A method of transitioning an unmanned aerial vehicle from a vertical flight mode to a horizontal flight mode, the method comprising: providing a UAV having: a wing structure comprising elongated equal first and second wings, a support structure comprising first and second sections coupled to a middle position of the wing structure and extending in opposite directions perpendicular to the wing structure, four propellers, each mounted to a respective one of the first and second wings, and first and second sections and arranged to power the UAV during both the vertical and horizontal flight modes; operating the propellers to perform a vertical take-off; once a predetermined height and a speed equal to at least a stall speed of the UAV is reached, initiating a circular manoeuvre with aerodynamic lift developed in the radially inward direction; performing a momentary inverted flight of the UAV at a maximum point of the circular manoeuvre; performing a downward flight of the UAV; and completing the transition to start flying in a horizontal flight mode, wherein the circular manoeuvre has a radius controlled by adequately powering and controlling the speed of the UAV so that the angles-of-attack of the wing structure are within a linear, pre-stall range throughout the transition. providing a UAV having: a wing structure comprising elongated equal first and second wings, a support structure comprising first and second sections coupled to a middle position of the wing structure and extending in opposite directions perpendicular to the wing structure, four propellers, each mounted to a respective one of the first and second wings, and first and second sections and arranged to power the UAV during both the vertical and horizontal flight modes; operating the propellers to perform a vertical take-off; once a predetermined height and a speed equal to at least a stall speed of the UAV is reached, initiating a circular manoeuvre with aerodynamic lift developed in the radially inward direction; performing a momentary inverted flight of the UAV at a maximum point of the circular manoeuvre; performing a downward flight of the UAV; and completing the transition to start flying in a horizontal flight mode, wherein the circular manoeuvre has a radius controlled by adequately powering and controlling the speed of the UAV so that the angles-of-attack of the wing structure are within a linear, pre-stall range throughout the transition. a wing structure comprising elongated equal first and second wings, a support structure comprising first and second sections coupled to a middle position of the wing structure and extending in opposite directions perpendicular to the wing structure, four propellers, each mounted to a respective one of the first and second wings, and first and second sections and arranged to power the UAV during both the vertical and horizontal flight modes; operating the propellers to perform a vertical take-off; once a predetermined height and a speed equal to at least a stall speed of the UAV is reached, initiating a circular manoeuvre with aerodynamic lift developed in the radially inward direction; performing a momentary inverted flight of the UAV at a maximum point of the circular manoeuvre; performing a downward flight of the UAV; and completing the transition to start flying in a horizontal flight mode, wherein the circular manoeuvre has a radius controlled by adequately powering and controlling the speed of the UAV so that the angles-of-attack of the wing structure are within a linear, pre-stall range throughout the transition.