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Pratt & Whitney is backing the geared fan
By Patrick Hoeveler
If Pratt & Whitney wants to stay in the civil propulsion business, it is imperative that it is involved in production of the engine for the next narrowbody aircraft. After all, with the exception of the Airbus A380, the company has been left out in the cold, excluded from all the new commercial aircraft programmes. According to a P&W internal paper, it is therefore viewed as vital that the company wins a role on the 737 and A320 successor models, preferably through the IAE consortium. In its proposal for the Next-Generation Single-Aisle (NGSA) aircraft, Pratt & Whitney is backing the concept of a geared fan in which the fan is connected to the rest of the low-pressure system by the use of a reduction gear. Through the interposition of gearing each component can work at its own optimal speed, resulting in greater efficiency. The thrust bracket of interest here is the range 90kN to 160kN, the main focus being on aircraft with around 150 seats. However, the company strategists also have their eye on scaled-down versions suitable for 100-seat regional jets.
The engineers are hoping to reduce fuel consumption with the Geared Turbofan (GTF) by up to twelve percent compared with the V2500 or CFM56. Other targets include a 40 percent reduction in maintenance costs, noise emissions 30 dB below the Stage 3 noise limit and exhaust gas emissions as much as 70 percent below the CAEP2 limit. Robert Saia, Vice President Commercial Engine Programs at Pratt & Whitney, explained the advantages of the geared fan to FLUG REVUE in East Hartford as follows. “Decoupling by means of gearing enables the fan to rotate at only one third of the speed of the rest of the low-pressure system. This means we can raise the speed of the low-pressure compressor and turbine. It allows us to have just three stages in the low-pressure compressor instead of six for a given thrust.”
The concept also makes it possible to increase the diameter of the fan, which in turn allows a higher bypass ratio and hence lower fuel consumption. “We are trying to make the fan as large as we can,” Saia explains. “The slower we can rotate it, the more efficient and the quieter it gets.” Hence the gearing is central to the new design. It consists essentially of an inner ring and an outer ring, between which a set of planetary gears is interposed. A highly efficient design in this area is essential. “Otherwise, the oil temperatures will be driven up to the region of 180º Celsius, which in turn will make cooling necessary.” The gearing also produces higher rotational speeds, which in turn expose the seals and bearings to greater loading. Even so, Saia is still assuming a maintenance interval of up to 25,000 flight cycles.
But despite all the benefits, the geared fan must not cost more than today's V2500. The engineer is hoping that the additional maintenance costs caused by the gear will be offset by the smaller number of stages in the low-pressure area. Along with greater use of composite materials, this should compensate for the extra weight. According to Saia, the total gear assembly will weigh around 68kg. “Besides, each extra centimetre of fan diameter costs on average almost 18kg per engine.”
To demonstrate this architecture, Pratt & Whitney is working with MTU Aero Engines, Avio and Volvo Aero on a demonstrator based on the PW6000, which should be running by the end of the year. The existing core will be supplemented by a new fan with hollow titanium blades and a three-stage low-pressure compressor. Avio is supplying the gear assembly, while MTU will contribute the high-speed low-pressure turbine. The three-stage turbine is an aerodynamically modified derivative of the design already tested in the CLEAN programme. With up to 9000 rpm, it offers twice the speed of normal turbines of this size.
The demonstrator produces 133kN of thrust, with a bypass ratio of 11:1. The fan diameter is 203.2cm. “The production version will probably have a diameter of only 190.5 centimetres,” says Saia. To put this in perspective, the fan on the V2500 has a diameter of 161cm. The next step will be to install the eight-stage high-pressure compressor, which Pratt & Whitney is developing jointly with MTU. It is currently undergoing assembly in Munich, where testing is expected to commence shortly after publication of this issue.
Next year airborne testing of the demonstrator will be carried out on a Boeing 747SP, albeit still with the six-stage MTU compressor from the PW6000. Here the engine nacelle and fastenings play an important role, as the engine will be shorter and at the same time its centre of gravity will move forwards due to the lighter low-pressure turbine with its fewer stages. “That is bad for the airframe and raises the question of how to install the engine,” Saia admits. Moreover, the cowling needs to have minimal drag to avoid the greater resistance of the considerably larger nacelle cancelling out the savings. The greater volume of exiting cold air will be directed by an adjustable nozzle.
The designers still have time to puzzle out detailed solutions. Saia is working on the assumption that the relevant aircraft will enter into service in the period 2013 to 2015. “We expect the aircraft manufacturer to issue crisp engine requirements at the end of 2008.” According to P&W's internal plans, the provisional development phase will start at the beginning of next year. The first full engine test would then take place at the end of 2009, followed by certification in July 2011. The timescale will move in line with what Airbus and Boeing are planning. In any event the market is huge: Pratt & Whitney believes there is a requirement for over 30,000 engines in the period 2013 to 2032.
From FLUG REVUE 2/2007
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