Effects of Off-takes for Aircraft Secondary-Power Systems on Jet Engine Efficiency

Abstract

Power and bleed-air engine offtakes have a detrimental effect on the efficiency of the engine. This paper presents a thermodynamic analysis that relates the offtakes specific fuel-consumption penalties to the design parameters of the engine. A set of equations is derived in order to determine the fuel-consumption penalties of known power and bleedair offtakes. Their predictions are compared against numerical results performedwith an in-house engine simulation tool. The equations include design parameters that are known in the early preliminary design stages and can be a useful tool for aircraft, engine, and secondary-systems designers. Furthermore, the analytical expressions offer physical insight on the parameters that drive the magnitude of the specific fuel-consumption penalties. It has been found that the main factor driving the magnitude of the penalties is the size of the offtakes relative to the core power. This leads to the conclusion that future engineswith a lower specific thrust factorST=V0, amore efficient fan and lowpressure turbine, and a higher power factor Ppo= T V0 will face increased offtake penalties.