Incremental Nonlinear Fault-Tolerant Control of the Variable Skew Quad Plane with Loss of Two Opposing Rotors

Abstract

The operation of heavy hybrid Unmanned Aerial Vehicles (UAVs) in populated areas demands robust Fault Tolerant Control (FTC) strategies to ensure safe landings following actuator failures. This paper presents the first successful reallife demonstration of stable relaxed hover on a large hybrid drone, the Variable Skew Quad Plane (VSQP), subject to the total loss of control authority in two opposing multirotor motors while in forward-flight mode. A modified Incremental Nonlinear Dynamic Inversion (INDI) controller is derived to regulate the vehicle's principal axis of rotation. The framework is extended to actively incorporate aerodynamic control surfaces (flaperons), which are shown to significantly improve the rejection of unmodelled aerodynamic and reaction moments. Furthermore, alongside a standard position controller, a novel pusher-motor guidance strategy is proposed and flight-tested. By exploiting the pusher motor the control-authority requirements on the primary stabilization actuators are reduced. The controller achieves fully autonomous tracking of a square trajectory. These results confirm the efficacy of INDI for executing relaxed hover on large quad-planes equipped with aerodynamic surfaces, and demonstrate the benefits of leveraging the full suite of hybrid UAV actuators.