Turboprop

A schematic diagram showing the operation of a turboprop engine.

The P&W PT6, one of the most popular turboprop engines.

A Turboprop (Turbo-propeller) or turboshaft engine is a type of gas turbine engine which uses its power to drive a propeller. The propeller of a turboprop looks much the same as for a piston or reciprocating engine, but the engine is often longer, and usually provides more power and uses less fuel. Turboprop engines are generally used on small or slow subsonic aircraft. A turboprop is related to a Turbojet which derives its power in thrust by exhaust of high speed gases expelled through the exhaust rather than producing rotating shaft power. Turboprop can only refer to turboshaft engines in aircraft. Turboshaft powerplants can be used to power ships, generators, automobiles, locomotives, helicopters tanks and just about any other vehicle that does not use propellers ('props).

A turbojet consists of an intake, compressor, combustor, turbine and a propelling nozzle. Air drawn into the intake is compressed by the compressor. Fuel is burnt with the compressed air in the combustor. The hot combustion gases expand through the turbine, to provide power to the compressor. Further expansion of the gases occurs in the propelling nozzle; the high velocity jet produced providing forward thrust.

In a turboprop much of the jet thrust is sacrificed in favor of shaftpower, which is obtained by extracting additional power (to that necessary to drive the compressor) from the turbine expansion process. 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. Owing to the additional expansion in the turbine system, the residual energy in the jet is fairly low (<10% of total thrust, including that of the propeller).

Because the propeller is very much larger in diameter than the power turbine, the tip speed of the propeller can become supersonic. Consequently, to prevent this, a speed reduction gearbox is inserted between the power turbine and propeller shafts. The gearbox is part of the engine, whereas in a turboshaft the (helicopter) rotor reduction gearbox is remote from the engine.

Residual thrust on a turboshaft is avoided by further expansion in the turbine system and/or truncating and turning the exhaust through 90 degrees, to produce two opposing jets.

Apart from the above, there is very little difference between a turboprop and a turboshaft.
Turboprops are very efficient at modest flight speeds (below 724 km/h or 450mph), because the jet velocity of the propeller (and exhaust) is relatively low. Consequently, small commuter aircraft and military transports tend to feature turboprop engines. Although turboprops are used in some General Aviation applications, their high price deters more widespread acceptance except for very high performance STOL applications. One of the most common turboprop military aircraft is the C-130 Hercules transport.

While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compression, because of the small size of the engines.

Propellers lose efficiency as aircraft speed increases, which is why turboprops are not used on higher-speed aircraft. However, turboprops are far more efficient than piston-driven propeller engines.

History

A Rolls-Royce RB.50 Trent on a test rig at Hucknall, in March 1945

The world's first Turboprop was the 'Jendrassik Cs-1' designed by the Hungarian engineer György Jendrassik. It was produced and flown briefly in Czecho-Slovakia between 1939 and 1942. The engine was fitted to the Varga XG/XH twin-engined reconnaissance bomber but proved very unreliable. Jendrassik had also produced a small-scale 75 kW turboprop in 1937.
The first British turboprop engine was the Rolls-Royce RB.50 Trent, a converted Derwent II fitted with reduction gear and a Rotol 7' 11" five-bladed propeller. Two Trents were fitted to Gloster Meteor EE227 — the sole "Trent-Meteor" — which became the first relatively reliable turboprop powered aircraft. From their experience with the Trent, Rolls-Royce developed the Dart, which became one of the most reliable turboprop engines ever built. Dart production continued for more than fifty years.

When the Soviet Union did have the technology to create a jet powered strategic bomber comparable to the Boeing B-52, they instead produced the Tu-95 Bear which uses 4 contra-rotating propellers with supersonic tip speeds to achieve maximum cruise speeds in excess of 575 mph, faster than many of the first jet aircraft, and comparable to jet cruising speeds for most missions. The Bear would serve as their most successful long range combat and surveillance aircraft and symbol of Soviet power projection throughout the end of the 20th century. The USA would incorporate contra-rotating turboprop engines in experimental tail-sitting VTOL and fighter aircraft during the 1950s, but none would be adopted into service.

The first American turboprop was the General-Electric T-31. America skipped over turboprop airliners in favor of the 707, but the technology of the Lockheed Electra would be used in both the long-lived P-3C Orion as well as the classic C-130 Hercules, one of the most successful military aircraft in terms of length of production ever produced.

A European consortium is currently developing the 11,000 shp TP400-D6 turboprop for the Airbus A400M military transport. The engine is all-axial and has a two shaft core, with a free power turbine mounted on a third coaxial shaft.

The most popular turboprop engine ever built is the legendary Pratt & Whitney Canada PT6.

Turboprop: Encyclopedia BETA

Contents

Turboprop

History

See also