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Convertible unmanned aerial vehicle (UAV) combines advantages of convenient autonomous launch/recovery and efficient long range cruise performance. Successful design of this new type of aircraft relies heavily on good understanding of powered lift generated through propeller-wing interactions, where the velocity distribution within propeller slipstream is critical to estimate aerodynamic forces during hover condition. The present research studied a propeller-wing combination with a plain flap. A 5-hole probe measurement system was built to construct three-dimensional (3D) velocity field at a survey plane after wing trailing edge. The study has found that significant deformation of propeller slipstream was present in the form of opposite transverse displacement on extrados and intrados. The deformation could be enhanced by flap deflections. Velocity differences caused by the slipstream deformation could imply local variation of lift distribution compared to predictions from conventional assumptions of cylindrical slipstream. An analytical method was developed to reasonably estimate the position of deformed slipstream centreline. The research underlined that the mutual aspect of propeller-wing interaction could be critical for low-speed aerodynamic design.

Convertible UAVs unlock a new range of applications, by combining the flight features of vertical take-off and landing (VTOL) and fixed-wing UAVs. Tilt-rotor UAV (TRUAV) is a popular category, in which switching from one flight mode to another is achieved by tilting some or all the rotors. In this work, we consider a new TRUAV design that does not include any control surfaces. This design, which we named control-surface-free TRUAV (CSF-TRUAV), exploits only propellers to control the drone’s position and attitude in both VTOL and fixed-wing modes. We also consider a control scheme that, unlike existing works, uses a single controller to handle both flight modes. This makes the transition from VTOL to cruise mode no longer an issue. This control scheme is implemented using a sliding mode controller (SMC), and validated on the full nonlinear model of the CSF-TRUAV, including all coupling and aerodynamic effects. The obtained results show the incapacity of a first-order SMC in dealing with the aerodynamic forces and moments, which act as external perturbations if they are not accurately estimated and fed to the controller. To deal with this issue, a super-twisting SMC (ST-SMC) is designed. The ST-SMC was capable of accurate trajectory tracking in both VTOL and fixed-wing modes.