Model-following control of a military helicopter with damage mitigation

This paper investigates the feasibility and potential benefits of damage mitigating control (DMC) for rotorcraft. The controller is applied to regulate rotor speed and control vertical speed and pitch attitude on a military helicopter, with the objective of minimizing damage to the main rotor transmission, while maintaining Level 1 handling qualities over a range of flight conditions. An explicit model-following control scheme is used, in which the feedback portion is designed using an LQR solution. The controllers are gain scheduled with damage reduction and total airspeed as parameters. The damage is represented by the length of a crack in the main bevel pinion of the helicopter transmission. The results of nonlinear simulations over a range of airspeeds show good tracking performance with no damage mitigation, and an acceptable tradeoff of performance for damage reduction at higher levels of damage mitigation. Evaluation of handling qualities against the ADS-33E specification shows excellent performance (Level 1) with no damage mitigation. As the amount of damage mitigation increases, handling qualities degrade, moving closer to, or into, Level 2 for large amounts of damage mitigation. The controller has been integrated with an upper level supervisory control system that selects the damage mitigation level based on the current damage level and mission scenario of the aircraft.