Turboprop

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Not to be confused with propfan.
Schematic diagram showing the operation of a turboprop engine
An ATR-72, a typical 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.

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]

Turboprop engines are generally used on small subsonic aircraft, but some aircraft outfitted with turboprops have cruising speeds in excess of 500 kt (926 km/h, 575 mph).[citation needed] Large military and civil aircraft, such as the Lockheed L-188 Electra and the Tupolev Tu-95, have also used turboprop power. The Airbus A400M is powered by four Europrop TP400 engines, which are the third most powerful turboprop engines ever produced, after the Kuznetsov NK-12 and Progress D-27.[citation needed]

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.

Turboprops are most efficient at flight speeds below 725 km/h (450 mph; 390 knots) because the jet velocity of the propeller (and exhaust) is relatively low. Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required. The most common application of turboprop engines in civilian aviation is in small commuter aircraft, where their greater power and reliability than reciprocating engines offsets their higher initial cost and fuel consumption. Turboprop airliners now operate at near the same speed as small turbofan-powered aircraft but burn two-thirds of the fuel per passenger.[2] However, compared to a turbojet (which can fly at high altitude for enhanced speed and fuel efficiency) a propeller aircraft has a much lower ceiling. Turboprop-powered aircraft have become popular for bush airplanes such as the Cessna Caravan and Quest Kodiak as jet fuel is easier to obtain in remote areas than is aviation-grade gasoline (avgas).[citation needed]

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.[3][4][5] Consequently, the exhaust jet produces (typically) less than 10% of the total thrust,[citation needed][6] and 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]

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]

Since propellers are not efficient when their tips reach or exceed supersonic speeds,[5] reduction gearboxes are placed in the drive line between the power turbine and the propeller to allow the turbine to operate at its most efficient speed. The gearbox is part of the engine and contains the parts necessary to operate a constant speed propeller. This differs from the turboshaft engines used in helicopters, where the gearbox is remote from the engine.[citation needed][3][4]

Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft[3][4][5] above Mach 0.6-0.7.[6] 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]

Some commercial aircraft with turboprop engines include the Bombardier Dash 8, ATR 42, ATR 72, BAe Jetstream 31, Beechcraft 1900, Embraer EMB 120 Brasilia, Fairchild Swearingen Metroliner, Dornier 328, Saab 340 and 2000, Xian MA60, Xian MA600, and Xian MA700, Fokker 27, 50 and 60.[citation needed]

History

File:Rolls-Royce RB50 Trent Turboprop On Test Rig At Hucknall.jpg
A Rolls-Royce RB.50 Trent on a test rig at Hucknall, in March 1945
Kuznetsov NK-12M Turboprop, on a Tu-95
Rolls-Royce Dart turboprop engine

Alan Arnold Griffith had published a paper on turbine design in 1926. Subsequent work at the Royal Aircraft Establishment investigated axial turbine designs that could be used to supply power to a shaft and thence a propeller. From 1929, Frank Whittle began work on centrifugal turbine designs that would deliver pure jet thrust.[16]

The world's first turboprop was designed by the Hungarian mechanical engineer György Jendrassik.[17] Jendrassik published a turboprop idea in 1928, and on 12 March 1929 he patented his invention. In 1938, he built a small-scale (100 Hp; 74.6 kW) experimental gas turbine.[18] The larger Jendrassik Cs-1, with a predicted output of 1,000 bhp, was produced and tested at the Ganz Works in Budapest between 1937 and 1941. It was of axial-flow design with 15 compressor and 7 turbine stages, annular combustion chamber and many other modern features. First run in 1940, combustion problems limited its output to 400 bhp. In 1941,the engine was abandoned due to war, and the factory was turned over to conventional engine production. The world's first turboprop engine that went into mass production was designed by a German engineer, Max Adolf Mueller, in 1942.[19]

The first public mention of turboprop engine in a general public press, was in the British aviation publication, Flight, in February 1944 issue, which included a detailed cutaway drawing of what a possible future turboprop engine could look like. The drawing was very close to what the future Rolls-Royce Trent would look like.[20] 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.[21][22] It first flew on 20 September 1945. From their experience with the Trent, Rolls-Royce developed the Rolls-Royce Clyde, the first turboprop engine to be fully type certificated for military and civil use,[23] and 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.[24] It was also the first four-engined turboprop. Its first flight was on 16 July 1948. The world's first single engined turboprop aircraft was the Armstrong Siddeley Mamba-powered Boulton Paul Balliol, which first flew on 24 March 1948.[25]

The Soviet Union built on German World War II development by Junkers Motorenwerke, while BMW, Heinkel-Hirth and Daimler-Benz also developed and partially tested designs.[citation needed] While the Soviet Union had the technology to create the airframe for a jet-powered strategic bomber comparable to Boeing's B-52 Stratofortress, they instead produced the Tupolev 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 twin-turbine Allison T40 — essentially a twinned up pair of Allison T38 turboprop engines driving contra-rotating propellers — into a series of experimental aircraft during the 1950s, with aircraft powered with the T40, like the Convair R3Y Tradewind flying boat never entering U.S. Navy service.

The first American turboprop engine was the General Electric XT31, first used in the experimental Consolidated Vultee XP-81.[26] The XP-81 first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. The technology of the Allison's earlier T38 design evolved into the Allison T56, with quartets of the T56s being used to power the Lockheed Electra airliner, its military maritime patrol derivative the P-3 Orion, and the widely-produced C-130 Hercules military transport aircraft. One of the most produced turboprop engines used in civil aviation is the Pratt & Whitney Canada PT6 engine.[citation needed]

The first turbine-powered, shaft-driven helicopter was the Kaman K-225, a development of Charles Kaman's K-125 synchropter, which used a Boeing T50 turboshaft engine to power it on 11 December 1951.[27]

Current engines

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manufacturer Country designation dry weight (kg) takeoff rating (kW) Application
DEMC  People's Republic of China WJ5E 720 2130 Harbin SH-5, Xi'an Y-7
Europrop International  European Union TP400-D6 1800 8203 Airbus A400M
General Electric  United States CT7-5A 365 1294
General Electric  United States CT7-9 365 1447 CASA/IPTN CN-235, Let L-610, Saab 340, Sukhoi Su-80
General Electric  United States T64-P4D 538 2535 Aeritalia G.222, de Havilland Canada DHC-5 Buffalo, Kawasaki P-2J
Honeywell  United States TPE331 Series 150 - 275 478 - 1650 Aero/Rockwell Turbo Commander 680/690/840/960/1000, Antonov An-38, Ayres Thrush, BAe Jetstream 31/32, BAe Jetstream 41, CASA C-212 Aviocar, Cessna 441 Conquest II, Dornier Do 228, Fairchild Swearingen Metroliner, General Atomics MQ-9 Reaper, Grumman Ag Cat, Mitsubishi MU-2, North American Rockwell OV-10 Bronco, RUAG Do 228NG, Short SC.7 Skyvan, Short Tucano, Swearingen Merlin, Fairchild Swearingen Metroliner
Honeywell  United States LTP 101-700 147 522 Air Tractor AT-302, Piaggio P.166
KKBM  Russia NK-12MV 1900 11033 Antonov An-22, Tupolev Tu-95, Tupolev Tu-114
Progress  Ukraine TV3-117VMA-SB2 560 1864 Antonov An-140
Klimov  Russia TV7-117S 530 2100 Ilyushin Il-112, Ilyushin Il-114
Progress  Ukraine AI20M 1040 2940 Antonov An-12, Antonov An-32, Ilyushin Il-18
Progress  Ukraine AI24T 600 1880 Antonov An-24, Antonov An-26, Antonov An-30
LHTEC  United States LHTEC T800 517 2013 AgustaWestland Super Lynx 300 (CTS800-4N), AgustaWestland AW159 Lynx Wildcat (CTS800-4N), Ayres LM200 Loadmaster (LHTEC CTP800-4T) (aircraft not built), Sikorsky X2 (T800-LHT-801), TAI/AgustaWestland T-129 (CTS800-4A)
OMKB  Russia TVD-20 240 1081 Antonov An-3, Antonov An-38
Pratt & Whitney Canada  Canada PT-6 Series 149 - 260 430 - 1500 Air Tractor AT-502, Air Tractor AT-602, Air Tractor AT-802, Beechcraft Model 99, Beechcraft King Air, Beechcraft Super King Air, Beechcraft 1900, Beechcraft T-6 Texan II, Cessna 208 Caravan, Cessna 425 Corsair/Conquest I, de Havilland Canada DHC-6 Twin Otter, Harbin Y-12, Embraer EMB 110 Bandeirante, Let L-410 Turbolet, Piaggio P.180 Avanti, Pilatus PC-12, Piper PA-42 Cheyenne, Piper PA-46-500TP Meridian, Shorts 360
Pratt & Whitney Canada  Canada PW120 418 1491 ATR 42-300/320
Pratt & Whitney Canada  Canada PW121 425 1603 ATR 42-300/320, Bombardier Dash 8 Q100
Pratt & Whitney Canada  Canada PW123 C/D 450 1603 Bombardier Dash 8 Q300
Pratt & Whitney Canada  Canada PW127 481 2051 ATR 72
Pratt & Whitney Canada  Canada PW150A 690 3781 Bombardier Dash 8 Q400
PZL  Poland TWD-10B 230 754 PZL M28
RKBM  Russia TVD-1500S 240 1044 Sukhoi Su-80
Rolls-Royce  United Kingdom Dart Mk 536 569 1700 Avro 748, Fokker F27, Vickers Viscount
Rolls-Royce  United Kingdom Tyne 21 569 4500 Aeritalia G.222, Breguet Atlantic, Transall C-160
Rolls-Royce  United Kingdom 250-B17 88.4 313 Fuji T-7, Britten-Norman Turbine Islander, O&N Cessna 210, Soloy Cessna 206, Propjet Bonanza
Rolls-Royce  United Kingdom T56-14 3433 P-3 Orion
Rolls-Royce  United Kingdom T56-15 828 3424 C-130 Hercules
Rolls-Royce  United Kingdom T56-27 880 3910 E-2 Hawkeye
Rolls-Royce  United Kingdom AE2100A 715.8 3095 Saab 2000
Rolls-Royce  United Kingdom AE2100J 710 3424 ShinMaywa US-2
Rolls-Royce  United Kingdom AE2100D2, D3 702 3424 Alenia C-27J Spartan, Lockheed Martin C-130J Super Hercules
Rybinsk  Russia TVD-1500V 220 1156
Saturn  Russia TAL-34-1 178 809
Turbomeca  France Arrius 1D 111 313 Socata TB 31 Omega
Turbomeca  France Arrius 2F 103 376
Walter  Czech Republic M601E 200 560 Let L-410 Turbolet UVP-E
Walter  Czech Republic M601F 202 580 L420-UVP
Walter  Czech Republic M602A 570 1360 Let L-610
Walter  Czech Republic M602B 480 1500

See also

References

Notes

  1. ↑ "Turboprop", Pilot's Handbook of Aeronautical Knowledge, Federal Aviation Administration, 2009.
  2. ↑ More turboprops coming to the market - maybe
  3. ↑ 3.0 3.1 3.2 "Turboprop Engine" Glenn Research Center (NASA)
  4. ↑ 4.0 4.1 4.2 "Turboprop Thrust" Glenn Research Center (NASA)
  5. ↑ 5.0 5.1 5.2 Variations of Jet Engines; Turboprop Engines
  6. ↑ 6.0 6.1 "The turbofan engine", page 7. SRM University, Department of aerospace engineering
  7. ↑ Ilan Kroo and Juan Alonso. "Aircraft Design: Synthesis and Analysis, Propulsion Systems: Basic Concepts" Stanford University School of Engineering, Department of Aeronautics and Astronautics Main page
  8. ↑ Prof. Z. S. Spakovszky. "11.5 Trends in thermal and propulsive efficiency" MIT turbines, 2002. Thermodynamics and Propulsion
  9. ↑ 9.0 9.1 Nag, P.K. "Basic And Applied Thermodynamics" p550. Published by Tata McGraw-Hill Education. Quote: "If the cowl is removed from the fan the result is a turboprop engine. Turbofan and turboprop engines differ mainly in their bypass ratio 5 or 6 for turbofans and as high as 100 for turboprop."
  10. ↑ "Propeller thrust" Glenn Research Center (NASA)
  11. ↑ Paul Bevilaqua : The shaft driven Lift Fan propulsion system for the Joint Strike Fighter page 3. Presented 1 May 1997. DTIC.MIL Word document, 5.5 MB. Accessed: 25 February 2012.
  12. ↑ Bensen, Igor. "How they fly - Bensen explains all" Gyrocopters UK. Accessed: 10 April 2014.
  13. ↑ Johnson, Wayne. Helicopter theory pp3+32, Courier Dover Publications, 1980. Accessed: 25 February 2012. ISBN 0-486-68230-7
  14. ↑ Wieslaw Zenon Stepniewski, C. N. Keys. Rotary-wing aerodynamics p3, Courier Dover Publications, 1979. Accessed: 25 February 2012. ISBN 0-486-64647-5
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ Gunston Jet, p. 120
  17. ↑ Gunston World, p.111
  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ Green, W. and Swanborough, G.; "Plane Facts", 'Max'Air Enthusiast Vol. 1 No. 1 (1971), Page 53.
  20. ↑ "Our Contribution - How Flight Introduced and Made Familiar With Gas Turbines and Jet Propulsion" Flight, 11 May 1951, p. 569.
  21. ↑ James p. 251-2
  22. ↑ Green p.18-9
  23. ↑ http://www.flightglobal.com/pdfarchive/view/1950/1950%20-%202035.html
  24. ↑ Green p.82
  25. ↑ Green p.81
  26. ↑ Green p.57
  27. ↑ Lua error in package.lua at line 80: module 'strict' not found.

Bibliography

  • Green, W. and Cross, R.The Jet Aircraft of the World (1955). London: MacDonald
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  • James, D.N. Gloster Aircraft since 1917 (1971). London: Putnam & Co. ISBN 0-370-00084-6

Further reading

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External links

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