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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.

Phase 2 work included developing a detailed finite element model (FEM) of the structure and conducting aeroelastic testing in the Transonic Dynamics Tunnel (TDT) at NASA Langley Research Center. “Out of Phase 1, the big uncertainty was wing weight for the truss-braced wing. With higher fidelity in Phase 2, we hoped that would come down to the more favorable end of the uncertainty band, and it did. Plots now put wing weight near where we hoped it would be to show benefit from the configuration,” says Rich Wahls, NASA Fixed Wing project scientist. “We've done the analysis, and weight is down to the lower end of the band,” Bradley confirms.

Testing of the 15%-scale semi-span aeroelastic model began in the TDT late last year and is to be completed in February. The goal is to establish the flutter boundaries for the flexible wing and show that active control can suppress flutter and provide gust-load alleviation, allowing the weight penalty for a slender wing to be minimized and the drag reduction realized. “We went into the wind tunnel to verify our modeling, predict flutter boundaries and calculate if the wing is as light as we think,” says Bradley.

“Aeroelasticity was one of the concerns, so we wanted them to take the FEM and design tools and create a wind tunnel model, then predict with those tools how it would react in the tunnel so that we can get confidence in the full-scale FEM and weight estimate,” Wahls says. “Phase 2 analysis and testing will allow us to verify our structural prediction of the wing weight, and take the performance credit for going to higher span with less weight penalty,” Bradley says.

For Phase 2, Virginia Tech joined Boeing and Georgia Tech and its multidisciplinary optimization tool—developed over 15 years of on-and-off research on strut- and truss-braced wings—was used to refine the Sugar design. “Georgia Tech and Virginia Tech set up a design environment to optimize the wing. Boeing worked on the detailed design and interface with the FEM,” says Bradley.

The optimized truss-braced wing (TBW) has a span of more than 173 ft., compared with 113 ft. for the 737, and an aspect ratio of around 19. Where the Phase 1 Sugar design had a single strut supporting the wing on each side, the Phase 2 design has a truss with one major member and one jury member. “We did a lot of structural modeling, and the truss looks better, with less buckling,” Bradley says.

The wind-tunnel model, built by NextGen Aeronautics, is scaled for testing in heavy gas, R134A refrigerant, rather than air. Heavy gas provides a Reynolds number (a measure of aerodynamic scale) closer to full size, but the greater density also makes the model easier to build. “If we scaled it for air, it would be really light,” says Robert Scott, a senior aerospace engineer with the aeroelasticity branch at NASA Langley. “We can make a stiffer model for heavy gas, but it's still pretty flexible. This is far from a traditional metal model.”

The wing and strut are dynamically scaled to behave as similarly to full-size as possible, while the pylon and flow-through nacelle have the right mass and inertia. “We are interested in the wing and strut behavior; the fuselage is effectively rigid,” says Scott. The model is instrumented with strain gauges and accelerometers on the wing and strut. Two high-bandwidth ailerons provide active flutter suppression and gust-load alleviation.

Testing is also unusual. Instead of a detailed test matrix, the first phase involved exploring the flutter boundaries of the wing “open loop,” without active control. This required a careful eye on the model and a quick hand on the kill switch to shut down the tunnel before flutter onset could cause structural failure. “The model is very flexible, with a lot of dynamic responses. We have to monitor it very carefully and find where the instabilities are,” Scott says.

Tests were conducted from below the Sugar's Mach 0.7 design cruise speed to above Mach 0.9. “We tested for flutter outside the normal operating regime,” says Scott. The data are being used to develop flight-control laws for flutter suppression and load alleviation. Closed-loop testing is planned for February. The model will fly through gusts generated by an airstream oscillation system in the TDT.

Because of the strut, the wing does not respond linearly. “Stiffness properties depend on how the wing is loaded,” says Scott. “We change the angle of attack to put an upload or download on the wing. That puts the strut in tension or compression. We are seeing pretty meaningful differences in the flutter boundary as a function of angle of attack,” he says.

Another area of uncertainty is the lift-to-drag ratio of a wing with a truss, because of interference drag at the junctions. ”In Phase 2, they are doing some computational fluid dynamics work with a bit more fidelity,” says Wahls. “It's analogous to a pylon, but a bit more complicated. The strut has its own flow separation, which is hard to predict.” The next step, if NASA can find the money, would be traditional high-speed and low-speed wind-tunnel testing to validate cruise and high-lift performance.

Boeing will wrap up Phase 2 this year by updating the technology road maps and research recommendations that came out of N+3 Phase 1 in 2010. “We will recommend more wind-tunnel testing, and more wing design work,” says Bradley.

Phase 2 also includes further study of the hybrid propulsion technology in Boeing's Sugar Volt concept (see article below). While the recommendation there is to wait for battery technology to improve, “we know how to do a truss-braced wing, and it does not involve waiting for technology,” he says. “Even with existing engines we can get a 5-10-percent improvement in fuel burn with TBW if we can make it work.”

NASA wants to look at pushing speed up to the Mach 0.8 range, says Wahls. Virginia Tech has studied a Boeing 777-like, Mach 0.85-cruise TBW design. “We did not encounter any show-stoppers at higher speed, and showed substantial benefit,” says Joseph Schetz, professor of aerospace engineering. He sees the Sugar work as a validation of the university's TBW research. “To say folks were skeptical is an understatement, but the numbers keep working,” he says.

One advantage of the truss-braced wing is that it scales to different sizes, which is harder with other fuel-saving configurations proposed for the N+3 time frame, such as the blended wing body. “N+3 is aimed at entry into service in 2030-35; that is not at all hard for the TBW,” says Bradley.