A helicopter, or auto-gyro, is a type of aircraft that derives lift and propulsion from one or two horizontal rotors. While it is possible to have more than two, no working design in use today does this.
The main advantage of the helicopter is that the horizontal rotor(s) provide lift without moving the aircraft forward. This allows a helicopter to take off and land vertically, without a runway, giving the vehicle type a flexibility that was unknown in conventional, fixed wing aircraft. Even VTOL jets and tilt-rotor aircraft are not as capable in the vertical take off and landing role as helicopters, and these two technologies are unknown outside of military applications.
A helicopter works by means of a horizontally-aligned rotor, spinning from a central mast. In most modern designs, the rotor is partially or fully articulated, allowing the rotor to be tilted to a limited degree, transferring some propulsion from a vertical to a horizontal direction. This allows these helicopters to make more rapid changes in direction (front, back, side to side). In most one rotor helicopters, a small tail rotor is present to counteract the natural effect of having the helicopter turn in the opposite direction as the rotor. One of the advantages of the dual rotor system is that the rotors can spin in opposite directions, counterbalancing each other and avoiding this counter-rotational problem.
Helicopters lose some features of fixed-wing aircraft in gaining their vertical take-off and landing capability. They have limited speed, mostly due to their aerodynamic features and the stall characteristics of their rotor configuration. They also cannot roll or do loops. They typically have low service ceilings (maximum altitude), are noisy, and prone to heavy vibration.
Although there were earlier designs of helicopters that did take flight, these were all impractical. They flew, but did not have all the working control features necessary for practical use. The first example of a practical helicopter was Juan de la Cierva's prototype, first flown in Spain in 1923. The first helicopter sound enough to be mass-produced was the R-4 of the famous Igor Sikorsky, first manufactured in 1942. The first helicopter cleared for civilian use in the US was the Bell 47, introduced in 1946.
