Boeing and NASA have completed two initial wind-tunnel tests using small-scale models of X-66, the in-development trussed-braced-wing demonstrator.
The company disclosed the milestone on 4 March, saying the tests bring it closer to validating the technological feasibility of a design it and NASA have said could enable next-generation passenger jets to burn much less fuel.
Boeing says the team has finished testing an X-66 model with a 1.8m (6ft) wingspan at NASA’s Langley Research Center in Virginia. Those demonstrators involved evaluating “forces such as lift and drag throughout many aerodynamic configurations and flight conditions”.
The team also completed high-speed wind-tunnel studies at NASA’s Ames Research Center in California using an X-66 “semi-span model” – meaning a model of one side of the aircraft, as if it had been cut in half lengthwise.
“This test replicated expected flight conditions to obtain engineering information to influence design of the wing and provide data for flight simulators,” Boeing says.
“The data from these tests informed any design adjustments necessary before advancing to additional tests,” it adds.
Boeing and NASA are meanwhile still completing wind-tunnel testing of a full-scale X-66 model at NASA’s Ames site. The partners are developing X-66 under NASA’s Sustainable Flight Demonstrator project and aim for first flight in 2028.
This fiscal year, NASA aims to complete X-66’s “system-level” preliminary design review and to determine if the programme is sufficiently mature to transition into the first stage of actual development, say NASA budget documents.
Boeing last year also said it had started dismantling an MD-90 airliner, which it intends to modify into the X-66 demonstrator.
The X-66’s stand-out feature will be its truss-braced wings. Such wings are much longer than traditional wings, translating into a greater aspect ratio (which denotes the relationship between a wing’s span and area) and less drag, meaning more efficiency. But the wings, to be made from composite materials in X-66’s case, are so long that they require trusses for support.
The design is not a slam dunk. Long, flexible wings can pose aero-elasticity challenges – they can bend and distort in flight, increasing drag and potentially causing flutter.
Still, NASA has estimated truss-braced wings could make a future new narrowbody jetliner 10% more efficient than current aircraft, with other technological advancements bringing total potential efficiency gains to 30%.
While X-66 progress continues, Boeing has not committed to actually developing a truss-braced-wing passenger jet, though the company’s former chief executive David Calhoun had expressed optimism that the design would be a winner.
Some analysts are doubtful. They question whether the design will prove feasible and whether financially and operationally challenged Boeing will be willing to take the risk of breaking from traditional airliner designs in favour of a complex new concept.
