Ground testing is imminent on the open-rotor engine portion of the European Union’s Clean Sky initiative, with Snecma and partners now putting the finishing touches to assembling the demonstrator. The noise issues that have plagued open-rotor engines in the past appear to have been largely overcome, and the key questions once ground and flight tests are complete will be if, how and when the technology finds its way onto the future aircraft fleet.

Earlier this year, Clean Sky Joint Undertaking director Eric Dautriat described the design and development by Safran's Snecma propulsion unit of an open-rotor engine as being by far the programme’s “most exciting challenge”.

The technology, which features two unshrouded, counter-rotating blades, promises to cut fuel consumption by up to 30% compared with today’s aircraft engines – potentially offering a lifeline to an industry that has vowed to halve its carbon dioxide emissions by 2050, from their 2005 levels. GKN Aerospace, which is providing the majority of the engine’s rotating module, delivered the front and aft rotating frames for the demonstrator to Snecma in February, in preparation for assembly and testing.

“We are at the engine assembly stage now and our engineers are supporting Snecma with this, both on-site and from our operation in Sweden,” says Henrik Runnemalm, director of advanced engineering at GKN’s engine systems division. “Next we will commence the ground-based demonstrator phase with the test programme continuing for the remainder of this year at Snecma’s test facility in France.

“We will then study the results that emerge on the performance of the different engine modules.”

Most of the manufacturing work on the demonstrator has now been carried out and Snecma is looking to late summer to begin ground tests, as the manufacturer’s open rotor chief engineer, Olivier Jung, explains: “Regarding the whole test, manufacturing is completed for more than 80% of the components. Instrumentation and assembly are ongoing in the Vernon facility in Normandy. The open air test bench has been erected and it will be fully operational by the end of summer, in order to start engine testing.”

The timeframe for flight tests, however, is less clear. Flight-testing on an Airbus A340 was originally scheduled to take place by 2019 under Clean Sky 1, but development delays have pushed this part of the project into the second phase of the Clean Sky initiative and no new flight-test date has been provided.

Earlier this year, Dautriat told Flightglobal flight-testing would take place “at least four years” after ground-testing. However, Jung does not rule out the possibility of a flight test occurring in 2019.

“With our partner Airbus, we are assessing the value of such a test in that timeframe,” he says. “What is mostly important right now, after demonstration of acoustic compliance, is to validate mechanical integration of this powerplant in order to open the range of its potential selection for regional or SMR [short/medium-range] applications in the next decade.”

One of the key hurdles that open-rotor research has had to clear is the fact engine noise cannot be contained in the same way as it is in a regular turbofan encased by a nacelle. In addition to meeting its ambitious emissions reduction targets, the aviation industry must also work within the confines of strict regulations on noise.

ICAO’s latest noise standard, Chapter 14, states that from 2017, new, large commercial aircraft types must be at least 7 EPNdB (Effective Perceived Noise in Decibels) quieter than the stipulations set out in the current Chapter 4 standard. Smaller aircraft types will have to comply by 2020.

Tim Johnson, director of the UK-based Aviation Environment Federation (AEF), is concerned that while modern-day open rotors might be able to meet noise requirements in decibels, the differences in this type of engine’s tonal characteristics could prove intolerable to people on the ground. He says the question remains unanswered as to whether open-rotor engines “would be perceived as more or less annoying” to the ears than regular turbofans.

However if these concerns prove unfounded, Johnson would embrace the efficiency gains associated with open-rotor engines. “We would welcome the ability to have greater efficiency, providing it didn’t have an impact on noise,” he says. “We always prefer emissions reductions to come at the source, rather than through off-setting.”

Snecma’s Jung concurs that open-rotor engines have different tonal characteristics, but is confident these issues can be overcome. “Thanks to intensive use of CFD [computational fluid dynamics] and aero-acoustic wind tunnel testing of advanced propeller blade concepts, Safran demonstrated that this architecture is not only compliant with the new noise standards for certification (Chapter 14) but also consistent with expected performance levels,” he says.

“It is true that with the improvements we are still foreseeing in our design, the engine will be quieter decibel-wise. It is also true that open-rotor and turbofan sound signatures can vary quite significantly, in particular considering their tonal contents. However, acoustic perception studies performed by Snecma have not shown notable differences in the unpleasantness levels estimated by the subjects for these two engine technologies.”

GKN had to overcome challenges related to the design of the frame which, unlike regular turbofans, must have the ability to move. “The engine propulsion system is counter-rotating, with the new rotating frames in the back of the engine. Frames are usually static but here, the design solution is both acting as a static frame and, at the same time, rotating with the blades of the engine,” says Runnemalm.

“It is this combination of static and rotative loads which has created new challenges. The rotating frames are also, unlike static frames, considered as safety-critical parts.” The rotating module must provide sufficient design space and grounding to pitch and twist the angle of the front and rear blades to maximise engine performance in all phases of flight.

“This ability to alter pitch angle is key to ensuring the optimum performance of the open-rotor engine,” says Runnemalm. “The GKN Aerospace rotating module design must allow the pitch mechanism to do its job – transferring control of the blade angle/pitch from the control mechanism at the centre of the engine.”

To move to a rotating frame from a static one required a change of materials, and GKN used nickel-based alloys because they can withstand the effect of extremely hot gases moving across their surface. Runnemalm says the quality and consistency of functionality have been the main development challenges, and GKN has had to adopt new processes to overcome this.

“We have used a set of novel manufacturing processes, including laser welding – which is more typically used in static parts – and electric discharge machining, which ensures the efficient and non-abusive removal of large parts of material in an efficient way,” he says.

Open-rotor engines are expected to enter commercial service by 2030-2035, but this is dependent on the introduction of an all-new airframe. “Our view is that the open-rotor configuration is likely to enter the market powering a smaller, shorter-distance airframe – perhaps a regional jet – rather than a larger, long-distance passenger airliner,” says Runnemalm.

Close co-operation between the engine’s manufacturers and the airframers is essential to decide how open rotors can be integrated into future aircraft designs, as Snecma’s Jung explains: “For more than a decade we have been collaborating with airframers on this topic and we are currently still assessing different configurations of open rotors.

“The test that will be performed before the end of the year will help with selecting the most interesting configuration.”