Turboprop

Schematic diagram showing the operation of a turboprop engine
A Fairchild F-27 representative of the 2nd generation of modern Rolls-Royce Dart turboprop powered aircraft, after the initial success of the 1950s era Vickers Viscount
An ATR-72, a typical modern turboprop aircraft

A turboprop engine is a turbine engine that drives an aircraft propeller. [1] In contrast to a turbojet, the engine's exhaust gases do not contain enough energy to create significant thrust, since almost all of the engine's power is used to drive the propeller.

In its simplest form a turboprop consists of an intake, compressor, combustor, turbine, and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop.

Technological aspects

Flow past a turboprop engine in operation

Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (up to that necessary to drive the compressor) from turbine expansion. Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low. [2] [3] [4] Consequently, the exhaust jet typically produces around or less than 10% of the total thrust. [5] A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds. [6] Turboprops can have bypass ratios up to 50-100 [7] [8] [9] although the propulsion airflow is less clearly defined for propellers than for fans. [10]

The propeller is coupled to the turbine through a reduction gear that converts the high RPM/low torque output to low RPM/high torque. The propeller itself is normally a constant speed (variable pitch) type similar to that used with larger reciprocating aircraft engines.[ citation needed]

Unlike the small diameter fans used in turbofan jet engines, the propeller has a large diameter that lets it accelerate a large volume of air. This permits a lower airstream velocity for a given amount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree, [11] [12] a low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use. [13] [14]

Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft [2] [3] [4] above Mach 0.6-0.7. [5] However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching Mach 0.75. To increase propeller efficiency, a mechanism can be used to alter their pitch relative to the airspeed. A variable-pitch propeller, also called a controllable-pitch propeller, can also be used to generate negative thrust while decelerating on the runway. Additionally, in the event of an engine outage, the pitch can be adjusted to a vaning pitch (called feathering), thus minimizing the drag of the non-functioning propeller.[ citation needed]

While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compression. Centrifugal compressors have the advantage of being simple and lightweight, at the expense of a streamlined shape.[ citation needed]

While the power turbine may be integral with the gas generator section, many turboprops today feature a free power turbine on a separate coaxial shaft. This enables the propeller to rotate freely, independent of compressor speed. [15] Residual thrust on a turboshaft is avoided by further expansion in the turbine system and/or truncating and turning the exhaust 180 degrees, to produce two opposing jets. Apart from the above, there is very little difference between a turboprop and a turboshaft.[ citation needed] [9]

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