Reconfigurable helicopter flight control methods for main rotor actuator failures

While reconfigurable flight control has been demonstrated on fixed-wing aircraft using control surface redundancies, such failure accommodations were widely believed to be impossible for single main rotor helicopters. In fact, on any such existing helicopter the failure of a main rotor actuator is catastrophic, resulting in a complete loss of control. This paper presents an actuator geometry that enables reconfigurable helicopter flight control and, from this, develops reconfigurable flight control methods to accommodate main rotor actuator failures. The geometry provides control axes coupling which is key to solving the reconfigurable flight control problem. Such coupling enables the development of a swash-plate reconfiguration strategy where any two of the three control axes can be controlled at a time, despite a failure in any one of the main rotor actuators. Of most interest is the particular swashplate reconfiguration solution where attitude control (pitch and roll) is retained, while sacrificing vertical control. Vertical control is then achieved by one of two methods: flying to a longitudinal velocity that supports the desired vertical velocity, or by using rotorspeed to change the aircraft's lift in order to perform limited closed loop vertical control. These methods are then combined to form a robust reconfigurable flight control method. All of these methods are independent of the underlying flight control system but are demonstrated here using a classic PID controller. All designs are tested using FLYRT, a sophisticated nonlinear validated model of the Apache helicopter.