Modeling and flight control simulation of a quad rotor tail-sitter VTOL UAV

This paper presents the modeling and simulation of a quad rotor tail-sitter VTOL UAV. The VTOL UAV is configured with four rotors for full attitude control and thrust generation for both vertical and forward fight. In the modeling process, a full attitude wind tunnel test was performed on the full scale UAV to capture the UAV aerodynamics. In our model, we explicitly consider the degradation of motor thrust and torque at the presence of forward speed, which is usually neglected in most multi-rotor models. Quaternion based attitude representation is utilized to drive the dynamics of the aircraft. When monitoring and commanding the UAV attitude, ZXY Euler angles are used to avoid the singularity at pitching angle of 90 degrees of commonly used ZYX Euler angles. Other minor considerations include motor dynamics, gyroscopic effect, etc. Such a model captures the UAV kinematics, dynamics, aerodynamics and actuator dynamics, and serves a good reference model for flight controller design and verification. As a testimony of the presented model, an attitude controller with an altitude holding loop will be presented.