Helicopter Engine

an aircraft engine designed to drive one or more main rotors on a helicopter. It may be a piston or jet engine. Rocket engines may also be used as auxiliaries to increase power temporarily when the helicopter is taking off or landing. Single-shaft turboprop airplane engines have been used in some helicopters, but they are being supplanted, especially in multiengine helicopters, by two-shaft turboprop engines with a so-called free turbine. In such engines the turbocompressor has no mechanical connection to the main rotor. The use of a two-shaft engine increases the efficiency of use of the helicopter’s power plant, which—irrespective of the speed of rotation of the turbocompressor—establishes the most favorable speed of rotation of the main rotor for all flight conditions. Two-shaft engines with a free turbine ensure the highest operating reliability for the propulsion system.

A jet drive for the main rotor is also possible. In this case the circumferential force is applied directly to the blades of the main rotor without using a heavy and complicated mechanical transmission. The circumferential force is developed either by independent jet engines mounted on the main rotor blades or by the discharge of a gas (compressed air) from nozzle openings located at the ends of the blades. A jet drive is less economical than the mechanical drive. The jet drive with turbojet engines on the rotor blades is the most economical of the jet-propulsion systems, but it has not come into practical use because of the complicated construction.