January 27, 2014
Tap the icon in the digital edition of AW&ST for a look at the history of the truss-braced wing, or go to AviationWeek.com/truss
Look at any sailplane and it is clear—long, thin wings make flying more efficient. But sailplanes are light and slow; for heavier, faster airliners, there is a limit beyond which conventional cantilevered wings, supported only at their roots, cannot be pushed without becoming too flexible to fly.
Boeing's 787 and Bombardier's CSeries take slenderness to new lengths in search of fuel savings. But meeting the efficiency and emissions requirements anticipated for the next generation of all-new airliners could push designers beyond the limits of conventional wing configurations.
A wing's slenderness is expressed as its aspect ratio: span squared divided by area. Most of today's jetliners have an aspect ratio around 9; thanks to their stiff carbon-fiber wings, the 787 and CSeries push this to around 11. NASA thinks up to 15 is possible for cantilevered wings using active control of flexible structures to suppress flutter—the aeroelastic coupling of aerodynamic loads with structural modes that can become unstable and cause catastrophic failure.
Slender wings are desirable because increasing aspect ratio reduces the lift-induced component of drag. Longer spans, and lower drag, are possible if the wing is supported by a strut or truss. This was done successfully in the 1950s by French manufacturer Hurel-Dubois, but at the expense of greater wing weight as well as drag from aerodynamic interference where the struts joined the wing.
In 2010, under its NASA-funded Subsonic Ultra Green Aircraft Research (Sugar) project, Boeing produced the design for a 737-size aircraft with a strut-braced wing. The high-aspect-ratio wing, combined with hybrid turbine/electric propulsion and other advanced technologies, was needed to meet NASA's so-called N+3 goal of reducing fuel burn by 60% for an airliner entering service in 2030-35.
The design looked promising, but the biggest uncertainty was in estimating wing weight, which had a large impact on the calculated performance. “The weight range of uncertainty in Phase 1 was really big, from better to worse than a conventional wing,” says Marty Bradley, Sugar principal investigator at Boeing Research & Technology. “The strut allowed us to increase span with less weight penalty, but there were a lot of uncertainties.” So for Phase 2, NASA funded Boeing to do detailed structural analyses and wind-tunnel testing to get a better idea of the wing weight.