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Turboshaft and Turboprop

- question from name withheld

This question makes reference to two past articles discussing the types of jet engines and the turboprop. We have seen that both the turboprop and the turboshaft are types of jet engines because they use the gas turbine cycle. A gas turbine engine works by combusting a mixture of air and fuel to generate a high-speed exhaust. As this exhaust escapes, it passes through a series of blades called a turbine causing the blades to rotate. This rotation converts the thermal energy of the exhaust gases into mechanical energy.

Schematic of a turboshaft engine

The turboshaft engine receives its name from the fact that this rotation is used to turn a shaft of some kind. In the aerospace field, the turboshaft engine is used on helicopters. The rotating shaft of the engine is used to turn the rotor blades that provide lift and forward motion of the vehicle. However, this application is only one of many to which the turboshaft has been applied. The power industry also makes use of turboshaft engines to generate electricity. In this case, the rotating shaft is used to rotate a coil through a magnetic field to generate electrical current. Turboshafts are also used in ships and boats where the rotating shaft turns the vehicle's propeller. Turboshafts can even be used on ground vehicles, particularly military tanks and some racecars.

Schematic of an electrical generator

You are correct that there is a strong relationship between the turboshaft and another type of jet engine called the turboprop. The turboprop also uses a gas turbine core to turn a shaft. However, the turboprop gets its name from the fact that the shaft is connected to an aircraft propeller causing it to turn. Nevertheless, you are incorrect in saying that a turboprop must have a direct connection to the propeller. Perhaps your confusion is based on the following example provided in one of our previous explanations about this subject. This particular design does indeed illustrate a propeller that is directly connected to the engine shaft.

Schematic of a turboprop engine

However, most turboprops, like that below, are not directly connected. The engine shaft is instead attached to a gearbox that is connected to the propeller. This system includes reduction gears that allow the engine shaft and propeller to spin at different rates.

Schematic of a turboprop engine with a gearbox

It would be incorrect to call these engines turboshafts just because of how the engine shaft and propeller are connected. Any jet engine that turns a propeller is called a turboprop for that reason. However, it is correct to say that the turboprop is a subcategory or variant of the more generic class of engines known as turboshafts. In fact, a turboshaft is defined as any type of gas turbine engine that turns a shaft connected to something other than an aircraft propeller.

Additional information on how a turboprop engine works, including several animated images, can be found at NASA's Learning Guide on Propulsion.
- answer by Greg Alexander, 2 January 2005

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