An Analysis of Classical and Alternate Hexacopter Configurations with Single Rotor Failure

This study examines the operation of a hexacopter in hover and forward flight conditions with a single rotor failure. A classical configuration, with adjacent rotors spinning in opposite directions, and an alternate configuration are considered. The simulation model used in this study 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. For the alternate configuration hexacopter, if one of four rotors (out of six) fails, the aircraft can be trimmed in hover as well as in forward flight, and is fully controllable. But recovery from failure of one of the other two rotors for fully trimmed flight is impossible. For the classical configuration hexacopter, if any of the forward facing rotors fail, the aircraft can be trimmed in forward flight and is fully controllable. If an aft rotor fails, the aircraft cannot be trimmed, but could be turned around to orient the failed rotor forward. While the classical configuration can always be trimmed in hover by turning off the rotor diametrically opposite rotor to the failed rotor, the aircraft is not independently controllable about all axes due to a rank deficient control matrix. Thus, in the event of a rotor failure, the classical configuration hexacopter could cruise back to base and land, but not maintain a sustained hover. Power penalties of up to 23% were observed in the event of failure due to the increased induced and profile drag on the operational rotors.