Engines

simple mechanical devices, such as rubber bands or spindles, which relegated the size of helicopters to toys and small models. For a half century steam engines were used to forward the development of the understanding of helicopter aerodynamics, but the limited power did not allow for manned flight. The introduction of theinternal combustion engine at the end of the 19th century became the watershed for helicopter development, as engines began to be developed and produced that were powerful enough to allow for helicopters able to lift humans.

Early helicopter designs first utilized custom-built engines or rotary engines designed for airplanes, but these were soon replaced by more powerful automobile engines and radial engines. The main issue of the power produced by an engine versus the engine's weight was the single most limiting factor of helicopter development. When the compact, flat engine was developed, the helicopter industry found a powerplant easily adapted to small helicopters, although radial engines continued to be used for lager helicopters.

Turbine engines revolutionized the aviation industry, and the turboshaft engine finally gave helicopters an engine with a large amount of power and a low weight penalty. The turboshaft engine was able to be scaled to the size of the helicopter being designed, so that all but the lightest of helicopter models are powered by turbine engines today.

Special jet engines have been developed that drive the rotor from the rotor tips. Tip jets powered by a remote compressor are referred to as cold tip jets. Tip jets powered by combustion exhaust are referred to as hot tip jets. An example of a cold jet helicopter is the Sud-Ouest Djinn, and an example of the hot tip jet helicopter is the YH-32 Hornet.

Some radio-controlled helicopters and smaller, helicopter unmanned aerial vehicles, such as Rotomotion's SR20 use electric motors.^[34] Radio-controlled helicopters may also have piston engines that use fuels other than gasoline, such as Nitromethane.