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Aviation History
1958
1958 - 0148.PDF
152 FLIGHT, 31 January 1958 Aircraft Electrical Systems A CRITICAL APPRAISAL - - - PART 2 By A CHARTERED ELECTRICAL ENGINEER IF the papers and discussions at the May 19561.E.E. Conventionon Electrical Equipment* are anything to go by—and if thespeakers adhere to the views then expressed—it must be admitted that the position in regard to aircraft electrical systemsis somewhat confusing. It would seem (once again to an electrical engineer outside the aircraft industry) that the logical line ofdevelopment was towards a standardized constant-frequency A.C. system for that class of aircraft which includes Vanguards,Britannias and Comets. Perhaps the A.C. system, having been proved in a British Service aircraft or two, could find its wayinto a large civil transport and then by a process of development into a medium transport, so that eventually most electrical require-ments could be met from ranges of standardized A.C. components in various sizes. The few odd D.C. components that might still beessential could be supplied from small rectifier units. It sounds far too simple, but wouldn't it be handy for everyone concerned! Actually that is what has been happening in the United Statessince 1949, particularly with regard to A.C. systems making use of the Sundstrand drive. This unit was proved in such aircraftas the B-36 and is now being adopted for the two big civil trans- ports mentioned in Pt. 1 of this article. Having found thatthe constant-frequency system really works the Americans are going ahead. As related in detail in Flight for November 15 last,late-model B-52G aircraft have Sundstrands in place of turbo- alternators energised by bleed air. The American General ElectricCompany have also developed a constant-speed drive in the form of a hydraulic ball-piston unit. It is incorporated in complete air-craft generating systems placed on the market in 1956. G.E. also have a new constant-frequency power package using the G.E.drive and a statically excited alternator; they claim: "smaller and lighter than conventional drive and generator combinations of thesame rating, the 20kVA, 8,000-r.p.m. drive delivers a full 32 h.p. over an input speed range of 4,300 to 7,500 r.p.m." The powerpackage is used on the Grumman F11F Tiger. In this country the English Electric Company have been respon-sible for the development of an A.C. constant-frequency system with alternators driven through Sundstrand units, but it does notseem to be at all popular with British civil aircraft constructors. Neither have they taken any interest in bleed-air turbo-alternators.Maybe the British constructors have their own good reasons for not wanting constant-speed drives and A.C. systems; possiblythey do not like taking a chance with something new, preferring to stick to equipment which now after several years of service isreasonably free of "bugs" and on the whole pretty satisfactory. Apart from the drive units themselves the A.C. system does, ofcourse, involve new and relatively untried components. It may well be that aircraft constructors don't want to start from scratchwith these components. Perhaps they are right to be cautious, having regard to the possible troubles they might be letting them- *Extracts were quoted in Part 1 of this article, last week. Miniature A.C. induction motors are widely used in aircraft, taking supplies from variable-frequency alternators or inverters. The units illustrated are by B.T.H. Now in production by the English Electric aircraft equipment division in Bradford, the Sundstrand hydraulic constant-speed drive is already well proven in American military service. The photograph shows the standard British model, rated at 40kVA. selves in for; but since it looks as though the constant-frequencysystem really does have advantages, which become more pro- nounced with increasing power demand, then it is a pity that thesystem has yet to go into a British civil aircraft. It seems that the English Electric constant-frequency system is being putthrough its paces in a V-bomber, but unfortunately no informa- tion will be forthcoming about its performance in service. An interesting alternative to the Sundstrand-drive (producedunder licence by the English Electric Company in Great Britain) is the hydraulic constant-speed drive and alternator developedby H. M. Hobson, Ltd., in conjunction with Integral, Ltd., and the British Thomson-Houston Co., Ltd. Described briefly inFlight for November 15, this unit offers the advantage that after one engine has been started by some sort of conventional startingunit, each of the other alternators can be operated in turn as an A.C. motor running at synchronous speed, starting torque beingapplied to the engine through the C.S.D., its ratio being controlled automatically by the starting current input to meet the startingrequirement of the engine. Starting time is relatively long, but admissible for transport and bomber aircraft. Making one machine do the work of two always pleases theelectrical engineer, apart from saving weight; which brings to mind that the D.C. starter-generator is another reversible set-upthat on the face of it looks like an attractive engineering proposition. But although so-called starter-generators have been developed,the arrangement does not appear to have got into any aircraft. It seems that at one time the Vanguard was to have starter-generatorsbut plans were changed and a rectified A.C. system decided upon. One of the difficulties seems to be handling the starting currentwithout the possibility of brush deterioration likely to affect ser- viceability of the machine as a generator. One is inclined to wonder what aircraft constructors really thinkare the snags about the constant-frequency A.C. system. Is it that the constant-speed units are considered unsatisfactory? Genera-tors have been driven at variable speed from accessory gearboxes for so long now that perhaps British aircraft designers don't fancyany other sort of drive—apart from any alleged weight penalties. If designers are shying away from constant-frequency systemsbecause they think, or know, that constant-speed drive units are not likely to be as satisfactory in practice as some people claim,then perhaps they might be interested in the range of variable- speed constant-frequency alternators under development by Rotax,Ltd. Examples of these alternators were shown at Farnborough forthe first time last year and a publicity release implied that a range of these machines was more or less available. It seems, however,that at the beginning of September these alternators were still under development. Rotax designers were being somewhat cautiousabout making claims—and rightly so, for claims have been made by other people for things that didn't work out in practice. Butthis Rotax alternator seems to be on the right lines and with the obvious advantage that it can be driven from an accessory gear-box. A provisional specification of the performance of a typical 60kVA alternator was available at Farnborough. Over the variablespeed range of 4,500-13,500 r.p.m. a supply is given at 208 volts ± 1.5 per cent at a frequency of 400 c/s ±0.5 per cent. Operatingtemperature range is — 40 deg C to -I- 50 deg C and it is stated that the maximum altitude is dependent upon cooling, there being both
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