A turboprop is a gas-turbine engine that drives an aircraft propeller. Most of the propulsive force comes from the propeller, with a smaller contribution from the engine exhaust. Turboprops are particularly associated with commuter, utility and transport aircraft whose operating requirements favour efficient flight at comparatively modest speeds.[1]
From combustion to shaft power
The core contains a compressor, combustion chamber and turbine. Rather than retaining most of the available exhaust energy for a high-speed jet, the engine extracts substantial mechanical power through its turbines. A shaft and reduction gearbox transmit that power to the propeller, allowing the propeller to rotate at a suitable speed.[1]
This explains the distinction between a turboprop and a piston-powered propeller aircraft. Both can obtain thrust from a propeller, but they use different machinery to turn it. It also distinguishes a turboprop from a turbojet: the turbine engine is being used chiefly as a source of shaft power rather than direct exhaust thrust.[2]
How the propeller creates thrust
A propeller blade acts as a rotating wing. Its outer sections travel faster than sections near the hub, so blades are twisted along their length to accommodate the changing local airflow. As the engine turns the propeller, the blades do work on the air and produce a pressure difference across the propeller plane.[3]
The flow eventually returns toward ambient pressure farther downstream, but retains increased rearward velocity. A simplified momentum model relates thrust to the mass flow and its change in velocity. Real propellers also have aerodynamic losses, so that ideal model explains the principle without providing a complete performance prediction for a particular blade design.[3]
Exhaust contribution
The total thrust of a turboprop includes both propeller thrust and residual jet thrust. NASA's thrust explanation treats these as separate contributions and shows why the propeller term normally dominates: the propeller handles a much larger airflow, while relatively little exhaust energy remains after the turbines have extracted shaft power.[2]
Related systems
A turboshaft uses a related gas-turbine principle but supplies mechanical power to another device, commonly a helicopter rotor transmission. The distinction lies in the intended output and installation. Turboprop and turboshaft are therefore useful engineering categories, even though both convert combustion energy into shaft power and may share elements of their underlying design.[1]