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Aviation History
1957
1957 - 1839.PDF
FLIGHT, 13 December 1957 929 SUPERSONIC FLIGHT TESTING Techniques and Procedures Described by English Electric s Chief Test Pilot The author in the cockpit of the first prototype P.IB. AS briefly recorded in our issue of November 29, a remarkably informa-tive lecture was given on November 27 to the Graduates and Students Section of the Royal Aeronautical Society by W/C. R. P. Beamont,manager of flight operations and chief test pilot, English Electric Co., Ltd. Below is given a more extensive version of the lecture. FOR many years [said W/C. Beamont] we in this countrylagged behind in experience of supersonic flight. True super-sonic flight began in Britain in 1954, when the P.I prototype exceeded Mach 1 in level flight in August, and since then morethan 700 flights have been made by P.Is, mostly at supersonic speed. The Fairey Deltas, which first exceeded Mach 1 in levelflight in 1955, have also completed some hundreds of flights, including the establishing of their magnificent world speed recordunder F.A.I, rules in 1956, which remains unbroken today. The P.I programme—which has completely covered stability,performance, and operational fighter handling up to maximum design speed—has, together with the Fairey Delta programme, putour practical experience and knowledge of this subject on a par with that of the other leading world authorities.Broadly* the supersonic aeroplane is just another aircraft from the test panning aspect and must be put through the same logicalsequence of tests to prove firstly its flight safety and secondly its ability to achieve its design mission; so the prototype programmeis made up as follows : — Handling checks:—(a) Systems functioning. (b) Taxying for ground controllability, brake operation and energyabsorption, ground-roll measurement, and tail-parachute functioning. (c) First and initial flights for handling checks of functioning andresponse of controls, stability on all axes, trimmer ranges and effective- ness. Engine suitability and response to controls. Stall approaches andassessment of landing technique. Plus auto-observer recording of engine bay and jet-pipe-cooling suitability, electrical power supplyconditions and any other critical systems items on which initial ground running has focussed attention. In other words, it is just another aeroplane, and the tests con-tinue within the initially set flight limitations until snags are met or until these limits are reached. The initial flight limits are set to ensure that there is no possi-bility of entering into a critical "flutter" region inadvertently, but as with aircraft of this performance a relatively low indicated air-speed of 400-500 knots can still allow a high Mach number to be reached at altitude, if (as can be the case) there is no Mach flutterlimit, this does not necessarily hinder early handling investigation at high Mach numbers. This was the case with the first prototypeP.1A, which was handled supersonically with ease and comfort on its third flight, and also with the first P. IB, which reachedM=1.2 on its first flight. During this phase there is likely to be some preoccupation withminor systems defects in radio, warning systems, etc. By Ministry edict, these aeroplanes are now fitted with the most impressiverows of warning lights, doll's-eye indicators and flashing "attention getters." These are excellent devices, for once the pilot has learnedto control his initial impulse to climb over the side on seeing the flashing reds he is presented with detailed information on whichof the complex systems is malfunctioning. In the early stages, of course, it is not uncommon for a pilot to absorb the shock of thered flashes only to receive another on finding the cause to be an ominous fire warning. He then has an interesting thought processwhich involves deciding whether he has in fact a fire, or a faulty warning system—if the latter of course, he can get home; if theformer, at which stage in the return will he find out definitely one way or the other whether it is a serious fire? On occasions thisstate of uncertainty can continue until the aircraft is safely on the ground, in which case the pilot will then be quickly cheered up bythe design gentleman, who is almost sure to say to him during de-briefing: "Well, what are you worrying about? It wasn'tactually on fire." However, once these complex warning systems are properly Right" photograph developed they are comforting aids to confident operation of whatare, after all, quite complex machines. But there is a case in point here which is worth consideration. In the event of many of thesystems in these aircraft giving a failure warning, the only action and decision that can be taken is by the pilot; but in the event ofan indicated fire warning, it is almost always possible to detect the presence of a true fire visually by the trail of smoke or fuelif this can be seen. Almost invariably it cannot be seen from the cockpit, and for this reason as much as for any other the U.S.A.F.find it necessary to maintain units of squadron strength manned by experienced pilots and equipped with die latest high-speed jetaircraft, purely for the purpose of flying "chase" on experimental aircraft on test. This practice is not followed as a definite policyin this country, and it still continues to prove difficult to obtain adequate chase facilities to cover the full range of early experi-mental testing on high-speed prototypes. That airborne observation of early prototype test flying is essen-tial is borne out by almost every new aircraft that flies. In the case of the P.I A, undercarriage sequencing trouble was traced bythis method, and a different type of undercarriage trouble experi- enced on the P. IB was also filmed from another aircraft and thecause traced down from this evidence. In this country, however, we normally have to experience trouble first before we can obtaina chase aircraft to observe a subsequent flight for the purpose of reproducing the defect. This is wasteful, and also fails to coverthe most important aspect of chase flying, namely, that of pro- viding the test pilot with immediate confirmatory advice whenthings go wrong in the first instance. It is obviously not possible to obtain chase aircraft with adequateperformance to cover all aspects of new type testing, but the American principle of employing the fastest aeroplane availablein a proved and reliable state for the job is, I am sure, the right one. Once the initial handling is complete, and a satisfactory standardof reliability has been obtained in all the aircraft services, and the standards of control response and stability have reached anadequate stage, attention is turned to raising the flutter limits in order to push out the proved performance envelope to the designlimits under all conditions. Until recent years it was normal practice to increase indicatedspeed gradually and to excite structural oscillation by stick-jerking or flying through turbulent air in order to attempt to excite anddetect the early stages of flutter. Many incidents have occurred in which unpleasant flutter conditions have been experienced and,of course, in not a few cases loss of the aircraft has resulted. Various means are now employed of exciting structures, in knownareas of possible trouble, by various mechanical means. The method employed for flutter clearance of the P.I series has beenby the R.A.E.-developed "bonking" method. By this means small explosive charges are fired, generally at 90 deg to the surface ofthe structure concerned, and impart a jolt to that structure. The damping of the resulting oscillation is then measured by recordingequipment, and subsequent study can determine if the order of damping corresponds to die calculated value. Theoretically this method should give a margin of safeguardagainst inadvertent experience of flight flutter, but although it has contributed very considerably to the rate of progress at whichflutter testing has been completed on the P.Is and to the store of knowledge now available on the subject, it did not in our caseprevent running into fin-rudder flutter precisely between one scheduled stage of the investigation and the next. This particularflutter condition was corrected by altering the fin stiffness, and as an added safeguard fitting as standard a viscous damper to dierudder. These mediods have been most satisfactory and complete flutter clearance has been obtained throughout the, even by modernstandards, impressively high design limits of the aircraft. While flutter-clearance work is progressing, every opportunityis taken to fill in every moment of each flight not associated with the actual flutter test point with the other essential investigations,such as engine handling and suitability, single-engine handling,,
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