As a young aerospace engineer, Zach Hoisington built his own wind tunnel — in a courtyard at his apartment building in California — to test sections of his paraglider prototype.
Years later, in 2007, Hoisington felt right at home in a room with nine like-minded Boeing engineers in Huntington Beach, California, looking for novel ways to lessen the environmental impact of commercial aviation.
Long drawn to lighter, externally braced structures, Hoisington floated the idea of an airplane configuration with high, long, thin wings that could be braced by diagonal struts, or trusses, attached to the lower part of the fuselage.
NASA requests ideas
In early 2008, since-retired Boeing Technical Fellow Marty Bradley reconvened the same Boeing Research & Technology team to refine their ideas. They dubbed their project the Sustainable Ultra Green Aircraft Research program, or SUGAR.
Later that year, NASA Aeronautics issued a solicitation for advanced-aircraft research ideas. Each concept was evaluated for its potential to influence reductions in fuel burn and emissions.
“This seemed like a perfect fit for what we had funded internally and what NASA was now asking for,” Bradley said.
In late 2008, NASA awarded initial research contracts to a Boeing-led team that included various industry and university partners. NASA adopted the SUGAR moniker for this first phase, though the “Sustainable” in Boeing’s version of the acronym was switched to “Subsonic” to match NASA’s solicitation language.
Engineers Marty Bradley, Christopher Droney and Zach Hoisington review an early idea around 2010. (Boeing photo)The work lasted through March 2010, at which point the Boeing-led team focused on the two most promising approaches, each coupling a truss-braced wing configuration with other carbon-reducing technologies. The concepts were called SUGAR High and the hybrid-electric SUGAR Volt, which helped popularize the idea of pursuing hybrid-electric technology in aviation.
Within Boeing, the aircraft concepts became synonymous with their airframe configuration and the speed at which they would be flown — becoming known as the Transonic Truss-Braced Wing, or TTBW.
Studies around the SUGAR High and SUGAR Volt efforts became the basis for a second NASA contract, SUGAR Phase II.
Model behavior in the wind tunnel
From 2012 to 2014, the Phase II team continued to define the TTBW airplane’s architecture and configuration. With a better understanding of its drag and structural characteristics, they built a first wind tunnel model out of fiberglass.
At the Transonic Dynamics Tunnel at NASA’s Langley Research Center in Hampton, Virginia, Boeing tested a 15% scale model without trusses to understand the wing design’s baseline aeroelasticity, or flexibility, in flight conditions. Afterward, the trusses were secured, and the same tests were performed again.
“Adding the truss underneath the wing makes a big triangle,” Bradley explained. “It stabilizes the whole inner part of the wing. The part that’s deflecting the most is the part outboard of that. The center is more rigid and the outboard part is more flexible, which mirrors what you see on regular airplanes with cantilevered wings that come out of the fuselage.”
In 2014, Boeing began studying the idea of a TTBW-based flight demonstrator under NASA’s Ultra-Efficient Subsonic Transport plan, setting the stage for future developments.
Idea comes to life
In Phase III, it was time to translate textbook theory into a functional design — and during wind tunnel testing, to see whether it could actually demonstrate the expected performance gains.
From 2014 to 2016, researchers used digital modeling to advance the shape of the wings, the struts and their interaction — including airflow in the narrow channel between them — to ensure aerodynamic efficiencies were in the target range without incurring extra drag.
Boeing designers tested a new model during a low-speed test at NASA’s Ames Research Center, but at Mach 0.745, slower than the typical cruising speed of today’s airliners.
Speed surprise
As the configuration concept matured, the SUGAR team optimized the aircraft cruise speed to better match the demands of the market. By Phase IV, conducted from 2016 to 2020, the team again tested wind tunnel models at Mach 0.80 — including a first test of a high-lift system.
“We surprised ourselves because the performance actually got better,” said engineer Christopher Droney. “That’s a classically harder thing to go do, the faster you go. But we kept getting smarter on the problem.”
Boeing engineers Neal Harrison, Niko Intravartolo and Eric Dickey with NASA aerospace engineer Greg Gatlin in 2021 at NASA’s Langley Research Center. (Photo: NASA)Detailing a demonstrator
SUGAR Phase V included high-speed buffet testing to check against vibrations incurred at Mach 0.80 and faster. Given the configuration’s unusual geometry, it was important to assess airflow between the wing and the truss.
Phase V also included more low-speed testing, which looked at the effects of icing and the performance at takeoff and landing.
With this phase complete, Boeing anticipated an opportunity to validate years of development and testing with a full-scale demonstrator vehicle through NASA’s Sustainable Flight Demonstrator (SFD) program.
Flight tests this decade
In January 2023, NASA announced its award of a $425 million Funded Space Act Agreement to Boeing and industry partners — to design and build an SFD with a TTBW configuration.
In June, the U.S. Air Force conferred an X-plane designation for the demonstrator vehicle, with the name X-66A. The letter designation has been dropped since then.
Years in the making, a demonstrator airplane with a truss-braced wing configuration may go big in the next decade. (Image: NASA)The TTBW airframe allows the demonstrator to have more underwing space than a typical airliner, allowing it to accommodate an advanced propulsion system. Combined with expected improvements in propulsion, materials and systems architectures, the TTBW configuration could yield up to a 30% reduction in fuel consumption and emissions relative to today’s most efficient single-aisle airplanes.
Learn more about the early ideas behind the sustainable flight demonstrator inside Innovation Quarterly.
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