As preparations proceed for running a full open-rotor engine demonstrator in 2016 under Europe’s Clean Sky research effort, French engine maker Snecma sees the program’s participants reaching a consensus over whether or not to proceed in the 2017-to-2019 period. 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.
Emphasizing that open-rotor architecture amounts to only one option, Snecma director for product strategy and markets Vincent Garnier said it presents the greatest potential for fuel-burn reduction but also the most difficult challenges.
Advancement in modeling will play a vital role, he asserted. “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.
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. Moreover, he said, designers will consider “psychoacoustics,” which study those transient noises most unpleasant to human ears. Onera plans on simulating open rotor noise by next year.
Airframe-engine integration represents another criterion in the decision, 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 that airframers will decide whether or not to go ahead with a new architecture in a more collaborative process than used in the past. For example, airframe surfaces that block sound waves can cut noise, making airframer participation crucial.
Other options include various intermediate architectures between a conventionally ducted turbofan and the open rotor. All would enter into service around 2030.
By then, Snecma plans on a continuing improvement of the Leap’s fuel performance. CFM56 upgrades have introduced fuel-burn improvements of .5 to 1 percent per year and Garnier said he is betting on a similar curve for the Leap.