YawSitter: Modeling and Controlling a Tail-Sitter UAV with Enhanced Yaw Control

TL;DR

This work tackles the challenge of achieving precise lateral dynamics and yaw control for hover in tail-sitter UAVs by introducing a differential slipstream-based lateral force model that leverages asymmetric propeller thrust. The authors combine a gravity-aware YXZ attitude representation with a cascaded nonlinear PID framework to enable yaw-based lateral tracking without roll coupling, supported by CFD-derived aerodynamic coefficients. Validation is conducted in a Python–Unity simulation environment across rectangular and circular hover trajectories, showing low mean absolute position errors and yaw deviations constrained to $"5.688^{\circ}"$ (max observed in circular tests). The approach offers a practical route to agile, hover-capable tail-sitter UAVs and lays groundwork for robust real-time control, with future work aimed at eliminating attitude singularities via quaternion-based control and expanding verification through SIL testing and physical prototyping.

Abstract

Achieving precise lateral motion modeling and decoupled control in hover remains a significant challenge for tail-sitter Unmanned Aerial Vehicles (UAVs), primarily due to complex aerodynamic couplings and the absence of welldefined lateral dynamics. This paper presents a novel modeling and control strategy that enhances yaw authority and lateral motion by introducing a sideslip force model derived from differential propeller slipstream effects acting on the fuselage under differential thrust. The resulting lateral force along the body y-axis enables yaw-based lateral position control without inducing roll coupling. The control framework employs a YXZ Euler rotation formulation to accurately represent attitude and incorporate gravitational components while directly controlling yaw in the yaxis, thereby improving lateral dynamic behavior and avoiding singularities. The proposed approach is validated through trajectory-tracking simulations conducted in a Unity-based environment. Tests on both rectangular and circular paths in hover mode demonstrate stable performance, with low mean absolute position errors and yaw deviations constrained within 5.688 degrees. These results confirm the effectiveness of the proposed lateral force generation model and provide a foundation for the development of agile, hover-capable tail-sitter UAVs.

Figures (5)

• Figure 1: Tail-sitter frames description in UNITY simulation
• Figure 2: Aerodynamic coefficient variation with angle of attack and flap deflection: (a) Moment coefficient $C_m$, (b) Lift coefficient $C_L$, and (c) Drag coefficient $C_D$.
• Figure 3: Controller diagram logic. The trajectory generator provides reference positions and attitudes to the position and attitude controllers. The position PID outputs desired linear velocities, while the attitude PD outputs desired angular rates. Velocity and rate controllers compute the required forces and moments, which are mapped by control laws to actuator commands $(\delta_R, \delta_L, T_L, T_R)$.
• Figure 4: Rectangular trajectory tracking of tail-sitter,(a) Positions responses(m) for $(x, y, z)$, (b)Attitude response of $(\phi, \theta, \psi)$$^\circ$, and (c) Real-time simulation in UNITY.
• Figure 5: Circular trajectory tracking of tail-sitter,(a) Positions responses for $(x, y, z)$, (b)Attitude response of $(\phi, \theta, \psi)$$^\circ$, and (c) Real-time simulation in UNITY.