ocr: HELICOPTER, neavier-than-air craft that derives its lift not from fixed wings like those of conventional airplanes, but from a power-driven rotor or rotors, revolving on: a vertical axis above the fuselage. Helicopters can rise or descend BK-105 (civillan) vertically, hover, and move forward, backward, or sideways. The helicopter was the first type of heavier-than-air craft capable of vertical flight. It differs from the AUTOGIRO (q.v.), another form ofrotating-wing aircraft, in that its rotor gives both lift and propulsion. The rotor ofa helicopter usually has two, three, or four blades radia ...
