This paper was published as Control Engineering Practice, 2001, 9 (1), pp.1-10. It is available from http://www.sciencedirect.com/science/journal/09670661. Doi: 10.1016/S0967-0661(00)00088-5This paper describes the development and validation of a nine-degree-of-freedom (DOF) helicopter model, and the use of this model in designing H∞ optimal controllers for testing on an experimental fly-by-wire helicopter. The model includes rotor flapping dynamics and an inflow correction factor. Its use has led to improvements (with respect to earlier designs based on a different model) in the robustness properties of the resulting controller. For example, the need for predictor-type filters in the feedback path has been eliminated. Flight-test results are presented. Discrepancies still remain between predicted and achieved performance, but overall handling quality ratings have improved with respect to a previous H∞ design, and clear improvements have been obtained in pitch and roll attitude control: e.g. an increase in roll axis bandwidth from 1.9 to 2.5 rad/s
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