Comparative Analysis of the Windmilling Performance of Turbojet and Tbrbofan Engines

The windmilling performance of turbojet and turbofan (TF) engines is investigated numerically by means of a performance synthesis program. Different types of TF engines are taken into consideration, including mixed and unmixed exhaust configurations and configurations with and without a booster compressor on the low-pressure shaft. Furthermore, different design bypass ratios are investigated. The free windmilling performance is calculated for various ram pressure ratios, and, in addition, power input and offtake on the high-pressure shaft are considered. The calculated results indicate a rather strong dependency, not only on the design bypass ratio, but also on the engine type. For example, the internal drag and the maximum power offtake for a given ram pressure ratio differ considerably when comparing mixed and unmixed exhaust configurations. However, for the relight capability, the design bypass ratio is identified as the most important parameter. Nevertheless, the results of this investigation make evident that the rather widespread procedure to present the windmilling performance of engines dependent on ram pressure ratio and design bypass ratio is insufficient.