Turboprop

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Schematic diagram showing the operation of a turboprop engine
An ATR-72, a typical turboprop aircraft.

A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.[1]

The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller. The engine's exhaust gases do not contain enough energy, compared to a jet engine, to create significant thrust in the propulsion of the aircraft.

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). 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 very 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 reliability than reciprocating engines offsets their higher initial cost. 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[edit]

Flow past a turboprop engine in operation

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. 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.[citation needed]

Propellers are not efficient when the tips reach or exceed supersonic speeds. For this reason, a reduction gearbox is 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]

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]

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]

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. 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]

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[edit]

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.[3]

The world's first turboprop was designed by the Hungarian mechanical engineer György Jendrassik.[4] Jendrassik published a turboprop idea in 1928 and on March 12, 1929 he patented his invention. In 1938, he built a small-scale (100 Hp; 74.6 kW) prototype of his patent.[5] The Jendrassik Cs-1, was produced and tested in the Ganz Works in Budapest between 1939 and 1942. It was planned to fit to the Varga RMI-1 X/H twin-engined reconnaissance bomber in 1940, but the program was cancelled.[citation needed]

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.[6] 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.[7][8] 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,[9] 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.[10] 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.[11]

The Soviet Union built on German World War II development by Junkers (BMW and Hirth/Daimler-Benz also developed and partially tested designs).[citation needed] 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 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 Allison T40, into a series of experimental aircraft during the 1950s, but none would be adopted into service.[citation needed]

The first American turboprop engine was the General Electric XT31, first used in the experimental Consolidated Vultee XP-81.[12] The XP-81 first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. The technology of the Lockheed Electra airliner was also used in military aircraft, such as the P-3 Orion and the C-130 Hercules, using the Allison T56. One of the most produced turboprop engines 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 December 11, 1951.[13]

Current engines[edit]

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

manufacturerCountrydesignationdry weight (kg)takeoff rating (kW)Application
DEMC People's Republic of ChinaWJ5E7202130Harbin SH-5, Xi'an Y-7
Europrop International European UnionTP400-D618008203Airbus A400M
General Electric United StatesCT7-5A3651294
General Electric United StatesCT7-93651447CASA/IPTN CN-235, Let L-610, Saab 340, Sukhoi Su-80
General Electric United StatesT64-P4D5382535Aeritalia G.222, de Havilland Canada DHC-5 Buffalo, Kawasaki P-2J
Honeywell United StatesTPE331 Series150 - 275478 - 1650Aero/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 StatesLTP 101-700147522Air Tractor AT-302, Piaggio P.166
KKBM RussiaNK-12MV190011033Antonov An-22, Tupolev Tu-95, Tupolev Tu-114
Progress UkraineTV3-117VMA-SB25601864Antonov An-140
Progress UkraineTV7-117S5302100Ilyushin Il-112, Ilyushin Il-114
Progress UkraineD-271,65010440Antonov An-70
Progress UkraineAI20M10402940Antonov An-12, Antonov An-32, Ilyushin Il-18
Progress UkraineAI24T6001880Antonov An-24, Antonov An-26, Antonov An-30
LHTEC United StatesLHTEC T8005172013AgustaWestland 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 RussiaTVD-202401081Antonov An-3, Antonov An-38
Pratt & Whitney Canada CanadaPT-6 Series149 - 260430 - 1500Air 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 CanadaPW1204181491ATR 42-300/320
Pratt & Whitney Canada CanadaPW1214251603ATR 42-300/320, Bombardier Dash 8 Q100
Pratt & Whitney Canada CanadaPW123 C/D4501603Bombardier Dash 8 Q300
Pratt & Whitney Canada CanadaPW1274812051ATR 72-200
Pratt & Whitney Canada CanadaPW150A6903781Bombardier Dash 8 Q400
PZL PolandTWD-10B230254PZL M28
RKBM RussiaTVD-1500S2401044Sukhoi Su-80
Rolls-Royce United KingdomDart Mk 5365691700Avro 748, Fokker F27, Vickers Viscount
Rolls-Royce United KingdomTyne 215694500Aeritalia G.222, Breguet Atlantic, Transall C-160
Rolls-Royce United Kingdom250-B1788.4313Fuji T-7, Britten-Norman Turbine Islander, O&N Cessna 210, Soloy Cessna 206, Propjet Bonanza
Rolls-Royce United KingdomT56-158283424C-130 Hercules
Rolls-Royce United KingdomT56-278803910E-2 Hawkeye
Rolls-Royce United KingdomAE2100A715.83095Saab 2000
Rolls-Royce United KingdomAE2100J7103424ShinMaywa US-2
Rolls-Royce United KingdomAE2100D2, D37023424Alenia C-27J Spartan, Lockheed Martin C-130J Super Hercules
Rybinsk RussiaTVD-1500V2201156
Saturn RussiaTAL-34-1178809
Turbomeca FranceArrius 1D111313Socata TB 31 Omega
Turbomeca FranceArrius 2F103376
Walter Czech RepublicM601E200560Let L-410 Turbolet UVP-E
Walter Czech RepublicM601F202580L420-UVP
Walter Czech RepublicM602A5701360Let L-610
Walter Czech RepublicM602B4801500

See also[edit]

References[edit]

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