Turboprop

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GE T64 propeller at left, gearbox with accessories in the middle, turbine core at right JMSDF P-2J T64-IHI-10E turboprop engine left side view at Kanoya Naval Air Base Museum April 29, 2017.jpg
GE T64 propeller at left, gearbox with accessories in the middle, turbine core at right

A turboprop engine is a turbine engine that drives an aircraft propeller. [1]

Contents

In its simplest form a turboprop consists of an intake, compressor, combustor, turbine, and a propelling nozzle. [2] 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. Thrust is obtained by the combusting gases, pushing toward a (vectored) surface in front of the expanding gas.

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. [3]

In contrast to a turbojet, the engine's exhaust gases do not generally contain enough energy to create significant thrust, since almost all of the engine's power is used to drive the propeller.

Technological aspects

Schematic diagram showing the operation of a turboprop engine Turboprop operation-en.svg
Schematic diagram showing the operation of a turboprop engine
Propulsive efficiency comparison for various gas turbine engine configurations Gas turbine efficiency.png
Propulsive efficiency comparison for various gas turbine engine configurations

Exhaust thrust in a turboprop is sacrificed in favour 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. [4] [5] [6] Consequently, the exhaust jet typically produces around or less than 10% of the total thrust. [7] A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds. [8]

Turboprops can have bypass ratios up to 50-100 [9] [10] [11] although the propulsion airflow is less clearly defined for propellers than for fans. [12] [13]

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) propeller type similar to that used with larger aircraft reciprocating engines.[ citation needed ]

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, [14] [15] a low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use. [16] [17]

Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft [4] [5] [6] above 0.6-0.7 Mach. [7] However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching 0.75 Mach. To increase propeller efficiency across a wider range of airspeeds, turboprops are typically equipped with constant-speed (variable-pitch) propellers. The blades of a constant-speed propeller increase pitch as aircraft speed increases, allowing for a wider range airspeeds than a fixed-pitch propeller. Another benefit of this type of propeller is that it can also be used to generate negative thrust while decelerating on the runway. Additionally, in the event of an engine failure, the propeller can be feathered, thus minimizing the drag of the non-functioning propeller. [18]

While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compressor which 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. [19] 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. [11]

History

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. [20]

The world's first turboprop was designed by the Hungarian mechanical engineer György Jendrassik. [21] 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. [22] 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[ specify ] was designed by a German engineer, Max Adolf Mueller, in 1942. [23]

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

The first mention of turboprop engines in the general public press was in the February 1944 issue of the British aviation publication Flight , 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. [24] 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 (2.41 m) 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. [25] [26] 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, [27] 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. [28] 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. [29]

The Kuznetsov NK-12 is still the most powerful turboprop Kuznetsov NK-12M turboprop on Tu-95.jpg
The Kuznetsov NK-12 is still the most powerful turboprop

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. [30] 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. [31]

Usage

A military transport aircraft, over 2,500 Lockheed C-130 Hercules have been built Lockheed C-130 Hercules.jpg
A military transport aircraft, over 2,500 Lockheed C-130 Hercules have been built

Compared to turbofans, 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. Modern turboprop airliners operate at nearly the same speed as small regional jet airliners but burn two-thirds of the fuel per passenger. [32] However, compared to a turbojet (which can fly at high altitude for enhanced speed and fuel efficiency) a propeller aircraft has a lower ceiling.

The Beech King Air and Super King Air are the most-delivered turboprop business aircraft, with a combined 7,300 examples as of May 2018 Svensk flygambulans97.jpg
The Beech King Air and Super King Air are the most-delivered turboprop business aircraft, with a combined 7,300 examples as of May 2018

Compared to piston engines, their greater power-to-weight ratio (which allows for shorter takeoffs) and reliability can offset their higher initial cost, maintenance and fuel consumption. As jet fuel can be easier to obtain than avgas in remote areas, turboprop-powered aircraft like the Cessna Caravan and Quest Kodiak are used as bush airplanes.

Turboprop engines are generally used on small subsonic aircraft, but the Tupolev Tu-114 can reach 470 kn (870 km/h, 541 mph). Large military aircraft, like the Tupolev Tu-95, and civil aircraft, such as the Lockheed L-188 Electra, were also turboprop powered. The Airbus A400M is powered by four Europrop TP400 engines, which are the second most powerful turboprop engines ever produced, after the 11 MW (15,000 hp) Kuznetsov NK-12.

In 2017, the most widespread turboprop airliners in service were the ATR 42/72 (950 aircraft), Bombardier Q400 (506), De Havilland Canada Dash 8-100/200/300 (374), Beechcraft 1900 (328), de Havilland Canada DHC-6 Twin Otter (270), Saab 340 (225). [34] Less widespread and older airliners include the BAe Jetstream 31, Embraer EMB 120 Brasilia, Fairchild Swearingen Metroliner, Dornier 328, Saab 2000, Xian MA60, MA600 and MA700, Fokker 27 and 50.

Turboprop business aircraft include the Piper Meridian, Socata TBM, Pilatus PC-12, Piaggio P.180 Avanti, Beechcraft King Air and Super King Air. In April 2017, there were 14,311 business turboprops in the worldwide fleet. [35]

Reliability

Between 2012 and 2016, the ATSB observed 417 events with turboprop aircraft, 83 per year, over 1.4 million flight hours: 2.2 per 10,000 hours. Three were “high risk” involving engine malfunction and unplanned landing in single‑engine Cessna 208 Caravans, four “medium risk” and 96% “low risk”. Two occurrences resulted in minor injuries due to engine malfunction and terrain collision in agricultural aircraft and five accidents involved aerial work: four in agriculture and one in an air ambulance. [36]

Current engines

Jane's All the World's Aircraft . 2005–2006.

ManufacturerCountryDesignationDry weight (kg)Takeoff rating (kW)Application
DEMC Flag of the People's Republic of China.svg  People's Republic of China WJ5E 7202130 Harbin SH-5, Xi'an Y-7
Europrop International Flag of Europe.svg  European Union TP400-D6 18008203 Airbus A400M
General Electric Flag of the United States.svg  United States CT7-5A3651294
General Electric Flag of the United States.svg  United States CT7-93651447 CASA/IPTN CN-235, Let L-610, Saab 340, Sukhoi Su-80
General Electric Flag of the United States.svg  United States Flag of the Czech Republic.svg  Czech Republic H80 Series [37] 200550 - 625 Thrush Model 510, Let 410NG, Let L-410 Turbolet UVP-E, CAIGA Primus 150, Nextant G90XT
General Electric Flag of the United States.svg  United States T64-P4D5382535 Aeritalia G.222, de Havilland Canada DHC-5 Buffalo, Kawasaki P-2J
Honeywell Flag of the United States.svg  United States TPE331 Series150 - 275478 - 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, Grum Ge man, Mitsubishi MU-2, North American Rockwell OV-10 Bronco, Piper PA-42 Cheyenne, RUAG Do 228NG, Short SC.7 Skyvan, Short Tucano, Swearingen Merlin, Fairchild Swearingen Metroliner
Honeywell Flag of the United States.svg  United States LTP 101-700147522 Air Tractor AT-302, Piaggio P.166
KKBM Flag of Russia.svg  Russia NK-12MV 190011033 Antonov An-22, Tupolev Tu-95, Tupolev Tu-114
Progress Flag of Ukraine.svg  Ukraine TV3-117VMA-SB25601864 Antonov An-140
Klimov Flag of Russia.svg  Russia TV7-117S5302100 Ilyushin Il-112, Ilyushin Il-114
Progress Flag of Ukraine.svg  Ukraine AI20M10402940 Antonov An-12, Antonov An-32, Ilyushin Il-18
Progress Flag of Ukraine.svg  Ukraine AI24T6001880 Antonov An-24, Antonov An-26, Antonov An-30
LHTEC Flag of the United States.svg  United States LHTEC T800 5172013 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 Flag of Russia.svg  Russia TVD-20 2401081 Antonov An-3, Antonov An-38
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PT-6 Series149 - 260430 - 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-6 Porter, Pilatus PC-12, Piper PA-42 Cheyenne, Piper PA-46-500TP Meridian, Shorts 360, Daher TBM 700, Daher TBM 850, Daher TBM 900, Embraer EMB 314 Super Tucano
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PW120 4181491 ATR 42-300/320
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PW121 4251603 ATR 42-300/320, Bombardier Dash 8 Q100
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PW123 C/D4501603 Bombardier Dash 8 Q300
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PW126 C/D4501950 BAe ATP
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PW127 4812051 ATR 72
Pratt & Whitney Canada Flag of Canada (Pantone).svg  Canada PW150A7173781 Bombardier Dash 8 Q400
PZL Flag of Poland.svg  Poland TWD-10B 230754 PZL M28
RKBM Flag of Russia.svg  Russia TVD-1500S 2401044 Sukhoi Su-80
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom Dart Mk 5365691700 Avro 748, Fokker F27, Vickers Viscount
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom Tyne 215694500 Aeritalia G.222, Breguet Atlantic, Transall C-160
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom 250-B1788.4313 Fuji T-7, Britten-Norman Turbine Islander, O&N Cessna 210, Soloy Cessna 206, Propjet Bonanza
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom Allison T56 828 - 8803424 - 3910 P-3 Orion, E-2 Hawkeye, C-2 Greyhound, C-130 Hercules
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom AE2100A715.83095 Saab 2000
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom AE2100J7103424 ShinMaywa US-2
Rolls-Royce Flag of the United Kingdom.svg  United Kingdom AE2100D2, D37023424 Alenia C-27J Spartan, Lockheed Martin C-130J Super Hercules
Rybinsk Flag of Russia.svg  Russia TVD-1500V 2201156
Saturn Flag of Russia.svg  Russia TAL-34-1 178809
Turbomeca Flag of France.svg  France Arrius 1D111313 Socata TB 31 Omega
Turbomeca Flag of France.svg  France Arrius 2F103376
Walter Flag of the Czech Republic.svg  Czech Republic M601 Series [38] 200560 Let L-410 Turbolet, Aerocomp Comp Air 10 XL, Aerocomp Comp Air 7, Ayres Thrush, Dornier Do 28, Lancair Propjet, Let Z-37T, Let L-420, Myasishchev M-101T, PAC FU-24 Fletcher, Progress Rysachok, PZL-106 Kruk, PZL-130 Orlik, SM-92T Turbo Finist
Walter Flag of the Czech Republic.svg  Czech Republic M602A5701360 Let L-610
Walter Flag of the Czech Republic.svg  Czech Republic M602B4801500

See also

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References

Notes

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  5. 1 2 "Turboprop Thrust" Glenn Research Center (NASA)
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  7. 1 2 ""The turbofan engine Archived 18 April 2015 at the Wayback Machine ", page 7. SRM Institute of Science and Technology, Department of aerospace engineering.
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  12. "Propeller thrust" Glenn Research Center (NASA)
  13. Philip Walsh, Paul Fletcher. "Gas Turbine Performance", page 36. John Wiley & Sons, 15 April 2008. Quote: "It has better fuel consumption than a turbojet or turbofan, due to a high propulsive efficiency.., achieving thrust by a high mass flow of air from the propeller at low jet velocity. Above 0.6 Mach number the turboprop in turn becomes uncompetitive, due mainly to higher weight and frontal area."
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  21. Gunston World, p.111
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  25. James p. 251-2
  26. Green p.18-9
  27. "rolls-royce trent - armstrong siddeley - 1950 - 2035 - Flight Archive". flightglobal.com. Retrieved 31 August 2016.
  28. Green p.82
  29. Green p.81
  30. Green p.57
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Bibliography

Further reading