CFM International, the GE Aviation/Snecma joint venture, is beginning testing on its first demonstration LEAP-X engine core as it aggressively develops a powerplant for a potential next-generation narrowbody entering service by 2018 (ATWOnline, May 15). The LEAP-X does not feature an open rotor design, but GE and Snecma nevertheless are engaged actively in testing the technology first conceived in the 1980s. "All [of the research] that we are doing with the LEAP-X could be used for the open rotor," Snecma VP-Product Strategy and Market Fabienne Lacorre explained during a recent LEAP-X briefing near GE's Cincinnati headquarters.

While CFM says the LEAP-X would provide 16% lower fuel consumption than the CFM56-7 powering the 737NG, it asserts that an open rotor engine could give a 26% fuel efficiency gain. "The 10% lower fuel burn is really a low number," Laccorre said. Indeed, Airbus Director-Product Marketing-A320 Family Stuart Mann said earlier this month that a next-generation replacement will require "game-changing" technology--meaning a 20% efficiency improvement--to "justify changeover costs" for airlines.

GE and NASA last month jointly launched a wind-tunnel test program to evaluate counterrotating fan-blade systems. Testing at NASA's Glenn Research Center in Ohio is expected to continue into mid-year (ATWOnline, Oct. 27, 2008). Snecma simultaneously will conduct research and testing on fan blade designs that could mitigate the open rotor's considerable noise.

"It's true that specialists say we will not reach the LEAP-X [noise] level, but we could have quite a good result," Lacorre commented. She explained that blade size and spacing could be adjusted "to find the best acoustic design." The composite three-dimensional modeling-designed LEAP-X fan blades, on which Snecma has a patent, could provide valuable information regarding open rotor fan blade designs, she said.

CFM President and CEO Eric Bachelet told ATWOnline that the open rotor's viability ultimately will depend on whether Airbus and/or Boeing is willing to work closely with an engine manufacturer to make it feasible. A successful open rotor application will be "tied to an innovative airplane design," he said.

He conceded that any application for the technology is "further down the road," but added: "It's important to keep going [with research]. There is a very strong potential. You can't just wait until you've solved all the problems." He said CFM will have a "much better picture" regarding the open rotor's possibilities in 2011, by which time it will have evaluated fully the testing being conducted by GE/NASA and Snecma. He emphasized that he does not believe noise will be enough of a factor to scuttle the open rotor because ways will be found to make it quieter and its potential to lower fuel burn is so great.

A similar belief has led Rolls-Royce to study the open rotor (which it describes as a possible "game changer") under its Vision 20 program. Open rotor research is additionally a priority under Europe's €1.6 billion Clean Sky joint technology initiative. Pratt & Whitney, however, has dismissed the concept, arguing that it likely would reverse the substantial gains made in lowering aircraft noise and even could lead to regulators imposing "noise restrictions for cruise."

"The good news with an open rotor is you have unlimited bypass ratio," CFM Executive VP Chaker Chahrour said. "But you also have the noise. Based on what we know today, we don't know how to get the noise level down to a turbofan. But a lot of work is being done." He conceded that the noise emanating from an open rotor is "pretty annoying. It's a different type of jet engine noise. It's quite a buzz and it's a lower pitch."

Beyond the noise issue, he added, "The installation challenge is very real. Where do you put this? It obviously can't go underneath the wing." Because of the challenges, airlines are wary, he admitted. "To be honest about it, I have not seen many airlines jumping around looking for an open rotor. Most of the airlines I have talked to have not been enamored with an open rotor."

But he believes CFM must continue exploring the technology because the 16% fuel burn gain promised by the LEAP-X may not be enough to convince Airbus and Boeing to move ahead with a new narrowbody in the next decade. "If the timing works out and there's not going to be [a next-generation] airplane in the 2018 timeframe and the open rotor becomes the answer, we want to make sure we're ready. If the aircraft is in a 2025 timeframe, we want to keep all options open."