ACT/FHS System Identification Including Rotor and Engine Dynamics

Abstract

At the German Aerospace Center (DLR) Institute of Flight Systems, models of the Active Control Technology/Flying Helicopter Simulator (ACT/FHS), an EC135 with a fly-by-wire/light flight control system, are needed for control law development and simulation. Thus, models are sought that cover the whole flight envelope and are valid over a broad range of frequencies. Furthermore, if the models are to be used in the feedforward loop of the model following the control system, they have to be invertible and thus should not have any positive transmission zeros. For rotor flapping, the explicit formulation with flapping angles was modified slightly to avoid positive transmission zeros. For the regressive lead–lag, a simple model formulation was found that needs only one dipole with two states. The engine dynamics were first modeled separately and then coupled to the body/rotor model. The final integrated model has 17 states and yields a good match for frequencies up to 30 rad/s. All system identification was performed using the maximum likelihood method in the frequency domain.