Unified Engineering Spring Term 2005

Problem P7. (Propulsion) L.O. F

Use ideal cycle analysis to estimate how engine design and performance are related to aircraft performance. Assume the supersonic aircraft in Problem 5 is powered by an ideal turbojet engine. The engine uses a conventional hydrocarbon fuel (h = 42.8 MJ/kg), g = 1.4, c_p = 1000 J/(kg-K), the turbine inlet temperature is 1900K, and the compressor pressure ratio is 15. Assume an altitude of 11km where the pressure is 22.6 kPa, the temperature is 217 K, the density is 0.34 kg/m³, and the speed of sound is 295 m/s.

a) What are the overall efficiency, thermal efficiency and propulsive efficiency of the engine at M = 2.0 and M = 0.8? Explain why each of the efficiencies changes for the two flight speeds.

b) Assuming the weight fraction is the same (but drag and overall efficiency change), by what percent would the range change if the airplane was flown at these two speeds for each of these ideal cycles?

c) If you were asked to change the engine design to improve the range at M= 2 at fixed altitude by 10% what things might you do?