On the surface it may seem like a “me-too” effort a quarter-century after the U.S. demonstrated in flight that open-rotor engines can deliver dramatic fuel-burn savings over conventional turbofans. But Europe's research is tightly focused on the two barriers that could prevent open rotors from being considered for the next generation of all-new narrowbody airliners: noise and safety.

Under Europe's Clean Sky program, research conducted by teams led by Airbus and Snecma has concluded that a 2030-timeframe short/medium-range airliner with counter-rotating open-rotor engines is technically feasible and will meet new International Civil Aviation Organization Chapter 14 noise limits, the companies declared to the Greener Aviation 2014 conference here this month.

Design studies have shown open-rotor engines are the best candidates for low fuel burn and emissions, says Airbus powerplant safety engineer Charlie Rulleau. The next hurdle is to improve the aircraft's economics, in particular reducing the weight penalty of meeting blade-off safety requirements, which could negate the fuel-burn benefit of open rotors.

“We have confirmed the feasibility of the design. Now we need to improve the economic viability to be able to propose a product,” Rulleau says.

Under Clean Sky, Snecma is developing a geared open-rotor demonstrator engine that is scheduled for ground testing at the end of 2015. Airbus is working on engine-airframe integration, alternative configurations and certification issues. The goal is to establish economic viability by 2017, then proceed into an open-rotor flight demonstration, planned by 2023 under the follow-on Clean Sky 2 program.

Flight tests of Snecma's demonstrator engine mounted on the aft fuselage of an Airbus A340-600 were planned under Clean Sky, which wraps up in 2017. But delays have deferred the flight demonstration to Clean Sky 2, which will begin this year, and the objectives have been modified to focus more on engine-airframe integration and certification issues. The demonstrator platform is now planned to be an A340-300, but that could change as a result of Airbus's continuing studies.

Open rotors burn less fuel than turbofans because they can have large diameters for ultra-high bypass ratios without the drag and weight penalties of a large nacelle. General Electric demonstrated the GE36 Unducted Fan in flight in late 1980s, but the concept was shelved when the fuel crisis ended. Work was revived earlier this decade and, with funding from NASA, GE wind-tunnel-tested refined blade designs for increased performance and reduced noise. But NASA did not continue the project, leaving Snecma—GE's partner in narrowbody-engine joint venture CFM International—and Europe's Clean Sky program to take the lead in advancing the maturity of the open rotor as a possible successor to CFM's Leap engine.

Key to economic viability will be the weight penalty incurred to protect the aircraft from damage caused by a rotor burst or blade release. A turbofan can contain a released blade, but an open rotor will require shielding of the airframe and systems. In Airbus's baseline concept, which has pusher open-rotor engines mounted on the aft fuselage and a conventional T tail, shielding of the rear fuselage and tail adds about 0.5 metric tons (1,100 lb.) to the aircraft's weight.

Too high a weight penalty would negate the open rotor's fuel-burn advance over turbofans. “The design is at low maturity; it is not a good solution,” says Rulleau. “The next step is to improve the engine and shielding design to reduce weight.” Initial rulemaking for certification of open rotors requires a fail-safe hub to prevent a rotor burst and blade-release mitigation “at the aircraft level” through shielding, he says, adding that the fail-safe hub design is still at low maturity, but tests have shown the blades can withstand bird impacts.

Airbus studied but rejected an aircraft configuration with “puller” open rotors mounted in the conventional underwing location because cabin noise “was above the target by more margin than available technology can mitigate,” Rulleau says. The aft-mounted location “is not far from the target,” he says. Airbus is studying three possible configurations, including rear-fuselage and tail designs that would acoustically shield the open rotors to reduce the noise levels reaching the ground.

“Buyers of next-generation short/medium-range airliners will expect big steps in aircraft economics, at least a 40-percent fuel-burn-per-passenger improvement,” says Vincent Garnier, Snecma vice president of marketing strategy for civil engines. “That is a very high mountain to climb.” Options under evaluation are turbofans with bypass ratios beyond 15 and the counter-rotating open-rotor (CROR) engine. “They offer different mixes of fuel-burn and noise benefits, and the engine and airframe architecture will be very closely coupled,” Garnier told the conference, organized by the Council of European Aerospace Societies and France's 3AF.

To power a 2030-timeframe narrowbody, Snecma is studying both a counter-rotating turbofan with 16% lower specific fuel consumption and 20 dB lower noise than the CFM Leap-1 and the CROR, which can reduce fuel burn by 26% but noise by only 10 dB.

“Why is Snecma pushing open rotor? Because it has the strongest propulsive efficiency potential and poses the greatest challenges and uncertainties,” Garnier says. “Clean Sky is a great learning vehicle, and what we learn will have wide application. It is helping build a team of partners to build a consensus and a community.”

Snecma has confirmed the geared open-rotor's target efficiency and that it is “Chapter 14-compliant with margin,” Garnier says. “It is still a long road to the next-generation short-/medium-range airliner, and we will need absolute confidence before we change architectures. We will need more demonstration steps, and possibly another build of the CROR.”

Snecma validated its efficiency and noise projections with tests of a scaled CROR model in French research agency Onera's SM1A wind tunnel last year. Tests of the HERA propulsor rig at speeds up to Mach 0.30 measured noise at the three certification points: takeoff, sideline and approach. “We assessed noise level versus Chapter 14, blade performance and validated successive blade design optimizations,” says acoustic engineer Rasika Fernando.

Three generations of blade sets were tested, HERA1, 3 and 5. Snecma achieved a 5.2 EPNdB noise reduction and a 1.1% efficiency increase between the HERA1 and 3 blade sets, both for the original direct-drive CROR design. Another 2.7-EPNdB noise reduction and 0.5% efficiency improvement were reached with the HERA5 blade set, the first for the geared CROR.

“We achieved good optimization of performance and certification noise. Our blade designs are now compliant with Chapter 14,” Fernando says. The CROR ground-demonstration engine now under construction will use the HERA5 blade set. Meanwhile, “more blade design optimization is underway to further improve efficiency and reduce noise,” he says.

Airbus's design studies under Clean Sky have included three large wind-tunnel test campaigns, involving isolated and semi-installed open-rotor propulsor tests and a 1/7th-scale full aircraft model. The tests, in the large low-speed facility (LLF) at DNW in the Netherlands, involved blade designs from Snecma, Rolls-Royce and Airbus itself. There were “only slight differences” between the HERA results and Airbus's tests in the DNW-LLF, “a cumulative 1-dB difference when projected to flight,” says Fernando.

Clean Sky's goal is to enable open rotors to be a viable option to power the next generation of single-aisle airliners, but in the end operators, regulators and the public will decide whether they are prepared to trade lower noise for higher fuel efficiency and lower emissions, the conference was told. “The next generation will have to bring something massive [in terms of improvements], or the trend of aviation will be altered,” Garnier said.