Development of a VTOL mini UAV for multi-tasking missions

Recent developments in the field of Mini-UAVs lead to successful designs in both hovering rotorcraft and fixed wing aircraft. However, a polyvalent MAV capable of stable hovering and fast forward flight is still expected. A promising candidate for such versatile missions consists of a tilt-body tail-sitter configuration. That concept is studied in this paper both from the flight mechanics and control points of view. Developments are based on an existing prototype called Vertigo. It consists of a tail sitter fixed-wing mini-UAV equipped with a contra-rotating pair of propellers in tractor configuration. A wind-tunnel campaign was carried out to extract experimental results front the Vertigo aerodynamic characteristics. A 6-component sting balance was fitted in the powered model enabling excursion in angles of attack and sideslip angles up to 90 degrees. Thus, a detailed understanding of the transition mechanism could be obtained. An analytical model including propwash effects was derived from experimental results. The analytical model was used to compute stability modes for specific flight conditions. This allowed an appropriate design of the autopilot capable of stabilisation and control over the whole flight envelope. A gain sequencing technique was chosen to ensure stability while minimising control loop execution time. A MATLAB-based flight simulator including an analytical model for the propeller slipstream has been developed in order to test the validity of airborne control loops. Simulation results are presented in the paper including hover flight, forward flight and transitions. Flight tests lead to successful inbound and outbound transitions of the Vertigo.