Flexible, Slender, Active Wings Lie Ahead For Airliners
Wings will get longer, lighter and more flexible, with passive tailoring and active control
July 1, 2016
The distinctive upward curve of the Boeing 787's wing is the shape of the future. The graceful lines are a consequence of the wing's more flexible carbon-fiber structure for light weight and long, slender planform for low drag. These are characteristics that will become more pronounced with the next generation of wings as designers push for even higher levels of performance.
The 787 wing has a higher aspect ratio (AR)—the relation of span to chord—than its predecessor, the 767: 11 versus 8. A more slender wing is more aerodynamically efficient. Wings now on the drawing board have even higher aspect ratios for lower drag. But the 787 wing's curved profile is a result of its flexibility, and future high-AR wings will be lighter and more flexible, requiring new approaches to manufacturing and to alleviating loads and suppressing flutter in flight.
Under its High Aspect Ratio Wing project, NASA is investigating both the passive and active aeroelastic tailoring of future wings by optimizing structural stiffness and the scheduling of control surfaces. The work began with system studies of so-called N+3 aircraft for entry into service in 2030-35, for which different companies proposed different configurations.
"We distilled these down to their common elements and saw aspect ratio was increasing," says Karen Taminger, technical lead for high-AR wings within NASA's Advanced Air Transport Technology project. "Wings were longer and thinner for improved aerodynamics and laminar flow. They were also more flexible, which was an unintended outgrowth of trying to keep weight down."
But designers exploited the flexibility with real-time wing shaping, using the flight controls to change camber, bending and twist. "There is a tension between structural efficiency and aerodynamic efficiency, so we are looking at optimizing high-AR wings for drag and weight at the same time," she says.
One technique is passive aeroelastic tailoring. "A wing will naturally turn up and out under load. Tailoring is biasing the wing through material selection and structural design so that it will naturally turn in to compensate for the load," says Taminger.
Aeroelastic tailoring was used in the past to prevent the Grumman X-29's forward-swept wing from diverging and to enable NASA’s X-53, a modified Boeing F-18, to roll by twisting its flexible composite wing. Tailoring has not been used in a commercial aircraft, but NASA studies show potential for a 5-10% weight saving.
For the past 18 months, NASA has been working with Aurora Flight Sciences, Boeing and the University of Minnesota to demonstrate technologies for flexible, high-AR wings. As a reference, these studies are using NASA’s open Common Research Model (CRM), which is similar to a Boeing 777 but with the aspect ratio increased to 13.5 from 9.
Aurora is looking at two approaches to a passive aeroelastic tailored wing. The first is to use tow-steered composites in which, instead of laying plies at standard angles of 0, 90 and ±45 deg., the automated fiber-placement machine steers the carbon-fiber tows along paths that provide directional stiffness to bias the wing’s response under load. The goal is to provide passive load alleviation and improve aeroelastic stability to design out flutter.
Under the project, Aurora will build a 39-ft.-long composite wingbox with tow-steered skins, at 27% scale to the 13.5-AR CRM wing. Delivery is scheduled for March 2017, for static and ground-vibration testing at NASA Armstrong Flight Research Center. Initial optimization studies show a roughly 13% reduction in wingbox mass for tow-steered versus the baseline, says Taminger.
Under the second part of Aurora’s contract, Georgia Tech will demonstrate a 3-D-printed “topology optimized” wingbox. This technique gets rid of the traditional spars and ribs and places material only where it is needed to carry loads, and it is enabled by additive manufacturing.
The outer mold line is unchanged, but the interior of the wingbox could look like the inside of a bird bone, Taminger says. “Topology optimization takes all the loading conditions, cuts the wing into cubes, decides in each if material is needed or not, and starts hollowing out the wingbox. You need a supercomputer to do the optimization at the scale of a whole wing,” she says.
“The expectation is that instead of seeing ribs aligned with the airflow and spars aligned with the span you will see things that are more diagonal, more organic-looking internal truss-like structures that connect the upper and lower skins.” Because of the limited size of 3-D printers, Georgia Tech will build a 12-ft.-span wingbox in fused-deposition-modeling (FDM) plastic, for static and modal-response testing.
Boeing, meanwhile, is working toward wind-tunnel tests of an active aeroelastic wing concept under the
NASA-funded Integrated Adaptive Wing Technology Maturation (IAWTM) project. This is evaluating control-law algorithms for maneuver- and gust-load alleviation, flutter suppression and fuel-burn minimization, as well as novel control-effector concepts.
The IAWTM wing has three ailerons and three flaps—two outboard and one inboard in each case—plus 11 separately actuated “mini plain flaps” on the trailing edge of these surfaces. “In addition to higher aspect ratios and more flexible, lighter-weight wings, we are seeing significantly more control surfaces,” says Taminger.
“There ends up being a weight-reduction value both in the structures— the forces on each control surface are smaller, requiring less structure to react to those forces—as well as lighter actuators, and a control benefit that allows us to tailor the wing shape throughout the flight envelope for maximum aerodynamic efficiency and reduced drag,” she says.
Boeing looked at both its Variable- Camber Continuous Trailing Edge Flap (VCCTEF) concept and a rotating raked tip before selecting the IAWTM concept, Taminger says. VCCTEF has 42 flap sections, each commanded individually to change wing camber as aircraft weight and cruise conditions change. Fitting all the actuators into a 10% semi-span model was impractical, so the simpler IAWTM was selected for testing in the Transonic Dynamics Tunnel at NASA Langley Research Center in 2018.
NASA’s third effort is the five-year Performance Adaptive Aeroelastic Wing (PAAW) project with the University of Minnesota to develop a flexible wing for flight testing on the X-56. The surviving X-56, of two built by Lockheed Martin Skunk Works for the Air Force Research laboratory (AFRL) to demonstrate active flutter suppression, is now at NASA Armstrong, where it has completed initial carbon-fiber “stiff-wing” tests.
The stiff-wing flights demonstrated NASA’s open controller, developed to replace Lockheed’s proprietary system so that data can be shared. The glass-fiber “flex” wing, the third of three produced for AFRL, is now undergoing ground vibration testing while NASA completes development of its flex-wing controller.
NASA has integrated its fiber-optic sensing system (FOSS) on to the wing to measure its shape in flight. “Initially, the flex wing will use model-based control, with FOSS for monitoring and data collection. After that we will integrate fiber-optic feedback into the controller for structural mode suppression based on real-time shape awareness,” she says.
When the aircraft will return to flight depends on completion of the investigation into the November 2015 accident when, on its first attempt at a flex-wing flight, the first X-56 pitched up and crashed on takeoff. While the accident involved an aircraft flying for AFRL, NASA had its own incident when a coupling between the aircraft and its landing gear resulted in a “pogo” characteristic and hard landing that ended the stiff-wing flights. The gear is being redesigned and NASA plans to use a conventional controller for takeoff and landing, switching to the higher-speed active structural mode controller once up and away.
The University of Minnesota, meanwhile, continues to work toward rewinging the X-56 with the PAAW in fiscal 2018. The university has already built a series of smaller low-speed flying models similar to the X-56 to demonstrate flutter suppression, with flight tests planned for this year.
This control work will enable flexible wings to be designed to a lower flutter-speed boundary to save weight. “It would still not be inside the flight envelope, but instead of today’s 15% margin we could design to dive speed and use the flight controls to regain flutter margin,” says Taminger. “We could take weight out and use the controls to redistribute loads so we don’t get overstress or instability.”