Turboprop

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Image:Turboprop P&W PT6A-67D.jpg A Turboprop engine is a type of gas turbine engine which uses most of its power to drive a propeller. The propeller of a turboprop is very similar to that used by piston or reciprocating engines, but turboprops usually use a constant velocity propeller. Turboprop engines are generally used on small or slow subsonic aircraft, but some aircraft outfitted with turboprops have cruising speeds in excess of 500 kts (925 Km/h).

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. The hot combustion gases expand through the turbine, to provide power to the turbine. Part of this power goes to the compressor, and the rest, through the reduction gearing, to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gasses are expelled, providing approximately 25% of the thrust.

A turboprop engine is similar to a turbojet, but has additional fan blades in the turbine stage to recover more power from the engine to turn the propeller.

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.

[edit] 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 machanical engineer György Jendrassik. It was produced and tested in the Ganz Factory in Budapest between 1939 and 1942. It was planned to fit to the Varga RMI-1 X/H twin-engined reconnaissance bomber designed by László Varga in 1940, but the programme was cancelled. Jendrassik had also designed 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.

While the Soviet Union had 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.