Power Extraction in Military Aircraft

Abstract: Military aircraft require more and more power. Pneumatic and hydraulic systems are being replaced by electrical equipment and new, power consuming equipmentis introduced in the aircraft. Increased power extraction brings new challenges for the aircraft jet engine, both in terms of operability and with regards to engine performance. This thesis describes how the engine performance of a conventional low bypass ratio mixed flow turbofan engine is affected by power extraction from the highpressure shaft, the low-pressure shaft or a combination of the two. A twin-spool low bypass ratio mixed flow turbofan engine has been developed in a Chalmers in-house tool to evaluate engine performance in different parts of the flight envelope. In order to evaluate the impact of aircraft/engine interaction on flight performance, an aircraft performance tool has been developed as well. A turbine inlet gas temperature increase is required when power is extracted from the HP or LP shaft. This temperature increase is more considerable if power is extracted from the high-pressure shaft, increasing specific thrust and specific fuel consumption. When the engine is running close to, or at the maximum turbine inlet temperature limit, power extraction will have a detrimental impact on the engine performance, whether power is extracted from the high-pressure shaft or the low-pressure shaft, but the thrust reduction will be more substantial if power is extracted from the high-pressure shaft. When the engine is running close to or at the maximum overall pressure ratio limit, the thrust reduction due to high-pressure shaft power extraction is more moderate compared to the low-pressure shaft power extraction case, provided that the required temperature increase is acceptable from an engine operability perspective.

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