In 1922, George de Bothezat built a helicopter with four six-bladed rotors, under the sponsorship of the U.S. Army. Powered with a 180-hp (134-kW) engine and weighing 3,600 pounds (1,633 kilograms), it carried up to three passengers in more than 100 flights at McCook Field near Dayton, Ohio. However, the helicopter never rose higher than 15 feet (4.6 meters). [ larger image ]

Credits - Library of Congress