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Aviation History
1939
1939 - 1031.PDF
358 FLIGHT. APRIL 6, 1939 The lecturer next considered an aspect which had a vital bearing on the whole subject, that of using a separate auxiliary power unit comprising a petrol engine of, say, 30 h.p. coupled to an electric generator and perhaps a hydraulic generator. Possibly the advantages were con siderable and fairly obvious, but it could not be said that it was of much advantage to the aircraft designer ; all he wanted was a service, and the less interference with the layout of the aircraft the better. Such schemes had been applied to certain aircraft in America, and he believed he was correct in saying that they had not proved an un qualified success. He felt that their greatest disadvantage would be in connection with small- and medium-sized aircraft. Intimately bound up with this question was that of the claims being made for the use of high-frequency high- voltage alternating current. After setting out these claims Mr. Chaplin proceeded to put forward what he considered to be some disadvantages, and urged that it would not be wise to stray far from accepted standards without extreme caution. He also put forward objections to the use of alternating current of normal voltage and frequency. He submitted that the case for alternating current in any form was not good enough—it was better to deal with the devil they knew than with the devil they did not know. Returning to more general problems, they might assume they had an electrical generator and possibly a hydraulic generator driven off the mam power unit or units. Broadly speaking, there appeared to be two methods of arranging this: one was to have a single drive to an electric generator to provide all the energy necessary to operate the other power services, and the other was to have two drives on the engine for electrical and hydraulic generators respec tively. Examining details, the lecturer said that it hardly seemed reasonable to have a relatively large generator on the engine and then install an electric motor of almost the same power as the generator to operate the hydraulic pump. These observations brought them automatically to the second alternative, which seemed by far the most practical compromise. Mr. Chaplin continued: — " The suggestion is to drive two generators, namely, an electric and a hydraulic one, from the engine. Preferably these should be mounted on the engine itself provided the overall shape and dimensions of the engine unit were not unduly affected. One of the main reasons for the separate gear box unit disappears, as the number of service connec tions to be broken when an engine is removed is so few as to be little trouble. Should, however, the installing of the units on the engine itself prove an appreciable source of trouble to the engine people, these same two generators would have to be mounted on a separate gear box. "Almost immediate application of the scheme can be achieved by utilising small electric motor-driven units in Both lecturers discussed the claims of the auxiliary engine for supplying service power. This is a well known British example, the A.B.C. flat twin. One neat approach to the problem—the Bristol six-unit accessory gear box. the aircraft for vacuum and compressed air where there has not been sufficient time to redesign certain mechanisms to operate electrically and ^or hydraulically, and there is no satisfactory existing design so operated. "The fact, also, that there are in existence certain hydraulically operated units designed to work off a low- pressure hydraulic service at 300 lb./sq. in., in addition to the more general hydraulic service at up to 1,400 lb./sq. in., need not constitute a difficulty. These two services working at such different pressures are not generally to be found on any other than multi-engined aircraft, in which case a high- and a low-pressure hydraulic pump would be driven respectively by each of the engines of a twin-engined machine." Mr. Chaplin said that it might come as a surprise to many to know that aircraft were under construction to-day which carried hydraulic equipment weighing nearly half a ton and comprising as many as five independent hydraulic services. Surely something should be done to make it possible to avoid this sort of thing. As regards future development, the necessity for standardisation came first. Next was the necessity for higher pressures, which would permit a reduc tion in the size of the fittings and the amount of oil carried. Instantaneous Response Another point for attention was to ensure that energy should be available for immediate operation without time lag. In other words, it was desirable that, running through the aircraft, there should be a hydraulic main always main tained slightly above the working pressure, oil being diverted to the working circuits when required. Certain disadvantages associated with this scheme could be over come by the use of a self-regulating pump working on the variable-stroke principle. As regards emergency supplies, the pneumatically loaded hydraulic accumulator provided a ready solution, and in any case it was a comparatively simple matter to arrange a cylinder of compressed gas to provide the necessary motive power for a jack. To obtain a similar result by storage batteries in an electric circuit would involve prohibitive weight. Manually operated emergency mechanism had grave shortcomings for high-speed operation. Mr. Nixon, speaking as an engine designer, said that he intended to make his views complementary to those of Mr. Chaplin. In preparing the paper he had been greatly assisted by accessory firms in this country, France, Germany and the United States, and their extremely generous re sponse to requests for information had indicated that the firms themselves were anxious to arrive at a satisfactory solution of the problems of mounting accessory drives 011 power plants. After reviewing, as had Mr. Chaplin, the ever-increasing demands for power services, Mr. Nixon said that about four years ago it became evident that with the increased adop tion of retractable undercarriages, of flaps and of gun turrets, much more power would be required. With engine
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