Hybrid UAV Attitude Control using INDI and Dynamic Tilt-Twist

The increased search for the performance of Unmanned Aerial Vehicles (UAVs) has led to an interest in hybrid concepts like the tail-sitter UAV. A tail-sitter UAV is capable of combining vertical take-offs and landings (VTOL) with efficient long-endurance forward flights. During hover, the wings do not provide lift but instead act as disturbance and limit the yaw response. Attitude control based on direct quaternion feedback does not take the differences in reaction speed for the three axes into account. Tilt-twist control has been proposed to overcome this problem as it splits the faster tilt (pitch and roll) from the slower and less important twist (yaw) and is successfully applied to quadrotor control. This paper proposes a novel tilt-twist controller based on Incremental Nonlinear Dynamic Inversion (INDI). But in tail-sitter UAVs, the lift vector can differ a lot from the tilt angle, especially when partly or fully transitioned to forward flight. To address this, a dynamic tilt-twist controller is proposed that redefines the twist according to the transition angle. Simulations and test flight tests are performed with the Neder- Drone hybrid tail-sitter to show the increased performance.