Preliminary Design Method and Prototype Testing of a Novel Rotors Retractable Hybrid VTOL UAV

Conventional hybrid VTOL (Vertical Take-Off and Landing) aircraft are always designed by directly mounting multicopter rotors onto a fixed-wing plane. This type of VTOL aircraft may suffer from terrible drag due to the copter rotors. To tackle this problem, this paper describes a novel rotors retractable hybrid VTOL UAV (RR VTOL) that combines the hovering capability of multicopters and the high cruise velocity property of pure fixed-wing aircraft. Computational fluid dynamics (CFD) analysis on rotors folding and unfolding models proves that the retractable rotors can increase the lift-to-drag ratio by approximately two times compared to that of the common hybrid VTOL UAV. Moreover, some aerodynamic performance data of the rotors folded model are also acquired. A preliminary design method for this creative UAV is proposed with new empirical equations derived from many hybrid VTOL UAVs’ data. Emphasis is given to the method for determining the center of gravity position and horizontal tail dimensions as they are the unique aspects of the RR VTOL configuration. Subsequently, the manufacturing process and rotor retraction mechanism of two UAV generations are reviewed and a rapid prototyping method for RR VTOL is summarized. Finally, a successful flight test confirms that the proposed VTOL concept is practical and the drag reduction of rotors retracting function is remarkable.