Snecma eyes leap to open rotors for next-gen narrowbodies

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Despite owning half of CFM International, the world’s most successful commercial engine manufacturing joint venture, Snecma parent Safran is not about to rest on its laurels when it comes to technology. As CFM conducts the first full tests of its next generation Leap turbofan for the Airbus A320neo and Boeing 737 Max families, Snecma is already looking to deliver the next step-change in engine technology in the form of an open rotor powerplant.

The technology building blocks have been in development for several years, supported by European Union research programmes such as Clean Sky. Even so, Snecma’s announcement last month that it aims to test-fly a full-scale prototype open rotor engine mounted on an A340 testbed by as early as 2019 took many in the industry by surprise.

Snecma argues that such an aggressive timetable is necessary to ready the open rotor concept for service-entry by 2030, given the time it will take to mature the required technologies and agree and implement new certification requirements with airworthiness authorities.

The ultimate goal is to produce an engine for the 150-seat mainline twinjets that are – and are expected to remain – the work-horses of the global commercial airliner fleet, but which will burn 30% less fuel than today’s powerplants. Snecma describes this ambition as “bold but realistic” despite the challenges associated with introducing such a radical engine architecture into airline service, not least of which is the requirement for the configuration of the airframe itself to be completely rethought.

An open rotor engine marries a conventional gas generator with a turbine driving a pair of counter-rotating unducted fans. The advantage of dispensing with the duct that surrounds a traditional turbofan is that the amount of bypass air drawn past the engine core can be increased, boosting efficiency.

One of the principal challenges is noise, but Pierre Guillaume, Snecma research and technology director, says tests conducted in July last year at the Onera windtunnel in Modane, France gave the manufacturer confidence that this obstacle can be overcome. The open rotor’s overall noise footprint should be similar to that of the CFM Leap, at about 10dB below the level of today’s in-service engines, which is “very important for us”, he says.

“Of course we know that the noise regulation will still evolve in the future and that we’ll still have to work on it.”

The tests at Modane used a 1:5 scale model of Snecma’s open rotor design and also validated the performance of the counter-rotating fan design.

“We’ve been using our most advanced simulation code that we’ve been developing with our partners such as Onera and Cenaero in Belgium,” says Guillaume. “Working with these codes has been very successful as the final design and optimisation has been confirming what the simulation was showing.”

Snecma is now focused on building a full-scale prototype for bench-testing by the end of 2015, based on the core of the Dassault Rafale fighter’s M88 engine. The first long-lead parts will begin production in the middle of this year.

This demonstrator – being built with partners Avio Aero, GKN and Aircelle – will facilitate testing of the propulsion module (all the components of which rotate), including the control units and software governing blade pitch.

Avio is providing the power turbine and reduction gearbox, GKN supplies the main rotating frame and Dowty the pitch control mechanism, while Snecma is responsible for integration, major static frames, plates, carbon-fibre composite propeller blades and all the components required for integration with the core.

“Using the composite 3D woven carbon fibre that we are embedding on the Leap engine [for the fan] is a real technology enabler, so the same technology will be used for the open rotor and this is critical to ensure the success of this architecture in terms of weight and performance,” says Guillaume.

The full-scale demonstrator will have a tip-to-tip diameter of 4.2m (14ft), and be in the 20-25,000lb thrust range to meet the power requirements of a 150-seat narrowbody.

“Using the [software] codes has allowed us to generate a different profile of the blades and select the one that would be the most interesting. It could still be possible to get some improvement for the blade that we will be testing on the engine in 2015,” says Guillaume.

Noise mitigation is “purely an optimisation of the blade design right now”, he adds. “We still have to test it on some models with our partners to test some interactions with the fuselage and with the pylon, but we are very confident from the simulation that we’ve achieved, that it will be confirming what we’ve been observing in the windtunnel. There can be a kind of shielding effect coming from the aircraft itself and this is fully taken into account.”

Flight tests aboard the Airbus-owned A340 testbed will be crucial to further validate noise assumptions and investigate the aerodynamic effect of the unducted fans on the aircraft’s fuselage. They will also support efforts to agree the new regulatory framework that will be required.

“We are expecting some evolution of the regulations in the future and so that means that we will have to keep on improving our design and to work with the airframers – especially Airbus within the scope of the Clean Sky project – to be sure that we will be compliant with future regulations,” says Guillaume. “We still have work to do, but we are very confident with our simulation that we have been able to test and to confirm.”

It is expected that open rotor engines will be treated in the same way as turboprops regarding the “blade-off” scenario. Turbofan engines are required to show that they can retain their structural integrity in the event of a blade loss at take-off power.

“We consider that certifying this kind of engine is really similar to what is performed today on turboprops,” says Guillaume. “It has nothing to do with the turbofan where the blade out is considered at certification, as the design of these propeller blades cannot be compared to fan blades. They are mechanically completely different, especially in terms of compliance with bird strike.

“We are convinced it is possible to demonstrate that the open rotor concept is completely similar to a turboprop concept in terms of certification. Today the regulations are not fully defined, but there is a working group looking at it. Our position today is that it is possible to go forward in this direction.”

For the A340 flight tests it is yet to be determined whether the open rotor demonstrator will be mounted on one of the aircraft’s existing wing pylons or on the fuselage, to better represent the likely installation of an eventual production engine.

“Of course we will try to test it in the most representative conditions that we can,” says Guillaume. “Especially as we are exploring a pusher configuration more than puller, as this is the option that would be the best fit for us. But it still has to be discussed within the framework of the Clean Sky 2 programme.

“In terms of integration we will learn a lot about the behaviour of the contra-rotating propeller blades. Flight testing this kind of engine is really critical, but also for some performance measurement it is important to have this flight test demonstration in the most representative configuration.”

Looking ahead to potential series production of an open rotor, Snecma is already discussing options with its CFM partner General Electric, but emphasises that the architecture is one of several options for a future engine to succeed the Leap.

“We are discussing this kind of architecture, as others, with GE in the frame of our CFMI agreement and collaboration. We haven’t defined yet the spilt between GE and Snecma if such a configuration were to be selected by an aircraft manufacturer, but these discussions are in progress with our partners.”