This invention relates to ring-wing aircraft and is suited particularly, although not exclusively, to use in micro-unmanned air vehicles (UAV's) with ring-wings. An aircraft (10) according to the invention comprises a ring-wing (11) defining a duct (16) with a longitudinally-extending central axis (31), propulsion means (15) located within the duct and moveable aerofoils (13, 18) for controlling the aircraft in flight, the ring-wing being truncated obliquely at one end, that end being the rear (11b) when in horizontal flight, to form a ring-wing with opposed sides of unequal length. This arrangement produces centre of mass offset from the central axis of the ring-wing, the pendulum effect will ensure that the aircraft will roll so that its centre of mass will always be at the lowest height possible when the aircraft is airborne. Therefore the aircraft has a preferred orientation, and the control surfaces can be oriented with respect to this preferred orientation. In addition, the oblique truncation at the rear keeps the centre of mass towards the front of the aircraft thereby giving improved stability in all three axes.