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Aviation History
1961
1961 - 0520.PDF
528 FLIGHT, 20 April THE LONG VIEW... would be no need to spend many expensive hours training pilots tomaster supersonic flight at 70,000ft. It might be hard to convince pilots that they could undertake without doubt or mental uneasi-ness a completely blind landing with fully automatic equipment. And there would be a particular reluctance among pilots, in thespeaker's opinion, if automatic landing systems were not absolutely reliable when first introduced into operational service. Anyunserviceability restrictions or reservations placed on the equipment in the early stages of pilot indoctrination would be disastrous; theywould not only disturb public confidence in the vehicle, but would force the airlines into the unenviable position of trying to operatehighly expensive equipment by means of old operational techniques and philosophies. Such a condition invariably led to accidents. In closing, the Qantas representative noted an operational re-quirement which was considered essential for completely safe operation of modern aircraft. There was, he said, an urgent needfor a take-off monitor for all large transport aircraft, particularly the jets. The take-off monitor was a "must" for the next generationof aircraft ""and it is to be hoped that the reliability of jet engines stands us in good stead until such time as this equipment is developedand fitted. It is inconceivable that we should press on with the development of a highly sophisticated vehicle such as the supersonicaircraft unless we can solve the two fundamental problems of ensuring maximum safety during the critical stages of take-off andlanding." Big-jet Handling One of the most interesting technical papers yet presented on thesubject of supersonic transports—and one which gave keen insight into the problems of piloting the big American jets—was deliveredby three men who must be amongst the world's most experienced jet transport pilots. The work, entitled "The Supersonic Transport:Pilot Views on Operational Goals, Handling Characteristics, and Related Subjects,'" was given by Capt W. W. Moss, one ofPanAm's most qualified pilots, J. R. Gannett, chief engineering pilot at Boeing, and P. H. Patten, an engineering test pilot atDouglas. As the authors of the paper saw it, "the ultimate goal that we are all striving for is an aeroplane that can carry passengersfrom point A to point B with the ultimate of safety, reliability, punctuality, comfort, and economy at supersonic speeds." Thehandling characteristics of supersonic types was of vital importance and the purpose of the subject paper was to indicate some problemareas where attention to the piloting aspects could go a long way towards attaining the above simple objectives. Contrary to theideas expressed by Mr Howson, it was the authors' contention that fully automatic control for a whole flight was at best years away—and was not really necessary, anyway. Human control of super- sonic types should be perfectly feasible, since all flight below 35,OOOftwould be accomplished subsonically, because of noise difficulties. The authors warned, however, that although present stability andcontrol criteria appeared to be valid "as far as they went," there were specific areas where one should endeavour to define additionalsuitable design objectives. "Modern swept-wing jet aircraft have a tendency to exhibit poorlow-speed lateral-directional handling qualities," continued the paper. Because of these characteristics, oscillations were frequentlyencountered during manoeuvres, making the pilot's job more difficult. In answer to the question of what should be optimum lateral-directional control characteristics for supersonic aircraft, the following recommendations could be made: ""Aileron control dis-placement and force should command roll-rate primarily. Elevator control displacement and force should define pitch-rate or anincrement of airplane speed change from the original trim point. Rudder control displacement should define yaw rate or sideslipangle. Any noticeable amount of lag or overshoot after a control input in roll-rate, pitch-rate or yaw-rate detracts from good con-trollability and is very undesirable. The cross-coupling associated with the Dutch-roll mode is an excellent example of undesirablemotion in any airplane.'' Pilots were, in the authors' opinion, "rather simple human beings" in that when they made a simplecontrol motion they were most happy when the response was singular and proportional to the input. Control effects that causedlag, overshoot and cross-coupling were partially responsible for the problems frequently encountered by pilots of today's jet aircraft."A moderately damped Dutch roll in the take-off, approach, or landing configuration is tolerable, but undesirable." The authors observed that it was possible for a pilot to sit in asimulator and indicate his reaction to certain sets of stability derivatives and comment on their desirability; but it was quiteanother matter to witness the instinctive reactions of a pilot with 20 to 25 years of airline experience when he was making an instru-ment approach in rough air and had to land successfully and smoothly on a 150ft-wide, wet, slippery runway. "Under thesemoments of stress, the pilot reverts to instinctive and conventional control handling and the airplane must respond in an acceptablemanner." It was important that the designer should not settle for minim urnacceptable handling qualities. It went without saying that linearity of response was important. Further, it was imperative that the pilot/aeroplane loop be made extremely tight, with no noticeable amount of lag or spurious aircraft motion introduced as a result of a con-trol input. "With present jet transports, the engine locations on the wings, and the yaw-roll characteristic, introduce a situationwhich occasionally can be somewhat bothersome, particularly during an instrument approach. It is possible when all four enginesare advanced simultaneously that one will put out more thrust than another, due to individual engine response and trim character-istics. Consequently a yaw motion was introduced, followed by the associated roll motion due to yaw, which was mentioned previouslyas one of the undesirable cross-coupled effects." The condition was prevalent and especially undesirable duringILS approaches, when the pilot's attention was largely given to fiighi instruments and where thrust adjustments were usually made cm a""feel"' basis rather than by reference to engine thrust indicators. The result was that pilots in some cases never really had theopportunity to settle down directionally and had to make small bank angle and directional corrections which would not otherwisebe necessary. The need for decoupling yaw from roll was quite evident. The three authors next turned their attention to rudder require-ments. The use of a rudder on today's jet aircraft was largely con- fined to three situations: (1) keeping the aircraft straight on theground during take-off and landing; (2) kicking-off the drift during the flare for a crosswind landing; and (3) controlling the yaw due toasymmetric thrust (in the engine-out case). "The need for this precise directional control during landing and take-off has becomemore apparent recently because of the shortcomings of differential braking on wet runways and the poor effectiveness of differentialreversing." To the authors' way of thinking there was a tendency for thedesign engineer to satisfy the static control requirements without providing two very important additional features. One was to havesufficient excess control left over to satisfy the dynamic requirements which were always present in line operations, and the other was tohave an aircraft response-rate which was capable of providing precise control. "The rudder is a prime example where excesscontrol is required to cope with the dynamic conditions and the response-rate is of great importance to provide good controllability.It is one thing for an experimental pilot to take an engine-cut six or eight times a day in a test programme to demonstrate a givenairplane. It is another thing for an airline pilot—who may never have had an engine failure on take-off except during training(where he was alert to the possibility)."' The case of two engines out on the same side (a rarity withtoday's engine reliability) posed special handling problems, in that when it did occur the pilot was called upon to perform amanoeuvre which he did rarely—and then only in training. From long training experience it appeared desirable that the VMCA speedwith the two most unfavourable engines inoperative should not exceed the approach speed with full flaps. Control Characteristics In general, the longitudinal control characteristics of present-dayjets were quite good; however, the stabilizer-elevator relationship was worthy of comment. Under trimmed conditions there were noundesirable features, "but there have been incidents wherein the aircraft was misloaded to the point where it would not unstick ontake-off with full-up elevator." During one incident it was necessary ""to ease the elevator pressure and retrim the stabilizer hurriedlywhile progressing toward the end of the available concrete at 250ft/sec." After this hurried re-trim, a second unstick attemptproved satisfactory. "'In a second case a normal lift-off was made. but immediately upon becoming airborne the aircraft assumed aprogressively nose-up attitude in spite of corrective force of both pilots on the yoke and stabilizer control." Luckily the aircraft hadbeen light and climbed rapidly until the crew was able to deduce that the stabilizer was trimmed fully nose-up. However, they werestill unable to immediately re-trim the aircraft, because of the out- of-trim stabilizer-elevator relationship that then existed. Fortun-ately, one of the pilots then reduced thrust and allowed the nose to drop to the point where the aircraft could be appropriately trimmed. According to the authors, another look should be taken at therequirements that pertained to the approach problem. "When the pilot wishes to increase his glide-slope angle he should be ableto ease the nose down and accomplish just this. When he wants to decrease the angle he will instinctively raise the nose and he normallyexpects the airplane to attain a flatter approach. If other things happen, such as raising the nose causing a steeper approach angle,we have an obviously intolerable situation which could lead to nothing but disaster." Any reversal of conditions from normalpiloting techniques were highly undesirable. The pilot must
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