TABLE OF CONTENTS "The shortest and surest way to live with honor in this world is to be in reality what we would appear to be."
Socrates
Accessory units provide the power for 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.
The drive for the accessory units is typically taken from a rotating engine shaft, via an internal gearbox, to an external gearbox that provides a mount for the accessories and distributes the appropriate geared drive to each accessory unit. A starter may also be fitted to provide an input torque to the engine. An accessory drive system on a high bypass engine takes between 400 and 500 horsepower from the engine.
The location of the internal gearbox within the core of an engine is dictated by the difficulties of bringing the driveshaft radially outwards and the space available within the engine core.
Thermal fatigue and a reduction in engine performance, due to the radial drive shaft disturbing the gas flow, creates 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 located within a static...
