Abstract: The rotor consists of two airfoils symmetrically disposed around an axis of rotation. Each airfoil consists of an outer vane having a leading edge and a trailing edge and an inner vane having a trailing edge and having a leading edge attached to the trailing edge of the outer vane. The outer vane has a radius R and extends not more than 90 degrees around the axis of rotation. The inner vane extends 180 degrees around an axis that is parallel to but displaced from the axis of rotation. The radius of the inner vane is a fraction, between 0.6 and 0.8, of the radius of the outer vane. Automatic limitation of the speed of rotation is achieved in one embodiment by letting centrifugal force deform the inner vanes so as to reduce the cross sectional area of the air flow path through the rotor.