Aircraft

Boeing to build braced-wing airliner, shooting for 30% efficiency gain

Boeing to build braced-wing airliner, shooting for 30% efficiency gain
NASA has awarded Boeing US$425 million towards building and testing a full-sized prototype of its transonic truss-braced wing airliner concept
NASA has awarded Boeing US$425 million towards building and testing a full-sized prototype of its transonic truss-braced wing airliner concept
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NASA has awarded Boeing US$425 million towards building and testing a full-sized prototype of its transonic truss-braced wing airliner concept
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NASA has awarded Boeing US$425 million towards building and testing a full-sized prototype of its transonic truss-braced wing airliner concept
The extra-long, slim wings generate extra lift while reducing drag. NASA and Boeing are hoping to see a 30% reduction in fuel burn as a result
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The extra-long, slim wings generate extra lift while reducing drag. NASA and Boeing are hoping to see a 30% reduction in fuel burn as a result
Under development for more than a decade, the truss-braced wing concept has been extensively tested in CFD and at subscale in wind tunnels
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Under development for more than a decade, the truss-braced wing concept has been extensively tested in CFD and at subscale in wind tunnels
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With a US$425-million cash injection from NASA, Boeing will build and test a full-sized airliner based on its transonic truss-braced wing (TTBW) concept, using long, thin, strut-braced wings to add lift, reduce drag, and burn an impressive 30% less fuel.

When you burn as much fuel as an airline does, a single-digit fuel efficiency tweak adds up to massive savings. Take the drag-reducing Aeroshark film Swiss Airlines has stuck all over its 12 Boeing 777s – it delivers a 1% efficiency gain, and as a result, over just 12 aircraft, Swiss expects to use 4,800 tonnes less jet fuel every year, saving nearly half a million dollars per year, per plane at today's prices. That'd be closer to half a billion a year for an operator like American Airlines, closing in on 1,000 planes in its fleet, from a 1% efficiency gain.

So you can see how an airliner 30% more efficient than today's best single-aisle machines could be a bit of a big deal. We first ran across Boeing's "truss-braced wing" design concept back in 2010, as a part of the "Subsonic Ultra Green Aircraft Research" (SUGAR) Volt concept it designed as part of a NASA research program.

The idea takes advantage of the higher lift and lower drag you get with longer, slimmer, high aspect ratio wings – the sort you might find on an unpowered glider. A concept Boeing was testing in 2016, for example, had wings some 50% wider than comparable standard aircraft.

Structurally, that kind of thing simply doesn't work without reinforcement. So Boeing's design hangs the wings from the top of the fuselage, and braces them with long trusses coming up from the belly of the plane. These too are carefully shaped airfoils, adding extra lift as well as strength and stability.

The extra-long, slim wings generate extra lift while reducing drag. NASA and Boeing are hoping to see a 30% reduction in fuel burn as a result
The extra-long, slim wings generate extra lift while reducing drag. NASA and Boeing are hoping to see a 30% reduction in fuel burn as a result

As a subsonic concept cruising at around Mach 0.70 to 0.75 (519 to 556 mph, 835 to 895 km/h), Boeing estimated these braced-wing airliners could burn 50% less fuel than a regular plane. In 2019, the concept was redesigned to cruise at the edge of transonic speed, around Mach 0.8 (593 mph, 955 km/h), and whether because of the added speed or simply from a better understanding of the aerodynamics, Boeing has walked the efficiency claims back.

"When combined with expected advancements in propulsion systems, materials and systems architecture," reads a Boeing press release, "a single-aisle airplane with a TTBW configuration could reduce fuel consumption and emissions up to 30% relative to today's most efficient single-aisle airplanes, depending on the mission."

It's been a long time in digital modeling and subscale wind tunnel testing, but NASA has now awarded Boeing funding through the SFD Space Act agreement to the tune of $425 million, to be thrown in with some $725 million from Boeing and various other business partners, to actually go and build the thing at full scale and get it properly flight tested.

NASA says it plans to complete testing on the Transonic Truss-Braced Wing demonstrator aircraft "by the late 2020s, so that technologies and designs demonstrated by the project can inform industry decisions about the next generation of single-aisle aircraft that could enter into service in the 2030s."

Under development for more than a decade, the truss-braced wing concept has been extensively tested in CFD and at subscale in wind tunnels
Under development for more than a decade, the truss-braced wing concept has been extensively tested in CFD and at subscale in wind tunnels

There will certainly be challenges. For starters, these super-long wings might simply not fit with existing airport terminals or hangars. Boeing hasn't said anything about the demonstrator aircraft, but on the 2019 concept it spoke of using foldable wings to get around this issue on the ground.

And then there's the fact that the huge, thick, lower aspect ratio wings on standard airliners create a perfect hollow space for their fuel tanks. Keeping the fuel out in the wings places a lot of weight out wide, closer to the center of lift, reducing engineering stresses where the wings meet the body. It contributes to safety somewhat in a crash, keeping burning fuel further from the passengers. And from a pure brass-tacks perspective, it frees up room in the cabin for extra money-making seats. The truss-braced design uses such slim wings that fuel tanks will likely have to go back into the fuselage.

On the other hand, Boeing says that the high-mounted, braced wings "could eventually accommodate advanced propulsion systems that are limited by a lack of underwing space in today's low-wing airplane configurations" – although this demonstrator won't be testing any fancy new motors straight away.

And it goes without saying, anything that can take planes further on a given amount of energy has exceptional relevance to decarbonization efforts. Battery-electric, hydrogen-electric, hydrogen-combustion, ammonia and other clean powertrain technologies are all constrained by lower range figures than traditional jet fuel power, and designs like these could definitely make a big contribution.

You can see some of the CFD (Computational Fluid Dynamics) and wind tunnel work that's gone into this design in the video below, including a rather gnarly looking flutter simulation that makes us feel like trains might be a pretty good option.

Transonic Truss-Braced Wing: Wind Tunnels Enabling the Next Gen of Transport Aircraft Technology

Sources: NASA, Boeing

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26 comments
26 comments
maross600
if the efficiency is true that’s great for sales, but damn that thing is butt ugly. the 737 looks 1000x better
Pression de Gonflage
Yes, but when will they unveil the Triplane ?
pmshah
Why does it look so similar to the very long range Russian bombers with almost identical wings concept?
michael_dowling
Sort of brings to mind aircraft of the 20s or 30s,when material strength did not allow unbraced wings. Maybe if carbon nanotubes get easier to manufacture..
martinwinlow
Interesting... I rather like the look of it... and find it actually more graceful than ordinary wing arrangement/design.

I wonder if the new wings would lend themselves to the concept of a 'structural battery' (ie here the wings *would be* the battery) as is already making headway in road vehicles (EVs, I mean)?
Joe123
Congratulations to Boeing on this one. This concept is so much better than the wide body, hydrogen power, or other pie-in-the-sky concepts. This one can actually work and is viable. If this is the path to modernization, then keep up the great work!
MarylandUSA
I woudn't be surprised if the braced design saves some weight, too. In the 1980s, I worked at Lockheed Aircraft in the factory that was building the C-5B, a giant military transport. The wingspan was so wide (223 feet, or 68 meters) that the plane had to use one center wing, two inner wings, and two outer wings. The bolts that secured each inner wing to the center wing were 4 inches wide. Their tolerances were so fine, the bolts were dipped in liquid nitrogen to shrink them a tiny bit just before the bolts were installed.
Of course, Antonov was able to miss an entire 120-foot (36.6 -meter) left wing (and right wing) out of a single aluminum billet.
Maboomba Maboomba
Your tax dollars at work. :0.... I'd recommend building smaller versions first to save money. There are many holes and unsolved major issues in this concept. One more "press-release breakthrough"...
David Ingram
This design might be the route to easier load/unloading for both passengers and cargo, small field use, and even a gravel runway option. Lots of possibilities. Go for it.
Edward Vix
Maross is right, it's not good-looking.
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