Control Reconfiguration for a Hexacopter Experiencing Single Rotor Failure

This study examines the operation of a classical hexacopter (with adjacent rotors spinning in opposite directions) in hover and forward flight after a single rotor failure. The simulation model used calculates aerodynamic forces (thrust, drag and side force) and moments (pitching and rolling moments, and torque) at the rotor hub using blade element theory coupled with a finite-state dynamic inflow model to capture the rotor induced velocities. Failure of various rotors is considered individually and an understanding is developed of how the aircraft trims post-failure in both vertex-first and edge-first orientations. The results show that the classical hexacopter trims in hover by turning off the rotor diametrically opposite the failed rotor, and equally speeding up the remaining four rotors for poweroptimal operation. The resulting power penalty is 22% relative to the fully operational hexacopter. In hover, the control sensitivity matrix, however, is rank-deficient, implying that independent control about different axes is not available (affecting its ability to hold position or compensate for disturbances). In cruise flight, the classical hexacopter can be trimmed if any of the forward facing rotors fail. The rotor diametrically opposite to the failed rotor is slowed down but, unlike hover, does not turn off. The power penalty, varying between 20-27%, increases as the position of the failed rotor moves from forward-most to a lateral location on the aircraft. Importantly, the control sensitivity matrix in cruise flight is full-rank, implying the aircraft is fully controllable in cruise. If a rotor aft of the pitch axis fails the hexacopter cannot be trimmed in cruise, owing to the inability to provide nose-down pitching moments while simultaneously satisfying equilibrium about the other axes. Thus, in the event of a forward rotor failure, the classical hexacopter could cruise back to base, momentarily hover into a landing, but not maintain a sustained hover. For an aft-rotor failure, the best solution would appear to be turning the aircraft around so that the failed rotor faces forward.

Reference

McKay, M., Niemiec, R., and Gandhi, F., "Control Reconfiguration for a Hexacopter Experiencing Single Rotor Failure ,"

Proceedings of the 27th International Conference on Adaptive Structures and Technologies, Lake George, New York, Oct 3-5, 2016.