A contribution to the development of a full flight envelope quasi-nonlinear helicopter simulation

Helicopter modeling is a complex task as helicopters consist of many subsystems (nacelle, rotor, engine, … ) that are described by coupled differential equations. Controller design requires a model that is accurate over a certain frequency range. Usually, system identification is used to derive the models from flight test data and the model complexity depends on the frequency range of interest. As helicopters have eigenvalues that vary depending on airspeed, system identification is performed at different operating points. Each identified linear model is then valid only in a small region around the corresponding operating point. To arrive at a model that covers the whole operational envelope of the helicopter, the individual models are stitched together. This paper describes the derivation of a quasi-nonlinear model accounting for known nonlinear terms, such as gravity, inertia or trim curves. In addition, the coefficients of the identified linear matrices are interpolated depending on airspeed. Finally, the quasi-nonlinear as well as the linear model are compared against flight test data using model validation techniques. As the quasi-nonlinear model of the Flying Helicopter Simulator is validated, it can be used for a sophisticated controller design.