Abstract: First and second windmill wheels are mounted on and non-rotatably connected by shaft means, which define a vertical axis. Each of said windmill wheels has rigidly interconnected vanes, which are adapted to return opposite to the direction of the wind. Said two windmill wheels define axially outermost flight circle planes which intersect at an acute angle. The vanes of each of said windmill wheels are adapted to describe an orbital line, which is tangent to a corresponding orbital line described by the vanes of the other windmill wheel at an apex, which is disposed between said flight circle planes. Control means are operable to move said windmill wheels relative to each other so as to move said apex about said vertical axis and thus to vary said angle when measured in a vertical plane that is at right angles to the direction of the wind.