Turboshaft

A turboshaft is a gas turbine arranged to deliver mechanical power through an output shaft. In helicopters, that power passes through the transmission to the rotor systems. Most useful energy is extracted through turbine machinery, while the exhaust leaves after performing that work.^{[1]Helicopter Flying Handbook: Helicopter Components, Sections and SystemsFederal Aviation Administration · faa.gov · Accessed 2026-09-13Open source details}

Gas generator and power turbine

The compressor raises inlet-air pressure before fuel burns continuously in the combustion chamber. Hot gas then passes through turbine stages. One turbine drives the compressor and engine accessories; another extracts power for the aircraft's transmission. ^{[1]Helicopter Flying Handbook: Helicopter Components, Sections and SystemsFederal Aviation Administration · faa.gov · Accessed 2026-09-13Open source details}

In a free-turbine engine, the gas-generator and power-turbine shafts are mechanically independent. Gas flowing between the turbine sections transfers energy even though their shafts can rotate at different speeds. A fixed-turbine arrangement instead couples the compressor and output system mechanically. Gas-generator speed is commonly labelled N1 or NG; power-turbine speed is N2 or NP.^{[1]Helicopter Flying Handbook: Helicopter Components, Sections and SystemsFederal Aviation Administration · faa.gov · Accessed 2026-09-13Open source details}

A modern example

Safran's Arrano uses two centrifugal compressor stages, a high-pressure turbine and a single-stage free turbine. The manufacturer selected this architecture to combine performance with a relatively simple arrangement. The example demonstrates how a helicopter engine divides compression, self-sustaining turbine work and useful output between distinct components.^{[2]Arrano: the cutting-edge engine for helicoptersSafran · safran-group.com · Accessed 2026-09-13Open source details}

Gearing and rotor drive

A reduction gearbox adapts high engine speed to the driven machinery. Safran's CTS800 gearbox transmits mechanical power while reducing rotational speed in the Super Lynx helicopter installation. The gearbox must therefore handle torque and the engine installation's temperature environment as well as provide the required speed relationship.^{[3]CTS800 reduction gearboxSafran · safran-group.com · Accessed 2026-09-13Open source details}

The helicopter transmission distributes power to the main and tail rotors. A freewheeling unit can disconnect an engine that slows below the equivalent rotor speed, allowing the rotor system to continue turning during autorotation. This transmission function is distinct from the engine's free power turbine.^{[1]Helicopter Flying Handbook: Helicopter Components, Sections and SystemsFederal Aviation Administration · faa.gov · Accessed 2026-09-13Open source details}