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Aviation History
1958
1958 - 0142.PDF
A curve of pitch-damping, plotted from flight tests with the Avro 707'A and 707B. Damping remains positive at all speeds. THE VULCAN STORY... provided by a single Rolls-Royce Derwent fed from a dorsalintake between the canopy and the fin. Unlike the bomber the controls were manually operated, with inset hinges, and thefuselage was proportionately oversize. The 707 bore the serial number VX 784 and it was taken intothe air for the first time by F/L. S. E. Esler on September 4, 1949 (and thus was the second delta-wing aircraft to fly in theworld). During the following week the aircraft flew about three hours and the handling characteristics were proved to be quitesatisfactory—in fact little different from those of conventional aircraft. It was exceedingly unfortunate that the aircraft crashedshortly afterwards, killing Eric Esler, but the cause of the acci- dent was in no way traceable to the delta configuration. It was at once decided to construct two further 707s. The firstto be completed was the 707B (VX 790) which was flown by W/G R. J. "Roly" Falk, Avro's superintendent of flying untilthe beginning of this year, on September 6, 1950. This aircraft likewise retained the original design of wing, with a constant t/cratio of 10 per cent. It was, however, much modified in detail, having an ejection seat, elevator balance and modified airbrakes;later, as a result of flight tests, the intake was preceded by an N.A.C.A. divergent ramp to give improved pressure recovery.The 707B flew many hundreds of hours and has been responsible for a great deal of valuable research. With it Avro establisheda self-contained team at Boscombe Down to handle the delta research flying. VX 790 is still on trie inventory of the R.A.E.at Bedford, and was in use as a research machine until 1956. Chronologically later came the so-called "high-speed" machinedesignated 707A (WD 280). This aircraft was the first to be fitted with the modified wing section which had previously beenfound to be the optimum, the engine being fed through wing- root intakes and the peak-thickness line being swept forwardat the roots. Another major improvement was the incorporation of spring-tab controls with internal sealed balances. W/C. Falkflew WD 280 on July 14, 1951, and thereafter hundreds of hours of intensive testing laid the bogey of pitch-damping once andfor all. The aircraft was retro-fitted with fully powered irrever- sible controls on the elevators and ailerons and has now com-pleted well over 500 research flights. The aircraft is still actively flying, in Australia. So valuable was the 707 A that Avro manufactured two more707s with virtually identical wings and control systems. The first was WZ 736, which was almost a straight copy of WD 280. Thisaircraft was built for the R.A.E., who employed it in the develop- -6 -5 -4 -3 -2 -1 m POWE POWER OFF: (BOTH JET PIPES! R ON: ORIGINAL JET MODIFIED . ^ PIPE O-l 0-2 LIFT O-3 COEFFICIENT O-4 As described in the text, the jet-pipe nozzles of the Vulcan were turned slightly to minimize trim changes due to variation in engine power. The curves above were originally plotted from 707 flying. On the right are comparative curves, actually for the 7078, indicating the reduction in nosewheel-raising speed resulting from a lengthened nose leg. 146 FLIGHT, 31 January 1958 ment of an automatic throttle system. The second was a side-by-side two-seater designated 707C (WZ 744), which was used by the A. and A.E.E. at Boscombe Down as a general appreciationand pilot-familiarization aircraft. The 707C has also been used for comparative gun-aiming trials and is now engaged in thedevelopment of a new control system for the I.A.P. department of the R.A.E. Between 1949 and the first flight of the prototype 698 inAugust 1952 the experimental data obtained with the 707s indicated that the delta configuration was perfectly satisfactoryunder all flight conditions; in fact, the 707 has been described as "considerably more docile than current swept-wing aircraft ofmedium aspect ratio." An accompanying set of curves show that the damping ofshort-period pitching oscillations is always positive. Admittedly the damping falls to a minimum at a Mach number slightly abovethe design cruising value for the 698, but at no point does it become unsatisfactory and it increases again as sonic velocity isreached. On the 707B an undamped pitching oscillation which did manifest itself was traced to an out-of-phase elevator motionjust sufficient to produce a destablizing pitching moment. This undesirable phenomenon was almost eliminated by the spring-tab controls of the faster 707 A, and the irreversible controls of the 698 obviated its occurrence on that aircraft. Typical of the type of characteristic uncovered by the 707aircraft which might otherwise have remained ui.deiected was the precise relationship between wing incidence and the speedat which the nosewheel could be raised from the ground during the take-off run. As originally built, the 707B had to reach aspeed almost equal to the take-off value before the nosewheel could be lifted clear of the runway, which then caused the air-craft to jump off the ground abruptly. This shortcoming was alleviated by increasing the wing incidence during the groundrun by lengthening the nose leg, whereupon it became possible to lift the nose at less than 80 per cent of the unstick speed.Had the 707B not been built, a major modification to the 698 would undoubtedly have been necessary. Another important effect investigated with the 707B wasthat, by suitably adjusting the line of action of the propelling nozzle, variation of longitudinal stability and trim with enginethrust could be virtually eliminated. By this time the 698 had in- takes parallel with the leading edge (as shown on p. 143, the intakewas originally well forward) yet even this aft intake location was appreciably ahead of the e.g. and the vertical component of theintake momentum drag produced a considerable nose-up change in trim. Accordingly the inboard jet nozzles were angled down-wards through between 4 and 5 deg until change in power had almost no effect on trim. At the same time the outboard nozzleswere turned through the same angle and also rotated, so that the thrust lines of the outer engines passed no further from the e.g.than those of the inners. By this means a worthwhile reduction in engine-out speed was achieved, the yawing moment of an outerengine being made the same as that of an inner. This in turn permitted a reduction in the size of the vertical tail. One of the most remarkable characteristics of the subsonicdelta wing developed for the 698 was found to be that, when its angle of attack was progressively increased, no sudden loss of liftoccurred, as is suffered at the stalling point of a conventional -3O GROUND ATTITUDE 6 deg 9O IOO HO TA.S.(kt) 12O I3O
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