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Aviation History
1949
1949 - 0524.PDF
334 FLIGHT MARCH 24TH, 1949 Short-range Airline Speed Pistons versus Jets : The High Cost of Saving a Few Minutes By L. H. W. HARRIS AIR COMMODORE SIR FRANK WHITTLE is to becongratulated on his recent thought-provoking lec-k ture, London to Paris by Jet Plane,* which will, no doubt, be studied by everybody interested in the development of air transport. His conclusions asserting so definite a superiority of the pure jet aircraft for short ranges are startling, and not yet widely held. Whilst, therefore, it may seem foolhardy to cross swords with the man whose pioneering work in the field of jet propulsion has made his name a household word, I nevertheless venture to take up cudgels to protect the decrepit, present-day aircraft with its "nasty up-and-downer" engine, from his sus- tained attack. In order to arrive at a figure for the structure-weights of piston-engined aircraft and of a pure-jet-engined type, the Air Commodore takes as respective patterns the Airspeed Ambassador and Avro C-102. He works on a figure of 35 for the percentage structure-weight of the piston-engined aircraft at cruising speeds of 200 m.p.h., 250 m.p.h., 300 m.p.h., and 350 m.p.h., and a figure of 34 for the jet types at cruising speeds of 300 m.p.h., 350 m.p.h., and 400 m.p.h. However, it seems hardly fair to assume that the struc- ture-weight remains constant over such a wide range of speeds, since, to take one example, the air loading of the 400 m.p.h. machine will be 1.77 times that of the 300 m.p.h. type. Readers will doubtless recall that the Tudor VIII is restricted to a speed of 350 m.p.h. due to the strength limitations of the airframe, whereas the four Nenes would be capable of giving it a speed of some- thing like 480 m.p.h. In any case, the choice of air- craft for the estimation does not seem very suitable, since the high aspect ratio of 11.5 for the Ambassador's wing, and its high position relative to the fuselage, are both features which make for increased structure- weight as compared with the moderate aspect ratio (8.3) and low wing-position of the C-102. In other words, in the writer's opinion, if the specification to which the Ambassador was built had been fulfilled by a design of similar layout to the A.W. Apollo, a percentage structure-weight of about 28 could have been realized as compared with the figure of 35 quoted by Sir Frank for the Ambassador. These conclu- sions are borne out by data published for the Apollo, Lin- coln and Constellation. Incidentally, in his paper read at the Anglo-American Conference in 1947, Mr. A. E. Raymond, of the Douglas Aircraft Company, gave an estimated 5,650-lb increase for a "high-wing" D.C.-6. Speed Turning to the question of speed, a little arithmetic shows that to cover the London-Paris trip in 30 minutes, as suggested by A. Cdre. Whittle, would, assuming most efficient stacking, demand a maximum cruising speed of 600 m.p.h.! Assuming that an operating height of 25,000ft would be demanded in the interests of fuel economy, a • Reported iti "Flight" of February 10th. The Mach number of 0.87 would be attained, bringing with it a host of problems. If we consider an aircraft of 55,000 lb gross weight—to take a similar figure to that used by Sir Frank—it appears doubtful whether, with our present knowledge, such a machine could be designed for economical'operation; it is interesting to note that the Meteor IV has a limiting Mach number of 0.80. Moreover, although the Boeing XB-47 Stratojet has been designed for a Mach number of 0.95, there is no indication that it has approached this figure and, in any case, its fuselage is hardly suitable for the economical carriage of passengers. Weight A rough estimate indicates that, assuming a very clean aircraft, with very thin swept-back wings, a total of 32,000 lb of thrust would be required to give such a per- formance (this assumes the retention of a reasonably low stalling speed). Such a preponderance of thrust would provide an installed weight of 17,000 lb, working on the figure of 0.53 lb/lb thrust which obtains in the case of the Derwent V installation for the Meteor, and this figure would represent a percentage power-unit weight of 30. A figure of 36 per cent for the structure-weight of such a machine appears to be a reasonable '' guestimate'' when allied to the figure of slightly below 34 quoted by Sir Frank for the C-102, which is designed to cruise at 400 m.p.h. To see how uneconomical such a machine would be, we shall compare it with a piston-engined aircraft of the same gross weight and designed for a cruising speed of 250 m.p.h. As discussed previously, a percentage structure - weight of 28 would seem to be readily attainable for the latter machine, whilst a figure of 18 per cent for the installed power of some 4,300 b.h.p. seems reasonable, assuming a supercharge to 20,000ft. Thus it can be seen that the disposable load of the piston-engined machine is so much greater than that of the jet aircraft, even ignor- ing the large difference in fuel weights, that there can be little doubt of its ability to operate a commercial service at a lower cost. In the above considerations, it has been thought desir- able to keep the stalling speeds down to a figure of 78 m.p.h. at the gross weight, with flaps down, in order to provide some measure of safety in event of a "ditching" or forced landing. Moreover, a reasonably low minimum flying speed is likely to be an asset on a route where foggy conditions are often met. This condition is, of course, a disadvantage to the high-speed aircraft, since it is well known that the use of very thin swept-back wings implies a reduction in the value of the maximum lift coefficient and therefore a larger wing area has to be provided to give the same stalling speed as a normal plan form. Unfortunately, Sir Frank has nothing to say regarding the economics of the 600 m.p.h. machine or even a 500 m.p.h. one; therefore, to discuss the points raised, we shall need to talk in terms of 400 m.p.h. The befct idea seems to be to compare the jet aircraft with the 250 m.p.h. piston-engined machine, representa- B 4 CeiiiQiitiii-engined Airspeed Ambassador.
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