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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]

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.

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

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.^[2][3][4] Consequently, the exhaust jet typically produces around or less than 10% of the total thrust.^[5] A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds.^[6]

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][11]

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]}

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,^[12][13] a low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.^[14][15]

Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft^[2][3][4] above Mach 0.6-0.7.^[5] 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 failure, 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.^[16]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]}

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.^[17]

The world's first turboprop was designed by the Hungarian mechanical engineer György Jendrassik.^[18] 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.^[19] 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.^[20]

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.^[21] 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.^[22][23] 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,^[24] 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.^[25] 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.^[26]

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.^[27] 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.^[28]

Usage

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.^[29] 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 in history, with a combined 7,300 examples as of May 2018^[30]

The most common application of turboprop engines in civilian aviation is in small commuter aircraft, where their greater power and reliability offsets their higher initial cost and fuel consumption. 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 avgas.^{[citation needed]} 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.

Turboprop engines are generally used on small subsonic aircraft, but the Tupolev Tu-114 can reach 470 kt (870 km/h, 541 mph). 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 second most powerful turboprop engines ever produced, after the eleven megawatt-output Kuznetsov NK-12.

In 2017, The most widespread turboprop airliners in service were the ATR 42/72 (950 aircraft), Bombardier Q400 (506) and Dash 8-100/200/300 (374), Beechcraft 1900 (328), de Havilland Canada DHC-6 Twin Otter (270), Saab 340 (225).^[31] 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.^[32]

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.^[33]

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  Czech Republic	H80 Series[34]	200	550 - 625	Thrush Model 510, Let 410NG, Let L-410 Turbolet UVP-E, CAIGA Primus 150, Nextant G90XT
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, GrumGeman, 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	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-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	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	PW126 C/D	450	1950	BAe ATP
Pratt & Whitney Canada	Canada	PW127	481	2051	ATR 72
Pratt & Whitney Canada	Canada	PW150A	717	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	Allison T56	828 - 880	3424 - 3910	P-3 Orion, E-2 Hawkeye, C-2 Greyhound, C-130 Hercules
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	M601 Series[35]	200	560	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	Czech Republic	M602A	570	1360	Let L-610
Walter	Czech Republic	M602B	480	1500

See also

• Jet engine


• Jet aircraft


• Jetboat


• Propfan


• Ramjet


• Scimitar propeller


• Supercharger


• Tiltrotor


• Turbocharger


• Turbofan


• Turbojet


• Turboshaft

References

Notes

Bibliography

• Green, W. and Cross, R.The Jet Aircraft of the World (1955). London: MacDonald


• 
  Gunston, Bill (2006). The Development of Jet and Turbine Aero Engines, 4th Edition. Sparkford, Somerset, England, UK: Patrick Stephens, Haynes Publishing. ISBN 0-7509-4477-3.
  


• 
  Gunston, Bill (2006). World Encyclopedia of Aero Engines, 5th Edition. Phoenix Mill, Gloucestershire, England, UK: Sutton Publishing Limited. ISBN 0-7509-4479-X.
  


• James, D.N. Gloster Aircraft since 1917 (1971). London: Putnam & Co.
  ISBN 0-370-00084-6
  

Further reading

• 
  Van Sickle, Neil D.; et al. (1999). "Turboprop Engines". Van Sickle's modern airmanship. McGraw-Hill Professional. p. 205. ISBN 978-0-07-069633-4.
  

External links

• 
  Jet Turbine Planes by LtCol Silsbee USAAF, Popular Science, December 1945, first article on turboprops printed


• Wikibooks: Jet propulsion


• 
  "Development of the Turboprop" a 1950 Flight article on UK and US turboprop engines