A Turboprop (Turbo-propeller) or turboshaft engine is a type of gas turbine. It differs from a jet engine in that the design is optimized to produce rotating shaft power in order to drive a propellor, instead of thrust from the exhaust gas.
A jet engine consists of a set of compressor fans that compress the intake air, a flameholder where the combustion happens, and another set of fans (a set of turbine stages) at the rear to catch the some of the hot exhaust and use it to drive the initial compressor fans.
By adding another turbine stage to the engine, all of the jet exhaust can be used for rotary force rather than jet thrust. Coupling this second (or third) turbine stage to a propeller makes for a very efficient engine due to the inherent efficiency of a propeller at low speeds. This is called a turboprop, and can be found on many smaller commuter planes, cargo planes, and helicopters (where it is often known as a turboshaft).
Propellers lose efficiency as aircraft speed increases, which is why turboprops are not used on higher-speed aircraft.
The first turboprop engine was the Rolls-Royce RB.50 Trent, a converted Derwent II fitted with reduction gear and a Rotol 7' 11" five-bladed propeller. Two Trents were fitted to Gloster Meteor EE227 - the "Trent-Meteor" - which became the first turboprop powered aircraft.
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