A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors. Helicopters are classified
as rotary-wing aircraft to distinguish them from conventional fixed-wing aircraft. The word helicopter is derived from the
Greek words helix (spiral) and pteron (wing). The first single-rotor, fully-controllable helicopter to enter large full-scale
production was made by Igor Sikorsky in 1942.
Compared to conventional fixed-wing aircraft, helicopters are much more complex, more expensive to buy and operate, and
are more limited in speed, range, and payload. The compensating advantage is maneuverability: helicopters can hover in place,
reverse, and above all take off and land vertically. Subject only to refueling facilities and load/altitude limitations, a
helicopter can travel to any location, and land anywhere with enough space (approximately twice the area of the rotor disk).
Compared to other vertical lift aircraft like tiltrotors (V-22 Osprey for example) and vectored thrust airplanes (AV-8
Harrier for example), helicopters are very efficient, carrying more than twice the payload, consuming less fuel in hover and
costing considerably less to buy and operate. However these other configurations have a much higher cruise speed than a helicopter
(270 km/h for a helicopter, 460 km/h for a tiltrotor, 900+ km/h for a vectored thrust airplane).