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Aviation History
1953
1953 - 1582.PDF
736 FLIGHT Accessory Drive by Air Turbine Rotol Licensing Agreement with AiResearch of the U.S.A.: The First Application UNTIL quite recently, direct mechanical operation, by shaft coupling to the main engine, has been the widely accepted method employed for engine-accessory driving. In some applications it will no doubt continue to be the method used, but in others an improvement is called for. Two factors make a change particularly desirable, namely, the increasing demands on electric power in modern aircraft and the wide differences between r.p.m. at take-off and cruise, and on the approach and landing, in die case of turbine power- units. In addition, the problem of positioning shaft-driven accessories becomes increasingly difficult. Briefly, the requirement is for a constant-speed drive unit, and the air-turbine motor, deriving its motive force from a tapping from the main engine compressor and having an associated governor mechanism, is one method of meeting this requirement irrespective of the main engine operating condition. If a constant- speed drive is provided, alternator output is at a constant fre quency, with resultant simplification of the aircraft electrical system. Additionally, if a number of separate alternators have to be driven independently at a constant speed they can operate in parallel. A second important advantage is that the installation of the turbine unit and gearbox is much easier, because a simple air duct replaces the auxiliary drive shaft. An incidental advantage which is nevertheless important is that in many cases a lighter installation can be achieved with air-turbine drive. Gearbox accessibility in flight is relatively simple to arrange should it be desirable. A licensing agreement has recently been concluded with the AiResearch Division of the Garrett Corporation in America. It allows Rotol, Ltd., "to utilize the extensive and proved technique of AiResearch" in the design and development of Rotol air-turbine units. AiResearch are regarded as the leading producers of air- turbines (and a variety of other items) in America, although other companies in the United States—and at least one in England— are making a somewhat similar approach to the constant-speed drive requirement. The American company state that they believe they were first in the field and that the idea originated from the application of large-turbine technique in the hydro-electrics industry. Their repre sentatives are at present visiting the Rotol factory at Cheltenham Road, Gloucester, and we believe they have been impressed with what they have seen of the facilities and developments there. First employment of the licensing agreement will be for a 100 h.p. Rotal accessory gearbox operating a 15 kVA alternator and a 3 kW generator. A fan is also driven, to cool not only the electrical accessories but the gearbox lubrication system as well. The air-turbine motor driving these accessories is maintained at a constant speed by means of a pneumatically operated gover nor system. Rotol explain that this comprises a speed-sensing Two views of the first Rotol air-turbine unit, to drive a 15 kva alternator and 3 kW generator. The unit has integral lubrication and cooling. device connected to a servo actuato.. The purpose of the actuator is to move variable-area nozzles (in operation, reminiscent of the iris diaphragm of a camera lens) to the correct setting to maintain constant r.p.m. They further point out that variable-area nozzles are an attractive feature of the design because they permit high turbine efficiencies to be maintained irrespective of the condition of the air supply from the engine. Air consumption is less than with a throttle-type control. A high degree of stability is achieved by the governing system and an over-speed cut-out is provided as a safety measure in the event of mal-functioning. Detail of AiResearch's variable-area nozzle design, a diminutive application of large steam- and water-turbine practice. An electrically operated air valve is fitted to the turbine inlet for starting and stopping and, in addition to its primary purposes, it serves as an automatic pressure-regulating valve. Pressure regu lation is required when the aircraft is operating at low altitudes to limit the pressure of the air supplied to the turbine. The accessory gearbox embodies an oil sump with dip-stick and the usual filling, overflow and drain plugs. Provision is made for negative-g conditions. The drives are pressure lubricated with the aid of a pump in the gearbox and the cooler for the oil is provided as an integral part of the sump. After this first application of the air-turbine motor, Rotols plan to develop a range of air-turbine accessory drive units with powers of from 50 h.p. to over 100 h.p. This will be in addition to the accessory drive equipment with which they have hitherto been concerned and which now covers more than 150 different aircraft installations. Although the first Rotol unit is particularly concerned with an alternator and a generator, air-turbine driven accessory gearboxes are suitable for use with other accessories in the same way as their shaft-driven counterparts. Rotol developments which will be announced next year will certainly include gearboxes with drives for a variety of accessories. (See also references to air-turbine motors on page 726.)
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