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Aviation History
1957
1957 - 0116.PDF
118 FLIGHT,25 January 1957 A wing of 8-475, (like that above) represents an investment of £50m for the aircraft alone. Oyer 40 such wings are currently active OPERATIONAL ANGLES ON THE B-47... There are two basic reasons why the jet landing operation callsfor greater precision than does the conventional landing: (1) jet engines accelerate slowly, and (2) jet aircraft decelerate slowly.The J47 requires from 12 to 20 sec to accelerate from 40 (idle) to 100 per cent r.p.m. Most of this time is required to acceleratefrom idle to 55 per cent, and it is desirable to carry 55 per cent until the landing is assured. At the same time, however, it is con-versely desirable to reduce power to a minimum during the approach, in order to steepen, as much as possible, the charac-teristic, flat glide angle of the jet. Because the airplane is stream- lined and has no windmilling propellers, it does not want to stopflying, and, once on the runway, it does not want to stop rolling. Ten knots excessive speed at flare-out will increase the requiredrunway length by 1,700ft. On a wet runway, at 3,000ft elevation and 80 deg F, a 120,000-lb B-47 will stop, using drag chute andbrakes, in 5,400ft. With a G.C.A. touchdown point 2,000ft down the runway and 15 kt excess speed on final approach, it will stopjust 50ft short of the end of a 10,000ft strip. If the chute had failed under these circumstances, the aircraft would have required14,200ft on the wet runway, 11,000ft on a dry runway. Approach speeds for conventional heavy bomber and transport-type aircraft may be 30 per cent higher than the stall speed—i.e., 100-kt stall, 130-kt approach speed. To keep from floating righton by the airfield the jet not equipped with dive brakes, spoilers or approach chutes must reduce this margin over stall by one-half.For example, the 100,000-lb B-47 stalls at 114 kt; the best approach speed is 128 kt, with 14 kt separation. Working closer to the stall speed is not dangerous providedtwo important factors are considered: the increase in stall speed due to bank, and the increase due to g forces. If the 100,000-lbairplane, approaching the runway at 128 kt, banked in excess of 35 deg, it would stall. This places -a definite restriction on theamount of manoeuvring which can be done after breaking a particularly low ceiling, makes a point of killing drift early duringG.C.A. and I.L.S. approaches, and emphasizes the need for approach lights out to 3,000ft. If 0.25g force is placed on the100,000-lb B-47 flying at its approach speed, it will stall. The down-wind leg is flown at best approach speed plus 30 kt. Thisexcess is systematically bled off during the turn into final. The induced stall is, therefore, not a problem unless there is turbulenceor gusting winds or possible downdrafts at the runway threshold. There is nothing unconventional about jet traffic patterns. Infact, one pattern is standard at all S.A.C. stations—for jet and conventional traffic alike. Traffic altitude is 1,500ft above theterrain, and the turn onto base leg is started 45 sec after passing the end of th- runway on the downwind leg, with an adjustmentfor higher-than-normal wind velocities. Most pilots "eyeball" this point, by starting [he turn when the sweptback wing tippasses the end of the runway. Power is adjusted to lose 700ft in the 180 deg turn, rolling out on final at 800ft. The bicycle gear was found to facilitate cross-wind landingswhile the sweptback wing and the approach and brake chutes complicated the cross-wind landing. It was found that the B-47could be landed without undue risk if the 90-deg cross-wind com- ponent was not in excess of 25 kt. The accepted procedure wasto track directly into the runway by holding the wing down on the windward side until the aircraft was actuaUy on the ground. Theupwind outrigger gear would touch first. There was no- reason, with the bicycle gew, to straighten up before landing, and therewas good reason for not yawing excessively. Because of the geometry of the sweptback wing, airflow over the advancing wingduring a yawed condition becomes more chordwise, resulting in increased lift for that wing, while flow over the retreating wingbecomes more spanwise, thus resulting in a loss of lift and, at extremely low speeds, possibly a stall for that wing. The per- missible 90-deg cross-wind component is reduced to 20 kt when the approach chute is deployed. Bodi the approach and the brake chutes tend to hold the rear gear off the ground and "weathercock" the airplane. If die front gear strikes the ground ahead of the rear gear andif there is excessive speed, the airplane will bound back into the air in a nose-high attitude. If the power has been reduced to theidle position, it is not possible to catch the airplane by sudden use of the throttles. In fact, diis procedure will only aggravate thecondition. The power will not take effect until die aircraft has flown through a second bounce. If it should happen to be at thepeak of the second bounce and if the power comes up unevenly, there may be insufficient aileron to prevent die outboard enginepod on the low-power side from striking the ground. We flew over a million hours in the B-29 during 1950, 1951 and1952. We had 189 accidents, resulting in a rate of 17 accidents per 100,000 flying hours. Of these, 68 (36 per cent) were landingaccidents. We flew the B-47 a little less than one-half million hours during1952, 1953 and 1954. We had 75 accidents, resulting in the same rate as the B-29's—17 accidents per 100,000 hours. Thirty-five(48 per cent) of the B-47 accidents occurred during the landing phase. We have indeed matured, and we have learned manylessons. We have applied these lessons, and the state of the art has advanced. The larger B-52, weighing over 400,000 1b, flies higher, fasterand farther than the B-47. With wing spoilers, it lands more slowly than the B-47 with better control. The J57 engine acceler-ates in half the time required by the J47. When thrust reversal is added to the next line of jet aircraft, we will drop the brakechute, and jet flying will have become a relatively simple and safe operation. But commercial and military jet operators will both have oneunsolved problem for a long time to come. Jets are noisy, and Americans prefer their decibels in modest quantity and theircycles-per-second in the ranges of the audio spectrum below 15,000. Low approaches for landing with the throttles retarded are noproblem, but heavyweight take-offs with 100 per cent of power from six and eight jet engines are another matter. If the airlinescan solve the noise problem, their transition to jets will be a breeze. A NEW RESEARCH FELLOWSHIP AMONG prominent industrialists concerned at the increasing1 *• shortage of young scientists and technicians in this country is Sir Alan Cobham, chairman and managing director of FlightRefuelling, Ltd. His particular interests include the Wessex Technical Education Association, a body being formed to createfunds for the furtherance of such education in the West of England. It is now announced that to advance these aims Sir Alan hassponsored a Fellowship at Southampton University. To be known as the Sir Alan Cobham Research Fellowship, it will beof approximately three years' duration and will be open to candi- dates with industrial or research experience who are members ofthe British Commonwealth and are preferably British, Canadian or Australian. It is envisaged that the Fellow will be between 25and 30 years of age. Value of the Fellowship is £800-£900 a year. The first appointment is likely to be made shortly, selectionbeing by a committee of representatives from Flight Refuelling, Ltd., and Souuiampton University. The University has a considerable reputation for aeronauticalstudies, current work including investigations of aircraft noise and its ettect on fatigue, of helicopter vibration, and of structural damp-Hj«- rfle Professor of Aeronautical Engineering is Prof E J Richards. .. .>
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