Noise – and lots of it – helped kill open-rotor research 25 years ago.
The Pratt & Whitney/Allison 578-DX and the General Electric GE36 proved during a series of high-profile demonstrates in the late-1980s that the unshrouded fan is more fuel efficient than a conventional turbofan, but only with an enormous cost in noise emissions.
The collapse of oil prices at the end of that decade did nothing to motivate engine manufacturers to conquer the significant issue of noise.
Complying with then-distant ICAO Chapter 4 standards – finally adopted in 2006 – seemed impossible. The Chapter 3 noise standard adopted in the mid-1980s proved too high for first-generation open-rotor technology. ICAO set the bar even higher in 2013, requiring new commercial aircraft certificated after 2020 to be 7dB quieter than the Chapter 4 standard.
It was no surprise, then, when reactions to GE’s decision to revive open rotor-research in 2007 were, generally, pessimistic.
Seven years later, the collaboration between GE, NASA and the Federal Aviation Administration has produced the first evidence that a noise-compliant open-rotor engine is possible even after ICAO’s Chapter 14 regulations take effect in 2020.
“We’ve come a tremendous distance from where we were at in the late 1980s to the point that we think you could do an open-rotor aircraft that is as quiet or quieter than much of what’s flying today,” says Dale Van Zante, an aerospace engineer at NASA Glenn Research Center.
The Cleveland, Ohio-based laboratory starting doing windtunnel tests in the fall of 2009 on a rig equipped with a new generation of open-rotor fan blades designed by GE.
“It’s safe to say many people were extremely sceptical when GE approached us in 2007-2008 about bringing back an open rotor because everyone had that unducted fan experience kind of burned in their brain,” Van Zante says.
“There was a lot of deep scepticism that we could really make any significant progress on noise,” he says.
But GE had developed a new method to muffle the buzzing roar produced by the open rotor engine’s contra-rotating fan blades. The engine industry’s engineers in the mid-1980s lacked modern design tools, such as computational fluid dynamics and aeroacoustics. Such tools, Van Zante says, allow GE to evaluate a large number of possible blade shapes to optimise for aerodynamics and noise.
An open-rotor engine uses two sets of unshrouded blades to produce more thrust in a smaller package than required by a single set of blades. But it also creates the noise problem.
“The rear blade is always cutting through the wakes of the forward blade,” Van Zante says. “Most of the modern designs, if you look at them closely, you’ll see that the aft rotor is slightly smaller in diameter than the forward rotor. That’s so the tip vortex that goes off the forward rotor actually goes over the top of the aft rotor, so that the aft rotor misses chopping through that vortex.”
The FAA had set a programme goal of achieving a 15-17 effective perceived noise in decibels (EPNdB) reduction in noise emissions relative to Chapter 4 standards and a 25% reduction in fuel burn. Van Zante believes that is possible: “We think the new blade designs could achieve the acoustic target and achieve the fuel burn target at the same time.”
However, while the windtunnel tests of the open-rotor fan blade rig produced a library of aerodynamic and acoustic data, the research project is not quite complete, and NASA declined to fund a second round of windtunnel testing on open-rotor engines in the next budget cycle.
But there are still areas of research that Van Zante and his team at the Glenn Research Center wish to explore, especially integration of the engine with the aircraft. Open-rotor engines, by their nature, produce vortices that bounce off the airframe in ways that ducted turbofans do not. Those interactions between the air bouncing between the engine and the aircraft can cause extra noise or reduce aerodynamic efficiency.
“It would be good to choose a configuration and get some detailed information about that flow field and then go back around and do a third generation of open rotors where aerodynamics are tailored for the particular installation,” Van Zante says. Such a third-generation blade design may not be able to improve on the 17EPNdB reduction claimed in previous tests of the fan blades alone – but NASA’s goal in this testing programme at Glenn is to keep that same acoustic margin with an installed system.
In any case, the windtunnel tests carried out by Van Zante and his colleagues confirm that open-rotor engine designs have come very far from the mid-1980s vintage. “Once we started seeing the test data, and doing some of our systems analysis internally, it’s, like, wow, these things really are a lot different than your grandfather’s open rotor or your father’s open rotor,” he says.
Ultimately, NASA’s goal for aircraft noise reduction is to move well beyond 17EPNdB. The windtunnel testing at Glenn Research Center was funded within NASA’s Environmentally Responsible Aviation project, a five-year effort with lofty goals for noise reduction and aerodynamic efficiency. The ERA project established a 2025 baseline for a 42dB noise reduction compared with Chapter 4 levels, or 25dB less than achieved by the open-rotor engine at Glenn. A follow-on goal is to achieve a 52dB reduction by 2035, which would keep the noise signature of an aircraft within the airport boundary.
The open-rotor engine’s performance on a standard, “tube-and-wing” aircraft still impressed NASA officials. “I wouldn’t say there was any disappointment because the challenge of open rotor has always been that it shows this promise for fuel-burn reduction, which is really significant,” says Russ Thomas, an aerospace engineer at NASA’s Langley Research Center.
Thomas’s team led a separate effort on open rotor research. The team used the database created by the windtunnel data by Van Zante’s team at Glenn, and analysed how the aerodynamic and acoustic effects would change on various aircraft configurations.
Although a conventional aircraft can achieve a respectable noise margin with the new open-rotor engine technology, it will take a more radical approach to achieve NASA’s long-term noise-reduction goals. In a conventional “tube-and-wing” layout, the engines are hung on either side of the aft fuselage or under the wing, which provides no opportunity to use elements of the airframe to act as a noise shield. But NASA and Boeing’s military aircraft division have been pursuing hybrid wing-body vehicles for more than two decades. Thomas’s team combined the research on hybrid wing-body and open-rotor engines.
In a new paper published in January, Thomas concluded that pairing an open-rotor engine with a hybrid wing-body configuration can produce substantial noise reductions up to 38dB below Chapter 4 levels.
“What the paper was trying to convey was that there is a huge opportunity to reduce the noise of an open-rotor aircraft configuration like that,” Thomas says. “You could put the aircraft with the engines in these locations, and if you apply this additional technology – if you were able to invest and develop the technology to that level – you could see as much as a 32dB reduction.”
What the NASA research shows is while open-rotor technology would require the industry to switch to a new transport aircraft fuselage configuration, it does provide a growth path from an acoustic performance baseline that is much better than that of open rotors of the mid-1980s. There are, however, a few critical questions that have not been answered by the NASA projects.
Perhaps the most significant unknown facing the technology is the certification issue. A turboprop engine is certificated with the hubs and blades as “prime reliant” components, meaning they can never fail. The blades in a turbofan engine spin faster than a turboprop’s propellers, so making them prime reliant would make the engine prohibitively heavy. As a result, the blades in a turbofan engine can fail in very rare circumstances, but they can never punch through the fan case and pose a risk to other aircraft systems and the passenger cabin. An open rotor combines elements of both types of engine and no certification standard currently exists to guarantee the safety of the aircraft and its passengers in the event of a blade failure.
The certification issue is currently being considered within EASA’s Clean Sky initiative, but a final decision is not expected for several years.
There are ways that aircraft manufacturers can mitigate the effects of a blade-out certification issue. Van Zante notes that installing the engine in a position where the fan blades are located behind the aft pressure bulkhead inside the fuselage is a possibility. The likely carbonfibre composition of the fan blades is also easier to protect against, as the material usually shatters upon impact, he says.
But there are more mundane challenges for the open-rotor engine’s designers to conquer.
For instance, the exposed fan blades of an open-rotor engine include a pitch-change mechanism, but the blades tend to produce significantly more noise with the airflow entering the engine at a high angle of attack, Van Zante says.
“You can imagine placement on the airframe is going to make a large difference to the angle of attack into the engine,” he says. “GE has been thinking about ways to try to further desensitise the blade designs to angle-of-attack effects. These are the kind of research topics we would go after going forward with open rotor.”
Despite the lingering issues, European engine manufacturers seem most eager to pursue the technology. Rolls-Royce still lists an open-rotor engine as parts of its Vision 20 project. Snecma is working under the Clean Sky project to develop a full-scale open-rotor fan derived from the M88 core for testing on an Airbus A340 by 2019.
The US manufacturers, however, do not seem quite so eager.
Pratt & Whitney claims a practical open-rotor engine will be inferior to a next-generation geared turbofan with a higher gear ratio than the 3:1 scaling available today on the PurePower PW1000G series.
“We’re pretty confident that with a geared architecture… we can provide the same fuel efficiency as an open rotor at the aircraft level without the debits associated with some of the operational elements, like noise,” says Bob Saia, P&W’s vice-president of next-generation programmes.
The fuel efficiency of an open rotor could also be reduced by the additional weight of an installed system, he says.
“When you have an open rotor and you’ve got the engine positioned where it’s 10ft away from the fuselage or 15 away from the fuselage, you still have to have a pretty strong pylon or attachment to get to the airplane,” Saia says.
Indeed, the NASA research studies assumed a 4.27m (168in) fan on each open-rotor engine factored in extra weight for the pylon and 953kg (2,100lb) for the hydraulics to move the pitch change mechanism for the blades. However, the open rotor still met its fuel-efficiency target and exceeded the performance of an ultra-high bypass, ducted turbofan.
The debate on open rotor is unlikely to end soon. Meanwhile, one of its biggest champions has gone eerily silent. GE revitalised open-rotor technology in 2007, commissioned the NASA research and celebrated its results. Asked to comment for this story, however, the company demurred.
“The programme is going through a period of transition”, GE says, “that precludes discussion of the engine at this time.”