Adaptive model inversion control of a helicopter with structural load limiting

An adaptive control system capable of providing consistent handling qualities throughout the operational flight envelope is a desirable feature for rotorcraft. The adaptive model inversion controller with structural load limit protection evaluated here offers the capability to adapt to changing flight conditions along with aggressive maneuvering without envelope limit violations. The controller was evaluated using a nonlinear simulation model of the UH-60 helicopter. The controller is based on a well-documented model inversion architecture with an adaptive neural network (ANN) to compensate for inversion error. The ANN was shown to improve the tracking ability of the controller at off-design point flight conditions; although at some flight conditions the controller performed well even without adaptation. The controller was modified to include a structural load-limiting algorithm to avoid exceeding prescribed limits on the longitudinal hub moment. The limiting was achieved by relating the hub moment to pitch acceleration. The acceleration limits were converted to pitch angle command limits imposed in the pitch axis command filter. Results show that the longitudinal hub moment response in aggressive maneuvers stayed within the prescribed limits for a range of operating conditions. The system was effective in avoiding the longitudinal hub moment limit without unnecessary restrictions on the aircraft performance.