CFM International CFM56

CFM56
CFM56-5
Type	Turbofan
National origin	France and United States
Manufacturer	CFM International
First run	June 1974 [1]
Major applications	Airbus A320 family Airbus A340 Boeing 737 KC-135R Stratotanker
Number built	~20,000 (as of Sep 2009)[2]
Developed from	General Electric F101
Developed into	CFM International LEAP-X

The CFM International CFM56 series (US military designation F108) are a family of high-bypass turbofan engines made by CFM International with a thrust range from 18,500 to 34,000 lb_f (82 kN to 151 kN). CFM International is a 50-50 joint company of SNECMA, France and GE Aviation, USA. Both companies are responsible for producing various components and each has their own final assembly line. GE is responsible for the high pressure compressor, combustor and high pressure turbine, and SNECMA is responsible for the fan, low pressure turbine, the gearbox and the exhaust. The engines are assembled by GE in Evendale, Ohio, USA and by SNECMA in Villaroche, France.

The CFM56 is one of the most prolific engine types in the world with nearly 20,000 built.^[2] The engine is most widely used on the Boeing 737. It is also the sole powerplant of the A340-200 & -300, and the CFM56 (designated F108) replaced the Pratt & Whitney JT3D engines on the US KC-135 Stratotanker in the 1980s to create the KC-135R. The engine is also used on some on some Airbus A320s. As of September 2009, the CFM56 had flown over 450 million cumulative hours (the equivalent of more than 51,000 years) across all platforms.^[2]

Development

In the late 1960s, it was becoming clear that the the next generation of commercial engines would be high bypass ratio engines in the "10-ton" (or 20,000 lbf) class. SNECMA was the first to act, and began searching for a partner with commercial experience (they had mostly built military engines to this point) to design and build the engine. After considering Pratt & Whitney, Rolls-Royce, and GE Aviation, SNECMA selected GE. The two main reasons GE was likely selected was that they needed an engine in this market class (Pratt & Whitney dominated the commercial market at this point), and that SNECMA had experience with them, collaborating on the production of the CF6-50 turbofan for the Airbus A300.^[1]

A major reason that GE was so interested in the collaborative project, rather than building a 10-ton engine on their own, was that the SNECMA project was the only source for development funds for an engine in this class at the time. GE was initially considering only contributing technology from its CF6 engine rather than its much more advanced F101 engine, developed for the B-1 Lancer supersonic bomber. However the US Air Force announced its Advanced Medium STOL Transport project in 1972 which included funding for the development of a 10-ton engine. GE was now faced with a dilemma - should they build two 10-ton engines, one with "limited" technology with SNECMA and one with "advanced" technology on their own, or should they try to develop a single, advanced, engine? Worried that if they did not win the Air Force contract with the advanced engine (in which they were competing with Pratt & Whitney and GM), they would be left with only the "limited" engine in their portfolio, GE decided to apply for an export license for the F101 core technology. ^[3]

Export issues

In 1972, GE applied for an export license for its F101 core technology as GE's primary contribution to the "10-ton" engine project. However, the United States Department of State's Office of Munitions Control recommended the rejection the application on national security grounds, specifically because the core technology was an aspect of a strategic national defense system (the B-1 bomber), it was built with Department of Defense (and therefore American taxpayer) money, and that exporting the technology to France would limit the number of American workers on the project. ^[4] The official decision was made in a National Security Decision Memorandum signed by the National Security Advisor Henry Kissinger on 19 September 1972.^[5]

While national security concerns were cited as the grounds for rejections, it was reported that high level politics played an important role as well. The project, and the export issue associated with it, was considered so important that French President Georges Pompidou appealed directly to President Richard Nixon in 1971 to approve the deal, and that Henry Kissinger brought the issue up with President Pompidou in a 1972 meeting. GE reportedly argued at the highest levels that having half of the market was better than having none of it, which they argued would happen if SNECMA pursued the engine on their own without GE's contribution. However, Nixon administration officials feared that this project could be the beginning of the end of American aerospace leadership.^[6]

There was also speculation that the rejection may have been, in part, retaliation for French involvement in convincing the Swiss not to purchase American-made A-7 Corsair II aircraft that had been competing against a French design (in the end the Swiss decided not to buy either of the aircraft). ^[6]

1973 Nixon/Pompidou meeting

US President Nixon and French President George Pompidou prior to the 1973 US-French summit in Reykjavik, Iceland.

Despite the export license being rejected, both the French and GE continued to push the Nixon Administration for permission to export the F101 technology. Efforts continued throughout the months following the rejection, culminating in the engine being an agenda topic during the 1973 meeting of Presidents Nixon and Pompidou in Reykjavik. Discussions at this meeting resulted in an agreement, allowing the development of the CFM56 to proceed. At the time it was reported that the agreement was based around assurances that the core of the engine, the part GE was developing from the military F101, would be built in the United States and then transported to France in order to protect the sensitive technologies.^[7] However, documents declassified in 2007 revealed that a key aspect of the CFM56 export agreement was that French government agreed not seek tariffs against American aircraft being imported into Europe.^[8]

CFM International

With the export issue now settled, GE and SNECMA worked out the agreement that founded CFM International (CFMI), which was the 50-50 joint venture that would be responsible for producing and marketing the 10-ton engine, the CFM56. The venture was officially founded in 1974.^[9] The two primary roles for CFMI were to manage the program between GE and SNECMA and to market/sell/service the engine at a single point of contact for the customer. CMFI was responsible for the day-to-day decision making for the project, while major decisions (developing a new variant, for example) required the go-ahead from GE and SNECMA managment.^[1]

The CFMI Board of Directors is split evenly between SNECMA and GE (5 members each). There are two Vice Presidents, one from each company, who support the President of CFMI. The President tends to be drawn from SNECMA and sits at CFMI's headquarters near GE in Cincinnati, Ohio.^[1]

Work Split

The work split, in general terms, gave GE responsibility for the high pressure compressor (HPC), the combustor, and the high pressure turbine (HPT), while SNECMA was responsible for the fan, the low pressure compressor (LPC), and the low pressure turbine (LPT).^[10] SNECMA was also responsible for the initial airframe integration engineering, mostly involving the nacelle design. SNECMA was initially responsible for the gearbox, but shifted that work to GE when it became apparent that it would more efficient for GE to assemble that component along with their other parts.^[11]

Early development

Once the export agreement was settled, development work on the CFM56 began in earnest, even before CFM International was formally created. While most work proceeded smoothly, the international arrangement led to some unique working conditions. For example, both companies had assembly lines, so some of the engines were assembled and tested in the US, and some were assembled and tested in France. Those engines assembled in France had to work around the initially strict export agreement, which meant GE's core was built in the US, then shipped to the SNECMA plant in France, and then placed in a locked room that the President of SNECMA wasn't even allowed in. The SNECMA components (the fore and aft sections of the engine) are brought into the room, where GE employees mounted them to the core. Then the assembled engine is taken out to be finished.^[12]

Despite challenges like this, work on the first CFM56 engine proceeded, and first ran at GE in June 1974. The second engine also first ran at GE in October 1974. It was then shipped to France in December, and first ran there on December 13 1974. These first CFM56 engines were considered "production hardware", not just test articles, and were designated as CFM56-2 engines, which was the first variant of the CFM56.^[11]

Three years later, in February 1977, the engine flew for the first on the McDonnell Douglas YC-15, an entrant in the Air Force's AMST competition. It replace one of the 4 Pratt & Whitney JT8D engines the aircraft used.^[13] Soon after the engine first flew on the YC-15, the next CFM56 was mounted on a Sud Aviation Caravelle at the SNECMA flight test center in France. This engine had a slightly different configuration (a long bypass duct with mixed exhaust flow^{[nb 1]}, rather than a short bypass duct with unmixed exhaust flow^{[nb 2]}), and was the first to include a "Thrust Management System" to maintain engine trim. ^[14]

First customers

After testing the engine for several years, both in the air and on the ground, CFM International was really looking for customers outside of a possible AMST contract. The main targets were re-engine contracts for the Douglas DC-8 and the Boeing 707, including the military tanker based on the the 707, the KC-135 Stratotanker. After announcing that a 707 would be configured with the CFM56 engine for flight tests in 1977, Boeing officially offered the 707-320 with the CFM56 engine as an option in 1978.^[15] Having the commercial 707 available with the CFM56 helped the engine's competitiveness for the KC-135 re-engine contract.

KC-135R

An air-to-air left side view of a re-engined KC-135R Stratotanker aircraft on its maiden flight. The new engines are the CFM56-2 high bypass turbofan.

Winning the contract to re-engine the KC-135 tanker fleet for the US Air Force would be a huge boon to the CFM56 project (with 600+ aircraft available to re-engine), and CFM International aggressively pursued that goal as soon as the Request For Proposals (RFP) was announced in 1977. Like many other aspects of the program, international politics played their in this contract. In efforts to boost the CFM56's chances versus its competitors (the Pratt & Whitney TF33 and an updated Pratt & Whitney JT8D), the French government announced in 1978 that they would upgrade their 11 KC-135s with the CFM56 engine.^[16] Whatever the impetus for the French announcement, it was one of the first orders for the CFM56 engine.

The USAF announced the CFM56 as the winner of the re-engine contract in early 1980. Officials indicated that they were excited to replace the Pratt & Whitney J57 engines currently flying on the KC-135A aircraft, calling them "...the noisiest, dirtiest, [and] most fuel inefficient powerplant still flying" at the time. ^[17] The re-engined KC-135 was designated the "KC-135R".

The CFM56 brought many benefits to the KC-135, decreasing takeoff roll by as much as 3,500 ft (1,067 m), decreasing overall fuel usage by 25%, greatly reducing noise (24 dB lower), and lowering total life cycle cost. ^[18]

Soon after the CFM56 was selected for the KC-135, the United States Navy selected the CFM56-2 to power their variant of the the Boeing 707, the E-6 Mercury.^[18]

DC-8

By the end of the 1970s, many airlines were considering upgrading their venerable Douglas DC-8 aircraft as an alternative to buying new quieter and more efficient aircraft. After the French KC-135 announcement in 1978, the April 1979 decision by United Airlines to upgrade 30 of their DC-8-61 aircraft was important for the CFM56.^[19] That decision marked the first commercial purchase (rather than government/military) of the engine, and Delta Air Lines and Flying Tiger Line soon followed suit.^[1] Like the KC-135, the DC-8 was fitted with the CFM56-2 model.

The Boeing 737

Engine inlet of a Garuda Indonesia Boeing 737-400 series showing the non-circular inlet for the CFM56 engine.

In what was likely the most important early purchase for the CFM56, Boeing selected the CFM56-3 to exclusively power the latest Boeing 737, the 737-300, in the early 1980s. The small launch order for the 737-300, only 20 aircraft between two airlines, did little to foreshadow the success the aircraft and engine were going to have.^[1] The 737 had lower wings than the previous applications for the CFM56, so several modifications had to be made to the engine. The fan diameter was reduced, which reduced the bypass ratio. Additionally, the engine accessory gearbox was moved from the bottom of the engine (the 6 o'clock position) to the 9 o'clock position, giving the the engine nacelle its distinctive flat-bottomed shape. The overall thrust was also reduced, from 24,000 lbf (107 kN) to 20,000 lbf (89 kN), mostly due to the reduction in bypass ratio.^[20]

Continued development

Once the CFM56 was well established in both military and commercial applications, CFM International continued to improve the engine and market it for new aircraft, like the Airbus A320 and the Airbus A340. As of 2009, there are 6 major variants of the engine, each many sub-variants. (Details described below in the "Variants" section).

Design

The CFM56 is a high bypass turbofan engine with several variants generating a wide range of thrust, from 18,500 to 34,000 lbf (82 kN to 151 kN). All the variants share a common design, but the details differ. All the variants are two-shaft (or two-spool) engines, meaning that there are two rotating shafts, a high pressure and low pressure one. Each is powered by its own turbine section (the high pressure and low pressure turbines, respectively).

Fan and booster

View of the CFM56-5 front fan and fan case.

The CFM56 features a single stage fan and either a 3 or 4 stage booster (depending on variant) on the low pressure shaft.^{[nb 3]} The original CFM56 variant, the -2, featured 44 tip-shrouded fan blades^[21], although the number of fan blades dropped in later variants as wide-chord blade technology developed, down to 24 blades in the latest variant, the CFM56-7B.^[22]

The fan diameter varies with the different models of the CFM56, and that change has a direct impact on the engine performance. For example, the low pressure shaft rotates at the same speed for both the CFM56-2 and the CFM56-3 models. However, the fan diameter is smaller on the "-3", which lowers the tip speed of the fan blades. The lower speed allows the fan blades to operate more efficiently (5.5% more in this case), which increased the overall fuel efficiency of the engine (increasing specific fuel consumption nearly 3%).^[20]

The CFM56's bypass ratio ranges from 5 to 6 depending on variant, so most of the air accelerated by the fan is bypasses the core of engine and is exhausted out of the fan case. The air that does go through the core is first compressed by the booster (or low pressure compressor) before reaching the high pressure compressor. In most variants of the CFM56, the booster is 3 stages. In the -5B and -5C variants, the booster has 4 stages. ^[23]

Compressor

View of an opened CFM56-3 engine, focused at the high pressure compressor.

The high pressure compressor (HPC) in all variants of the CFM56 features 9 stages. As the design developed, the HPC was improved, mostly though better blade design. In 2007, CFM International offered an upgraded HPC, again improved with better blade aerodynamics, as a part of their "Tech Insertion" package.^[24]

Combustor

Most variants of the CFM56 features a single annular combustor. An annular combustor is a continuous ring where fuel is injected into the airflow and then ignited, raising the pressure and temperature of the flow. Other types of combustors include "can" combustors, where each combustion chamber is separate, and "can-annular", which is a hybrid of the two.

In 1989, CFM International began work on a new, double-annular, combustor. Instead of having just one combustion zone, the double annular combustor has a second combustion zone that is used at high thrust levels. This combustor lowers both nitrous oxide (NOx) and carbon dioxide (CO2) emissions. The first CFM56 engine with the double-annular combustor went into service in 1995, and the combustor is used on "Tech Insertion" CFM56-5B and CFM56-7B variants. ^[25]

Recently, GE has been testing a new type of combustor called the Twin Annular Premixing Swirler Combustor, or "TAPS". This combustor is similar to the the double-annular combustor in that it has two combustion zones. However, this combustor "swirls" the flow, helping to create an ideal fuel-air mixture. This difference allows the combustor to generate much less NOx than other combustors. Tests on a CFM56-7B engine demonstrated an improvement of 46% over single annular combustors and an improvement of 22% over double annular combustors. ^[26]

Some of the analytical tools developed for TAPS were also used to improve the single-annular combustors in some CFM56-5B and -7B engines.^[27]

Turbine

All variants of the CFM56 feature a single stage high pressure turbine (HPT). The HPT blades are "grown" from a single crystal, giving them high strength and creep resistance. The low pressure turbine (LPT) features four stages in most variants of the engine, but the CFM56-5C has a five stage LPT. This change was implemented to drive the larger fan on the -5C.^[28]

Exhaust

Although CFM International tested both a mixed and unmixed exhaust design at the beginning of development^[1], most variants of the engine feature an unmixed exhaust^{[nb 2]} nozzle. Only the high power CFM56-5C, designed for the Airbus A340 has a mixed flow exhaust^{[nb 1]} nozzle.^[28]

Additionally, GE and SNECMA tested the effectiveness of chevrons^{[nb 4]} on reducing jet noise. After examining several configurations in the wind tunnel, CFM International chose to flight test chevrons built into the core exhaust nozzle. The chevrons reduced jet noise by 1.3 perceived loudness decibels during takeoff conditions. The chevrons are now offered as an option with the CFM56 for the Airbus A321.^[29]

Variants

CFM56-2 series

This is the original variant of the CFM56. It is most widely used in military applications, where it is known as the F108, specifically in the KC-135, the E-6 Mercury, and some E-3 Sentry aircraft. The CFM56-2 comprises a single stage fan, with a three-stage LP compressor, driven by a four-stage LP turbine, with a nine-stage HP compressor, driven by a single stage HP turbine. The combustor is annular.^[21]

CFM56-3 series

CFM56-3 series engines mounted on a Boeing 737-300 airliner. Note flattening of the nacelle at the bottom of the inlet lip.

The first derivative of the CFM56 series, the CFM56-3 was designed for Boeing 737-300/-400/-500 series aircraft, with static thrust ratings from 18,500 to 23,500 lb_f (82 kN to 105 kN). A "cropped fan" derivative of the -2, the -3 engine has a smaller fan diameter at 60 in (1.5 m) but retains the original basic engine layout. The new fan was primarily derived from GE's CF6-80 turbofan, rather than the CFM56-2, and the booster was redesigned to match the new fan.^[20]

A significant challenge for this series was the size of the wing-mounted engine for ground clearance. This was partially overcome by mounting the accessories (like the gearbox) at the lower corners rather than the bottom, and flattening the nacelle bottom and intake lip, thus giving a distinctive appearance of the Boeing 737 with CFM56 engines.^[30]

Following the Kegworth air disaster of 1989, which was due to metal fatigue causing a fan blade failure in the engine, the entire 737-400 fleet was temporarily grounded. A correction was implemented, and the engine returned to service.

CFM56-5 series

The CFM56-5 series is designed for the Airbus aircraft. It has a very wide thrust rating of between 22,000 to 34,000 lb_f (98 kN to 151 kN). It differs from its Boeing fitted cousins by featuring a FADEC and incorporating further aerodynamic design improvements It has 3 distinct sub-variants, the CFM56-5A, CFM56-5B and CFM56-5C. The Airbus designator for any aircraft equipped with CFM engines is "1"; eg. A320-211 or A340-312.

CFM56-5A series

CFM56-5A series is designed to power the short-to-medium range Airbus A320 family, with thrusts between 22,000 to 26,500 lb_f (98 kN to 118 kN). This is the initial CFM56-5 series. Its design is derived from the CFM56-2 and CFM56-3 families. Aerodynamic improvements, such as an updated fan, low pressure compressor, high pressure compressor, and combustor, make this variant 10-11% more fuel efficient than its predecessors.^{[31] [32]}

CFM56-5B series

CFM56-5Bs power all of easyJet's A319s

An improvement of the CFM56-5A series, it was originally designed to power the A321. Today, it can power every model in the A320 family (A318/A319/A320/A321), with a thrust range is between 22,000 to 33,000 lb_f (98 kN to 147 kN), and it has superseded the CFM56-5A series. Among the changes from the CFM56-5A is the option of a double annular combustor that reduces emissions (particularly NOx), a new fan in a longer fan case, and a new low pressure compressor with a 4th stage (up from 3 in earlier variants). It is the most numerous engine supplied to Airbus.^[23][33]

CFM56-5C series

With thrust rating of between 31,200 to 34,000 lb_f (139 kN to 151 kN), CFM56-5C series is the most powerful of the CFM56 family. It powers Airbus' long-range A340-200 and -300 airliners, and entered service in 1993. The major changes are a larger fan, a fifth low pressure turbine stage, and the same four stage low pressure compressor found in the -5B variant.^[34]

Unlike all other variants of the CFM56, the -5C features a mixed exhaust nozzle^{[nb 1]}, which offers slightly higher efficiency.^[28]

CFM56-7 series

The CFM56-7 powers the Boeing 737 Next Generation series (737-600/-700/-800/-900). The CFM56-7 is rated with takeoff thrust from 18,500 to 27,300 lb_f (82 kN to 121 kN).

It has higher thrust ranges, improved efficiency, and lower maintenance costs than its predecessor, the CFM56-3 series. It incorporates many features from the CFM56-5 series such as FADEC, double annular combustor (as an option), and improved internal design. The basic mechanical arrangement is as in -3 series, but all aspects have been aerodynamically improved from the -3 model. For example, the improved wide-chord fan blades allow the overall number of fan blades to be reduced to 24, from 44. Other improvements come from material advances, such as the use of single crystal turbine blades in the high pressure turbine.^[35]

The CFM56-7-powered 737 is granted 180-minute Extended-Range, Twin-Engine Operations (ETOPS) approval by the U.S. Federal Aviation Administration. It also powers the military versions of the Next-Generation 737, the C-40 Clipper, the P-8 Poseidon, and Boeing 737 AEW&C.^[36]

Applications

• Airbus A320 family
• Airbus A340
• Boeing 707-700 (prototype only)
• Boeing 737 Classic
• Boeing 737 Next Generation
• Boeing 737 AEW&C
• Boeing Business Jet
• C-40 Clipper
• Douglas DC-8 Super 70
• KC-135R Stratotanker
• P-8 Poseidon

Specifications (CFM56-7B15)

Data from ^[35]

General characteristics

• Type: 2 Spool, High Bypass Turbofan
• Length: 98.7 in (2.5 m)
• Diameter: 61 in (1.55 m) (fan)
• Dry weight: 5216 lb (2365.9 kg) (dry)

Components

• Compressor: 1 stage fan, 3 stage low pressure compressor, 9 stage high pressure compressor
• Turbine: 1 stage high pressure turbine, 4 stage low pressure turbine

Performance

• Maximum Thrust: 19,500 lbf (86.7 kN)
• Overall pressure ratio: 32.8:1
• Bypass ratio: 5.5:1
• Thrust-to-weight ratio: 3.7:1

See also

Related development

• CFM International LEAP-X
• General Electric F101
  

Related lists

• List of aircraft engines
  

Notes

1. ^ ^{a b c} Mixed Exhaust Flow is a term used to describe turbofan engines (both low and high bypass) that exhaust both the hot core flow and the cool bypass flow through a single exit nozzle. The core and bypass flow are "mixed".
2. ^ ^{a b} Unmixed Exhaust Flow is a term used to describe turbofan engines (generally only high bypass, but not exclusively) that exhaust their cool bypass air separately from their hot core flow. This is visually distinctive, as you usually see the outer, wider, bypass section end mid-way along the nacelle and the core continue to "stick out". With two separate exhaust points, the flow is "unmixed"
3. ^ The Low Pressure Shaft, in a two shaft (or spool) engine, is the shaft that is turned by the low pressure turbine (LPT). Generally the fan section(s) and the booster section(s) (also known as the "low pressure compressor") are on the low pressure shaft.
4. ^ Chevron is the name given to sawtooth cut outs that are sometime applied to the exhaust nozzles of jet engines in order to reduce the jet noise. An example can be seen here. (Note that the pictured engine is not a CFM56.)

References

External links

Wikimedia Commons has media related to: CFM56

• Official CFM56 website

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