Abstract: A fan having a single degree of freedom to torsionally oscillate in a horizontal plane which in one version oscillates about a fixed vertical axis, self-excited by an aerodynamic couple created by feathering vertically disposed vanes which are symmetrically located adjacent the tips of the fan blades on the air inflow side thereof, and in another version which oscillates about a pendulous axis, self-excited by precession of the rotating fan and motor armature as a pendulous gyro.