First attempts to account for flexible modes in ACT/FHS system identification

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Seher-Weiss, S.
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At the DLR Institute of Flight Systems mathematical models of the ACT/FHS (Active Control Technology/Flying Helicopter Simulator), an EC135 with a highly modified control system, are needed for control system development and simulation. So far, the models that have been derived by system identification account for rotor and engine dynamics. For comfort of ride investigations, and to improve the model quality for frequencies above 20 rad/s, the influence of flexible modes also has to be modeled. For the ACT/FHS the largest effect is the influence of vertical tail bending on pitch rate. The investigation started with a single-input/single-output system for pitch rate response to collective control inputs that was extended by one structural mode for tail flexibility. As this approach was successful, next an identified 17th order model of the ACT/FHS was also extended by one flexible mode. In this model, the structural mode was still dynamically decoupled from the 17th order model and its influence on pitch rate and longitudinal and vertical acceleration was described by influence factors in the output equations. Finally, a one-way coupled hybrid model was identified that extends the influence of the structural modes to other input/output combinations. Accounting for tail flexibility in this way extended the range of validity of the identified model up to the nominal rotor speed of 41 rad/s.