Turboshaft engines are a type of jet engine, differing from the similar turboprop largely in name and use -- turboshaft engines are used in helicopters, the name referring to the way the torque of the engine drives the power shaft at 90 degrees to the engine mounting. For this reason the shaft is often mounted near the middle of the engine, to place equal stress on either side of the main power shaft through the engine.
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 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.
The term originally referred to the way the power-take-off was independant of engine speed, allowing the helicopter rotor to spin at any speed instead of being geared directly to the turbine. Today almost all engines are built in this fashion, as it allows the engine to be split into two section, 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 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. The name may no longer be useful, but it remains common.
