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Aviation History
1958
1958 - 0305.PDF
319 VARIABLE NO; >v BRUSH GEAR VARIABLE NOZZLE "X^ X ' 1'*^'^' E.MIPQENCY Fig. 'MOLYBDENUM BISULPHIDE Fig. 7. (Above) Air bearing. 5. (Left) Labinal magnetic constant speed gearbox. ROTAX LOOKS AHEAD . . . lines System. This firm uses a function of alternator output tobrake magnetically the annulus ring gear of an epicyclic gearbox, and so obtain a constant speed gearbox output (see Fig. 5). In Britain, Rotax are doing a great deal of work upon constantspeed bleed or ram-air turbine drives, which some engineers believe are the ultimate answer to the constant frequency alternatorproblem. For the moment, although they can be closely governed, their efficiency is rather poor. There have also, in the past, beenone or two rather serious incidents as a result of the turbine wheel bursting, and the U.S. Civil Aeronautics Administration has speci-fied that the wheel must fail-safe if turbine drives are used. Experience at Rotax has been that, although turbine wheels mayshed their blades, all have so far been contained within the casing or entered the silencer through the exhaust air pipe. But theyadmit that they may have been lucky, and are now engaged in the development of a "frangible wheel"—something with the shatter-ing characteristics of a toughened glass windscreen, where the fractured particles are all small enough to have a low momentum.An instance of the urgency of this work is a requirement for air turbine starters on military aircraft that, should the pressure-reducing valve in the engine-bleed-air line jam open (it is assumed that the first engine of a multi-engined installation is started elec-trically or by combustion starter) the sortie must not be aborted. This means that the turbine, instead of being subject to air pres-sure at 30 Ib/sq in for a few minutes, may well be subject to 80 lb/sq in for several hours. The air pressure reducing valve is clearly an important itemof equipment; Rotax now make their own which has proved suc- cessful. As a fail-safe measure it requires an electric current toopen it and is loaded into the closed position. It can operate in a temperature of 450 deg C. The major problems of an integral air turbine-alternator unitrunning at 24,000 r.p.m.—bearing and centrifugal stresses—have been overcome to the extent where a 15kVA two-pole machine (for example) is anentirely practical unit, with an adequateoverhaul life. One ad- vantage of this com-bined unit is that it is possible to savesome 15 or 20" lb weight on a gearboxor shafting. A pro- mising approach tothe air turbine prob- lem is to use bothram-air and bleed-air to drive the alterna-tor, with a control system that metersbleed-air only when the ram-air variablenozzles are adjusted for maximum flow. The advantage of a small bleed-air require-ment during cruise is apparent. Such a scheme is shown in Fig. 6. Some useful work has also been done on air-lubricated bearings,an example of which is shown in Fig. 7. The bearing is a loose fit in its housing and floats on a cushion of air. But no air bearingworks well under high g loadings because the non-viscous film breaks down; the solution proposed is a layer of molybdenumdisulphide lubricant on the outer race and the inner face of the housing. This type of bearing has been remarkably successful ontest. Normally, spring-loaded angular-contact bearings are used in the rotating assembly of an air-turbine drive, lubricated by anair force-fed plunger pump. Perhaps the most encouraging experience of air turbine-drivenalternators results from the development effort that Rotax have put Rotax air turbine starter. RAM AIR TURBINEWHEEL TEMPERATURE INDEX Fig. 6. An alternator driven by both ram air and engine bleed air develops its full output in all conditions of flight. into control techniques. A mechanical governor to adjust the statornozzles can be used where control to within ± 5 per cent is adequate. But for close control of constant frequency supply,a magnetic amplifier servo control is used that governs to within ±\ per cent in the steady state and ±2j per cent in thetransient condition, with a return to close control within one second. Present development is concerned with paralleling the outputof two or more air turbine-driven two-pole alternators, and Rotax are experimenting with techniques that have not been used pre-viously elsewhere; to a certain extent the air turbine is used as a vehicle for investigation into the parameters of protection, controland alternator matching. The "soft drive" characteristic of the air turbine-alternator canbe stiffened electrically by "doctoring" the output transformer current and using the mag-amp servo as an anticipatory device,and a tacho-generator is used to compare the alternator voltage and keep paralleled machines in phase; load sharing is by a com-parison of current components through paralleling networks. Glancing again over the whole field of alternator drives, theposition might be summed up in the following way: the variable speed, constant frequency alternator is probably the ideal if itsefficiency can be maintained over the necessary 3/1 speed range. The air turbine drive is promising and might be die best eventualsolution. But serious work on paralleling is still at an early stage, and the weight penalty of such an air turbine drive in additionalfuel required to compensate for the air bled from the engine is regarded by some engine manufacturers as putting the scheme outof court. Hydraulic constant speed drive is practical for any size of alternator—Rotax and Joseph Lucas (Gas Turbine Equipment),Ltd., have teamed up on this development—and has the advantage of being well suited to remote operation. The overall efficiencyis comparable with other constant speed drives. The hydro-mechanical drive is quite practical and many are inservice, but has to be most carefully made and scaled to match the alternator size. With large machines in very high speed aero-planes there may be overheating problems. The fuel sink can be used, of course, but there is a growing reluctance to advance thisas an automatic panacea for all cooling ills. Finally there is the electro-mechanical Labinal drive, to which similar remarks aboutcooling may possibly apply. Any review of Rotax activities would be incomplete withoutreference to their service and after-sales organization, of which they are particularly proud. Sound arguments can be advancedfor a very much more liberal attitude by British designers and operators towards a "dispose at overhaul" policy for small com-ponents and much more standardization for large ones: Rotax, for example, manufacture no less than 285 different actuators, andare required to carry enormous and uneconomical stocks of spares, overhaul manuals and so on. However this may be, the manpowerneeded for customer liaison is fairly considerable. Rotax parallel the largest aircraft and engine manufacturers in the organizationof their servicing arrangements; Lucas-Rotax have a large organ- ization in Canada, a spares depot is being established in New Yorkand joint arrangements are being made for the handling of Rotax units through various agents across the world. As a matter ofcourse, engineers are made available at very short notice to firms developing Rotax-based electrical systems. Commercial survival within the aircraft accessory industry ofthe next few years must depend upon very much higher standards than those adequate in previous years. The stimulant of closecompetition should improve the product—by accelerating its birth- pains, hastening its development, extending its overhaul life andthe service that goes with it. This article has tried to show the research and development background of one firm that realizesthe challenge is upon them now. A.T.P.
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