Editing Turbofan (section)

=== High-bypass turbofan ===
{{More citations needed section|date=December 2024}}
{{further|Bypass ratio}}
<!-- Old non-animated image.....[[File:tfan-schematic-kk-20090106.png|thumb|Schematic diagram illustrating a modern 2-spool, high-bypass turbofan engine in nacelle with an unmixed exhaust. The low-pressure spool is colored blue and the high-pressure one orange.]] -->

[[File:Turbofan operation.svg|thumb|Schematic diagram illustrating a 2-spool, high-bypass turbofan engine with an unmixed exhaust. The low-pressure spool is coloured green and the high-pressure one purple. Again, the fan (and booster stages) are driven by the low-pressure turbine, but more stages are required. A mixed exhaust is often employed.]]

To further improve fuel economy and reduce noise, almost all jet airliners and most military transport aircraft (e.g., the [[C-17 Globemaster III|C-17]]) are powered by low-specific-thrust/high-bypass-ratio turbofans. These engines evolved from the high-specific-thrust/low-bypass-ratio turbofans used in such aircraft in the 1960s. Modern combat aircraft tend to use low-bypass ratio turbofans, and some military transport aircraft use [[turboprop]]s.

Low specific thrust is achieved by replacing the multi-stage fan with a single-stage unit. Unlike some military engines, modern civil turbofans lack stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve the desired net thrust.

The core (or gas generator) of the engine must generate enough power to drive the fan at its rated mass flow and pressure ratio. Improvements in turbine cooling/material technology allow for a higher (HP) turbine rotor inlet temperature, which allows a smaller (and lighter) core, potentially improving the core thermal efficiency. Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average [[stage loading]] and to maintain LP turbine efficiency. Reducing core flow also increases bypass ratio. Bypass ratios greater than 5:1 are increasingly common; the [[Pratt & Whitney PW1000G]], which entered commercial service in 2016, attains 12.5:1.

Further improvements in core thermal efficiency can be achieved by raising the [[overall pressure ratio]] of the core. Improvements in blade aerodynamics can reduce the number of extra compressor stages required, and [[Axial compressor#Bleed air, variable stators|variable geometry stators]] enable high-pressure-ratio compressors to work surge-free at all throttle settings.

[[File:CF6-6 engine cutaway.jpg|thumb|Cutaway diagram of the [[General Electric CF6]]-6 engine]]

The first (experimental) high-bypass turbofan engine was the [[Lycoming Engines|AVCO-Lycoming]] PLF1A-2, a [[Honeywell T55]] turboshaft-derived engine that was first run in February 1962. The PLF1A-2 had a {{cvt|40|in|cm|adj=mid|diameter}} geared fan stage, produced a static thrust of {{cvt|4,320|lb}},<ref name="Boyne2002">{{cite book |url={{GBurl|FW_50wm8VnMC|p=235}} |title=Air warfare: An international encyclopedia: A–L |editor-first=Walter J. |editor-last=Boyne |publisher=ABC-CLIO |year=2002 |page=235 |isbn=978-1-57607-345-2}}</ref> and had a bypass ratio of 6:1.<ref name="NASM-PLF1A-2">{{cite web |title=Lycoming PLF1A-2 turbofan engine |url=https://airandspace.si.edu/collection-objects/lycoming-plf1a-2-turbofan-engine/nasm_A19890042000 |work=[[Smithsonian National Air and Space Museum]] |access-date=December 31, 2021}}</ref> The [[General Electric TF39]] became the first production model, designed to power the [[Lockheed Corporation|Lockheed]] [[C-5 Galaxy]] military transport aircraft.<ref name="Neumann_2004_1984_pp228-230"/> The civil [[General Electric CF6]] engine used a derived design. Other high-bypass turbofans are the [[Pratt & Whitney JT9D]], the three-shaft [[Rolls-Royce RB211]] and the [[CFM International CFM56]]; also the smaller [[TF34]]. More recent large high-bypass turbofans include the [[Pratt & Whitney PW4000]], the three-shaft [[Rolls-Royce Trent]], the [[General Electric GE90]]/[[GEnx]] and the [[GP7000]], produced jointly by GE and P&W. The Pratt & Whitney JT9D engine was the first high bypass ratio [[jet engine]] to power a wide-body airliner.<ref>{{cite book | url=https://books.google.com/books?id=kWJBDAAAQBAJ&dq=first+high+bypass+turbofan&pg=PA997 | title=Fundamentals of Aircraft and Rocket Propulsion | isbn=978-1-4471-6796-9 | last1=El-Sayed | first1=Ahmed F. | date=25 May 2016 | publisher=Springer }}</ref>

The lower the specific thrust of a turbofan, the lower the mean jet outlet velocity, which in turn translates into a high [[Thrust lapse|thrust lapse rate]] (i.e. decreasing thrust with increasing flight speed). See technical discussion below, item 2. Consequently, an engine sized to propel an aircraft at high subsonic flight speed (e.g., Mach 0.83) generates a relatively high thrust at low flight speed, thus enhancing runway performance. Low specific thrust engines tend to have a high bypass ratio, but this is also a function of the temperature of the turbine system.

The turbofans on twin-engined transport aircraft produce enough take-off thrust to continue a take-off on one engine if the other engine shuts down after a critical point in the take-off run. From that point on the aircraft has less than half the thrust compared to two operating engines because the non-functioning engine is a source of drag. Modern twin engined airliners normally climb very steeply immediately after take-off. If one engine shuts down, the climb-out is much shallower, but sufficient to clear obstacles in the flightpath.

The Soviet Union's engine technology was less advanced than the West's, and its first wide-body aircraft, the [[Ilyushin Il-86]], was powered by low-bypass engines. The [[Yakovlev Yak-42]], a medium-range, rear-engined aircraft seating up to 120 passengers, introduced in 1980, was the first Soviet aircraft to use high-bypass engines.

<gallery mode="packed" heights="120">
File:Sam146 1.jpg|[[PowerJet SaM146]] which powers [[Sukhoi Superjet 100]]
File:Ge cf6 turbofan.jpg|[[General Electric CF6]] which powers the [[Airbus A300]], [[Boeing 747]], [[Douglas DC-10]] and other aircraft
File:Airbus Lagardère - Trent 900 engine MSN100 (6).JPG|[[Rolls-Royce Trent 900]], powering the [[Airbus A380]]
File:PW4000-112 (cropped).jpg|[[Pratt & Whitney PW4000]], powering the [[Boeing 777]], [[MD-11]] and [[Airbus A330]]
File:CFM56 P1220759.jpg|The [[CFM International CFM56|CFM56]] which powers the [[Boeing 737]], the [[Airbus A320]] and other aircraft
File:Airbus Lagardère - GP7200 engine MSN108 (1).JPG|[[Engine Alliance GP7000]] turbofan for the [[Airbus A380]]
File:Engine Il-96 "Aeroflot" (3447358279).jpg|[[Aviadvigatel PS-90]] which powers the [[Ilyushin Il-96]], [[Tupolev Tu-204]], [[Ilyushin Il-76]]
File:ALF502.JPG|[[Lycoming ALF 502]] which powers the [[British Aerospace 146]]
File:MAKS Airshow 2013 (Ramenskoye Airport, Russia) (524-34).jpg|[[Aviadvigatel PD-14]] which will be used on the [[Irkut MC-21]]
File:D-436-148 MAKS-2009.jpg|Three shaft [[Progress D-436]]
File:Trent 1000 GoodwinHall VirginiaTech.jpg|[[Trent 1000]] powering the [[Boeing 787]]
File:General Electric GE90 displayed at Farnborough Air Show 2008.jpg|[[GE90]] powering the [[Boeing 777]], the most powerful aircraft engine
</gallery>