Designing engines that require more frequent overhauls is one option being looked at by Rolls-Royce as it accelerates efforts to deliver efficiency savings of up to 50% for the next generation of Airbus and Boeing 150-seat airliner families.
The UK manufacturer is taking a "profound and multi-dimensional" look at how to mitigate the impact of record oil prices under a project it calls "Option 15-50". The study encompasses more than just engine technology, says R-R civil aerospace president Mark King, as the design of the airframe and the way it is operated, and the maintenance regime for its powerplant, also have a major impact on fuel consumption.
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"I think you'd take a fundamental look at how you manage the engine on-wing, and look at the trade between maintenance cost and fuel burn," says King. Aiming to achieve 30-40,000 flight hours on wing is "not necessarily the most fuel efficient way to run you fleet", he adds.
Fuel savings could be achieved by using lighter components and operating the engine at higher combustion temperatures, in return for a reduction in time between overhauls.
"One per cent of fuel now is probably the same as about 10-15% of maintenance costs, depending on whether you're short range or long range," says King.
Getting the company's strategy for the 150-seat market right is so critical that R-R has assembled a 25-strong team to study the emerging requirements of the airlines and airframers, and the manufacturer has visited more than 50 customers to "understand how they use their aircraft, and how they will in the future", says King.
"We think there could be some quite fundamental changes going on in the market," he says. Although 150-seaters today serve as workhorses on short- and medium-haul routes, "we're not quite sure what it is those people are going to fly around in in 10 or 15 years time".
In terms of pure technology R-R is looking at two- and three-shaft engine architectures, as well as a potential open-rotor solution. But King says there are many more factors at play, such as the relative inefficiency of ultra-long-haul flying (given the large amount of fuel that must be carried over long distances), the relatively high operating speeds of current airliners and the fact that engines are being maintained in the same way that they were when oil cost $20 per barrel.
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"We think there's a range of options we can pursue here, starting with something that is of the order of 15% efficiency improvement on the next aeroplane, through to something that could be as big as 50%," says King.
"The trade between maintenance cost, time-on-wing and fuel is very different to what it was. You really have to go right back and look at first principles about how should you be operating an engine in service. What should you be doing in terms of the balance between, for example, time-on-wing and fuel?"
Having a long-range aircraft make a stop halfway between London and Singapore, for example, could yield a fuel saving of around 10%, and flying more slowly could save another 5%. If the aircraft was designed from the outset to fly slower and only halfway to Singapore, a further 10% saving could result.
"There is 20-25% worth of decision making that can be done within the aircraft design," says King.
Option 15 - denoting a 15% efficiency gain - would essentially comprise an advanced but conventional turbofan incorporating the latest technology being incorporated into the three-shaft Trent XWB and two-shaft RB282. This could be available by the middle of the next decade.
"If you were prepared to wait a bit longer, wrap in some of these service approaches, maybe move the aeroplane design to something a bit bigger, then we come up with something that is more than 15%, a little bit later in that decade, still based on an advanced turbofan," says King.
Achieving "Option 50" would require making the leap to open rotor, optimising airframe design, accepting reduced time on wing and possibly the adoption of alternative fuels. Earlier this month R-R ran a one-sixth scale model of an open-rotor engine in the Netherlands to investigate noise characteristics, and the company plans to eventually run a full demonstrator based on a BR700-series core.
"We don't think that 50% efficiency improvement for the next generation is ridiculous, given the number of options we have," says King. "We are essentially saying to the aircraft manufacturers that we are ready to go, as long as we understand the timeframe and exactly what this aircraft looks like."
R-R's partner in the International Aero Engines consortium, Pratt & Whitney, believes its geared-fan PW1000G-series engines can deliver significantly reduced fuel consumption without compromising durability.
"Cash operating cost is something that we always work to, and that balances fuel cost versus maintenance cost," says P&W senior vice-president engineering Paul Adams.
"We are seeing a trend towards much higher performance engines and better fuel consumption, which typically does have to run hotter," says Adams. "We have built this into our baseline engine and we honestly don't think that we need to substantially drive maintenance cost up to be able to get lower fuel burn."
Robert Saia, P&W commercial engines vice-president next-generation product family, adds that the PW1000G has around 1,500 fewer aerofoils as a result of the geared architecture, the majority of which have been removed from the relatively maintenance-intensive low-pressure turbine.
"We get the maintenance run without making trades," says Saia. "We don't have to trade fuel consumption versus maintenance cost because we have fewer parts."
Source: Flight International
