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Aviation History
1957
1957 - 1551.PDF
FLIGHT, 25 October 1957 639 AIR-TURBINE DRIVES Recent Developments by a British Manufacturer The Rotax EXP.8009 air- turbine drive. This relatively small unit provides 3 h.p. at 4,000 r.p.m. at a typical high-altitude supply pressure. UUTIL ten years ago accessory power systems for aircraftfollowed a generally established pattern. Some aircraftdesigners tended to favour one system at the expense of others, but electrics, hydraulics and compressed air were allemployed in a traditional manner. In nearly every case the system derived its power in the first instance from a direct shaftconnection to the main aircraft powerplant. To a considerable extent this still holds good today but, particularly in the mostadvanced and complex types of aircraft, there is an increasing tendency to adopt completely new types of system. One overall factor behind this trend is that accessory powerrequirements are becoming progressively more arduous and difficult to meet. Another factor is that the increasing use ofgas-turbine powerplants is making available ample supplies of compressed air—or, at least, can do so if the engine designerallows for it. As a starting point it is instructive to examine the accessory power system of a typical modern transport, and theDouglas DC-8 may be chosen as an example. In this large, long-range machine primary propulsion is pro-vided by four nacelle-mounted turbo jets rated at more than 15,000 lb thrust each. Most of the heaviest accessory loads aremet hydraulically, the system pressure being 3,000 lb/sq in. Each inboard engine has a mechanical drive to a multi-plunger,variable-stroke pump, and a stand-by system is driven electrically. All four engines are coupled to hydraulic drives on which aremounted 30 kVA alternators with three-phase, 400 c/s current at 115/208 V. Cabin air is supplied by turbo-compressor unitsdriven by air bled from the high-pressure compressor spool of each engine. Hot air bled from the engines is also employed forairframe and powerplant de-icing. Such a system is typical of a balanced layout not involving anyextremes (by 1956 standards) of temperature. For aircraft designed to maintain sustained speeds in excess of about 1,000m.p.h., kinetic heating may be sufficient to prohibit the use of any accessory systems employing oil, hydraulic fluid or conventionalelectric insulation—harsh restrictions indeed. Even where the complete airframe is still fairly cool, thermal limitations maymake it impossible to bring any part of such systems into the proximity of the powerplant. In such cases the designer is left with little to work with butstainless steel and hot air. Nevertheless, much can be done even The basic Rotax 75 kVA turbo-alternator. with these ingredients, and, particularly in the U.S.A., low-pressure air systems are already widely employed. In fact, such systems are even found in military subsonic (i.e., relatively low-speed) aircraft in which operating conditions are not particularly severe. The thinking behind such a system is as follows. Air at upto 300 lb/sq in and 500 deg C can be extracted from the com- pressors of the main aircraft powerplants and ducted to anyrequired pan of the machine; the working medium (air) is cheap, tight and continuously available; die ducting can have a thin walland be of moderate weight per foot run; no return line is needed; automatic control, synchronization and protection systems can readily be incorporated; at low engine r.p.m. (ground-idling forexample) it may in certain circumstances be possible to extract more pneumatic power than could be taken out through a shaft;the air supply, and hence the power and the r.p.m., of the accessory can be precisely controlled or automatically regulated; the lattercan be done independently of the r.p.m. or power setting of the aircraft powerplants; an air-turbine drive contains few movingparts and can be made simple, reliable and light in weight; acces- sories do not have to be located dose to the main powerplants butcan be stationed in any convenient part of the aircraft; and it is sometimes possible to match an air-turbine drive with a drivenaccessory and treat the unit as a self-contained package capable of installation wherever convenient. It is also possible to make the fullest use of the inherent flexi-bility of ducted-air systems by incorporating air-cycle machines or secondary combustion chambers by means of which the tem-perature, and hence the total energy of the flow, can be raised or lowered. "Bleed-and-burn" systems can, if properly engineered,be made to deliver very high power for only a slight increase in weight over that of a simple low-pressure air system. Again,although the engine designer must make full provision for bleed- ing off the requisite flow of air from the aircraft powerplant, sucha procedure is mechanically straightforward and simplifies the engine geometry; it eases the congestion produced by multipledrive pads and can obviate the need for a remote shaft drive entirely.Naturally there are factors to be weighed on the debit side, but the foregoing arguments are so powerful that bleed-air tur-bines are now being accepted for all classes of aircraft. They have been particularly favoured in the U.S.A., and the classic applica-tion at the present time is undoubtedly the Boeing B-52, in which the whole aircraft accessory power is energized by compressor- The EXP.8010 in-line control valve which gov- erns flow according to the pressure in a storage vessel. It is intended to control turbines driving intermittent-duty acces- sories. bleed air flowing to multiple turbo units. In Britain it is notpermissible to enumerate military applications of low-pressure pneumatic systems, but it is possible to describe such air-turbineaccessory units as have been placed on the open market. One of the most important British firms in this field is Rotax,• Ltd., of Willesden Junction, London, who have been engaged in the design and manufacture of air turbines and related equipmentfor airborne applications for several years past. Rotax air-turbine developments are entirely company managed and are not based onany previous American work. Rotax are developing several sizes of air-turbine drive, each ofwhich will eventually be offered as a rationalized source of shaft power suitable for a corresponding range of matched accessoriessuch as alternators. Owing to the flexibility of a low-pressure pneumatic system, and the ease with which the r.p.m. of the airturbine can be accurately controlled irrespective of the power- setting of the aircraft powerplants, such a system becomes afeasible arrangement for driving multiple synchronized alter- nators. This presents a straightforward modus operandi for afully paralleled A.C. system—which is now widely accepted as the optimum electrical system for large and advanced aircraft. Development was initially concentrated upon a unit of modestsize, matched with a 15 kVA alternator. A range of large units rated at 30, 60 and 120 kVA have been considered and develop-ment of the 15 kVA and 30 kVA turbo-alternator sets for air- borne use is now at in advanced stage.As a typical basic unit it is possible to describe the general engineering features of the 15 kVA machine. The basic alter-nator is a blast-cooled, two-pole machine incorporating an integral air-cooling fan and manufactured as a single unit complete withthe turbine drive. In a typical installation, the unit may be mounted within the
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