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Aerodynamic and structural data on the X-66 were obtained from wind tunnel tests of a 5.3%-scale model in the Transonic Unitary Plan Facility at NASA’s Ames Research Center in California.
Boeing is indefinitely pausing development of the X-66 full-scale Sustainable Flight Demonstrator and will instead double-down on alternative plans to perfect the thin-wing technology at the center of the project.
Engineering resources from the experimental program are being reallocated to support Boeing’s ongoing push to complete the delayed certification of the 777X and final 737 MAX variants, clearing the way for deliveries to begin in 2026.
Based on a heavily modified McDonnell Douglas MD-90, the X-66 was intended to demonstrate a truss-braced version of the thin wing at full scale. The experimental aircraft formed the flagship of NASA’s push to achieve net-zero aviation emissions by 2050 under the agency’s Sustainable Flight Demonstrator project.
NASA, which launched the initiative in January 2023 under a funded Space Act Agreement, says together with Boeing it is currently “evaluating an updated approach to the agency’s Sustainable Flight Demonstrator project that would focus on thin-wing technology with broad applications for multiple aircraft configurations.”
The transonic truss-braced wing (TTBW) aircraft was expected to make its first flight in 2028. As well as being NASA’s largest X-plane, the X-66 was due to provide Boeing with a test platform for a sustainable design that could form the basis for a single-aisle replacement for the 737 in the 2030s.
The announcement to put the MD-90 modification on hold does, however, align with Boeing CEO and President Kelly Ortberg’s strategy to cut costs and focus on near term execution of current programs. It also comes as NASA faces the specter of looming cuts in the agency’s fiscal 2026 budget.
But despite its apparent change of direction on X-66, Boeing insists it remains confident in the benefits of thin-wing technology for application to its next-generation single-aisle aircraft studies. The OEM has also indicated that any moves to alter its X-66 plan will not materially affect the overall timing of a new next-generation single aisle project in the next decade.
“We’ve done extensive additional wind tunnel testing that confirmed the aerodynamic benefits of the thin wing,” Boeing Chief Technology Officer Todd Citron says. The key benefit of the high aspect ratio thin wing is less lift-induced drag, while its reduced thickness ratio also decreases profile and transonic drag. Combined with advanced propulsion systems, Boeing believes a thin-wing next-generation airliner could be 30% or more fuel efficient than current single-aisle models.
In the build up to developing the X-66 wing—much of it conducted by Boeing subsidiary Aurora Flight Sciences—work on systems, structures and fabrication has “really confirmed the value of thin wings, be it with a truss or without,” Citron says. “The thin-wing technology is clearly a benefit, and there are also of course challenges with that, so based on that learning, we want to work together with NASA to focus the project going forward on that thin wing and thin-wing technology.”
Citron adds that “our intent is to build a thin-wing manufacturing demonstrator, together with a flight controls rig to really mature those technologies for those future generations of commercial aircraft.”
The effort will “be used for a continuing pipeline of technologies to advance the capabilities of our wings—our learning today is pointing us to doing even more on the wing itself and focusing our activity there,” Citron says.
Under the original X-66 agreement with Boeing, NASA was to provide $425 million in funding through milestone payments while the airframer and its industry partners would contribute $725 million. Beyond this, Boeing is responsible for further costs on its side of the agreement.
No details of how these contracts could change have been revealed, though Citron says initial discussions have started with NASA. “There’s more to be done to take that to the point where we’re all set to move out on this approach, but we’ve had good discussions getting started,” he says.
“I think our partnership with NASA has been definitely very beneficial, and I don’t want to put words in their mouth, but I expect that they will be interested in working together with us on this path forward, and they certainly have capabilities that would contribute to the partnership,” Citron continues. “Our intent is they would be part of this, but we have to finish the discussions to make that official.”
NASA says that “under this proposal, all aspects of the X-66 flight demonstrator’s design, as well as hardware acquired or modified for it, would be retained while the long, thin-wing technology is being investigated with more focus. NASA and Boeing also would continue to collaborate on research into the transonic truss-braced wing concept. The proposal is based on knowledge gained through research conducted under the Sustainable Flight Demonstrator project so far.
Work on the X-66 at Boeing’s Palmdale, California, facility is meanwhile set to wind down over the next few months. “Working together with NASA, we will take the flight demonstrator through milestones into the summer timeframe and complete those and put that activity on pause,” Citron says. Under the original Boeing-NASA schedule, this will take the program up to the final stage before preliminary design review.
“We’re going to continue to assess the performance benefits of the truss-based configuration, and as we get results from that, we would decide what to do going forward from a flight demonstrator,” Citron says. “In parallel with that, we would then develop this manufacturing demonstrator and stand up the flight controls rig.”
The X-66 project—which was into the first few phases of a 23-step modification, rebuild and completion process—will be mothballed, pending a future decision to restart or scrap it. “At this point, don’t have a specific date in mind,” Citron says. “We want to make it data driven.”
NASA concedes that “work on the X-66 flight demonstrator—which currently incorporates a more complex transonic truss braced wing concept that the same thin wing technology as well as aerodynamic, structural braces—would pause and be evaluated based on the thin-wing testbed results and further truss-braced configuration studies.”
While negotiations continue it remains unclear what the final impact on NASA research will be if the X-66, as originally planned, is substantially altered in scope. Aside from forming a major plank of NASA’s Sustainable Flight National Partnership, through which it seeks to protect the environment, grow the U.S. economy, and develop new technology, the X-66A is the focus for a long list of planned key agency aeronautics investigations ranging from propulsion and wing-fuselage integration to new structures work and aeroelasticity studies.
Design tools, models and static loads methods were to be validated on the X-plane project, which was to also include new opportunities to validate internal loads modeling and new structures concepts like the joints between the truss and the wing and fuselage.
Boeing’s move to reshape its thin-wing research comes as Airbus continues studies of advanced high-aspect ratio wings through its eXtra performance Wing (X-Wing) demonstration and Wing of Tomorrow programs. As well as the morphing X-Wing aircraft, which is due to fly in 2026, Airbus is evaluating a strut-braced concept under the Ultra Performance Wing (UPWing) initiative.
In addition to UPWing, which is supported by the European Clean Aviation research program, French aerospace research agency ONERA is leading the €15.3 million ($16.6 million) Advanced Wing Maturation and Integration effort, which is focused on studies of a cantilevered and strut-supported thin wings.
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