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The X-66 Sustainable Flight Demonstrator (SFD), an X-Plane project with a high-mounted truss-braced thin wing launched in January 2023, is being put on the back burner. Boeing’s (Arlington, Va., U.S.) April 24 announcement that it is making moves to pause the NASA-led (Washington, D.C., U.S.) project have been followed by  that both partners are “currently evaluating an updated approach ... that would focus on demonstrating thin-wing technology with broad applications for multiple aircraft configurations.”

To become NASA’s largest X-Plane, the demonstrator was specifically focused on helping the U.S. achieve net-zero aviation emissions by 2050, informing future single-aisles with a new wing rendering — extra-long, thin wings stabilized by diagonal struts known as the transonic truss-braced wing (TTBW) concept. Boeing was working with NASA to build, test and fly a full-scale version, with wing assembly supported by Boeing company Aurora Flight Sciences (Bridgeport, W.Va., U.S.). Though materials were never confirmed, both Boeing and Aurora have extensive composites expertise that could potentially inform the future wing design.

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According to an Aviation Week article, the X-66 was previously expected to fly in 2028. Now, Boeing has proposed work on the X-66 “would pause for later consideration based on thin-wing testbed results and further truss-braced configuration studies.” 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 would also continue to collaborate on research into the transonic truss-braced wing concept.

that from Boeing’s perspective, this move is part of a “pragmatic program revision,” one that simultaneously seeks to redirect resources to certify the delayed 737 MAX and 777-9 while avoiding “premature commitment to a thin-wing configuration that may not work out in the long term.”

In summary: While the X-66 program is in limbo, the thin-win seems to live on.

“We have learned a lot in the past few years partnering with NASA on the X-66 program that will influence the future generations of airplane design,” a Boeing spokesperson tells CW. “That learning has shown us the value of thin-wings and associated technology. Going forward, we will focus efforts on maturing the technology to support thin-wing design. What we learn with this approach can be applied across multiple products, including potential truss-braced wing configurations and beyond.”

There are still concerns that the program pause will be detrimental to the ambitious X-Plan project in the long-term. This is reinforced by the termination of the NASA X-57 experimental electric aircraft in 2023, followed by the agency’s X-59 supersonic demonstrator that is years behind schedule. But it is clear that there is still a lot to be determined. For the moment, Boeing’s X-66 facility in Palmdale, California, is set to wind down over the next few months.