Radical new engine architectures incorporating open rotors or counter-rotating fans will be required if airline expectations for fuel-burn improvements on the next generation of narrowbody airliners are to be met, according to operator feedback gathered by CFM International.
The engine venture is stepping up research work into open rotor concepts as it prepares to decide by 2011 on whether the technology can be readied in time for Airbus and Boeing's next-generation single-aisle airliners. Their respective A320 and 737 family replacements are not now expected to emerge until 2017 or later.
By 2011 CFM's research projects will be at "the point where we can say whether we have a path through open rotor", says Ron Klapproth, Leap56 project director.
The Leap56 project nominally aims to deliver by 2015 an engine based on a similar architecture to the current CFM56, but providing a 10-15dB reduction in noise, 10-15% lower specific fuel consumption, 25% longer on-wing life and a 60% reduction in NOx versus current requirements.
The specific fuel consumption reduction would contribute to an overall aircraft fuel-burn cut of around 20% when coupled with airframe improvements. However Klapproth says CFM has "spoken to over 30 airlines, and they want the engine to bring more than a 20% fuel burn improvement.
"To make a step change of 20-30% - now you're in new architectures," he adds.
Adoption of an open rotor design would require the introduction of complex new systems, such as a blade pitch-change mechanism. CFM rival Pratt & Whitney is developing a geared fan engine incorporating a reduction gearbox.
The General Electric/Snecma partnership has, meanwhile, revisited data acquired during flight tests of the GE36 unducted fan demonstrator carried out in the late 1980s using a McDonnell Douglas MD-80.
"When we run that data through our latest [software] codes we see opportunities for improved efficiency and acoustics," says Klapproth. "It's given us confidence that we can improve over what we had 20 years ago," he adds.
CFM is looking at an open rotor design that would provide a bypass ratio of about 35:1, around seven times higher than the current CFM56.