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Aviation History
1949
1949 - 1395.PDF
AUGUST IITH, 1949 FLIGHT 151 Replacing the Rapide A Critical Examination of Requirements for Particular British Internal Routes: ^ The Ideal Specification BEFORE the war the D.H.89, or Rapide, was widelyrecognized and adopted as an economical aircraft forpassenger operations, and even now, fifteen years after it was designed, it is being flown Successfully by several charter companies on regular services in the United Kingdom. Such a record, is probably unique. Changed conditions in the post- war period—particularly high" labour costs—and the develop- ment of larger aircraft have, however, reduced the earning capacity of the Rapide to an uneconomic level. It has long been apparent that a replacement was necessary if B.E.A. were to continue services to the more remote airfields of the British Isles. During the year 1949-30 B.E.A. will have iy Rapides in ser- vice, flying over 1,135 route-miles, chiefly in Scotland. During the year they will fly some 1,395,000 aircraft miles in just over 15,000 flying hours, at an average block speed of 92 m.p.-h. The total capacity of these aircraft will be 897,000-tonne-iilo- metres, which is some 2.3 per cent of the total B.E.A. opera- tions. According to B.E.A. calculations, the Rapide opera- tions account for 15 per cent of the Corporation's total losses and a load factor of 206 per cent would be necessary to break even. The factors which account for such uneconomic operation are considered to be the small size, low speed and high main- tenance costs. The average capacity payload offered by a B.E.A. Rapide amounts to '880 lb or five passengers, so that with an average load factor of 65 per cent the total contribu- tion to operating costs, including a crew of two, is the revenue irom three passengers. Incidentally, during the next 12 months B.E.A. Rapides will be converted for one-crew opera- tion by the introduction of full V.H.F. radio cover within the ]British Isles. Tin's modification is expected to reduce the deficit attributable to Rapides to about 11 per cent. For the \ear 1950-51 B.E.A. plan to further reduce the Rapide losses by detail improvements in operational methods. Although the cruising speed is normally 115 m.p.h., the average block speed over rhort distances is 92 m.p.h. As a result, comparatively lew revenue miles are covered in a given number of flying hours. Servicing costs are high because the fabric-covered wooden-braced biplane airframe of the Rapide is expensive to maintain. At an average of £3.27 per flying hour the main- tenance costs work out, per ton oi gross weight, at 40 per cent more than those of the Viking. Engine maintenance costs are reasonable. It follows, then, 1 hat "the ideal Rapide replace- ment for .B.E.A. should combine an economic size and pay- load with a higher cruising speed and low maintenance costs. At the same time, in ordei to serve the same remote areas it should be capable oi a short take-off and landing. Four major factors in deciding the most suitable aircraft are performance, layout, size and power units. The aircraft should be as large as possible consistent with a reasonable load factor and frequency of service. An analysis of traffic potentials by route and stages estimated for 1952 (the probable first year of operation) and 1955 (a mid-way year in the antici- pated life of the replacement) established a 10/15-passenger aircraft, with allowance fox mail and freight, as the l>est com- Z2 ZO 10 5o 510 l I a. 5u o DIVERSION EQUALS BLOCK DISTANCEJJ- SO RAPIDE (5,550 LB) ISO > MARATHON 2QOtoo 4OO ISO BLOCK DISTANCE 25O STATUTE MILES 2OO NAUTICAL MILES Payloadjrange characteristics of the Rapide and alternatives. 'our British e _l I I* z V) , UJ O53 a 2 * 1 i \ \ \ ^-—. • —•Ml — — saa: = — i- 55= ——m ' MA DOVE ^ATHOI r5 1 6O 100 NAUTICAL MILES (4O ISO Estimated direct costs for Rapide and possible replacements. B 15 promise. It has been further established that B.E.A. would require 15 aircraft of that size to meet estimated traffic demands. Volumetric capacity was also considered since it must be adequate to stow that load which could be lifted over the shortest stage (150 nautical miles in still air) without reserves or allowances. F"or the longest range requirements (700 nerutical miles in still air without reserves) a payload reduced below the maximum seating capacity would be accept- able The layout should be suitable for the type of traffic and maintenance facilities available and to give the lowest operating costs for size. Performance of the aircraft should natuially be suitable for the airfields and routes over which it would operate. An analysis of airfields and their dimensions shows that the critical take-off requirement to reach a height of 50ft on all engines is 600 yd on grass and that the landing distance from 50ft should not exceed 420 yd. Great importance would be attached to the power units selected particularly with regard to reliability, weight and cost. During the past year B.E.A. are understood to have delayed action to secure such a replacement in the hope that a satis- factory and economic aircraft might be obtained from among several prototypes and one production machine now flying. Four British types were considered, the Handlcy Page Mara- thon, Percival Prince and the two de Havilland types, the Dove and Drover, and an analysis of the characteristics of each was made. Except for the Marathon (two crew) each one was studied with single-crew operation, a fuel reserve, of three- quarters of an hour stand-ofl, plus 100 nautical miles diver- sion. On this basis, and assuming the direct operating costs of the Rapide per passenger mile, at 80 miles stage .length, to be 100 per cent, the direct operating costs of the four types
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