The trend in military aircraft is toward increasing thrust loading for improved maneuverability coupled with a requirement for extended subsonic cruise range at low power settings. Conventional turbine engines designed to meet these requirements must operate over large ranges of airflow between maximum power and cruise. As a result, the inlets and nozzles designed for these engines cannot perform efficiently with the low airflow rates typical of subsonic cruise operation. Variable turbine geometry, however, offers a promising approach for obtaining both high thrust loading and efficient cruise performance by permitting large amounts of thrust modulation at constant airflow rates. As an example, the performance of a turbojet engine, which provides efficient high-thrust maneuvering and supersonic operation, can be improved by variable turbine geometry to the point where it is competitive with a fixed-turbine-geometry turbofan engine in the low-thrust subsonic cruise regime. (Modified author abstract).

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The different aspects of overall performance of three variable cycle engines (VCE) candidates for future supersonic civil transport are analysed in this work. These aspects concern the design and off-design points performance, the airframe engine integration and variable geometry compressor and turbine design and performance. The three engines are compared to a traditional turbojet. The variable compressor maps were obtained with their running lines for the whole mission profile including the transition mode from medium býpass ratio to a lower bypass ratio turbofan. The specific fuel consumption (SFC) of the VCEs showed a significant improvement, especially at subsonic cruise, relative to a Turbojet engine. The extent of the variable geometry on the compressor stator angles, mixing area and the nozzle throat and exit areas is evaluated. The Fuel bill is estimated for two standard mission profiles. The effect of installation is estimated on an isolated nacelle. A sizing calculation is carried out for the whole nacelle including the intake and the nozzle. The drag due to the friction, pre-entry, afterbody and the shock waves is calculated in order to estimated the installed performance of the three engines. In the search of improving the VCE performance at subsonic cruise, the use of variable geometry at the low pressure turbine for the Turbofan-Turbojet engine is investigated. The effects of varying the LP turbine guide vanes stagger angle on the engine performance and component parameters are analysed. The turbine efficiency and non-dimensional mass flow changes due to the use of variable geometry are estimated. An updated version of the Turbornatch program was corrected and tested in order to study variable cycle engines, especially to simulate the transition from one mode to another.