The geared turbofan is a type of

turbofan The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...

aircraft engine, with a

gearbox Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term ''transmission'' properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), diff ...

between the fan and the low pressure shaft to spin each at optimum angular velocities.

Technology

In a conventional turbofan, a single shaft (the "low-pressure" or LP shaft) connects the fan, the low-pressure compressor and the low-pressure turbine (a second concentric shaft connects the high-pressure compressor and high-pressure turbine). In this configuration, the maximum tip speed for the larger radius fan limits the rotational speed for the LP shaft and thus the LP compressor and turbine. At high bypass ratios (and thus high radius ratios) the tip speeds of the LP turbine and LP compressor must be relatively low, which means extra compressor and turbine stages are required to keep the average stage loadings and, therefore, overall component efficiencies to an acceptable level. In a geared turbofan, a planetary reduction gearbox between the fan and the LP shaft allows the latter to run at a higher rotational speed thus enabling fewer stages to be used in both the LP turbine and the LP compressor, increasing efficiency and reducing weight. However, some energy will be lost as heat in the gear mechanism and weight saved on turbine and compressor stages is partly offset by that of the gearbox. There are manufacturing cost and reliability implications as well. The lower fan speed allows higher bypass ratios, leading to reduced fuel consumption and much reduced noise. The

BAe 146 The British Aerospace 146 (also BAe 146) is a short-haul and regional airliner that was manufactured in the United Kingdom by British Aerospace, later part of BAE Systems. Production ran from 1983 until 2001. Manufacture by Avro Internation ...

is fitted with geared turbofans and is still one of the quietest commercial aircraft. A large part of the noise reduction is due to reduced fan tip speeds. In conventional turbofans the fan tips exceed the

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as we ...

causing a characteristic drone, requiring sound deadening. Geared turbofans operate the fan at sufficiently low rotational speed to avoid supersonic tip speeds.

History

After considering a geared design, General Electric decided against it for its CFM LEAP due to weight and reliability concerns, postponing its use for a future application, after Pratt & Whitney began development of the geared PW1000G.

Engines

Garrett TFE731 The Garrett TFE731 (now Honeywell TFE731) is a family of geared turbofan engines commonly used on business jet aircraft. Garrett AiResearch originally designed and built the engine, which due to mergers was later produced by AlliedSignal and n ...

Lycoming ALF 502 The Lycoming ALF 502/LF 507 (now:Honeywell ALF 502/LF 507) is a geared turbofan engine produced by Lycoming Engines, AlliedSignal, and then Honeywell Aerospace. Development In mid-1970, Avco Lycoming was advertising two Lycoming T55-derive ...

/ LF 507 *

Pratt & Whitney PW1000G The Pratt & Whitney PW1000G, also called the Geared Turbofan (GTF), is a high-bypass geared turbofan engine family produced by Pratt & Whitney. After many demonstrators, the program was launched with the Mitsubishi MRJ's PW1200G in March 2008 ...

* Rolls-Royce UltraFan * Turbomeca Astafan * Turbomeca Aspin

Tentatives

* Rolls-Royce/SNECMA M45SD, a derivative of Rolls-Royce/SNECMA M45H turbofan, designed to demonstrate ultra-quiet engine technologies, needed for STOL aircraft operating from city centre airports * IAE SuperFan, an IAE V2500 derivative proposed for the Airbus A340 between 1987 and 1992 * Aviadvigatel PD-18R, a

derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...

of the gearless PD-14, based on its high-temperature core, to be completed by 2020

References

Notes

Bibliography

* * Francillon, René J. ''McDonnell Douglas Aircraft since 1920''. London: Putnam, 1979. . * {{refend Turbofan engines