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Aviation History
1955
1955 - 0023.PDF
IFLIGHT, 7 January 1955 IWING DROP AND PITCH-UP . . . too 100 120 140 160 180 100 120 140 PERCENTAGE OF STALLING SPEED (vs j Fig. 3. Rates of descent at idling power (left) and approach power (right) for hypothetical transonic (dotted line) and supersonic aircraft, plotted against speed shown as a percentage of the stalling speed in the landing configuration (VSL). even at constant Mach number, but it could be minimized andeven eliminated by proper selection of wing plan-form, wing leading-edge camber, slats set to open at these values of CL, andsometimes by proper location of the tailplane in relation to the wing. Tailplanes set below the wing were so placed in recentsupersonic aircraft primarily for this reason. Unaccelerated Stability. Unaccelerated stability implied highpositive stability at a constant Mach number, and positive un- accelerated static stability would be described by a pilot as acondition where increasing forward stick-movement would be required if the speed were increased. If cruising in the Machnumber range where this phenomenon occurred, the pilot might, for example, trim the aircraft "hands off" for Mach 0.95. If agust was then encountered which slowed the aircraft to Mach 0.93 (I.A.S. change of 7 kt at 40,000ft), the trimmed tailplaneposition would, with the pilot's hands off the stick, remain con- stant and cause the aircraft to start a slow pull-up to 1.2g. Thisincreased g would further slow the aircraft up and lead to yet further g. The aircraft would always wander away from itstrimmed cruising speed and accurate cruising would require continual attention. The severity of this characteristic depended mainly on thewing plan-form and the aerofoil section used, and to some extent also on the tailplane location. From the pilot's point of view,unaccelerated stability effects could be explained as follows. The speaker assumed the pilot intended to pull 3g at Mach 1.0,and set from —1.25 to —3 deg on the tailplane. If he could main- tain his Mach number by power or dive, no pitch-up wouldoccur but, if the Mach number decreased to about 0.95 with a constant tailplane angle, he would experience 4g instead of the3g applicable at constant Mach number. This would not be serious unless a structural limitation became involved, for ex-ample, in aircraft with low limiting load factors. This type of pitch-up, the speaker said, could probably not be entirely elimin-ated and very effective tailplane controls, preferably of the "all- flying" or "slab" type, would be required, as opposed to theerstwhile conventional tailplane and elevator. Landing Speeds. For pilots, one of the major drawbacks ofhigh-speed aircraft was their high landing speed. Yet the demand for higher speeds inevitably increased stalling speeds, as thegraph (Fig. 2) showed. Stalling speeds for many different types of aircraft, from the Mooney Mite to the F-86, were plotted. The speaker added that high landing speeds lead to high costs,particularly in tyre and brake wear, since modern braking systems provided little feel for the pilot at ground speeds of 100 to 110 kt;this low available braking power, combined with lift on the wings —which lessened the weight on the wheels—caused many pilotsto burst tyres before they became accustomed to the conditions. Furthermore, the cleanness of modern aircraft almost eliminatedthe aerodynamic drag which was a traditional source of landing- run deceleration. Also to blame were increased airfield sizes andgreatly increased take-off weights. Amongst various remedies, the speaker cited the tail parachute which, he said, worked verywell; turbojet reverse thrust would also prove useful. Flaps, on the other hand, were no longer sufficient to prevent the increaseof landing speeds as maximum level speeds grew higher. Boundary-layer control by blowing and/or sucking wassupposed to be the cure-all for the future, and appeared to be yielding something after years of research, although it might onlybe a 10 per cent decrease in stalling speeds. Pilots would, how- ever, have to be instructed in realizing this improvement, since '••••••""• • ' : ' ' '•• % • -S so many of them, especially in the Air Force, worked on the age-old principle of increasing the touch-down speed by 5 kt each for wife and children. The speaker remarked that, in his experience,most pilots appeared to have at least five children, and suggested that more training was required in low-speed flying. Trainingand indoctrination would, he thought, be the joint responsibility of the Air Force and the manufacturers, and if not carried outwould lead to a high landing accident rate in the future. Approach and Landing Rates of Descent. The speaker saidthat current wing designs militated against good landing charac- teristics because the wing loading was generally from 40 to 60Ib/sq ft in the landing condition, and aspect ratios about 6 for a transonic aircraft and 3 for a supersonic aircraft. Both thesefactors combined to make landings at idling power or with power off extremely difficult. Observation of operational flying hadshown that pilots liked to approach with approximately the same rate of descent regardless of the type of aircraft being flown.Acceptable rates of descent were usually between 8 and 25ft/sec, with an optimum of 17ft/sec. Preferred touch-down rate of sinkwas 2 to 3ft/sec. Most aircraft hitherto had employed an approach speed of 140 per cent of the stalling speed in thelanding configuration (VSL). Touch-down speed was 110 to 120per cent of the VSL, and it was desirable in consequence that the speed of minimum rate of sink should occur near the touch-down speed to ensure good round-out characteristics. The graph (Fig. 3, left) showed approach rates of descentin excess of the acceptable limits, and a supersonic aircraft sank even faster than a transonic aircraft. The transonic aircraft'sminimum rate of sink occurred at 1.2 Vsi, while this was 1.35 for supersonic aircraft. In both cases, a powered approach wouldbe required to obtain the desired rate of sink. The effect of a powered approach, however, was to raise the speed for minimumrate of sink to a higher value. In the cases shown, the effect was small but the trend was undesirable. The graph showed that the rate of descent increased at speedsless than 1.4 VSL. Thus, if power was held constant during the approach, the rate of sink would increase as the speed decreasedduring the round-out. The implication was obvious—namely, that as soon as the pilot attempted to round-out, his rate of sinkwould increase instead of decreasing. He would therefore have to approach at something over 1.4 VSL in order to be able to exchangekinetic energy for g to reduce his rate of sink by rounding-out. This would require an approach speed of 1.5 to 1.6 VSi.. Hewould also have to judge his round-out point very accurately and touch-down before the rate of sink reached an unacceptable level.He would therefore not be able to spend long at doing "a little stick-jockeying to feel for the ground." In any case, diis charac-teristic would imply a touch-down speed for a supersonic aircraft in excess of the normal 1.2 V^T . The speaker enumerated five devices which would assist inobtaining a lower speed of minimum rate of sink as follows: (1) wing flaps, (2) drag-producing features such as speed brakes anddrag parachutes, (3) high aspect-ratio, though this was undesirable on die score of weight and aero-elastic effects, (4) low wing-loading,very desirable but difficult to obtain, and (5) wing boundary-layer control. B-45 Engine Test-bed Experience. Mr. Hodder next mentionedsome jet-efflux characteristics experienced while testing large jet engines suspended under the bomb-bay of B-45s. Severe tail-buffeting had occurred while running the test engine with after- burner lit. This, he said, indicated a new problem in supersonicaircraft. During initial trials, no trouble had been experienced, but whenthe afterburner was first lit, die resulting tail buffeting was so severe that die test was discontinued, and subsequent inspectionshowed cracks in die tailplane ribs. These could not have been caused by the efflux directly striking the tail, since the two wereseparated vertically by 11.25ft. There could be only two other causes, firstly, that shock-waves originating at the tailpipe struckthe tail, and secondly, that high-energy pressure waves combined with the normal pressure pattern over the tail to cause unstableshock-waves in that region, togedier with flow separation. The tests were being conducted at Mach numbers where mild normalshock-waves were in any case occurring over the tail. Similar problems might arise in any supersonic aircraft where portionsof the aircraft extended beyond the tailpipe efflux. Mr. Hodder suggested that some research organization such as N.A.C.A. shouldinvestigate diis problem with a high-altitude test flight programme. Conclusions. The speaker said that many of the problems asso-ciated with supersonic flight would probably be solved within the next ten years. He suggested, however, that the greatest improve-ments should be sought in reducing landing speeds for the pilot's sake. V.T.O. and flying-saucer configurations might, he thought,provide some answer, and in this connection he suggested the quotation "although it may not be good aerodynamic design, youcan fly anything, even the kitchen stove, if you put enough power on it." The speaker predicted that though his audience might notfind this quotation humorous at the moment, they undoubtedly would do so if they read his paper in a hundred years' time.
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