Turboprop and turboshaft engines

The turboprop engine is constructed in much the same way as the turbojet engine, but more of the available energy in the exhaust is used to drive the turbine. The extra power produced by the turbine is used to drive a propeller. Some thrust is produced by the exhaust jet, but this is only a relatively small proportion of the total. The advantage of the turboprop engine over the pure jet is that it is much more efficient. As we will see later, it is more efficient to produce thrust by giving a small increase in momentum to a large amount of air (as with a propeller), than to give a large increase in momentum to a small amount (as with a pure turbojet). However, as mentioned previously, pro­pellers produce serious problems at high speed. They are also noisy, and require high maintenance, and require the addition of a heavy gearbox to reduce the turbine speed, which can be up to 100 000 rpm or more, down to the few thousand rpm required for a propeller. They are normally used for relatively low-speed aircraft such as small airliners or heavy-lift transport types (Fig. 4B).

Instead of driving a propeller, the gas-turbine may be used to drive the rotor-blades of a helicopter, and in this application it is normally known as a turbo-shaft engine. Most military helicopters use turboshaft engines. Turboshaft engines are also used for purposes such as the production of aux­iliary electrical power.