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 (or spool). This is opposed to the power being extracted from the turbine/compressor shaft via a gearbox.
The advantage of the free turbine is that the two turbines can operate at different speeds and that these speeds can vary relative to each other. This is particularly advantageous for varying loads, such as turboprop engines. [1]
A free-turbine turboshaft ingests air through an intake. The air passes through a compressor and into a combustor where fuel is mixed with the compressed air and ignited. The combustion gases are expanded through a compressor-driving turbine, and then through a "free" power turbine before being exhausted to the atmosphere. The compressor and its turbine are connected by a common shaft which, together with the combustor, is known as a gas generator, which is modelled using the Brayton Cycle. The (free) power turbine is on a separate shaft.
Turboshaft engines are sometimes characterized by the number of spools. This refers to the number of compressor-and-turbine assemblies in the gas generator stage and does not include the free power turbine assembly. [2] As an example, the General Electric T64 is a single-spool design that uses a 14-stage axial compressor; the independent power shaft is coaxial with the gas generator shaft. [3]
One particular failure scenario, a gearbox failure, showed a free-turbine arrangement to be more at risk than a single-shaft turboprop. It could suffer a turbine overspeed to destruction after losing its connection to the propeller load. (In a single-shaft arrangement with a similar gearbox failure the turbine would still have most of its load from the compressor). Such a failure resulted in the 1954 accident of the second prototype Bristol Britannia, G-ALRX, which was forced to land in the Severn Estuary. A failure in the Bristol Proteus propeller reduction gearbox led to an overspeed and release of the power turbine of Nº3 engine. It cut through the oil tank and started a fire that threatened the integrity of the wing spar. The pilot, Bill Pegg, made a forced landing on the estuary mud. [4] [5] The Proteus gears were redesigned and an emergency fuel shut-off device was fitted to prevent a similar reoccurrence. Writing in 1994, Gunston [1] found it remarkable that protection was not common on free-turbine engines. However, certification regulations allow other methods for preventing excessive overspeed such as disc rubbing and blade interference.
Most turboshaft and turboprop engines now use free turbines. This includes those for static power generation, as marine propulsion and particularly for helicopters.
Helicopters are a major market for turboshaft engines. When turboshaft engines became available in the 1950s, they were rapidly adopted for both new designs and as replacements for piston engines. They offered more power and far better power to weight ratios. Piston helicopters of this period had barely adequate performance; the switch to a turbine engine could both reduce several hundred pounds of engine weight, 600 lb (270 kg) for the Napier Gazelle of the Westland Wessex, [6] and also allow considerably more payload weight. For the Westland Whirlwind, this converted the inadequate piston-engined HAS.7 to the de Havilland Gnome turbine-powered HAR.9. As one of the first anti-submarine helicopters, the HAS.7 had been so weight restricted that it could carry either a search sonar or a torpedo, but not both.
The free-turbine engine was found to be particularly suitable. It does not need a clutch, as the gas generator may be started while the output shaft remains stationary. For the Wessex, this was used to give a particularly fast take-off from a cold start. By locking the main rotor (and the power turbine) with the rotor brake, the engine could be started and then, with the gas generator at a speed of 10,500 rpm, the brake released allowing the power turbine to accelerate and bring the rotor to its operating speed from stationary in just 15 seconds and a time from engine start to take-off of only 30 seconds. [6]
A further advantage of the free turbine design was the ease with which a counter-rotating engine could be designed and manufactured, simply by reversing the power turbine alone. [7] This allowed handed engines to be made in pairs when needed. It also allowed contra-rotating engines, where the gas generator core and power turbine revolved in opposite directions, reducing the overall moment of inertia. For the helicopter engine replacement market, this ability allowed previous engines of either direction to be replaced simply. [7] Some turboshaft engines' omni-angle freedom of their installation angle also allowed installation into existing helicopter designs, no matter how the previous engines had been arranged. [7] In time though, the move towards axial LP compressors and so smaller diameter engines encouraged a move to the now-standard layout of one or two engines set side-by-side, horizontally above the cabin.
Turboprop aircraft are still powered by a range of free- and non-free turbine engines. Larger engines have mostly retained the non-free design, although many are two-shaft designs where the 'power' turbine drives the propeller and the low-pressure compressor while the high-pressure compressor has its own turbine.
The first free-turbine gas turbine engine was the Bristol Theseus turboprop. [8] This was the first Bristol gas turbine and its broad design had been produced by Frank Owner at Tockington Manor. It first ran in July 1945 and in December 1946 was the first turboprop to pass a 100 hour type test. [8]
Some large turboprop engines, such as the original Bristol Proteus and the modern TP400 have free turbines. The TP400 is a three-shaft design, with two compressor turbines and a separate power turbine. Where the turbine is at the rear of the engine, a turboprop engine requires a long drive shaft forwards to the propeller reduction gearbox. Such long shafts can be a difficult design problem and must carefully control any shaft vibration.
For small turboprop engines, the free-turbine design has come to dominate and these designs are also mostly reversed overall, with their air inlet and compressor to the rear, feeding forwards to hot section and power turbine at the front. This places the turbine output close to the propeller gearbox, avoiding the need for a long driveshaft. Such engines are often recognisable externally, as they use external 'elbow' exhausts ahead of the main engine. A particularly common example of this is the PT6 engine, of which over 50,000 have been produced.
An attractively simple configuration making use of the free turbine is the propfan engine, with a rear-mounted unducted fan in pusher configuration, rather than the more familiar tractor layout. The first such engine was the very early and promising Metropolitan-Vickers F.3 of 1942 with a ducted fan, followed by the unducted and much lighter F.5. Development of these engines stopped abruptly owing to corporate takeovers, rather than technical reasons. Rolls-Royce continued with design studies for such engines into the 1980s, [9] as did GE, but they have yet to appear as commercial engines. [10]
The advantage of the pusher propfan with a free power turbine is its simplicity. The prop blades are attached directly to the outside of the rotating turbine disc. No gearboxes or drive shafts are required. The short length of the rotating components also reduces vibration. The static structure of the engine over this length is a large diameter tube within the turbine. In most designs, two contra-rotating rings of turbine and propeller are used. Intermeshed contra-rotating turbines can act as the guide vanes for each other, removing the need for static vanes. [9]
The M1 Abrams main battle tank is powered by a Honeywell AGT1500 (formerly Textron Lycoming) two-spool gas turbine engine. A commercial derivative has been designed as the TF15 for marine and railroad applications, [11] [12] and a flight-rated version, the PLT27, was also developed but lost a major contract to the GE T700 turboshaft. [13]
Turboshaft engines were used to power several gas turbine locomotives, most notably using the Turbomeca Turmo in Turbotrain (France) and Turboliner (United States) service.
A turboprop is a turbine engine that drives an aircraft propeller.
A gas turbine, gas turbine engine, or also known by its old name internal combustion turbine, 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:
The Bristol Proteus was the Bristol Engine Company's first mass-produced gas turbine engine design, a turboprop that delivered just over 4,000 hp (3,000 kW). The Proteus was a reverse-flow gas turbine. Because the second turbine drove no compressor stages, but only the propeller, this engine was classified as a free-turbine. It powered the Bristol Britannia airliner, small naval patrol craft, hovercraft and electrical generating sets. It was also used to power a land-speed record car, the Bluebird-Proteus CN7. After the merger of Bristol with Armstrong Siddeley the engine became the Bristol Siddeley Proteus, and later the Rolls-Royce Proteus. The Proteus was to have been superseded by the Bristol Orion which would have given a Britannia a 75% increase in power for cruising faster.
A propfan, also called an open rotor engine, open fan engine or unducted fan, is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. A propfan is typically designed with a large number of short, highly twisted blades, similar to the (ducted) fan in a turbofan engine. For this reason, the propfan has been variously described as an "unducted fan" (UDF) or an "ultra-high-bypass (UHB) turbofan".
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.
The Europrop International TP400-D6 is an 11,000 shp (8,200 kW) powerplant, developed and produced by Europrop International for the Airbus A400M Atlas military transport aircraft. The TP400 is the most powerful turboprop in service using a single propeller; only the Kuznetsov NK-12 from Russia and Progress D-27 from Ukraine, using contra-rotating propellers, is larger.
A turboshaft engine is a form of gas turbine that is optimized to produce shaft horsepower rather than jet thrust. In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. They are even more similar to turboprops, with only minor differences, and a single engine is often sold in both forms.
The Allison T56 is an American single-shaft, modular design military turboprop with a 14-stage axial flow compressor driven by a four-stage turbine. It was originally developed by the Allison Engine Company for the Lockheed C-130 Hercules transport entering production in 1954. It has been a Rolls-Royce product since 1995 when Allison was acquired by Rolls-Royce. The commercial version is designated 501-D. Over 18,000 engines have been produced since 1954, logging over 200 million flying hours.
The Bristol Orion aero engine is a two-shaft turboprop intended for use in later marks of the Bristol Britannia and the Canadair CL-44. Although the engine was built and underwent a development program, the BE.25 Orion project was cancelled in 1958 by the British Ministry of Supply in favour of the Rolls-Royce Tyne. In addition, interest in turboprop-powered aircraft was beginning to wane, because of the successful introduction of the Boeing 707 and Douglas DC-8 jetliners into airline service.
The Rolls-Royce Gnome is a British turboshaft engine originally developed by the de Havilland Engine Company as a licence-built General Electric T58, an American mid-1950s design. The Gnome came to Rolls-Royce after their takeover of Bristol Siddeley in 1966, Bristol having absorbed de Havilland Engines Limited in 1961.
The General Electric GE38 is a gas turbine developed by GE Aviation for turboprop and turboshaft applications. It powers the Sikorsky CH-53K King Stallion as the T408.
The Napier Eland is a British turboshaft or turboprop gas-turbine engine built by Napier & Son in the early 1950s. Production of the Eland ceased in 1961 when the Napier company was taken over by Rolls-Royce.
The Avco-Lycoming AGT1500 is a gas turbine engine. It is the main powerplant of the M1 Abrams series of tanks. The engine was originally designed and produced by the Lycoming Turbine Engine Division in the Stratford Army Engine Plant. In 1995, production was moved to the Anniston Army Depot in Anniston, Alabama, after the Stratford Army Engine Plant was shut down.
The General Electric T64 is a free-turbine turboshaft engine that was originally developed for use on helicopters, but which was later used on fixed-wing aircraft as well. General Electric introduced the engine in 1964. The original engine design included technical innovations such as corrosion resistant and high-temperature coatings. The engine features a high overall pressure ratio, yielding a low specific fuel consumption for its time. Although the compressor is all-axial, like the earlier General Electric T58, the power turbine shaft is coaxial with the HP shaft and delivers power to the front of the engine, not rearwards. Fourteen compressor stages are required to deliver the required overall pressure ratio. Compressor handling is facilitated by 4 rows of variable stators. Unlike the T58, the power turbine has 2 stages.
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.
The Pratt & Whitney/Allison 578-DX was an experimental aircraft engine, a hybrid between a turbofan and a turboprop known as a propfan. The engine was designed in the 1980s to power proposed propfan aircraft such as the Boeing 7J7 and the MD-91 and MD-92 derivatives of the McDonnell Douglas MD-80. As of 2019, it is still one of only four different contra-rotating propfan engines to have flown in service or in flight testing.
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 Bristol Phoebus was an early turbojet engine developed by Bristol Engines. It was based on the gas generator core of the Bristol Proteus. The Phoebus was used for development but only a handful were made. As with other Bristol engines, it took its name from classical mythology.
The Allison T56 turboprop engine has been developed extensively throughout its production run, the many variants are described by the manufacturer as belonging to four main series groups.
During World War 2, the Bristol Aero Engine part of the Bristol Aeroplane Company was pre-occupied with developing and manufacturing radial piston engines, such as the Bristol Hercules and the more powerful Bristol Centaurus. However, in 1944 the Company decided to form a Project Department to investigate the design of gas turbines. Initially the department was based at Tockington Manor, a large country house close to the main factory at Patchway, Bristol. A predominantly young team was formed and was initially tasked with studying turboprop engines.