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Aviation History
1950
1950 - 2073.PDF
FLIGHT, 7 December 1950 513 times deciding factor in the ultimate limit of swept-wing aircraft. In some cases it had been found possible to raise the downgoing wing by application of rudder towards that wing: the reverse of normal corrective action. Though it could be understood, the effect of yawing or unyawing swept wings at high Mach Numbers remained a new reaction for the pilot to master. Lateral control was bound to be more satisfactory and development to this end was in progress. Very large ailerons were an obvious solution. The other symptoms of compressibility mentioned were, individually, little more than a distraction at high altitude, where there was a small element of risk produced mainly by the nose-down trim-change. At low altitudes, where control-angles assumed the significance formerly held by stick-forces on manually controlled aircraft. Another leading requirement at present was for contin- uous transmission of all instrument-readings. The auto- matic observer was no longer sufficient. Continuous speech-recording was a step in the right direction, but until a telemetering system had been perfected the lesponsi- bility for missing nothing rested with the pilot. Present and future requirements of high-speed research aircraft: It was essential to have sufficient thrust to negotiate the transonic region in a climb; thus the first requirement was for a rocket motor, as there seemed no immediate hope of developing gas-turbine thrust to the extent required. The author's experience of transonic flight was built up largely in testing the third D.H.I08, the only British aircraft to have exceeded Mach I. misbehaviour stemmed from a combination of high dynamic pressure and compressibility, the most unacceptable form of vice was flutter, which the author took to embrace any undamped oscillation of control or aircraft with a frequency greater than V cycle per second. A good illustration of a pitching oscillation was provided by the 108, in which damping was reduced by increased Mach Number and I.A.S.: on one occasion, despite careful, gradual increases, an undamped oscillation built up to + 4 -3g at the rate of 3 eye/sec. The final loss in damp- ing from neutral to seriously negative occurred over a Mach range of 0.005—a variation which could occur through gusts, apart from the difficulty of maintaining Mach Number to such extreme accuracy. Controlled investigation was con- tinued safely over the same Mach range [M=0.87/0.875] at lower I.A.S. Only one form of vice was likely to cause structural failure —extremely violent flutter. Failures at high speed from other causes were, almost without exception, due to design failure or overstressing by the pilot—and not of any mysterious phenomenon hovering like a spectre at the " barrier." A very large proportion of the risk and discom- fort could be avoided by patient and painstaking build-up in the Mach/I.A.S. combination after thorough investigation of the Mach scale at maximum altitude. The approach to high-speed investigation and means of obtaining test results.—Techniques developed over the past few years would, the author hoped, bear little relation to those of the near future. On the Spitfire and, to a lesser extent, the early jets, very uncomfortable dives were neces- sary to obtain Mach Numbers of up to 0.88. The arrival of the swept wing at once put us in the M = 0.86/9 bracket in level flight. This seemed to be the answer but, of course, was not. The real value of the swept-wing aircraft was its ability to explore Mach regions of up to 1. Thus we found ourselves in a dive again. It was possibly vital to know the dive-angle required to obtain a range of speeds at full throttle, and dive-angles were always over-estimated by pilots, particularly when they were seated in the nose of their aircraft. The Desyn, an instrument previously familiar only to the aerodynamacist, was becoming an important source of information to the pilot, owing to the critical nature of control-angle variations. Powered controls, with their artificial feel, strengthened the case for the Desvn as Despite its short endurance, the rocket-powered Bell X-l had made a steady and complete test at transonic and supersonic speeds on almost every flight. Larger air- brakes would be needed with increase in thrust. Landing- speeds should be allowed to rise (S/L. Derry quoted 200 m.p.h. as an acceptable speed—given suitable runways and advanced methods of deceleration). Further requirements were power-boosted controls and movable tailplanes, the success of which had already been demonstrated : no aircraft without a trimming tailplane had exceeded Mach = l in full control; the D.H.108 could not have been used for investiga- tion beyond Mach = 0.88 without its powered elevons. The unfortunate decision to delay construction of a British supersonic piloted aircraft had arisen, said the author, from an incorrect appreciation of what the pilot could cope with in an unconventional aircraft, particularly on take-off and landing and had resulted in the type of test-work described in his paper. Ironically, the risk involved had been far higher than would otherwise have been so; reports of flights with the Bell X-l had left no doubts on this score. Concerning tailless aircraft, S/L. Derry said that pilots' impressions had, in general, been unfavourable. But, on the credit side, the D.H.108, which first flew in 1946, had had a level-speed performance equal to or greater than any other aircraft flying in this country—on 30 per cent less power. It was still the only British aircraft to have exceeded M = l, although it was only in full control up to M=0.98. In view of the aerodynamic and structural advantages of the tailless type, it was to be hoped that the delta-wing research aircraft now flying would give a better impression than the older configuration. Unfavourable impressions should not be too readily accepted at an early stage: in approaching supersonic flight, pilots would have to accept new standards. Some were already accepted. Pilot-safety measures.—The improvement in maximum lift with increased Mach Number allowed "g-stalling" above 30,000ft equal to or in excess of " blacking-out g." It was essential to provide g-suits for research pilots and, in the near future, for fighter-pilots also. The prospect of rocket-powered, vertical-climbing aircraft brought a need to seek a remedy for the effects of negative g—which was more uncomfortable and demoralizing than positive g. Tight, well-fitting harness was vital. The problem of low atmo- spheric pressure could, if respected, be safelv negotiated.
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