Boeing’s development of NASA’s X-66A truss-brace-wing experiment aircraft will either spawn an entirely new class of commercial jetliners or prove the design unfeasible.

But the US manufacturer’s chief executive David Calhoun thinks his company might have a winning concept.

“If it matures the way we think it will – and [the way] NASA thinks it will – I do think it will see service,” Calhoun said on 26 July of Boeing’s truss-braced-wing airliner concept.

NASA on 25 July 2023 revealed this planned livery for the X-66A demonstrator

Source: NASA

NASA on 25 July 2023 revealed this planned livery for the X-66A demonstrator

“If it behaves like it did in the wind tunnel, we are in a pretty good place,” he adds, speaking during the company’s second-quarter earnings call.

Boeing is widely viewed as on a path to bringing a new narrowbody aircraft – a 737 replacement – to market in the mid-2030s. Calhoun insists it must be significantly more fuel efficient that today’s narrowbodies – on the order of 25-30%, he says on 26 July.

The design of Boeing’s next aircraft remains uncertain, but the company has put significant resources lately behind advancing its understanding of a concept it calls the “transonic truss-braced wing”, a design involving a much longer wing, supported by trusses.

In January, NASA said it picked Boeing to develop just such an experimental aircraft, dubbed the X-66A. The agency is kicking in $425 million of the project’s expected $1.15 billion cost, with Boeing and other commercial partners contributing the balance. Boeing is developing the aircraft using an MD-90 fuselage as its structure, and NASA aims for it to fly in 2028.

The concept calls for the aircraft’s wings to be mounted atop its fuselage, and for those wings to have a significantly greater aspect ratio than conventional wings.

Aspect ratios denote the relationship between a wing’s span and area. Higher-ratio wings have comparatively longer spans and are generally more efficient. But long wings can need extra support from trusses, as with the X-66A.

NASA says a truss-braced wing could make the next narrowbody airliner 10% more efficient, with other technologies (such as advanced engines) bringing total efficiency gain as high as 30%.

“We are intent on proving this technology,” Calhoun says on 26 July. “We like what it could potentially deliver to this market.”

He adds that a truss-based-wing jetliner could be powered by either larger-diameter turbofans or by an open-rotor engine, which would also be wider than current narrowbody engines. The aircraft’s high-mounted-wing configuration would leave enough space for such powerplants.

Boeing truss brace concept 011823

Source: Boeing

Boeing has released images of several truss-brace-wing jetliner concepts

CFM International has bet big on open rotors. It is developing such a powerplant under its Revolutionary Innovation for Sustainable Engines programme, shooting for a 20% efficiency gain.

NASA’s X-66A development project is part of its Sustainable Flight Demonstrator programme, an effort to mature aircraft designs and technologies that can improve fuel efficiency.

Boeing is anything but new to truss-braced wing development, having worked with NASA for more than a decade to develop a workable design. It and NASA evaluated concept starting in 2008 as part of NASA’s Subsonic Ultra Green Aircraft Research project, evaluating wings with aspect ratios of 14 and higher, according to NASA.

By comparison, wings on current narrowbodies have aspect ratios of roughly 8-10, various sources show.

Such high-aspect ratios to pose challenges, including a propensity for the wings to wobble and develop a dangerous condition called flutter, which trusses can alleviate.