Air turbine drives: Performance and limitations

THE practice in modern jet aircraft of using axial-flow jet engine compressors has decreased the number of engine pads available for driving accessories. This, coupled with the presence of a high-pressure air supply as a source of power, has brought about the introduction of the air turbine as a prime mover. Since the energy level of the jet engine bleed air is dependent upon the engine setting, which is in turn dependent upon the airplane flight condition, the design of the turbine drive will be dictated by, first, the flight condition which results in a minimum energy supply to the prime mover and, second, the power that must be developed by the prime mover. These factors will influence the performance of the unit at the airplane cruise condition where air consumption is most critical. The first section of this discussion is concerned with the imposed requirements at the design point, and the penalty involved from the standpoint of performance, at the cruise condition. Subsequent sections will deal with the turbine configuration and the turbine control system response rate.