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Aviation History
1952
1952 - 1284.PDF
55« FLIGHT, 9 May 1952 FASTEST and HIGHEST . . . moment of 750 lb in, equivalent to an 80 lb pedalrforce. The lessons of this fifth flight were taken to heart and the sixth launch resulted in the most successful flight of all; it also produced the highest Mach number. In order to escape the high drag met at an immediately transonic speed, the angle of attack in the climb was increased as rapidly as the stall-warning buffet allowed. Once, when the warning boundary was entered too deeply, the starboard wing dipped in a partial stall. Buffet limitations prohibited a climb being made at the ideal Mach number of 0.8. A little oscillation occurred at the top of the climb; the pushover was entered gently at o.8g, which was then reduced to o.6g without ill effect. A pronounced left-wing heaviness then made a return to o.8g necessary, but lateral control returned and the aircraft was then left at o.6g until the "all-burnt" condition was reached. The actual figures obtained on this flight have not yet been released. Bridgeman says he was never able to make a climb that satisfied the aerodynamicist. The Mach number always crept ahead of him by 0.05, so that he reached sonic speed some 8,000ft too low. Every effort was always made to keep the climb sufficiently steep to hold the Mach number at about 0.85 as long as possible. At the beginning of the climb, with a wing loading of some 100 lb/sq ft, difficulty was experienced in pulling up to a steep attitude suffi ciently quickly without inducing a high-speed stall. The lag of 10 seconds before obtaining full thrust from all four rocket-barrels aggravated this, and the limiting load was found to be about i.6g. The best stall-warning was found to be the vibration of the long needle-nose boom, which was all that could be seen of the airframe by the pilot; Bridgeman says he had calibrated it as a stall-indicator. The climb was further complicated by, in the first place, the phenomenon of a rapidly rising Mach number and a falling I.A.S., and secondly, by the rapid fuel consumption. This latter caused great variation in wing loading and allowable angle of attack, while the I.A.S. eventually became so low that the aircraft had to be permitted to exceed Mach 1 while still in the climb. At this point, the elevators, being conventional surfaces, ceased to be effective; further longitudinal control was maintained by the "all-flying" tail, this being used to control the pushover. The speed-run after the pushover did not pose any great longitudinal problem. WORK OF REFERENCE to the work of the Aerodynamics Division of the National Physical Laboratory is made in the annual report' of the Department of Scientific and Industrial Research. It is stated that the new i8in x 14m subsonic and supersonic tunnel has been installed and has been found to operate satis factorily at atmospheric pressure. It will later be run at pressures up to three atmospheres and will provide facilities for high-speed research under conditions more nearly corresponding to full scale than those possible with the earlier equipment. In pointing out that the damping of the pitching motion of high-speed aircraft is of vital importance in aerodynamic design, the report says that experimental and theoretical research on oscillating wings in high-speed flow have been continued inten sively, and "important progress" made as a result of this work. Additional facilities have been installed for the study of the interaction of shock waves and the viscous layers near solid surfaces in high-speed flow. Experiments on the maintenance of non-turbulent flow over wing surfaces have also been continued as part of a concerted plan—including flight research elsewhere— to achieve some of the large economic gain theoretically possible. In a section dealing with the properties of materials at high temperatures, the report refers to the continued studies of the stress-short-time plastic strain relations and stress-creep relations under conditions of combined stress, attention being particularly directed to investigation of the effects of anisotropy produced by pre-straining in tension or torsion. An associated investigation concerned with the fracture under combined stress-creep conditions of a 0.5 per cent molybdenum steel at 550 deg C has shown that fracture conforms with a criterion of maximum principal stress. It is stated that work on the temperature dependence of creep for an aluminium alloy has reached the report stage, and the time dependence of creep is being currently studied. Further tests to obtain information on the behaviour of eccentrically loaded light- alloy struts have been made. Noting that theoretical and experimental studies of creep due to fluctuating stresses are proceeding, the report mentions that *H.M. Stationery Office, price js. 6d. (7s. lid. by post). The necessity of making a turn through 180 deg immediately the "all-burnt" condition was reached meant that the turn would be entered at the highest Mach number reached. Bridgeman always found these turns difficult and educational. He found that entering a vertical bank and applying full up-elevator did not alter the course of the aircraft by as much as a degree; on the other hand, application of 3 or 4 deg (or, on occasions, all that was available) of "all-flying" tail movement could bring the load factor to about 4. This control via the tailplane made it possible to enter a diving turn and return to base. This turn was made more difficult if rapid lateral oscillation was present, as previously described. An undesirable feature was the need for returning the tailplane to neutral soon after the turn had been started; this was due to the sluggishness of the electric actuator at the extreme air-loads encountered by the tailplane. This made it essential to relieve the load factor almost as soon as it began to take effect in order to prevent a stall. Bridgeman has mentioned an occasion when the angle of attack reached 18 deg in the early part of one of these turns; although the Mach number was greatly above unity, the I.A.S. was low. Before the trim could be centralized, violent pitching occurred and nearly produced a complete stall. The effect was aggravated on this occasion by the fact that the speed dropped to a subsonic value as the turn progressed; this caused a shift of centre of pressure by about 10 or 15 per cent forwards, imposing additional loading by trying to tighten the turn. Only a few air launches with the all-rocket aircraft have so far been announced. Bridgeman stresses the fact that a research programme must be a co-ordinated whole allowed to grow only in easy stages. He feels there is much untapped potential left in the Skyrocket and hopes to reach still greater speeds and altitudes. He makes a plea for a "genuine" supersonic aircraft capable of keeping above Mach r for a time measured in minutes, not seconds. It must, he says, have a true supersonic wing section and be equipped with adequate irreversible controls—and, per haps, a yaw damper. An efficient air-conditioning system is a further requirement, while he hopes that much attention will be paid to the problem of aerodynamic heating at high speeds. Evidence that this plea will not go unheeded is provided by the forthcoming Douglas X-3. This perhaps most advanced of all aircraft is the outcome of many design-studies with turbojet, rocket and ramjet propulsion, used singly or in combination. It may fly this year, and will present Bridgeman with an even more "man-size" job than has been described in this article. W.T.G. THE N.P.L. examination of the speed effect in fatigue between one cycle and 8,000 cycles per minute for a light alloy and a mild steel at various temperatures has shown the effect to be important. Interesting correlations of results in bending and direct stress fatigue have been obtained. It is stated the considerable creep and fatigue work has been undertaken in connection with material for use in gas turbines. Dealing with stress analysis investigations undertaken by the Engineering Division, the report mentions that an analysis of the elasticity of fibre matrices, originally made in 1942-1943, has been revised for publication. It relates to fibre-filled synthetic resin bonded materials, and is concerned in particular with the effect of varying the form and orientation of the fibre filling on the elastic properties of the materials. The work is stated to have a bearing on the design of plastic aircraft structures and its publica tion was requested by those engaged in this development. There is also a reference to the initiation and propagation of fatigue cracks in metals. Experiments were made with test pieces of mild steel subjected to combinations of alternating bending and torsional stresses in such a manner that the regions of high stress due to bending and those due to torsion were partially separated. The results indicated that fatigue cracks may develop either along planes of maximum shear or across planes of maximum tension, and that each type of crack tends to propagate in its own mode. This technique of testing will now be applied to other materials. It is mentioned that a second fatigue-testing machine of 1,500 lb capacity, operating on the slipping-clutch principle, has been constructed. A summary of work done by the Metallurgy Division shows that researches of a theoretical type have been undertaken on the constitution of uranium alloys for the Directorate of Atomic Energy, and on the constitution of magnesium alloys for the Ministry of Supply. Work on the high temperature properties of magnesium alloys used in aircraft engines has also been undertaken. The Physics Division reports that considerable attention has been given to the measurement of noise from aircraft, both on the ground and in the aircraft itself, and that a portable sound-level meter and octave-band analyser for measurements in aircraft have been designed and constructed.
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