A truss-braced wing can be much thinner and narrower than usual:
By making the wings longer and narrower, and thus higher in aspect ratio, the wing tip vortices generated by the wing are weakened. This reduces drag on the plane, and quite significantly so.
For a glider, which has no forward propulsion of its own, minimizing drag is a must. Hence, high aspect ratio wings are very useful. Similarly, the high-flying U2 spy plane had excellent range because its high aspect ratio wings were very efficient. Both of these types of planes have fairly limited payload requirements, and are specialized enough that high aspect ratio wings can work as-is.
Airliners have other concerns that make high aspect ratio wings impractical. They must carry huge payloads in order to make lots of money per flight from paying passengers. It would be great to have high aspect ratio wings on airliners, as the efficiency would slash fuel bills significantly. Unfortunately, it’s difficult to make them strong enough for such heavy-duty purposes. There simply isn’t room for the structure and material required.
Instead, where a glider might have an AR of 30 or more, an airliner will feature wings with an AR closer to 7 to 10. High-end gliders achieve lift-to-drag ratios of over 50. Airliners do much poorer in this regard. The original Boeing 747 achieved a L/D ratio of 15.3, for example. Decades of development have seen today’s modern airliners push that number closer to 20, like the Airbus A380 and the Boeing 777.
Throw on a brace for support, though, and suddenly the high-aspect ratio concept becomes plausible.
The current concept in development is called the Boeing Transonic Truss-Braced Wing, or TTBW, with a full-scale demonstrator expected to fly in 2028. It’s also been designated as the X-66A within US aviation circles. In development since 2010, the concept involves a plane with an ultra-thin, high aspect ratio wing, supported by a truss underneath. The demonstrator is being built on a shortened airframe from a McDonnell Douglas MD-90, and will be tested at the NASA Armstrong Flight Research Center. The concept is intended to suit either 130-160 passengers, or 180-210 passengers, depending on the exact configuration Boeing lands on.
The demonstrator is on the scale of single-aisle aircraft, albeit with a far larger wingspan, at 51 meters. This is still far narrower than dual-aisle aircraft like the 747, at 68 meters, but much greater than a single-aisle Boeing 737 MAX at 36 meters. To accommodate this extra width in existing narrowbody facilities, Boeing may explore the use of folding wingtips. These have already been used successfully on the Boeing 777X, to ensure the wider-than-usual type could access as many airports as possible. Thus far, the concept is primarily being considered a proposition for smaller narrowbody, single-aisle airliners.
Wind tunnel tests have suggested that the higher-efficiency design could reduce fuel burn by up to 10%, based on the gains from the wings alone.
No space at airports for that.
If only there were some way to make a high-aspect-ratio lifting body.