A quadrotor equipped with a fixed-wing (hereinafter referred to as the quadrotor tail-sitter UAV) has been developed by authors. The quadrotor tail-sitter UAV can hover like a quadrotor and can fly like a fixed-wing airplane. The remarkable characteristic of the developed quadrotor tail-sitter UAV is that it does not use any control surfaces during any form of flight; hovering, transition flight or level flight. This paper discusses an optimal transition from hovering to level flight for the quadrotor tail-sitter UAV because transition flight is most likely to be unstable throughout flight owing to stalling of the fixed-wing. The paper proposes three transition strategies: (i) normal transition (PID feedback to a step input), (ii) minimizing the transition time, and (iii) minimizing the transition time with keeping altitude constant. To realize the above optimal transition strategies, the aerodynamic parameters of the developed quadrotor tail-sitter UAV are measured by the wind tunnel test in advance. The three strategies are experimentally verified and the comparison among the three strategies is presented.
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