I. Introduction
Autonomous flight of Unmanned Aerial Vehicles (UAVs) remains an interesting and active research domain after decades of studies in the subject. The wide variety of missions involving UAVs, combined with advances in the fields of materials and computer science, have contributed to the design of new UAV configurations. Therefore, the drawbacks of rotatory-wing UAVs (e.g. helicopters, quad-rotors and multi-rotors) in terms of endurance and range, with the lack of capability to take-off and landing from small areas of fixed-wing UAVs have also encouraged the development of a new UAV class namely hybrid UAV. This hybrid UAV configuration is able to perform complex flight missions in windy environments through its large flight envelope, as described in Fig. 1, with vertical take-off and landing with fast and efficient forward flight to reach a distant position. Although the combination of two different UAV configurations in a single one provides a wider application field, the control system needs to consider the particularities of each one in order to properly carry out the position tracking, velocity control and attitude stabilization during the entire flight envelope for a given mission.
Typical flight modes of hybrid unmanned aerial vehicles: 1 - vertical take-off; 2 - transition; 3 - forward flight; 4 - hover flight; 5 - vertical landing. The vector represents the wind disturbances.