Technological aspects
Polish turboprop, single engine, two seat trainer PZL-130 Orlik TC-I of Orlik Aerobatic Team
In a turboprop, much of the jet thrust is sacrificed in favor of shaft power, 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 exhaust jet is low. Consequently, the exhaust jet produces (typically) less than 10% of the total thrust.
Because the propeller is much larger in diameter than the power turbine, the tip speed of the propeller can become supersonic. 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 180 degrees, to produce two opposing jets. Apart from the above, there is very little difference between a turboprop and a turboshaft.
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.
Twin turboprop ATR 72 of Aerocaribbean airlines
Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft. However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching Mach 0.75. To increase the efficiency of the propellers, a mechanism can be used to alter the pitch, thus adjusting the pitch to the airspeed. The variable pitch propeller, also called controllable pitch propeller, can also be used to generate negative thrust while decelerating on the runway. After an engine outage, the pitch can be adjusted to a vaning pitch (called feathering), thus minimizing the drag of the non-functioning propeller.
Some commercial aircraft with turboprop engines include the Bombardier Dash 8, ATR 42, ATR 72, BAe Jetstream 31, Embraer EMB 120 Brasilia, The Fairchild Swearingen Metroliner, and Saab 340 and 2000.
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 mechanical 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 program 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-ft, 11-in five-bladed propeller. Two Trents were fitted to Gloster Meteor EE227 — the sole "Trent-Meteor" — which thus became the world's first turboprop powered aircraft, albeit a test-bed not intended for production It first flew on 20th September 1945. 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. The Dart-powered Vickers Viscount was the first turboprop aircraft of any kind to go into production and sold in large numbers It was also the first four-engined turboprop. Its first flight was on 16th July 1948. The world's first single engined turboprop aircraft was the Armstrong Siddeley Mamba-powered Boulton Paul Balliol, which first flew on 24th March 1948
While the Soviet Union had the technology to create a jet-powered strategic bomber comparable to Boeing's B-52 Stratofortress, they instead produced the Tu-95 "Bear", powered with four Kuznetsov NK-12 turboprops, mated to eight contra-rotating propellers (two per nacelle) 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, such as the ill-fated Allison T40, into a series of experimental aircraft during the 1950s, but none would be adopted into service.
The first American turboprop engine was the General Electric T31, first used in the sole experimental Convair YP-81 It first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. America skipped over turboprop airliners in favor of the Boeing 707, but the technology of the unsuccessful Lockheed Electra was used in both the long-lived P-3 Orion as well as the classic C-130 Hercules, one of the most successful military aircraft ever in terms of length of production. One of the most popular turboprop engines is the Pratt & Whitney Canada PT6 engine.
The first turbine powered, shaft driven helicopter was the Bell XH-13F, a version of the Bell 47 powered by Continental XT-51-T-3 (Turbomeca Artouste) engine
Polish turboprop, single engine, two seat trainer PZL-130 Orlik TC-I of Orlik Aerobatic Team
In a turboprop, much of the jet thrust is sacrificed in favor of shaft power, 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 exhaust jet is low. Consequently, the exhaust jet produces (typically) less than 10% of the total thrust.
Because the propeller is much larger in diameter than the power turbine, the tip speed of the propeller can become supersonic. 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 180 degrees, to produce two opposing jets. Apart from the above, there is very little difference between a turboprop and a turboshaft.
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.
Twin turboprop ATR 72 of Aerocaribbean airlines
Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft. However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching Mach 0.75. To increase the efficiency of the propellers, a mechanism can be used to alter the pitch, thus adjusting the pitch to the airspeed. The variable pitch propeller, also called controllable pitch propeller, can also be used to generate negative thrust while decelerating on the runway. After an engine outage, the pitch can be adjusted to a vaning pitch (called feathering), thus minimizing the drag of the non-functioning propeller.
Some commercial aircraft with turboprop engines include the Bombardier Dash 8, ATR 42, ATR 72, BAe Jetstream 31, Embraer EMB 120 Brasilia, The Fairchild Swearingen Metroliner, and Saab 340 and 2000.
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 mechanical 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 program 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-ft, 11-in five-bladed propeller. Two Trents were fitted to Gloster Meteor EE227 — the sole "Trent-Meteor" — which thus became the world's first turboprop powered aircraft, albeit a test-bed not intended for production It first flew on 20th September 1945. 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. The Dart-powered Vickers Viscount was the first turboprop aircraft of any kind to go into production and sold in large numbers It was also the first four-engined turboprop. Its first flight was on 16th July 1948. The world's first single engined turboprop aircraft was the Armstrong Siddeley Mamba-powered Boulton Paul Balliol, which first flew on 24th March 1948
While the Soviet Union had the technology to create a jet-powered strategic bomber comparable to Boeing's B-52 Stratofortress, they instead produced the Tu-95 "Bear", powered with four Kuznetsov NK-12 turboprops, mated to eight contra-rotating propellers (two per nacelle) 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, such as the ill-fated Allison T40, into a series of experimental aircraft during the 1950s, but none would be adopted into service.
The first American turboprop engine was the General Electric T31, first used in the sole experimental Convair YP-81 It first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. America skipped over turboprop airliners in favor of the Boeing 707, but the technology of the unsuccessful Lockheed Electra was used in both the long-lived P-3 Orion as well as the classic C-130 Hercules, one of the most successful military aircraft ever in terms of length of production. One of the most popular turboprop engines is the Pratt & Whitney Canada PT6 engine.
The first turbine powered, shaft driven helicopter was the Bell XH-13F, a version of the Bell 47 powered by Continental XT-51-T-3 (Turbomeca Artouste) engine
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