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Aviation History
1951
1951 - 0643.PDF
6 April 1951 401 the system should help to stabilize the aircraft on its undercarriage under rough-sea conditions. Power Plants.—In the second part of his lecture, Mr. Hollis Williams said that it had been normal to design naval aircraft around engines designed initially to R.A.F. requirements, and it would appear that there was no requirement for a power plant specifically for naval use. Between the wars, as engine power increasedj attempts were made to break down the total power into self-contained units, so that the failure of one small accessory should not lead to total power failure. The Bugatti multi engine, the Fiat and Hispano double engines, and the small double Menasco were remembered. The Fairey Aviation Company had thought seriously about building contra-rotating airscrews for use with high-powered single engines and had the conviction, more- over, that everything possible should be done to save valuable crews, equipment and aircraft. This led to the idea of "duplicated power". Permissible folding dimensions were of the order of 13ft, and it was not possible to design an aircraft with twin, wing- mounted nacelles which would fold into such limited dimensions, even if the aircraft could be handled safely on one engine for an emergency deck-landing. In 1935, Mr. A. G. Forsyth, the Fairey engine designer, pro- posed a double engine with each unit completely self-contained and delivering its power through one half of a co-axial airscrew. This was known as the P.24, and when cleared for prototype flight- testing it was mounted in a Fairey Battle. Under pressure of war the decision was taken not to go ahead with production in Great Britain, but the P.24-Battle was shipped out to America for assessment at Wright Field and created a profound impression. The engine comprised two 1,000 h.p. units. There were in America at that time several 2,000 h.p. engines, so steps were taken to develop the P.24 up to the 3,000 h.p. class, with a view to instal- ling it in an operational fighter. The shock of Pearl Harbour, however, caused the emphasis to be transferred immediately to the mass-production of developed engines, and the project was shelved. It was, nevertheless, a highly promising venture, and the P.24 was probably the first engine designed specifically for naval requirements. The lecturer emphasized that there was no pressure from the Navy for such a development, the morale of naval pilots being then, as now, entirely proof against the prospect of an occasional ditching. During the war developments with this line of thought were in abeyance until 1944. At that time the Fairey company was anxious to pack more power into one of its types, and H. E. Chaplin proposed a scheme based on two Merlin engines in tandem, with shaft drives to a special gearbox mounting a co-axial airscrew. Work was started on this development, but was not completed, because of the advent of the gas turbine. Towards the end of the war the Fairey company was working on a projected naval strike-aircraft based on a turboprop unit, and difficulty was found in meeting the fuel-load requirements, both from the point of view of stowage and of the effect on total weight. It was realized that here was the outstanding reason for reverting to the double engine. The study of a typical turbine power/con- sumption curve showed that the specific consumption rose rapidly with throttling, and that it paid to operate the engine at, or near, full load. Thus, with the power derived from two independent units it would pay, under cruising conditions, to operate one unit close to full load, with the other shut down. Discussions were held with Rolls-Royce and a double power plant was schemed, based on two large turboprops. The type was ordered and a considerable amount of design work, and some manufacture, was done over a period of some eighteen months. At that time, however, the engine-development programme was so over-loaded that some items had to be deleted, and unfortunately this double engine was considered to be of limited application and work was stopped. It was generally considered unwise, said the lecturer, to associate a prototype engine with a prototype aircraft, though recent rapid developments in turbines had often made such a risk unavoidable. For the fighter aircraft the turbojet offered such overwhelming advantages that the choice had only to be made between the various types available. In the strike group the decision might rest with either the turbojet or the turboprop. In the anti-submarine group, with its free-take-off requirement, the choice fell between the piston engine and the turboprop. At about 3£ hr endurance, with allowances for warming up, take-off, climb, stand-off and so forth, the twin turboprop, plus fuel, balanced the weight of the single piston engine plus fuel. For 4 to 6 hr endurance the weight of the piston engine plus fuel was lighter than the weight of the twin turboprop plus fuel. For endurances of more than 6 hr it paid, on a weight basis, to compound the piston engine. The piston engine required high-octane fuel and was handicapped by the require- ments for flame damping, which added weight and reduced power j but the balance was sufficiently close to make decision difficult. Had there been a piston engine of just the right size, with a margin in hand for development, it was probable that the Fairey company would have made a conservative selection in favour of the piston engine. As it happened, the only piston engine available in the weight/power class required held no particular promise for increase of power, and though it would have satisfied performance require- ments for the anti-submarine type, at its starting weight, it was known from experience that all naval aircraft increased by at least 20 per cent in all-up weight, during their life. The Swordfish, operated at 40 per cent overload. It was necessary, therefore, to choose an engine, or engines, which had sufficient reserve to cope with this probable final weight. The Fairey company was attracted, when considering the special- ized anti-submarine requirements, to the Mamba turboprop. It was proposed to Armstrong Siddeley that two of these engines should be coupled and the full support of that company was secured. There was some doubt as to whether the right way to develop thrust was through a contra-rotating airscrew or co-axial airscrews. The contra-rotating airscrew meant clutches and free wheels, and introduced various unknown features, whereas, Fairey had had flight experience with the co-axial and were satis- fied that no unpleasant surprises would result from such a develop- ment. Therefore, the company encouraged Armstrong Siddeley to go ahead with the co-axial arrangement and stressed the import- ance of retaining complete independence of power units. The engine as designed and developed had complete independence of gear trains and so on : the only point at which there was dependence was where the co-axial shafts rotated one within the other. The development units had given an extraordinarily good account of themselves, with long periods of maintenance-free running. At the moment, they were sensitive to starting-up conditions, either on the ground or in the air, and a false start would give a rapid temperature rise, which might cause trouble. This would mean further development and the introduction of automatic means to avoid such critical conditions. Prototypes had been brought home with one turbine shut down; they might otherwise have been damaged in forced landings. fairey Swordfish fitted experimentally with a Leigh light under the port lower mainplane. The faired object under the fuselage is the battery.
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