Helicopter

A helicopter is an aircraft which derives most of its lift from a rotating wing system known as a rotor. Helicopters usually get their horizontal thrust from the same wing system by directing the overall rotor thrust away from the vertical.

The rotor is the defining feature of a helicopter, and the complexity of a spinning wing system is a serious limitation on the thrust which can be applied and the forward speed of the aircraft. Although some types have more than one rotor, most rely on an anti-torque system to counteract the spinning motion which the power unit imparts on the body of the helicopter.

Their relatively poor performance is more than compensated by their versatility. One of the few types of aircraft which can hover and perform vertical take-off and landing, helicopters are used for transport where no runway is available - in mountains, on oil platforms, in cities, and in undeveloped parts of the world, as well as police, surveillance, medical evacuation and rescue work.

Most helicopters are powered by turbine engines to take advantage of their superior power-to-weight ratio, although these engines are expensive. Cheaper aircraft such as the very widespread Robinson R22 use piston engines which are heavier, but much cheaper to build. The Robinson R22 is the world's most numerous helicopter and in widespread use for training, although its designer Frank Robinson stated that was never his intention.