Development of Fixed-Wing VTOL UAVs with the Four-Retractable Rotor Propulsion

The operational ease of fixed-wing vertical take-off landing (VTOL) unmanned aerial vehicle (UAVs) derives from their capability to take off and land vertically. This particular capability is achieved through the utilization of an additional rotor propulsion system that operates during take-off and landing. However, the rotor propulsion system, located externally to the airframe, contributes to increasing drag force, especially during the cruise phase, reducing the efficiency and flight time. To overcome the issues, this study proposes a design for a fixed-wing VTOL UAV with the four-retractable rotor propulsion, demonstrating its feasibility and performance through flow simulation and flight tests. During flight tests at a speed of 18 m/s, UAVs with (folded UAV) and without retractable propulsion systems (unfolded UAV) can maintain cruise speeds of 18 m/s at throttle openings of 56.3% and 72.1%, respectively. The energy consumption was reduced by 33.0% followed by increasing in endurance of 32.0%. It aligns with the simulation results, which show that the four-retractable rotor propulsion can reduce the drag coefficient by 35.9% and increase the aerodynamic efficiency (C L /C D ) by 58.3% compared to the unfolded UAV. Both results confirm that the four-retractable rotor propulsion significantly reduces the aerodynamic drag, and increases the efficiency and endurance.