A variable cycle engine (VCE) is a very complex concept. Extensive variable geometry features are required to ensure safe handling characteristics in the two or more operating modes that are envisaged. These may include variable compressor staters, variable nozzle areas and variable bypass valves.

In this paper a method for assessing the optimum setting of control variables is described. The starting point of the analysis is one of the operating modes of the engine, and an optimisation is carried out to decide the new values of the control variables within that operating mode or when changing to another.

The sensitivity of the cycle, in the initial mode, is assessed and an influence coefficient vector is created. This is then employed to decide what setting changes should be implemented to the control variables to achieve the desired result, respecting operating constraints such as surge margins, gas temperatures and rotor speeds.

The engine selected for analysis is the selective bleed engine, although the method could be used for other engine geometries after suitable adaptation. There are two very useful features of this method. The first is that it can give a view of the variable cycle capabilities of an engine with variable geometry features. The second is that it identifies the constraints, enabling the designer to focus the development effort where it is most needed.

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