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Aviation History
1960
1960 - 1967.PDF
FLIGHT, 16 September 1960 Round the Stands . . . 461 Constant speed drives. Left to right Plessey CSDS. Dowty/Rotol air turbine and Plessey constant-speed alternator gas turbine; response is said to be very good under continualalternator load change and rapid engine acceleration. The arrange- ment is shown in the photograph; the alternator is co-axial, withan internal shaft passing through it to the gearbox. The remaining units are the air motor, air control valve, start control and overspeedshut-off valve and overspeed/underspeed unit. The CSDS has been very rapidly developed; a certain amount of external plumb-ing suggests possibilities for refinement. Even newer than the unit just described is the Rotax CSDS,the first example of which arrived from the AiResearch division of the Garrett Corporation barely a week before the opening ofthe show. All the ground starting and check-out features are available and, should the primary power-supply source fail, bleedair from another engine can be switched to the unit so that it continues to operate on air alone. The pneumatic drive of theRotax CSDS is by a well-proven air turbine and it is said that this arrangement reduces bulk for a given power output. Weightquoted for a 60kVA size is 551b; for a 30kVA size 321b, and a 20kVA size 301b. To these must be added an allowance for theair control valve upstream of the turbine and for a small oil tank. Because an air turbine drive is inherently "soft," and cannotbe reversed, at the cruise point about 30 per cent of the total input to the alternator is supplied by the bleed air turbine. At this levelthe turbine remains highly sensitive to variation in the flow through the variable-area nozzle vanes which form part of thecontrol system. For speed control the pneumatic governor—a centrifugally actuated spring-loaded poppet valve—supplies asignal to an hydraulic seryomechanism governing the position of the nozzle vanes. The Rotax CSDS is currently being tested at the HemelHempstead laboratories prior to full demonstrations; it could be in production and available in 12-15 months.Since English Electric introduced the Sunstrand CSD some 500-600 units have been manufactured by the Bradford factoryand overhaul life is now at least 800hr. The firm service 30kVA US units on BOAC's 707s and also supply others built in England.English Electric have considerable hope that they will supply 40kVA drives for the VC10.A production flow of Hobson units for the D.H.121 suffered a setback when the aircraft was reduced in size and power ratherover a year ago, and the 22^kVA drive matched to the RB.163 engines is still in the prototype stage. Following increasing BEAand D.H. confidence in the design when the 30kVA unit was run the earlier 30kVA 6,000 r.p.m. unit was extensively reworked inthe reduction to 22^kVA. It now has an output r.p.m. of 8,000 with a consequent torque reduction by half. While developmentof the smaller drive has been proceeding alongside that of the engine and the airframe, the 30kVA drive has been used fordevelopment and components have reached l,000hr "without per- formance deterioration." There are indications diat initial over-haul life could be 2,000hr. Although Hobson were the first to propose using an alternator as a synchronous motor and startingthe engine by changing the ratio of the CSD, this arrangement has not so far been adopted on the D.H.121 because of uncertaintyabout an a.p.u. But the facility has been kept in mind. A second Plessey unit on show was the Labinal constant speedalternator. The constant speed unit here is an integral part of the rotor assembly and comprises an eddy current brake anddifferential gear train. Plessey have added a secondary input gearbox which raises the range of input speeds from 2,900-4,700r.p.m. to 2,900-8,000 r.p.m. Solely air turbine drive is used in the Dowty/Rotol 15kVAdesign and speed control is achieved through a governor and AiResearch-type variable-area nozzles. The turbine and alternatorrotate at 24,000 r.p.m., but the unit is lightweight and weighs 1121b complete with its integrated electrical machine. Transientresponse is ±3 per cent for a 50 per cent load change. The Hobson hydrostatic drive (left) and English Electric Sundstrand advantage of this type of drive is that it can be positioned any- where that an air duct can be led and cross-bleeding is simple, but of course large amounts of bleed air are required. Electrical Generators PLENTY of a.c. brushless generators are now being manufacturedin Great Britain, the only country on this side of the Atlantic to have such units in operation. Newest of all is the Rotax solid-rotor type, 15, 20 and 3O-35kVA sizes of which were on show. These machines are designed to replace salient pole types andalthough power/weight ratios are not so good, lack of centrifugal force on the insulation of wound poles allows a better materialto be chosen. Higher temperature operation is thus possible. For the VC10, brushless 40kVA alternators are being suppliedby Plessey. These are of Westinghouse design built under licence, as are the 30kVA units supplied to BOAC for the 707. Followingbearing changes the latter has in prospect an overhaul life of 3,000hr. Another manufacturer of brushless machines is EnglishElectric, whose 50kVA units are either air or, now, oil-cooled. The latter will be used on the Victor B.2, and oil-cooled units areexpected to supersede air-cooled designs on military aircraft. Air- cooled 50kVA units weigh 871b, oil-cooled units, 1031b. The samefirm are producing 9kW d.c. generators for Dart-engined aircraft and have received inquiries for these from the US. The speedrange is 5,000 to 10,000 r.p.m. but on a limited speed range the rating can be increased to 12kW. Alternators for the D.H.121 are being developed by AEIalthough no examples of the 22^kVA size were on show. The40kVA unit demonstrated is, in fact, similar to the 22^kVA machineexcept in size and output r.p.m.—raised from 6,000 to 8,000. The latter weighs 551b, is cooled by ram air and is lubricatedby oil circulated from the engine. A unit on test at Coventry will shortly be passed to de Havilland for rig testing and a furthertwo units are in use by H. M. Hobson on the associated CSD. Pneumatics Particularly intriguing pneumatic actuators havebeen developed by Rotax and by Plessey. The former is a linear or rotary actuator and, as the sketch shows, is a cam-piston design(from AiResearch) utilizing bleed air as a direct energy source. No "conventional lubrication" or artificial cooling is required.Power output of the rotary actuator is 7.8 h.p. and the linear actuator has a stall load of 2,5001b.Plessey's twin air-motor unit is associated with efflux control of a Bristol-Siddeley engine. The input lever acts as the displace-ment control of a shaft which can rotate in either direction through a selected number of degrees. Output torque is 2001b /hr at100 r.p.m. but the essence of the design is a fast response rate and full proportional feedback to give stiffness. Duplication of (continued on page 464) CONSTANT-SPEED DRIVES Manufacturer Type Nominalsize Outputr.p-m. Frequency control (per cent) Starter capability Full ground check-out capability Oil system Remarks fcnglish ElectricHobson Plessey Ptessey (Ubinal) KOCQX Bleed air only Hydromechanical Hydromechanical Mechanical 4- air motor Electromechanical Mechanical + air turbine 15kVA 40kVA 22ikVA 40-50kVA 4-15kVA 20-60kVA 24,000 6,000 8,000 8,000 2,900-8,000 6 or 8,000 <±2.0±0.5 <±1.0 <±1.0 ±1.0 ±1.0* No No provision •fNo provision Air No provision Air Possible No No Yes No Yes Separate Integral Integral Separate Weiqhs 1121b with alternator For VC10 Max hydraulic transmission 15 per cent. (For D.H.121) No bleed air at cruise and above Cross-bleed operation possible be <±0.25 per cent with special fine control. tHot>son are associated with AEI in the development of starting system using alternator as synchronous motor
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