Abstract: A drum is mounted for rotation about an axis. Its surface is substantially cylindrical about the axis, and axially mounted outside of the surface is a plurality of vanes each defining a slot with respect to the surface. The slot is an aerodynamic slot where accelerating air decreases pressure to produce thrust which is in the direction of rotation. The outer surface of the drum is not quite cylindrical, but is shaped to be of larger diameter at the slot to enhance Venturi action.