Will Boeing Embrace Braced Wings?

The cover story in the latest Aviation Week & Space Technology is an exclusive on tests of Boeing's Sugar truss-braced-wing (TBW) airliner concept in NASA Langley Research Center's Transonic Dynamics Tunnel.

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Photo: NASA Langley

You can read how the expensive and flexible 13ft-span, 15%-scale model is being used to validate wing weight estimates for a full-size aircraft. These are critical to determining whether the fuel savings promised by the drag reductions possible with a high-aspect-ratio wing are achievable.

Boeing and NASA are pretty convinced the wing can be made light enough, and that a 737-size TBW airliner would have a 5-10% lower fuel burn, even with the same generation of engines. They are also pretty confident a truss-braced wing can be an option for the next generation of all-new airliners, expected somewhere around 2030. 

The study work, being conducted under NASA's N+3 project, is also looking at hybrid turbine-electric propulsion for Boeing's Sugar Volt concept, and in the same issue of AW&ST you can read my colleague Guy Norris's article on General Electric's work on the hFan gas-turbine/battery-electric engine concept.

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Sugar Volt - the pods are battery packs (Concept: Boeing)

But why a high-aspect-ratio wing? Aspect ratio (span-squared divided by area) is a measure of a wing's slenderness. The more slender a wing is, the lower the lift-induced component of drag and the higher the lift-to-drag ratio (a measure of aerodynamic efficiency) -- think of a sailplane wing.

Most jetliner wings have an aspect ratio around 9. Thanks to their stiff carbon-fiber wings, the Boeing 787 and Bombardier CSeries push this to 11. With active flutter suppression and gust-load allevation to control the structural flexibility, you might get to 15 with a conventional "cantilevered" wing supported only at its roots.

To go higher and reduce drag further, to an aspect ratio of 20, or even 30, you need to support the wing with a strut (a single member) or truss (two or more members). Hurel-Dubois pioneered this with a series of designs in the late 1940s and 1950s - you can see our slideshow on the history of strut/truss-based wings here.

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Hurel-Dubois HD.31 (Photo: 1000aircraftphotos.com/Johan Visschedijk Collection)

Hurel-Dubois' aircraft were successful technically, but not commercially. They were piston-powered and slow. To make TBW work for a future jetliner the structure must be light, and drag from aerodynamic interference between the struts and the wing must be minimized. That is what the Sugar study is working to validate.

Interestingly, the Sugar study involves not only Boeing Research & Technology but also Boeing Commercial Airplanes -- which has a tendency to keep unconventional configurations (like blended wing body) at arm's length. Perhaps its involvement with Sugar is a sign that TBW is a viable option for a future airliner.

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Sugar Volt - with props (Concept: Boeing)

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