helicopter, type of aircraft in which lift is obtained by means of one or more power-driven horizontal propellers called rotors. When the rotor of a helicopter turns it produces reaction torque which tends to make the craft spin also. On most helicopters a small rotor near the tail compensates for this torque. On twin-rotor craft the rotors spin in opposite directions, so their reactions cancel each other. The helicopter is propelled in a given direction by inclining the axis of the main rotor in that direction. The helicopter's speed is limited by the fact that if the blades rotate too fast they will produce compressibility effects on the blade moving forward and stall effects on the rearward–moving blade, at the same time.
The method of flight used by the helicopter was considered by Leonardo da Vinci, in the 16th cent., who described its possibilities but could not provide a propulsion system. Best known among its developers are the French inventor Louis Breguet and the engineers Igor Sikorsky of the United States and Juan de la Cierva of Spain. The helicopter has become very popular for short-distance transportation, because of its maneuverability and ability to land and take off in small areas; it has been adopted for a wide range of services, including air-sea rescue, fire fighting, traffic control, oil platform resupply, and business transportation.
Helicopters have been widely adopted by the military since their first appearance during the Korean War. During the Vietnam War, they became the preferred platforms for transporting troops and evacuating wounded; in the Persian Gulf conflict helicopter gunships provided air cover for advancing tanks. Beginning in early 21st cent. some military helicopters were modified with stealth technology to make them more difficult to track on radar.
See A. Gessow and G. C. Myers, Aerodynamics of the Helicopter (1967); W. Johnson, Helicopter Theory (1984).
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