French group Safran on Tuesday inaugurated ground testing of the first open-rotor engine demonstrator. The company expects the new unshrouded engine architecture to deliver fuel savings of 15 percent compared with the new Leap series of turbofans, built by the CFM joint venture between Safran and GE.

Safran built an open-air test stand at its facility in Istres, in the south of France, and has run the open-rotor demonstrator since May. Developed within the framework of the European Clean Sky 2 research program, the open-rotor study carries a funding budget of €65 million over eight years. Tests will continue until the end of the year to validate the demonstrator. "So we will have all the technology on the shelf, available to aircraft manufacturers," said Safran CEO Philippe Petitcolin, who came to Istres to inaugurate the test bench.

The primary target market for the open-rotor concept, at least to start, centers on single-aisle aircraft, the heart of the CFM’s customer base. Safran has conducted its development research independent of open-rotor studies conducted by GE. The partners will coordinate under CFM in the event of a launch of an open-rotor program. Certification of an open-rotor engine would not occur before 2030, according to Safran.

The breakthrough of the open rotor stems from a significant increase in the bypass ratio, from 11:1 on the Leap to more than 30:1. The lack of a nacelle covering makes it possible to increase the size of fans, thus the increase in bypass ratio. The higher the bypass ratio, the better the energy efficiency of the engine.

The disadvantage lies with the need to completely reconfigure the aircraft. An open-rotor engine cannot mount on a wing, but must attach to the rear of the fuselage. The demonstrator features two contra-rotating fans, the blades of which consist of 3D woven carbon composite, such as those of the Leap. GE subsidiary Avio of Italy manufacturered the gearbox.

Wind tunnel tests carried out in 2013 helped solve the noise challenge, one of the main hurdles in open-rotor development. "Our demonstrator has the same sound levels as a Leap, thanks to an optimization of the aerodynamics of fan blades," explained Safran research and technology director Stéphane Cueille. The company has not scheduled flight tests.

Airbus, a partner of Safran in Clean Sky 2, has expressed more interest in more conventional engine architecture known as the Ultra High Bypass Ratio (UHBR) engine, which incorporates a nacelle but offers a bypass ratio of 15:1. The reduction in fuel consumption of the UHBR would total between 5 and 10 percent compared with the Leap, according to Safran. The UHBR, which could enter service by 2025, also does not require much adaptation of the aircraft configuration. "Everything depends on the choice of the aircraft manufacturers," said Petitcolin. "Either they want an evolution of the current single-aisle by 2025-2027, and the UHBR will be possible, or they want a new design beyond a more distant horizon. And that would allow open rotor.”

Safran is working in parallel with its partner GE on the UHBR, but on different technological building blocks. It expects to test the first UHBR demonstrator in Istres sometime after 2020.