Adaptive model inversion control of a helicopter with structural load limiting

An advanced control system capable of providing consistent handling qualities throughout the operational flight envelope is a desirable feature for future rotorcraft. Performance and agility requirements and ADS-33 handling quality requirements for military rotorcraft have driven the need for high bandwidth control systems. The present work evaluates an adaptive model inversion based controller to achieve these requirements. The controller was tested using a non-linear simulation model of the UH-60A helicopter. An Attitude Command Attitude Hold response type is used for roll and pitch axes and Rate Command Attitude Hold response type is used for yaw axis. A linear model at a single design point is used for the model inversion controller. An adaptive neural network (ANN) is used in the pitch and roll channels to compensate for errors when operating at off-design points. The ANN improves the tracking ability of the controller for all speeds. High bandwidth control can also lead to higher structural loads on rotorcraft. Therefore, a modification to the controller has been developed in order to avoid exceeding prescribed limits on longitudinal and lateral hub moment. The load limiting function is achieved by limiting internal variable within the pitch and roll axis command filter. Results show that the longitudinal hub moment response stays within the prescribed limits during aggressive maneuvers for a range of operating conditions. The lateral hub moment limiting system was not effective due to a second transient peak in the opposite direction of the pilot command.