A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle.[2] Air enters the intake and is 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 stages, generating power at the point of exhaust. Some of the power generated by the turbine is used to drive the compressor and electric generator. The gases are then exhausted from the turbine. In contrast to a turbojet or turbofan, the engine's exhaust gases do not provide enough power to create significant thrust, since almost all of the engine's power is used to drive the propeller.[3]
Technological aspects
Schematic diagram showing the operation of a turboprop enginePropulsive efficiency for different engine types and Mach numbers
Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (beyond 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 produces about 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 have bypass ratios of 50–100,[9][10] although the propulsion airflow is less clearly defined for propellers than for fans.[11][12]
The propeller is coupled to the turbine through a reduction gear that converts the high RPM/low torque output to low RPM/high torque. This can be of two primary designs, free-turbine and fixed. A free-turbine turboshaft found on the Pratt & Whitney Canada PT6, where the gas generator is not connected to the propeller. This allows for propeller strike or similar damage to occur without damaging the gas generator and allowing for only the power section (turbine and gearbox) to be removed and replaced in such an event, and also allows for less stress on the start during engine ground starts. Whereas a fixed shaft has the gearbox and gas generator connected, such as on the Honeywell TPE331.
The propeller itself is normally a constant-speed (variable pitch) propeller type similar to that used with larger aircraft reciprocating engines, except that the propeller-control requirements are very different.[13] Due to the turbine engine's slow response to power inputs, particularly at low speeds, the propeller has a greater range of selected travel in order to make rapid thrust changes, notably for taxi, reverse, and other ground operations.[14] The propeller has 2 modes, Alpha and Beta. Alpha is the mode for all flight operations including takeoff. Beta, a mode typically consisting of zero to negative thrust, is used for all ground operations aside from takeoff.[14] The Beta mode is further broken down into 2 additional modes, Beta for taxi and Beta plus power. Beta for taxi as the name implies is used for taxi operations and consists of all pitch ranges from the lowest alpha range pitch, all the way down to zero pitch, producing very little to zero-thrust and is typically accessed by moving the power lever to a beta for taxi range. Beta plus power is a reverse range and produces negative thrust, often used for landing on short runways where the aircraft would need to rapidly slow down, as well as backing operations and is accessed by moving the power lever below the beta for taxi range.[14] Due to the pilot not being able to see out of the rear of the aircraft for backing and the amount of debris reverse stirs up, manufacturers will often limit the speeds beta plus power may be used and restrict its use on unimproved runways.[14] Feathering of these propellers is performed by the propeller control lever.[14]
The constant-speed propeller is distinguished from the reciprocating engine constant-speed propeller by the control system. The turboprop system consists of 3 propeller governors, a governor, and overspeed governor, and a fuel-topping governor.[14] The governor works in much the same way a reciprocating engine propeller governor works, though a turboprop governor may incorporate beta control valve or beta lift rod for beta operation and is typically located in the 12 o'clock position.[14] There are also other governors that are included in addition depending on the model, such as an overspeed and fuel topping governor on a Pratt & Whitney Canada PT6, and an under-speed governor on a Honeywell TPE331.[14] The turboprop is also distinguished from other kinds of turbine engine in that the fuel control unit is connected to the governor to help dictate power.
To make the engine more compact, reverse airflow can be used. On a reverse-flow turboprop engine, the compressor intake is at the aft of the engine, and the exhaust is situated forward, reducing the distance between the turbine and the propeller.[15]
Unlike the small-diameter fans used in turbofan 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. Since 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,[16][17] a low disc loading (thrust per unit disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.[18][19]
Propellers work well until the flight speed of the aircraft is high enough that the airflow past the blade tips reaches the speed of sound. Beyond that speed, the proportion of the power that drives the propeller that is converted to propeller thrust falls dramatically. For this reason turboprop engines are not commonly used on aircraft[4][5][6] that fly faster than 0.6–0.7 Mach,[7] with some exceptions such as the Tupolev Tu-95. However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching 0.75 Mach. To maintain propeller efficiency across a wide range of airspeeds, turboprops use constant-speed (variable-pitch) propellers. The blades of a constant-speed propeller increase their pitch as aircraft speed increases. Another benefit of this type of propeller is that it can also be used to generate reverse thrust to reduce stopping distance 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.[20]
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.[21]
History
Drawings of the Hungarian Varga RMI-1 X/H – the world's first working turboprop-powered aircraft.
The world's first turboprop was designed by the Hungarianmechanical engineerGyörgy Jendrassik.[23] 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; 74.6kW) experimental gas turbine.[24] 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. First run in 1940, combustion problems limited its output to 400bhp. Two Jendrassik Cs-1s were the engines for the world's first turboprop aircraft – the Varga RMI-1 X/H. This was a Hungarianfighter-bomber of WWII which had one model completed, but before its first flight it was destroyed in a bombing raid.[25][26] In 1941, the engine was abandoned due to war, and the factory converted to conventional engine production.
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.[27] The first British turboprop engine was the Rolls-RoyceRB.50 Trent, a converted Derwent II fitted with reduction gear and a Rotol7ft 11in (2.41m) five-bladed propeller. Two Trents were fitted to Gloster MeteorEE227 — the sole "Trent-Meteor" — which thus became the world's first turboprop-powered aircraft to fly, albeit as a test-bed not intended for production.[28][29] 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 receive a type certificate for military and civil use,[30] 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.[31] 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.[32]
The Kuznetsov NK-12 is the most powerful turboprop to enter service
The Soviet Union built on German World War II turboprop preliminary design work by Junkers Motorenwerke, while BMW, Heinkel-Hirth and Daimler-Benz also worked on projected designs.[33] 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 575mph, 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 through to the end of the 20th century. The USA used turboprop engines with contra-rotating propellers, such as the Allison T40, on some experimental aircraft during the 1950s. The T40-powered Convair R3Y Tradewind flying-boat was operated by the U.S. Navy for a short time.
The first American turboprop engine was the General ElectricXT31, first used in the experimental Consolidated Vultee XP-81.[34] The XP-81 first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. The technology of Allison's earlier T38 design evolved into the Allison T56, used to power the Lockheed Electra airliner, its military maritime patrol derivative the P-3 Orion, and the C-130 Hercules military transport aircraft.
In contrast to turbofans, turboprops are most efficient at flight speeds below 725km/h (450mph; 390 knots) because the jet velocity of the propeller (and exhaust) is relatively low.[citation needed] Modern turboprop airliners operate at nearly the same speed as small regional jet airliners but burn two-thirds of the fuel per passenger.[38]
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.
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.[42]
A jet engine is a type of reaction engine, discharging a fast-moving jet of heated gas that generates thrust by jet propulsion. While this broad definition may include rocket, water jet, and hybrid propulsion, the term jet engine typically refers to an internal combustion air-breathing jet engine such as a turbojet, turbofan, ramjet, pulse jet, or scramjet. In general, jet engines are internal combustion engines.
A gas turbine or gas turbine engine is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part and are, in the direction of flow:
A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the additional fan stage. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Aircraft using power components are referred to as powered flight. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many small UAVs have used electric motors.
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and a turbine. The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.
The Pratt & Whitney Canada PT6 is a turboprop aircraft engine produced by Pratt & Whitney Canada. Its design was started in 1958, it first ran in February 1960, first flew on 30 May 1961, entered service in 1964, and has been continuously updated since. The PT6 consists of two basic sections: a gas generator with accessory gearbox, and a free-power turbine with reduction gearbox. In aircraft, the engine is often mounted "backwards," with the intake at the rear and the exhaust at the front, so that the turbine is directly connected to the propeller. Many variants of the PT6 have been produced, not only as turboprops but also as turboshaft engines for helicopters, land vehicles, hovercraft, and boats; as auxiliary power units; and for industrial uses. By November 2015, 51,000 had been produced, which had logged 400 million flight hours from 1963 to 2016. It is known for its reliability, with an in-flight shutdown rate of 1 per 651,126 hours in 2016. The PT6A turboprop engine covers the power range between 580 and 1,940 shp, while the PT6B/C are turboshaft variants for helicopters.
This article outlines the important developments in the history of the development of the air-breathing (duct) jet engine. Although the most common type, the gas turbine powered jet engine, was certainly a 20th-century invention, many of the needed advances in theory and technology leading to this invention were made well before this time.
The Cessna 441 Conquest II is the first turboprop powered aircraft designed by Cessna and was meant to fill the gap between their jets and piston-engined aircraft. It was developed in November 1974, with the first aircraft delivered in September 1977. It is a pressurized, 8–9 passenger turbine development of the Cessna 404 Titan. The ICAO designator as used in flight plans is C441.
A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel.
The Honeywell TPE331 is a turboprop engine. It was designed in the 1950s by Garrett AiResearch, and produced since 1999 by successor Honeywell Aerospace. The engine's power output ranges from 575 to 1,650 shaft horsepower.
The Pratt & Whitney Canada JT15D is a small turbofan engine built by Pratt & Whitney Canada. It was introduced in 1971 at 2,200 lbf (9,800 N) thrust, and has since undergone a series of upgrades to just over 3,000 lbf (13 kN) thrust in the latest versions. It is the primary powerplant for a wide variety of smaller jet aircraft, notably business jets.
The Lycoming ALF 502/LF 507 is a geared turbofan engine produced by Lycoming Engines, AlliedSignal, and then Honeywell Aerospace. The U.S. military designation for the ALF 502 is YF102.
The General Electric T31 was the first turboprop engine designed and built in the United States.
The Garrett TPF351 is a turboprop engine designed by Garrett Engine Division of AlliedSignal Aerospace Company. Initiated by Garrett in October 1987, the TPF351-20 engine was selected by Embraer to power the Embraer/FMA CBA 123 Vector, a high-speed commuter "pusher" aircraft. It was first tested on May 19, 1989 and then ground tested and flight tested on a Boeing 720 on July 9, 1990. The first prototype CBA 123 was tested on July 18, 1990, followed by a flight to the Farnborough Air Show in September of the same year. Both programs were cancelled in 1992, when the TPF351 was nine months from engine certification.
An airbreathing jet engine is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in, compressed, heated, and expanded back to atmospheric pressure through a propelling nozzle. Compression may be provided by a gas turbine, as in the original turbojet and newer turbofan, or arise solely from the ram pressure of the vehicle's velocity, as with the ramjet and pulsejet.
The period between 1945 and 1979 is sometimes called the post-war era or the period of the post-war political consensus. During this period, aviation was dominated by the arrival of the Jet Age. In civil aviation the jet engine allowed a huge expansion of commercial air travel, while in military aviation it led to the widespread introduction of supersonic aircraft.
A free-turbine turboshaft is a form of turboshaft or turboprop gas turbine engine where the power is extracted from the exhaust stream of a gas turbine by an independent turbine, downstream of the gas turbine. The power turbine is not mechanically connected to the turbines that drive the compressors, hence the term "free", referring to the independence of the power output shaft. This is opposed to the power being extracted from the turbine/compressor shaft via a gearbox.
The General Electric Catalyst is a turboprop engine by GE Aerospace. It was announced on 16 November 2015 and will power the Beechcraft Denali, it first ran on December 22, 2017, and should be certified in 2024. The 850 to 1,600 hp engine aims for 20% better efficiency than its competition thanks to a 16:1 overall pressure ratio, variable stator vanes, cooled turbine blades, 3D printed parts and FADEC.
Aircraft engine performance refers to factors including thrust or shaft power for fuel consumed, weight, cost, outside dimensions and life. It includes meeting regulated environmental limits which apply to emissions of noise and chemical pollutants, and regulated safety aspects which require a design that can safely tolerate environmental hazards such as birds, rain, hail and icing conditions. It is the end product that an engine company sells.
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. ISBN0-7509-4477-3.
Gunston, Bill (2006). World Encyclopedia of Aero Engines, 5th Edition. Phoenix Mill, Gloucestershire, England, UK: Sutton Publishing Limited. ISBN0-7509-4479-X.
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