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Pratt & Whitney bets on geared turbofan
By Patrick Hoeveler
This time Pratt & Whitney claim to know the answer. A geared turbofan demonstrator based on the PW6000 should be running this December, and next year it is scheduled to be tested in flight. MTU Aero Engines is also involved, and is supplying the high-pressure compressor and a high-speed low-pressure turbine. What is unusual about this geared turbofan is a reduction gearbox which comes between the fan and the low-pressure turbine. What are the advantages of this design? Dr. Jörg Henne, Head of Development and Technology at MTU, explains. “It offers a significant reduction in noise mainly on account of the fact that the fan rotates a lot more slowly. The tip speed is reduced, resulting in less noise.” In this way the engineers hope to halve the subjective noise level.
But the geared turbofan also offers savings potential in the areas of fuel consumption and carbon dioxide emissions, each of which are to be reduced by twelve percent. Emissions of nitrogen oxides could fall by 55 percent. “The fuel consumption depends on the bypass ratio, but there is a limit on how much that can be increased. At some point the fan speed is simply too high. The problem with reducing the fan rotational sped accordingly is that the low-pressure turbine also slows down. Today's turbines actually already rotate much too slowly. Compensating for this calls for many stages and larger diameters, resulting in greater weight.”
Dr Henne estimates the bypass ratio of a production geared turbofan at up to 14:1, depending on how much ground clearance the aircraft manufacturer allows. If a future aircraft were to be designed not much differently from today's A320 and 737, the maximum bypass ratio possible would be 10:1 to 12:1. Again, the weight of the larger nacelles involved has to be included in the calculations.
However, contrary to the arguments of the competition (see FLUG REVUE 7/2007), Dr Henne argues that a geared turbofan is lighter than a conventional turbofan. Bob Saia, Vice President Next Generation Product Family at Pratt & Whitney, believes this too. “The demonstrator will already be five percent lighter than, for example, the CFM56-7. The new engine will then be ten percent lighter and ten percent shorter than the Boeing 737's present powerplant.” The additional, approx. 130kg weight of the gearbox is offset by the low-pressure turbine, which is almost 400kg lighter, as it has fewer stages.
According to Saia, although the demonstrator is based on the PW6000, with an output of between 133 and 147kN it will deliver 40 percent more thrust than the engine on the Airbus A318. “However, the core of a product application will be smaller and more effective.” At the moment several different component tests are running in parallel. As well as the high-pressure compressor at MTU, the low-pressure compressor is also undergoing tests. Tests on the 2.03m diameter fan with its 18 blades were carried out last year. A later series application will use a fan 13cm bigger.
However, special attention is going into the gearbox. In August, Pratt & Whitney in Middletown commissioned a new $12 million test rig. “Here we can analyse the efficiency of the gearbox and its structural properties. With this equipment it is possible to simulate altitudes of up to 13,700m, as well as takeoff and cruise flight and even zero gravity,” explains Saia.
Another experimental gearbox layout has simulated 10,000 starts and stops since April this year in order to gain an insight into the gearbox interaction and the abrasion. Cooling of the oil below 150º Celsius is important here. “The key question is where we position the oil cooler to ensure that there is sufficient cooling air available.” The plan envisages ten gearboxes, to be supplied by Avio of Italy. “We will also deliberately allow the gearbox to fail in order to see how far we can go.”
The gearbox has a reduction ratio of 3:1, which means that the fan (2,500 to 3,000rpm) rotates at one-third of the speed of the turbine (9,000rpm). In the event that the gearbox should fail, the fan must not seize up. To prevent this, a bearing with a predetermined breaking point will serve as “decoupler” and thus separate the fan from the shaft.
Despite its advantages, the geared turbofan concept has never yet been implemented (see box). “But the attitude of potential customers has changed over the last year,” says Saia. After all, Pratt & Whitney holds technology symposiums with airlines at least once a year and has also visited some 35 airlines. “What they want to see is confirmation of the concept. For this reason, we intend to conduct the demonstration programme in flight as well.”
Goodrich is expected to deliver a nacelle which is streamlined as far as possible next February, in time for the first flight scheduled for June 2008. Provided that the reliability and durability of the components are in line with the projections, there will then be little more work to go before the engine is sufficiently mature to be manufactured. “Our plan implies that we have the basic technology available by 2012/13.”
Although the geared turbofan is primarily aimed at the successors of the A320 and 737, according to Saia, there will be an earlier opportunity to employ it on the Mitsubishi Regional Jet (see FLUG REVUE 8/2007) or the Bombardier CSeries. The President of Pratt & Whitney, Steve Finger, is assuming that there will be a whole product range. “We expect to employ the concept on larger engines too. It could be suitable for the next generation of widebodies at the end of the next decade.”
Looking further ahead, an unshrouded propfan, on which Pratt & Whitney is working could also be used. Such an open rotor concept would similarly possess an ingenious gearing system and might be available from 2020.
From FLUG REVUE 10/2007
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