trade name for a type of airplane invented in the early 1920s by the Spanish aeronautical engineer Juan de la Cierva (1895–1936). The term is commonly applied to all planes that use the principle of autogyration to provide lift. The wings of an autogiro are rudimentary or nonexistent; the lift during flight is supplied by a large, multibladed rotor mounted above the fuselage. The plane is equipped with a conventional engine and propeller that pull it forward through the air; the forward motion causes the rotor to gyrate automatically, like a windmill. The rotor of an autogiro, unlike that of a helicopter, is not connected with the engine during flight, except during takeoff in some models. If such a connection is present, the craft is sometimes called a gyrodyne.

As the rotor turns, each blade moves forward on one side of the aircraft and backward on the other, thus creating more lift on the forward-moving side. De la Cierva joined each blade separately at the hub of the rotor so that an individual blade could rise automatically to avoid producing too much lift or fall to avoid producing too little.

Because of the large torque (rotational inertia) produced by the rotor, an autogiro is difficult to maneuver. Because of gyroscopic precession, any attempt to turn the aircraft makes it roll, with possibly disastrous effect. To counteract the torque of the main rotor, de la Cierva mounted a small rotor at the tail. An aircraft so equipped can be directly controlled by varying the pitch or speed of the rotor blades or by tilting the rotors; thus, it requires no conventional rudders, elevators, or ailerons. More recent autogiros have used two main rotors that turn in opposite directions and are mounted either coaxially or at the tips of winglike struts projecting from the sides of the fuselage.

The autogiro can ascend or descend very steeply and can therefore take off and land on extremely small airfields; it cannot, however, climb or descend vertically or hover over one spot as can a helicopter. Autogiros were used during the 1930s for military liaison, delivering mail, exploration, and agricultural purposes. They have now been generally replaced by helicopters.        L.A.B., LESLIE A. BRYAN, M.S., Ph.D.