Will spiralling fuel costs and the prospect of carbon credits signal the return of the propfan? Engine makers are dusting off some old concepts

On display in Boeing's Future of Flight centre in Everett near Seattle sits a curious, multi-bladed contraption that often has visitors scratching their heads. Easily mistaken for a Henry Moore sculpture or a Heath Robinson egg-beater, this is one of a few surviving propfans and a potential bellwether of 21st century aerospace.

Although no work of art, this engineering achievement is the Pratt & Whitney/Allison 578-DX unducted fan, which, along with the similar General Electric GE36, successfully demonstrated the fuel miserly ultra-high bypass (UHB) propfan concept in the late 1980s.

© CFM International  
The LEAP56 programme is developing an engine with lower fuel burn and emissions than current engines

Now, with soaring fuel prices and an emphasis on reducing fuel consumption and environmental emissions, the dust is being blown off these and other advanced designs that first appeared at least two decades ahead of their time.

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P&W is leading the pack in terms of full-scale demonstrators, but its competitors believe the market will ultimately demand better improvements than the geared turbo­fan can deliver. P&W remains adamant that the GTF is the best answer. It says it is at the start of its development life and is therefore poised to take pole position in the next-generation engine race.

CFM is building on its massive CFM56 experience base to develop a two-part strategy for the next-generation engine. The first part is the LEAP56 programme launched in 2005 to develop the technical foundations. The second is a subset of LEAP56, based on a series of "game-changing" concepts that will eventually build upon the baseline improvements produced by LEAP56 itself.

Timed for a target service entry of around 2015, LEAP56 is aimed at producing an engine with 10-15% lower specific fuel consumption than current available engines, 15% lower maintenance costs, up to 15dB lower noise levels and 25% longer life-on-wing.

The engine would produce lower nitrous oxide and other emissions than the CAEP/6 standards due for introduction from 2008. It will also have a higher bypass ratio of 9:1 versus 5:1 on current engines, and an ultra-high-pressure ratio core of more than 15:1 against the 11:1 of today's high-pressure spools. Although CFM studied a two-stage HP turbine to achieve this, it believes this performance can be reached more effectively with a 15% higher loaded single HPT stage and an eight-stage HP compressor.

Advanced technologies include a resin transfer-moulded, 3D woven composite fan blade set, a composite fan case, next-generation 3D aerodynamically designed HPC and HPT, ceramic matrix composite turbine nozzles, advanced low-pressure turbine with titanium aluminide blades and a twin-annular pre-swirled (TAPS) II combustor. LEAP56 component and rig tests later this year and in 2008 will precede core and full engine tests in 2009-10. Product design could then be launched by around 2011.

However, it remains to be seen whether the LEAP56 baseline targets are good enough to meet the mid-term needs of the airlines for a next-generation Airbus A320 or Boeing 737 successor, says CFM executive vice-president Bill Clapper.

"By the end of 2011 we can launch the product design. By then we'll have validated the technology and can score it against whatever requirements we'll have for the applications we'll be studying. The tough thing is we have to anticipate those requirements for four years. A great example is the carbon dioxide discussion. When we laid LEAP56 out in 2005 that wasn't even there," he adds.

Looking further ahead

So where next for CFM? "We are starting to look at other architectures to see if we can make other significant steps," says CFM56 director of programmes, Francois Planaud. It is the direction of those steps, however, that lies at the centre of the debate. What will be paramount in the future - noise or fuel and emissions? CFM is looking at a counter-rotating fan concept to cut noise even further. A rig test of this is already under way to examine aero-acoustic performance. However, as Planaud says: "This is an [anti] noise machine, and would burn the same fuel as LEAP56."

To cut fuel burn beyond LEAP56 targets, CFM is examining open rotor designs that would produce roughly the same sort of noise signatures as current aircraft.

"We're looking at concepts which have maybe a 10% better fuel burn than LEAP56, but which come with challenges in terms of noise," says Clapper. "Can we meet the noise targets? We don't know yet."

The issue could hinge on which side of the environmental lobby wins through.

"Will the emissions guys back off for the noise guys? It could be a case of 'Can you listen to the birds around the airport?' trading against the melting polar ice caps," Clapper adds.

"We need to further understand CO2 and the impact of that on fuel, and to factor that into our studies. If CO2 becomes overwhelming, the question remains, would noise trade for it?"

The technical challenges of solving these conundrums mean game changer concepts are much further off than the LEAP56 baseline that precedes them. Any open rotor design would not be ready for service until "very late in the second decade at the earliest", says Clapper. Tests of the open rotor concept and its technology are spaced out over an eight-year period, with flight tests not even provisionally planned until around 2013-14.

Part of the reason is the configuration changes that are required to support open rotors sized for the 25,000lb-thrust (110kN) class. Compared to the GE90-115B, currently the biggest engine in the world with a fan diameter of 3.3m (128in), the LEAP56 open rotor is provisionally outlined with a 4.3m-diameter set of blades.

"So the other piece of technology is how do you install it, and how do you certify something like that?" asks Clapper.

"The installation has a variable pitch blade acting through 180°, so you have to have a reliable pitch change mechanism. We need to look at those sorts of technologies over the next four to five years so it can be as reliable as today's CFM56."

Rolls-Royce faces more choices than CFM over the future direction of its 150-seat engine strategy. As a major partner in International Aero Engines with P&W, it continues to support growth and upgrade initiatives such as the V2500 Select, and remains fully committed to the success of this steadily growing product line.

Power contest

However, it remains unconvinced, publicly at least, about the merits of the GTF. Given the existence of such fundamental philosophical divisions, questions continue to be asked about the longer-term future of R-R's relationship with P&W as the players shuffle position in readiness for the forthcoming power contest in the 150-seat market.

R-R prefers to talk more openly about its continued exploration of new two- and three-shaft concepts for all thrust classes, most of it within wider pan-European R&D initiatives. Its own Vision product strategy has for years been tied to the Advisory Council for Aeronautics Research in Europe goals covering environmental targets over a succession of milestone phases through to 2020.

Relative to a 2000 standard engine, the most ambitious of these are enshrined in Vision 20 and include reducing NOx emissions by 80%, fuel and CO2 by 20% and perceived external noise by 18dB. Technology from the nearer term Vision 5/Vision 10 efforts has already been implemented on the Trent 900 engine for the Airbus A380 and Trent 1000 for the Boeing 787, with ongoing work more likely to feature on the Trent XWB for the Airbus A350 XWB.

While some Vision 10 features are expected to straddle the next-generation narrowbody requirements, it seems the continuing squeeze on fuel and emissions could call for performance levels originally set for development under the longer-term Vision 20 phase.

"The bottom line is we're keeping our options open," says Colin Smith, director of engineering and technology at R-R.

"In the 150-seat market the lifecycle cost is the main driver, but as fuel prices increase and possibly as carbon costs come in, we have to ask if that will be a main parameter of the design. That's the uncertainty."

Traditionally R-R favours tackling this sector with a less complex, two-shaft design aimed at keeping lifecycle costs down. "However, another option could be a more complex, three-shaft design with a higher pressure ratio when fuel burn is more important," says Smith. "There's quite a lot of talk about the environmental factors, and the environmental debate may shape the agenda."

Environmental debate

Like his counterparts at other engine companies, Smith is registering the increasing tempo of political debate over aerospace and the environment. He is concerned that the direction and pace of development could be altered for political expediency.

Open rotor concept engine 
© CFM International  
Open rotor concepts have low fuel burn levels but produce similar noise signatures to current aircraft

"However, we have the fundamental research and demonstrator programmes ready to develop the appropriate engine at the appropriate time," he adds.

One such demonstrator programme getting ready is the full-scale test of R-R's new RB262 two-shaft engine as part of the German-led Efficiency, Environment and Economy (E3E) technology programme. The E3E's aims include demonstrations for a 20% reduction in fuel consumption compared with a typical 5:1 bypass-ratio turbofan, a 10dB reduction in noise levels and an 85% cut in nitrous oxide emissions compared with current International Civil Aviation Organisation standards by 2010.

Smith says that the engine is "BR715-sized" in the 16,000-18,000lb thrust (71-80kN) range, and will run in Germany by the end of 2007. "But we've got a concept that can be scaled up and down simply by adding or deleting a couple of stages," he says.

Outline configurations based on the same architecture could include, for example, an engine with a seven-stage HP compressor and single HP turbine stage aimed at the 6,000-12,000lb thrust range, and a version with a nine-stage compressor and a two-stage HP turbine and cover a 12,000-25,000lb thrust range.

As part of its work under the European Environmentally Friendly Aero Engine collaborative research programme, R-R is also looking at UHBs, geared fans and contra fan designs.

"All our research and development as part of the two- and three-shaft solutions remains completely applicable to any open rotor gas generators," says R-R marketing vice-president Robert Nuttall.

P&W is meanwhile approaching the most crucial phase of its GTF development strategy with the start of initial full-scale ground tests later this year. Apparently unmoved, and possibly even encouraged by the public response of its competitors to reveal new engine details countering the growing publicity over the GTF, P&W continues to be bullish over the development. Todd Kallman, president of commercial engines at P&W, says: "This is not just a one-step change. We're going to continue to improve on this in the future."

P&W believes the GTF's fuel burn, emissions and noise benefits will outweigh the complex airframe integration issues posed by the open rotor concepts. It plans to have the first full production standard engine to test by the end of 2009, and the GTF available for entry into service as early as 2012-13, whereas the earliest open rotor availability is not expected until 2019-20.

Initial design of the production engine, based around a new eight-stage HPC now undergoing tests, is expected to get under way in 2008. P&W engineering senior vice-president Paul Adams says the compressor is sized to be flexible, and to cover the 10,000-30,000lb thrust range.

"We believe the GTF provides almost all of the benefits that a propfan would offer. It's a couple of per cent different in fuel burn, but the other attributes will more than make up for that in terms of significantly lower noise, and with our improved combustor technology we can work the emissions," says Bob Saia, vice-president of commercial engine developments at P&W.

P&W has targeted a 12% fuel consumption reduction for the GTF against current engines, a 40% reduction in maintenance costs, noise levels of around 15dB below Stage IV and emissions as much as 70% below the CAEP/2 limit.

The LP spool and other "GTF unique elements" were due to be mounted on the PW6000-based core in late June. "After the Paris air show, we will start to put the low [pressure] turbine module on the assembly," says Saia. The addition of each element, including the Avio-built fan gear drive unit, LP compressor and the MTU-developed high-speed LP turbine, is being completed once rig tests on the relevant piece have been finalised, he adds.

Because of the gear system, Saia says the fan will rotate at only one third of the speed of the rest of the LP system, thereby enabling the LPC and LPT to run at higher speeds up to 9,000rpm. The slower fan speed also allows for a larger diameter unit, helping to boost efficiency and reduce noise.


Source: Flight International