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CFM is working on the next engine generation
By Patrick Hoeveler
There is no question, the future engine for the A320 and 737 families is certain to be the biggest selling commercial aircraft engine. CFM alone is planning to deliver up to 1,400 engines per year from 2008. There are currently no plans to terminate production of the CFM56. Airbus and Boeing are also finding the decision on a successor to their bestsellers difficult. Francois Planaud, Executive Vice President at CFM, knows this. “Both the A320 and 737 are very good aircraft. To justify the leap to the next generation, compelling arguments as to economy and reliability are needed.” William Clapper, Planaud's counterpart at GE, amplifies on this. “The engine technology constitutes an enormously important part of this, but not the only part.”
Unducted Fan concept
At a time when fuel costs $2 per gallon, fuel accounts for as much as 39 percent of the direct operating costs of a typical narrow-bodied aircraft over a 925 km sector. On top of this, engine maintenance and aircraft maintenance account for eight percent and eleven percent respectively. These three areas will have to pay for the future savings demanded, as there is little potential for savings in the areas of crew (26%) and other costs (16%). Five years ago the situation looked quite different: at a kerosene price of US 66 cents per gallon, fuel accounted for only 17 percent of operating costs. “This made the case for the aircraft succession even more difficult,” explains CFM President Eric Bachelet.
So where are we heading for? To find this out, over the last 18 months CFM has been consulting a large number of customers. Some of the results surprised the managers. “Fuel consumption is a lot more important than before, but not at the expense of maintenance costs and reliability. This poses a challenge to us,” says Planaud. Altogether the operators would like to reduce their operating costs by between 15 and 20 percent. “The corresponding requirement of a 14 to 16 percent reduction in fuel consumption for a successor product proved somewhat higher than we had thought,” says Clapper. But other factors also influence the situation, such as carbon dioxide emissions in the face of global warming. As Bachelet points out, “I am truly impressed by the momentum which these discussions have achieved in the USA in the last six months.”
The present goals which the two CFM partners, GE and Snecma, have set themselves for their Leading Edge Aviation Propulsion (LEAP56) technology programme are ambitious. The fuel consumption of a possible new engine in the thrust bracket of 89 to 133kN should be up to about 15 percent lower than the present CFM56, with the same reliability, and its maintenance costs should be 15 percent lower. Noise emissions have to fall by up to 15 decibels, while nitric oxide emissions have to be 60 percent below the CAEP-6 limit. At the same time the planners want to raise the service life of the time-limited parts from 20,000 flight cycles to 25,000.
How can this leap be achieved? Clapper explains. “The development teams of GE and Snecma have investigated different engine concepts and analysed them against a number of criteria, from component efficiency through to the entire system. One of the concepts examined was the geared turbofan pursued by competitor Pratt & Whitney, but this met with little favour at CFM. “The fuel consumption is relatively good. Noise emission values are low with a large fan diameter, but this brings with it more weight and more resistance, which negates a lot of the fuel consumption benefit. On top of that one has to consider the complexity of the engine. We don't see sufficient benefits to justify the risk posed by this approach.”
A design with a two-stage high-pressure turbine did not convince the engineers either, even though its efficiency could be a lot better than a one-stage variant due to the altered load distribution. According to Clapper, it offers a 1 to 2 percent reduction in fuel consumption. However, this can only be achieved with higher operating temperatures, which in turn would bring higher maintenance costs for the hot parts. The designers therefore decided on the classic CFM layout with a single-stage turbine which has a track record of 354 million flying hours. Compared with the component in the CFM56, this allows a 15 percent higher loading and has eight percent fewer blades.
To achieve the required savings, it also needs significantly higher overall pressure and bypass ratios. On the present CFM56, the bypass ratio is about 5: 1. The possible successor, with 9: 1, is already close to the giant GE90 and has a fan diameter of 1.79m. A larger fan would naturally also be heavier, as Clapper is well aware. “We are therefore backing a fan and fan casing constructed out of composite materials so as to achieve a lower weight, less maintenance and higher resilience.” With its 18 swept wide-chord blades, the LEAP fan would be the first application of a composite fan in this size category. LEAP56 programme manager Ron Klapproth explains why: “A small fan has to endure the same load on birdstrike, for example, as a larger design. That means it has to be a lot more stable.”
This is made possible by the Resin Transfer Moulding (RTM) technology contributed by Snecma, which interweaves dry fibres in three-dimensions. Resin is then squeezed in under high pressure and high temperatures. As well as greater stability, a further benefit is that the blades no longer have to be inserted in different layers by hand. Using a special loom, this process is automated. Admittedly, to achieve the high production rates, a lot of work still lies ahead of the development team. The first blades of the planned size are already being produced and will be tested later this year. Ground runs in a CFM56-5C are planned for 2008.
An eight-stage high-pressure compressor is used in the core, its first two stages consisting of blisks. This means it has one stage less than the present CFM56 compressor. The pressure ratio rises from 11:1 to 15:1 with a weight saving of around 90kg. The new compressor should be tested before the end of the year. New blades constructed out of new materials which promise greater efficiency with fewer blades are to be used in the turbines. Thus the inlet guide segments and the shroud rings in the high-pressure turbine are constructed from Ceramic Matrix Composites (CMC), which are a lot more heat-resistant. However, the challenge lies in producing high unit numbers.
The low pressure turbine, the shaft of which rotates against the high-pressure system, also incorporates some innovations. According to Planaud, an improved 3D design alone brings a fuel consumption reduction of over 4 percent and a smaller number of parts, compared with present turbines. One of the materials to be used is the light titanium aluminide. The precise number of stages has not yet been determined. The first tests will take place next year. A complete core could then be tested in 2009. “Product development could start in 2011,” says Klapproth. An in-service date of the middle of the next decade would then be feasible. However, LEAP56 is not just about one specific engine. The programme simply produces technologies which can be used as and when required. “We are trying to predict future changes in requirements.”
On the other hand even the CFM management is not sure whether these improvements will be sufficient. As Clapper points out, “Given the requirements facing us today, we believe the LEAP56 is a very good offering. But whether it will be good enough for the airlines in the future remains to be seen.” CFM President Bachelet is of the same opinion. “That is why we are looking a lot further.” Under the designation of “Gamechanger”, the engineers are looking at more radical designs such as an engine with two shrouded, contra-rotating fans. This holds out the prospect of less noise combined with lower weight and resistance. Snecma is already testing a reduced-size model of a fan with a diameter of 56 cm, part of the work being carried out under the European VITAL research programme. The fuel consumption of this model is similar to that of LEAP56 and it would only be used at CFM if the importance attached to the noise emissions were to rise significantly.
There are even more unusual ideas around if one postulates that carbon dioxide emissions are the most important criterion. For this case strategists on both sides of the Atlantic are looking at the concept of a non-shrouded propfan such as the one GE had tested back in the late 1980s on a McDonnell Douglas MD-80 under the unducted fan (UDF) concept. However, CFM's study is called “Open Rotor” as UDF is a protected trademark of GE. Such an engine would possess an overall pressure ratio of 30:1 and a bypass ratio of 35:1. This would enable it to reduce fuel consumption by up to 25 percent compared with today's engines. “A conventional propeller would allow a cruise speed of Mach 0.67 to 0.7. The Open Rotor achieves Mach 0.8,” says Clapper.
The main challenge is the noise, as Planaud confirms. “The predicted values lie more or less within the range of today's products. If Stage 5 were to impose a limit of ten dB less than Stage 4, we would not be able to meet this with the present state-of-the-art.” Two contra-rotating rows of propellers are necessary to balance the torque and thus to utilise all the thrust, as Klapproth explains. However, this would increase the noise. “The flows produced by the first propeller are critical here as they interact with the row behind. This is something we have to tackle.” The diameter of 4.2 m also raises new questions over installation on an aircraft. Moreover, the blade adjustment mechanism (radius 180 degrees) needs to be extremely reliable. They are therefore aiming for a possible in-service date at the end of the next decade at the earliest.+
From page 80 of FLUG REVUE 7/2007
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