In 1922, Henry A. Berliner demonstrated the first helicopter prototype for representatives of the U.S. Bureau of Aeronautics in College Park, Maryland. Berliner made this first ever controlled horizontal helicopter flight in a war-surplus Nieuport 23 fighter with tilting tail rotor, and a short-span upper wing with 14-ft helicopter blades at the tips. It was powered with a Bentley 220 hp engine to the front which turned the two counter-rotating lifting rotors through a series of geared shafts. The two rotors could tilt slightly in opposite directions to control yaw. Two sets of five 36-in x 8-in louvers, located below each rotor, opened and closed differentially to provide roll control by presenting a flat surface, which reacted against the rotor downwash.