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Gears Galore! OPEN ACCESS

[+] Author Notes
Lee S. Langston

Professor Emeritus of Mechanical Engineering, University of Connecticut

Langston is a former editor of the ASME Journal of Engineering for Gas Turbines and Power and has served on the IGTI Board of Directors as both Chair and Treasurer.

Mechanical Engineering 135(04), 51-54 (Apr 01, 2013) (2 pages) Paper No: ME-13-APR4; doi: 10.1115/1.2013-APR-4

Abstract

This paper presents a review of gas turbines and Honeywell, a company based in Phoenix, history. The article through the review and historical analysis intends to provide perspective on the status of geared fan engines. The addition of a fan to a jet engine, first proposed by Frank Whittle, one of the inventors of the jet engine, increases thrust and reduces fuel consumption. Pratt & Whitney and Rolls Royce were the first to develop a dual spool engine for more efficient operation over a range of flight conditions. Work started on the geared fan TFE731 at the Garrett AiResearch Phoenix Division in 1968. The TFE731 gearbox resulted in a gear reduction of 1.8:1, to power the fan for a 2.5 bypass ratio, which was very high for the 1960s. Honeywell also has another geared turbofan engine, the ALF502. It was developed by AVCO Lycoming in Stratford, Connecticut, and has a 6000–7000 lbt thrust range. Honeywell’s successful 45-year record of producing geared fan small gas turbines gives promise of a bright future for geared fans on large commercial jet engines, providing lower fuel consumption and less noise.

Recently, I wrote an article[1] on Pratt & Whitney’s new geared turbofan (GTF) engine, now scheduled for airline operation as early as 2014-2016.

Currently, P&W’s first generation GTF engines use a gear reduction of 3:1, with a resulting bypass ratio of 12:1. This yields a fuel consumption as much as 16% lower than with current conventional fan engines. Noise reduction is substantial, with the characteristic turbofan whine being replaced by a lower frequency “whoosh#x201D;. The thrust range on P&W’s first generation GTF engines is 18,000-30,000 pounds of thrust (lbt), a range that powers twin-engine single-aisle, narrow body 70-200 passenger aircraft – and the most lucrative market for engine companies.

After reading my GTF article, Fred Borns of Phoenix-based Honeywell, emailed to remind me that his company had been developing and producing geared turbofan engines for several decades. Their TFE731 geared fan engine program actually started in 1968 and grew in 1995 with the addition of the AVCO Lycoming ALF502/507 GTF series. Honeywell has produced well over 15,000 geared fan engines for business, airline and military jets, all in the 3,500-7,000 lbt range. Let us review some gas turbine and Honeywell history to gain more prospective on the status of geared fan engines.

The addition of a fan to a jet engine, first proposed by Frank Whittle, one of the inventors of the jet engine, increases thrust and reduces fuel consumption. In a 1940 patent[2] he envisioned the fan to be a set of impeller blades, turbine mounted and aft on the engine. In the late 1950s, Rolls-Royce with the Conway engine and Pratt & Whitney Aircraft with the JT3D developed their first turbofan engines, by adding a fan at the front end of an existing turbojet engine. At the same time, General Electric, initially following Whittle’s 1940 patent, had an aft fan mounted on the turbine of their CF700. GE switched to front mounted fans subsequent engines.

Now common in the jet engine industry, Pratt & Whitney and Rolls-Royce were the first to develop a dual spool engine for more efficient operation over a range of flight conditions. Concentric dual rotational shafts provide the means to have a high pressure (HP) spool and a low pressure spool (LP). (Some Rolls-Royce models have a third intermediate pressure spool.) On a turbofan engine, the LP spool shaft transmits power from the LP turbine to the LP compressor and the front mounted fan, at a compromise rpm, determined by the conflicting needs of the fan and the LP compressor/turbine.The fan operates most efficiently at lower rpms (lower noise levels and lower fan tip speeds at its greater diameter satisfy stress and supersonic flow limitations) while the LP compressor/turbine is more efficient at higher rotational speeds. Thus, by putting a concentric reduction gear box between the fan and the rest of the LP spool, it makes it possible for both components to run more efficiently, closer to their optimal operating speeds.

Work started on the geared fan TFE731 at the Garrett AiResearch Phoenix Division in 1968[3], some 45 years ago. Unlike many commercial engines that emerged from antecedent military designs, the TFE731 was derived from Garrett’s auxilary power unit (APU) programs. The LP compressor design was based on their APU for the Boeing 747 and two spool shaft dynamics from their APU for the Douglas DC-10. Given the high rotational speed of APU LP spool (about 20,000 rpm), to avoid fan tip excessive speeds, Garrett engineers developed a concentric epicyclic gear box to drive the fan. This was based on the company’s experience with turboprop gear trains. (A hand cranked pencil sharpener uses epicyclic gearing.)

The TFE731 gear box resulted in a gear reduction of 1.8:1, to power the fan for a 2.5 bypass ratio, which was very high for the 1960s. A modern TFE731 fan gear box, shown in Fig. 1, is about 8.5 inches in diameter and transmits on the order of 3,000 hp to the 29 inch diameter fan. By way of comparison, a Pratt & Whitney GTF hub-mounted epicyclic gear box is shown in

Photo: Honeywell

Grahic Jump LocationPhoto: Honeywell

Fig. 2. It is about 18 inches in diameter and transmits as much as 30,000 hp.

Photo: Pratt & Whitney

Grahic Jump LocationPhoto: Pratt & Whitney

The TFE731 was certified in 1972 and has since become one on the most successful small gas turbine aircraft engines, with over 11,000 produced. It is one of the most widely used engines in its class, powering such aircraft as Jetstars, Learjets, Dassault Falcons, Cessna Citations, Gulfstreams and many military trainers.

Honeywell also has another geared turbofan engine, the ALF502. It was developed by AVCO Lycoming in Stratford, Connecticut (followed by the ALF507), and has a 6,000-7,000 lbt thrust range. It entered commercial service in 1980 and, along with other aircraft, powers the British Aerospace BAE 146-100 four engine regional transport aircraft.

Honeywell’s successful 45-year record of producing geared fan small gas turbines gives promise of a bright future for geared fans on large commercial jet engines, providing lower fuel consumption and less noise.

References

Langston, Lee S., 2013, “Not So Simple Machines”, Mechanical Engineering Magazine, January, pp. 46-51.
Whittle, Frank, 1940, “Improvements relating to Apparatus and Systems for the Production of a Flow of Energised Gas, for example for the Propulsion of Aircraft”, Patent Specification 583,111, The Patent Office, London.
Leyes, R.A. II and Fleming, W.A., 1999, The History of North American Small Gas Turbine Aircraft Engines, AIAA/Smithsonial Institution, pp. 673-688. [CrossRef]
Copyright © 2013 by ASME
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References

Langston, Lee S., 2013, “Not So Simple Machines”, Mechanical Engineering Magazine, January, pp. 46-51.
Whittle, Frank, 1940, “Improvements relating to Apparatus and Systems for the Production of a Flow of Energised Gas, for example for the Propulsion of Aircraft”, Patent Specification 583,111, The Patent Office, London.
Leyes, R.A. II and Fleming, W.A., 1999, The History of North American Small Gas Turbine Aircraft Engines, AIAA/Smithsonial Institution, pp. 673-688. [CrossRef]

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