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Aviation History
1958
1958 - 0114.PDF
NAVIGATION UGHTS 116 FLIGHT Aircraft Electrical Systems A CRITICAL APPRAISAL AND SUMMARY OF OPINIONS PART I ty A CHARTERED ELECTRICAL ENGINEER V.HF/UHF RADIO COUJKON IMOAK WmOSOCEN WIPERS t OE-ONC PUMPS A diagram prepared by the English Electric Company showing disposition of major units in a paralleled A.C. system for a hypothetical transport aircraft. A, Doppler navigator (75OW, 115V, 400 c/s ±5 per cent, 3-phase); B, main power supply (four 40kVA drives and alternators, arrows indicating loads and synchronizing); C, cabin-temperature control (10 h.p., 200V, 400 c/s, 3 phase), D, galley (current for hot-plate ana* tor heated-food compartment). TO an electrical engineer who, though outside the aircraftindustry, nevertheless takes an interest in aircraft electricalsystems it seems rather odd thai by this rime designers of such equipment have not got round to deciding upon a more orless standard system for similar classes of aircraft. At the present time there are main systems operating at nominalvoltages of 28V D.C., 112V D.C., and 200V A.C. 400 c/s. In some cases the D.C. systems are supplied directly from generators,in other cases the supplies are derived from die rectified output of variable-speed alternators, which also provide a frequency-wildsupply for such applications as de-icing, galley equipment and water heaters. With D.C. systems, just to make things more com-plicated, inverters have to be used to provide a controlled-fre- quency supply for certain instruments, radar, etc. So in somecases, the system begins with frequency-wild alternators, rectifies a large part of the A.C. output, then uses a D.C. motor to drivean alternator at constant speed in order to get a controlled fre- quency. On die face of it, the A.C. constant-frequency mainsystem would appear to have all the advantages, but here again complications are supposed to arise because of die need to have arectified D.C. supply for powering certain components, and a battery or an auxiliary power unit for emergency supplies, orthe lot. But the point is diat since all these types of system are atpresent operational on aircraft of similar class they would all appear to be generally satisfactory, in which case could it bethat die choice depends more or less on the whims and fancies of the particular aircraft electrical engineer? Consider the systems of some of the British aircraft in service. The ftotol accessory gearbox of a Viscount, showing the D.C. generator, alternator, tachometer-generator and cabin supercharger. Viscounts have 28V D.C. generators supplying most services, fre-quency-wild alternators for de-icing, etc., and inverters. Vanguards will have frequency-wild alternators with rectifiers for the D.C.supply, so corning into line with Comets, Britannias and Victors. So it seems that the rectified A.C. system is becoming popularfor British aircraft. There does not appear to be any published evidence that any British aircraft is using a constant-frequencyA.C. system, but it looks as though English Electric are in pro- duction with the Sundstrand drive, for military applications. Although apparently no British civil aircraft has, or is going tohave, constant-frequency A.C., the Americans seem to be forging ahead in this respect, for A.C. systems are going into die Boeing707 and die Douglas DC-8, bodi aircraft being equipped with Sundstrand hydraulic drives. Odier American aircraft do, ofcourse, employ die usual variety of systems. It is also worthy of note that the Canadair Argus has a constant-frequency A.C. systemwith Sundstrand drive. Judging, dien, by die different main systems in use (not tomention the secondary supplies) it would appear that, regardless of all die pros and cons put forward by die experts trying to makea case for a particular system, there is really so little to choose between systems that the aircraft constructor, on die advice of hiselectrical engineer, just backs his fancy and hopes for the best. Aircraft electrical engineers do not as yet seem to have got togetherwith the object of attempting to agree upon something like a standard for aircraft of a similar class. Behind die scenes, perhaps,diey are doing somediing about sorting out one system from another, but on die last occasion when they aired their views inpublic (die Convention on Electrical Equipment for Aircraft, held at die Institution of Electrical Engineers in May 1956) considerabledifferences of opinion were revealed. In die course of his intro- ductory talk to die Convention, Sir George (then Mr. G. R.)Edwards had something to say "about the type of system that we are aiming at in these days": — "We have gone through all sorts of permutations and combinations.I see no reason for going away from 28 volts D.C. on an aeroplane of medium performance where it is the easiest, the lightest and the moststraightforward method. I do not see any magic in running away from something in order to do something different. I see great danger inrunning away to new generator systems which will produce high-voltage D,C. or high-voltage A.C. if you have not also made sure that there is afull range of equipment which can be operated off whatever the generating system is giving you. It always makes me laugh to see a very fancyA.C. system being rectified to enable great numbers of 28-volt D.C. pieces of equipment to be used. I think that you have enough informa-tion in these days to decide which' is the right system to use for any given job. I merely ask you not to change it for the sake of change,and, when you do change it, to make sure that the equipment is there to take care of it." The erudite papers presented at die Convention ranged far andwide, covering all die permutations and combinations referred to by Sir George; but the subsequent discussions seemed to empha-size die fact that certain aircraft electrical engineers had chosen a particular system and had every intention of sticking to it, comewhat might. Authors of general papers attempted to present an impartial survey of electrical systems, but. there was, perhaps, atendency to emphasize die advantages of a constant-frequency
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