Turboprop
A Turboprop 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, instead of thrust from the exhaust gas.
A turboprop builds onto a basic turbojet jet engine by only using the power of the jet engine to drive a propeller.
Remember that a jet engine consists of a set of compresssor 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 idea of grabbing the power from the exhaust of a jet engine and using it for something other than direct thrust is quite useful. Other applications include an APU (auxiliary power unit), where a generator creates electrical power.
APU's are used for many applications, and are excellent at being light-weight, very reliable power sources. Almost all large aircraft include a much smaller engine to provide power while parked at the airport, called an auxiliary power unit. The small pop-up doors often visible near the tail of an airliner are the air vents for these engines. APU's are found in many industrial applications, peak-demand power generation stations, and military ships.
