The different aspects of overall performance of three variable cycle engines(VCE) candidates for future supersonic civil transport are analysed in this work. Theseaspects concern the design and off-design points performance, the airframe engineintegration and variable geometry compressor and turbine design and performance.The three engines are compared to a traditional turbojet. The variablecompressor maps were obtained with their running lines for the whole mission profileincluding 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 variablegeometry on the compressor stator angles, mixing area and the nozzle throat and exitareas is evaluated. The Fuel bill is estimated for two standard mission profiles.The effect of installation is estimated on an isolated nacelle. A sizing calculationis carried out for the whole nacelle including the intake and the nozzle. The drag due tothe friction, pre-entry, afterbody and the shock waves is calculated in order to estimatedthe installed performance of the three engines.In the search of improving the VCE performance at subsonic cruise, the use ofvariable geometry at the low pressure turbine for the Turbofan-Turbojet engine isinvestigated. The effects of varying the LP turbine guide vanes stagger angle on theengine performance and component parameters are analysed. The turbine efficiency andnon-dimensional mass flow changes due to the use of variable geometry are estimated.An updated version of the Turbornatch program was corrected and tested inorder to study variable cycle engines, especially to simulate the transition from one modeto another.
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