Turboprop? |
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A diagram showing how a turboprop works.A Turboprop (Turbo-propeller) or turboshaft engine is a type of gas turbine. It differs from a Turbojet in that the design is optimized to produce rotating shaft power to drive a propeller, instead of thrust from the exhaust gas.
Basically, 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. Whilst the power turbine may be integral with the compressor turbine, most 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 (AND1lt;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 would be 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.
Turboprops are very efficient at modest flight speeds (below 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.
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.
A Rolls-Royce RB.50 Trent on a test rig at Hucknall, in March 1945Propellers 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.
The first 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. The first purpose-designed turboprop was the Rolls-Royce Dart.
A European consortium is currently developing the 11000shp TP400-D6 turboprop for the 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.
Residual thrust on a turboshaft is avoided by:
a) further expansion in the turbine system
and/or
b) truncating and turning the exhaust through 90degrees, to produce two opposing jets.
Apart from the above and the remote location of the gearbox, there is very little difference between a turboprop and a turboshaft.
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