratio, the highest possible sweep- back and the smallest possible planform area. Clearly, some of these requirements were contradictory, and a compromise must be sought. The method of reducing distortion by increasing structural stiffness on any given planform was ultimately useless, the lecturer continued. Some planform requirements could, therefore, be stipulated as the main alternatives. These were either (i) wings possessing large volumetric content, so that an equitable spanwise and chordwise distribution of weight could be achieved, or (ii) flat, straight wings with small chords, but able to possess high torsional stiffness, or (iii) swept wings of small plan, again with small chords and able to possess high flexural and torsional stiffness. The problem of balancing tail loads arose because of considerable shifts in the mainplane aerodynamic centre when the aircraft was passing through the transonic region. For the minimum change in tail trim load (as a relative figure) the wing aerodynamic centre should move as little as possible in passing from subsonic to supersonic flow. In order that the loss of lifting efficiency of the tailplane might be minimized, the tail moment arm should be as short as possible, and the tail area as small as possible compatible with adequate control. Finally, in order to facilitate the reduction of tail area, while still permitting adequate control, the tailplane itself should be an all- moving control surface with possibly the conventional elevator used as a trimming device. The phenomenon of flutter could be experienced on both main surfaces and control surfaces. In order to prevent main surface flutter, the mass distribution should be evenly disposed over the plan- form area of the aircraft, and the mean chord should be as large as possible compatible with small torsion loads at the wing roots in pitching oscillations. In addition, the fundamental and torsional fre quencies of vibration should be as high as possible, which demanded high flexural and torsional stiffness of the wing, and small span. Control surface flutter could be prevented in three ways: (a) the positive dynamic coupling between motions of the main and control surfaces should be reduced to a minimum, either by adequate mass balancing of conventional systems, or by the use of systems which did not exhibit such coupling; (b) alternatively, where hinged surfaces were used, irreversible control circuits should be aimed at; and (c) where coupling could exist, the main structure should be designed so that the modes of vibration in which the relevant surfaces exhibited the major motion possessed the highest possible frequency. The necessary requirements to avoid—or at least postpone—control reversal were that the supplementary loading on the aircraft structure due to the control surface should be sq arranged as to cause the minimum pitching moment on the main surfaces, and that the torsional stiffness of the main surface at the control-surface points of attach ment should be as high as possible. THE DEVELOPMENT OF A DESIGN Having indicated some of the most important requirements to be met in any high-speed aircraft design, it was now possible to develop one such design, in this instance by verbal compromise, although it was not beyond the bounds of possibility to do so entirely on a mathematical basis, if suitable importance were attached to each of the previously mentioned factors. The most important of all these factors was undeniably the drag/thrust ratio. It must, of course, be presumed that the thrust was prescribed, at least for any given power unit, and so in order that this ratio should be as small as possible, the emphasis must be placed on drag reduction. There was one fundamental question, however, which required immediate attention, and that was whether in a high-speed aircraft several small power units were better than one large one. The question of engine size and efficiency was outside the scope of this paper. The minimum cross-sectional area which the embryonic airframe could present to the airstream was that of the power unit. Also, the power units had usually the same order of weight as the bare airframe, and if only one was used (occupying inevitably a central position) the idea of mass distributed equitably over the span became virtually impossible. Having to accept the frontal area of the power unit as prescribed, the question of mass distribution took pre-eminence over the drag and, in order to fulfil this requirement, it followed that several units distributed spanwise were better from the loading point of view than a large one centrally placed. This argument could not be pursued indefinitely, and in this instance the number of power units would be restricted to two. Their disposition in relation to the rest of the aircraft was a problem which needed to be regarded from four points of view. The drag, of course, must take pride of place, then flutter, with accessibility and structural design coming third, and distribution of mass loading last. u a- §6n' UJ ii~40 UJ rO 15 20 25 MACH NUMBER 30 IB 22 MACH NUMBER Fig. 1. (Left) The lifting efficiency of a typical swept wing (aspect ratio 3, sweep-bock 15 deg), plotted against Mach number, at sea level. Fig. 2. (Right) That of a rigid tail- plane mounted on a typical flexible fuselage structure, showing the im provement in aerodynamic efficiency obtained by shortening the tail moment arm.
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