Turboshaft

Turboshaft engines and jet engines use a gas turbine to produce power. While jet engines use mostly the produced thrust as power, turboshaft engines use the thrust to drive a shaft that provides the power.

A turboshaft engine is similar to a turboprop. In principle, a turboprop derives power directly from the shaft to power a propeller mounted in front of the engine through a gearbox and a turboshaft uses a second "free" turbine stage to drive a remote gearbox and some form of power output. The name is most commonly applied to engines driving ships, helicopters, tanks, locomotives and hovercraft or those used as stationary power sources.

The first true turboshaft engine was built by the French engine firm Turbomeca, led by the founder, Joseph Szydlowski. In 1948 they built the first French-designed turbine engine, the 100shp 782. In 1950 this work was used to develop the larger 280shp Artouste, which was widely used on the Aérospatiale Alouette II and other helicopters. The distinct whine of the Artouste is familiar to all those who have watched a 1967 UK television series The Prisoner, since an Alouette was featured in many of the episodes. Note that Artouste is also the name of an unrelated English design, the Blackburn Artouste.

Major efforts were underway in the United States and England to build similar engines. In the US Anselm Franz followed the same principles of simplicity that he used to develop the Jumo 004 in Germany, producing the T53 engine at Lycoming in 1953, and following this with the larger T55. General Electric beat his design into operation with their T58 series.

Today almost all engines are built so that power-take-off is independent of engine speed, using the free turbine stage. This has two advantages:

It allows a helicopter rotor or propeller to spin at any speed instead of being geared directly to the turbine.
It allows the engine to be split into two sections, the "hot section" containing the majority of the engine, and the separate power-take-off, allowing the hot-section to be removed for easier maintenance.

This leads to slightly larger engines—compare the Pratt & Whitney PT-6 and similar models from Garrett Systems, for instance—but for the speed ranges served by these engines it is considered to be unimportant. Today practically all smaller turbine engines come in both turboprop and turboshaft versions, differing primarily in their accessory systems.

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