Guy Norris
Le Bourget
Although launch dates of next-generation single-aisle airliners are still uncertain and the engine race is just getting underway, airframers and propulsion players are set to conduct a series of pivotal near-term technology demonstrations to guide their decisions.
Airbus and Boeing differ on a preferred propulsion concept: Airbus publicly favors CFM International’s open fan, while Boeing leans toward advanced ducted engines. Whether this architectural divergence continues will depend heavily on the outcome of these key demonstration programs, some of which begin next year.
Because Airbus’ ambitious development plans have been linked since 2022 to the higher-risk open-fan design, the company is upping the scale and complexity of tests of the Revolutionary Innovation for Sustainable Engines (RISE) open fan with CFM through 2026. In conjunction with the GE Aerospace-Safran joint venture, Airbus is designing and manufacturing a pair of subscale full-aircraft models to assess how an open-fan propulsion system affects aircraft performance. The tests could prove crucial to Airbus’ plans— whether it adopts the concept for its next-generation single-aisle (NCSA) in the 2030s or pivots to powering its A320 successor with advanced ducted engines.
A 1:11 scale model is in development for high-speed tests at French aerospace research center ONERA, and a 1:14 scale model will be used for tests at Airbus’ low-speed facility in Filton, England, in 2026. Design and production are already in progress at Airbus, which also recently worked with Safran on tests of an open-fan demonstrator in ONERA’s large wind tunnel in Modane, France. This test phase focused on two “minimum body models”: a 1:5.5 scale model for highspeed testing and a 1:7 model for low-speed work, the latter of which was evaluated in the German-Dutch DNW wind tunnel facility in Marknesse, Netherlands.
To ensure adequate ground clearance for the RISE, which CFM has said will measure approximately 12.9 ft. in diameter for the production open fan, the scaled models planned for tests next year are configured with a low wing that has an increased inboard dihedral, also known as a “gull wing.” The unusual wing configuration is preferred over the use of longer landing gear legs, which add weight and require significant fuselage reconfiguration to accommodate the larger volume.
However, the gull wing has its challenges. The increased inboard dihedral causes a loss of vertical lift and an increase in drag, particularly during maneuvers. These losses could be exacerbated by interaction of the open fan’s rotor wash with the wing. Structurally, the gull wing would be complicated to build and heavier than a conventional wing.
Despite these potential shortcomings, a gull wing was once expected to provide greater design flexibility if Airbus selected higher-bypass advanced ducted engines, either as an option for the open fan or as an alternate. “Today, that is not the assumption,” says Bruno Fichefeux, head of future programs at Airbus. “It is really a combination of wing and engine that fits the purpose of each engine’s peculiarities and loads. We are pretty confident of the progress we are making on the open fan, and it shows a real ambition and performance delta. That’s why we are pursuing this seriously, and that’s currently our baseline view.”
Should the open fan, therefore, require a bespoke wing design, it would theoretically provide de facto exclusivity for CFM on the next-generation A320neo replacement, unless Pratt & Whitney and Rolls-Royce come up with competing non-ducted concepts. That seems unlikely, however.
Although Pratt confirms to Aviation Week that it has filed patents for open-rotor technologies going back several decades, “including as recently as last year,” the engine-maker adds that its own analysis continues to support ducted designs. “Based on our research, we concluded that the installation and integration challenges associated with an open-rotor architecture would reduce the potential fuel burn benefit, thus reinforcing our conviction that the second-generation ducted [geared turbofan] engine architecture remains the right path forward for NGSA,” the RTX owned company says.
Fichefeux acknowledges that Airbus will have to choose between a standard wing and the gull-wing design associated with the open fan. “At some point, we have to make that decision,” he says, adding that Airbus plans to take the time to let the design options mature. “The timeline of the launch of this program will depend on the maturity of these technologies. I like to use the analogy of the wine business: When you want a profitable wine stalk, you let all the branches grow, and over time you observe how they behave. Over time, you pick the one that has the most capacity to make fruit, and then you trim and you cut the others. You bet on the most promising one.”
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The process entails “a certain timing which is set by nature,” Fichefeux says. “Here, it is very similar. We have different branches growing for open fan and ducted. This is a very visible choice of engines, but we also have many others—hybridization, different materials, level of automation. We let these branches grow, mature them, invest in them, test and demonstrate from the lab to ground-testing and, down the road, flight-testing those that are relevant. Then we will see the branch emerging that brings the most benefit in terms of value—and to the airlines in terms of fuel efficiency and maintenance. Then we will start to cut the other branches, so you focus your investment on those that bring the best results.”
CFM has, meanwhile, completed more than 350 key RISE-related tests, including more than 3,000 hr. of high-pressure (HP) turbine and nozzle endurance tests by GE Aerospace. Safran has recently focused on blade tests as well as evaluations of the low-pressure (LP) turbine and compressor, bearing systems and equipment associated with the open fan’s variable pitch control functions. Safran’s blade test campaign has evaluated three different blade configurations to demonstrate mechanical strength as well as improved aerodynamic and acoustic performance.
“More than 175 ingestion and endurance tests were conducted in the Villaroche site’s test benches, specially configured to accommodate large parts, in addition to the 300 hr. of testing carried out on a scale model of the open fan in the wind tunnels of ONERA and DNW in partnership with Airbus,” the French engine-maker says. Safran’s new 8-m-dia. (26-ft.-dia) test chamber is under construction at its Villaroche site near Paris. “Starting next year, it will enable testing of large modules and will have the capacity to test the open fan’s pitch-control systems,” the company adds.
At the same time, Pratt is continuing development of technologies that will feed into an enhanced second-generation geared turbofan (GTF) targeted at up to 25% lower fuel burn at an integrated aircraft level than today’s engines. The company plans to move from the current GTF’s 81-in.-dia. fan to a bigger 85-90-in.-dia. composite fan contained within a composite case and shrouded within a compact, shorter duct nacelle. The engine will also incorporate a revised, smaller core with a higher overall pressure ratio, ultralow emissions combustor and, more than likely, hybrid-electric motor-generators.
As part of testing for the hybrid-electric development, GKN Aerospace-developed high-voltage wire harnesses have been delivered to RTX’s Collins Aerospace for integration into a powertrain for testing on an experimental PW1100G. Developed under the European Clean Aviation Switch (sustainable water-injecting turbofan comprising hybrid-electrics) program, the electrical wiring interconnection system was built at GKN Aerospace’s site in Papendrecht, Netherlands, and is slated for system-integration testing this year at Collins Aerospace’s advanced electric power systems lab, in Rockford, Illinois, called The Grid.
Following this, GKN adds that it will support future hybrid-electric Pratt & Whitney GTF engine demonstrator testing at EME Aero, a maintenance, repair and overhaul facility in Poland. Initial harness shipsets were delivered to Collins Aerospace’s Electronic Controls and Motor Systems Center of Excellence in Solihull, England, and additional units will be shipped in July for testing at The Grid. Collins is supplying two motor-generators for Switch, both of which are identical 1-megawatt machines. The unit connected to the high-pressure spool will be derated to 500 kW, while the shaft will be rated to 1 megawatt. In an operational version, the electric system will be used for taxiing and to boost power for takeoff and other transient phases, RTX says.
Meanwhile, Rolls-Royce’s small UltraFan 30 development plan is following a “very simple strategy,” says Simon Burr, group director of engineering, technology and safety. Targeting the broad 30,000-lb.-plus-thrust range expected for the NGSA, the initiative fulfills a strategic goal unveiled by CEO Tufan Erginbilgig in late 2023 and marks a crucial step in Rolls’ renewed ambition to reenter the narrowbody sector that it effectively abandoned 13 years ago when it sold its shares in the International Aero Engines collaborative venture to Pratt.
“We’ve got a great new core on the Pearl 10X, but the demonstration is really not about the core, where we could do all kinds of things,” Burr says. “It’s about demonstrating the low-pressure system.” Ground tests of the small, geared engine are targeted for 2028, partially backed by the Hydrogen Engine Architecture Virtually Engineered Novelly (Heaven) project, which is another Clean Aviation initiative. However, Burr is challenging his design team to accelerate this to 2027. “We think by 2028, you’ve got to be able to show something, not just arm-waving and a PowerPoint,” he says.
“The lead time is often not driven by the gas path,” Burr continues. “It’s driven by all the accessories. So maybe you can think differently and say: Tm going to drive my gas path generator first, and then separately, I’m going to demonstrate how I can dress an engine on a clean fan case. Where am I going to pull these accessories?’ Ultimately, my vision is that we take quite a lot of accessories off the engine and we put them somewhere else, like in the pylon or some other place in the aircraft, because that helps you aerodynamically. If you shrink the core dressing down, you shrink the nacelle down and get more air through for a given diameter.”
With a potential fan diameter of “well over 90 in.,” Burr says various approaches are being studied to overcome the installation challenge. “You can slim the nacelle down, so there’s a lot of work being done on that as well as shorter nacelles, reducing the overhung mass, the vetted area and all the drag interference. A lot of optimizations are still to come in that space, and it’s interesting engineering. So I’m kind of pretty excited about that.”
Jens Flottau
Thierry Dubois
at Le Bourget