A double airfoil mounted on a structure controlled angularly around a generally vertical axis. The double airfoil includes a fore flap and an aft flap at least one of which has a fore-to-aft asymmetry. Each flap includes a series of shape members distributed in height, wherein the structure includes a fore mast and an aft mast connected by a boom member and a gaff member. The shape members of the fore flap are traversed by the fore mast while being able to turn around an axis defined thereby, while the shape members of the aft flap are traversed by the aft mast while being able to turn around an axis defined thereby. The aft mast is twistable, the shape members of the aft flap are rotationally locked to the aft mast, and the controller can act on the bottom and the top of the aft mast.