(aviation) A type of control surface, usually found on flying wing aircraft, consisting of two pairs of panels (one mounted near each wingtip on the wing's trailing edge), with one lying on top of the other sandwich-style; to roll the aircraft, both panels on one wing deflect downwards (increasing the lift of that wing) and both panels on the othe wing deflect upwards (decreasing the lift of that wing), while, to yaw the aircraft, the panels on one wing split apart from each other (the upper panel deflects upwards, and the lower panel deflects downwards), increasing the drag on that wing.