A device for providing a variable retarding torque upon a rotating shaft includes an aerodynamic rotor and control means which is responsive to independent and external signals to continuously vary the ratio of the speed of the aerodynamic rotor as compared to the speed of the rotating shaft. The control...http://www.google.com/patents/US4994003?utm_source=gb-gplus-sharePatent US4994003 - Apparatus using aerodynamic rotors for exerting a braking torque upon a rotating shaft
