Current gas turbine auxiliary power units (APUs) for aircraft main engine starting and secondary power operation are configured with pneumatic and electric links, and in some applications with a direct shaft power link. Respective examples are the pneumatic/electric link used in the S-70B helicopter and the shaft power link in the FSX fighter aircraft.
Future high performance aircraft will require more compact and lighter weight APUs. These APUs will require higher thermal efficiency and durable self-sufficient secondary power equipment, capable of reliable faster starting over a wider aircraft operating range.
Gas turbine auxiliary power design configurations and technology requirements to meet these objectives are examined here for both commercial and military aircraft applications.
The trend from existing APUs supplying mixed pneumatic and electric power outputs for main engine starting, environmental control, and electrical services, to all-electric output is paced by high speed starter/generator technology and accessory drive requirements.
Extended operating range aircraft, plus secondary power system growth and the effect upon main engine operating economics also influence the continuous duty APU option.
This paper discusses the impact of these and other emerging application trends on APU system design. It presents candidate solutions with associated technology improvements for continuing research into the next century.