Thursday, January 12, 2012

Accessory drives

Accessory drives:

1. Accessory units provide the power for aircraft hydraulic, pneumatic and electrical systems in  addition to providing various pumps and control systems for efficient engine operation. The high level  of dependence upon these units requires an extremely reliable drive system.taken from a rotating engine shaft, via an internal  mount for the accessories and distributes the starter may also be fitted to provide an input torque   by-pass engine takes between 400 and 500
2. The drive for the accessory units is typically gearbox, to an external gearbox which provides a appropriate geared drive to each accessory unit. A to the engine. An accessory drive system on a high  horsepower from the engine.
Fig. 7-1 Mechanical arrangement of accessory drives.

Internal gearbox

3. The location of the internal gearbox within the core of an engine is dictated by the difficulties of
bringing a driveshaft radially outwards and the space available within the engine core.
4. Thermal fatigue and a reduction in engine performance, due to the radial driveshaft disturbing the gasflow, create greater problems within the turbine area than the compressor area. For any given engine, which incorporates an axial-flow compressor, the turbine area is smaller than that containing the compressor and therefore makes it physically easier to mount the gearbox within  the compressor section. Centrifugal compressor engines can have limited available space, so the internal gearbox may be located within a static nose cone or, in the case of a turbo-propeller engine, behind the propeller reduction gear as shown in fig. 7-1.
5. On multi-shaft engines, the choice of which compressor shaft is used to drive the internal gearbox is primarily dependent upon the ease of  engine starting. This is achieved by rotating the compressor shaft, usually via an input torque from the external gearbox (Part 11). In practice the high pressure system is invariably rotated in order to generate an airflow through the engine and the high pressure compressor shaft is therefore coupled to the internal gearbox.

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