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A gas-turbine aero-engine speed-control system has been investigated on an analogue computer to determine the possibility of using a model reference adaptive control device to maintain constant dynamic behaviour of engine speed irrespective of altitude and aircraft forward speed. A conventional type of speed-control arrangement has been used in the main loop, and an auxiliary feedback loop has been introduced which biases the flow-control unit by an amount which is proportional to the derivative of engine speed. The constant of proportionality is the parameter which is being changed by the adaptive loop. The adjustment is such that a minimum in the value of integral of error squared is sought, where the error signal used is the instantaneous difference between engine and model speed outputs. The results suggest that it would be physically reasonable to produce such a controller, and that substantially similar dynamic behaviour is obtainable under changing environmental conditions. Such a loop could be added to existing speed-control systems without major modifications in design.