As preparations continue for running a full open-rotor engine demonstrator in 2016 under Europe’s Clean Sky research effort, French engine maker Snecma (Hall 4 Stand B12) sees the program’s participants reaching a consensus as whether or not to proceed in the 2017-to-2019 time frame. Clean Sky, which also involves Airbus, Rolls-Royce and French research center Onera, has provided a relatively unexpected discussion platform, thus facilitating a general agreement.

The benefit of an open rotor is that it enables an increase in the optimum bypass ratio. On a ducted turbofan, beyond a certain fan diameter, the engine’s weight and drag become too much of a penalty, with the fuel burn they require superseding the benefit of the higher bypass ratio. Snecma research engineers estimate that for an ultra-high-bypass ratio (more than 15), an open rotor can be 10 percent better than a ducted turbofan.

According to Vincent Garnier, Snecma director for product strategy and markets, the demonstrator program that is part of Clean Sky has three goals: to evaluate and validate the architecture; to push the science and technology of all the main components of the propulsion system; and to build a team of partners.

Emphasizing that open-rotor architecture offers only one option, Garnier said it presents the greatest potential for fuel-burn reduction but is also the most difficult to bring to fruition. Progress in materials will be one of the three keys, along with architecture innovation and high-fidelity modeling, he added. He deems the open rotor “a challenging concept but without showstoppers” because he considers that noise and certification issues are being addressed successfully.

In materials, while the first 3-D-woven carbon fibers will enter service in 2016 on CFM Leap fan blades, Snecma is already working on the second generation. Both the fibers and the matrix (applied using the resin transfer molding process) will offer improved performance. The pattern will be less uniform so it adapts to local needs inside the part. “With its full potential, this combination of technologies will help designing an engine with ultra-high propulsive efficiency,” Garnier said.

In addition, Snecma is currently testing ceramic-matrix composites (CMC) for the low-pressure turbine. “CMC airfoils weigh 70 percent less than conventional airfoils and have excellent damage tolerance,” said Garnier. He emphasized that lighter airfoils yield, in turn, a lighter disk overall. With its CMC technology, Snecma is aiming at the Leap but the material will then be available for future engine designs.

High-fidelity Modeling

Advancement in modeling will play a vital role. “High-fidelity modeling is on the way and will give our engineers a lot more room for creativity,” Garnier said. Today, modeling the aerodynamics of a conventional propeller cannot go further than one rotation. “With high-fidelity modeling, if we could not introduce radically different engines, there would be something wrong,” he said.

Airframe-engine integration represents another criterion in the decision to go ahead, prompting Onera to invest in new wind-tunnel equipment to test an open-rotor engine and its accompanying fuselage section. Researchers have been testing one-fifth-scale open-rotor propellers since 2010, but outside an aircraft environment. Garnier said engine makers will either green-light or reject a new architecture in a more collaborative process than has been used in the past. For example, airframe surfaces that block sound waves can cut noise, making airframer participation crucial. “The industry has done little work on engine and aircraft architectures over the past 40 years,” he pointed out.

Noise, often cited as the main problem of an open rotor, now presents “a controlled risk,” he said. With modern technologies, it would match that of a CFM Leap, Garnier claimed, adding that foreseeable progress should keep open-rotor noise within future certification limits. In addition, he said, designers will consider “psychoacoustics,” which study those transient noises most unpleasant to human ears. Onera plans to simulate open-rotor noise by next year.

A weak point of the open rotor is it may be less suited to long-haul aircraft. The bypass ratio is an important parameter for climb, which is a shorter flight phase, in proportion, on an intercontinental flight. Also, an open rotor is a heavy engine (due to its diameter) and weight directly translates into fuel burn. All this makes this kind of turbine engine more attractive for medium-haul aircraft, as opposed to long-haul. Therefore, one could see a differentiation in aircraft architectures between short/medium- and long-haul, Garnier suggested.

Other options include various intermediate solutions between a conventionally ducted turbofan and the open rotor. One is a variable-pitch fan. All are envisioned to enter service in around 2030; in the interim, Snecma plans on continuing improvement of the Leap’s fuel performance. CFM56 upgrades have introduced fuel-burn improvements of 0.5 to 1 percent per year and Garnier said he expects to see a similar curve for theLeap.