RAH-66 COMANCHE SECONDARY POWER SYSTEM TRADE STUDY

The following work describes the trade study methodology and alternative system approaches considered in defining the secondary power system for the RAH-66 Comanche aircraft, The objective of the study centered on defining the optimum secondary power system considering fixed accessory power losses (and the impact of same on aircraft performance), weight, reliability, maintainability and life cycle cost, i.e., development, acquisition, and operation and support. The alternative systems presented include systems which utilize one of the aircraft main engines for ground power as well as systems which employ auxiliary power units to accomplish same. The type of air conditioning cycle, i.e., vapor cycle or air cycle, is considered as a variable. In all cases, the subsystems which in total comprise the secondary power system, i.e., electrical, hydraulic, pneumatic, transmission, power unit, starting and environmental control are all included in the discussion considering their individual design requirements and attributes. The study concludes that the best secondary power source for the Comanche is a dedicated power unit, called a secondary power unit or SPU, which is utilized to provide accessory power for ground operation as well as in flight power to selected secondary systems. This approach represents a departure from typical U.S. aircraft design practice in that auxiliary power units are typically used for ground operation and are turned off prior to or shortly after take-off.