Turboshaft engines
A turboshaft engine is a form of gas turbine which is optimized to produce free turbine (see graphic at right) shaft power, rather than jet thrust. In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power.
Turboshaft engines are commonly used in applications which require a
sustained high power output, high reliability, small size, and light
weight. These include helicopters, auxiliary power units, boats and ships, tanks, hovercraft, and stationary equipment.
Turboshaft engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants) and are used to power almost all modern helicopters. The first shaft bears the compressor and the high speed turbine (often referred to as "Gas Generator"), while the second shaft bears the low speed turbine (or "Power Turbine" or "free wheeling turbine" on helicopters specifically due to the fact that the gas generator turbine spins separately from the power turbine). This arrangement is used to increase speed and power output flexibility.
