Use of harmonic decomposition models in rotorcraft flight control design with alleviation of vibratory loads

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Saetti, U.
Horn, J.F.
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An Explicit Model Following (EMF) control scheme is designed to achieve stability and desired Rate Command / Attitude Hold (RCAH) response around the roll, pitch and yaw axes, while alleviating vibratory loads through both feed-forward and feedback compensation. First, the effect of command model tailoring is explored to understand the effect of feed-forward compensation on vibratory loads, with a focus on the main rotor pitch links. Secondly, the harmonic decomposition methodology is extended to enable optimization of primary flight control laws that mitigate vibratory loads. Specifically, Linear Time Periodic (LTP) systems representative of the periodic rotorcraft dynamics are approximated by Linear Time Invariant (LTI) models, which are then reduced and used in LQR design to constrain the harmonics of the vibratory loads. The gains derived are incorporated in the EMF scheme for feedback compensation. Finally, simulation results with and without load alleviation are compared and the impact of feed-forward and feedback compensation on handling qualities is assessed in terms of ADS-33E specifications.