To achieve sustainable growth in air travel, future airliner designers face challenges never seen by their predecessors. New concepts will not only have to meet unprecedented performance goals, but they must do so while striving for carbon neutrality.

NASA's goal to solve this conundrum takes on new significance in coming weeks as researchers across the U.S. begin a series of landmark tests under the next stage of the agency's subsonic fixed-wing program. Wide-ranging work will include refining a glider-like truss-braced wing and integrating it with a hybrid-electric propulsion system, wind tunnel tests of a multirole wing leading edge and evaluation of a protective outer skin that could enable lighter structures.

“Now it is getting exciting,” says NASA's subsonic fixed-wing program manager, Ruben Del Rosario. As preparations continue toward more extensive testing, the agency is also poised to review progress made so far by Boeing, Cessna, the Massachusetts Institute of Technology (MIT) and Northrop Grumman on the initial technologies under study for Phase 2 of the program's N+3 vehicles. These are a group of aircraft concepts three generations more advanced than today's airliners.

“Phase 2 is going strong, and we're making the progress we were anticipating,” says Del Rosario. Most of the contracts are coming up for the first yearly review following their start early in 2011, he says. Today's work builds on the development of N+3 concepts in Phase 1, completed in 2010, and aims to identify enabling technologies for airliners targeted to enter service in 2030-35. The Boeing-led team is also wrapping up a one-year N+4 study into more advanced technologies for 2040-45, with the final report due to NASA in April.

To help direct research, the program is divided into six main technical challenges to reduce drag, weight, energy consumption, emissions and noise, as well as the development of revolutionary tools and methods to bring it all together. In addition, strategic thrusts include the development of economically practical approaches to improving energy efficiency and environmental compatibility.

The array of novel N+3 vehicle concepts that emerged in 2010—ranging from the hybrid wing-body and truss-braced wings to double-bubble lifting body fuselages—generated seven key subsystem concepts which helped fine-tune research goals. These included special “tailored” fuselage structures, high-aspect-ratio “elastic” wings, new and quiet high-lift systems, high-efficiency but small engines, hybrid electric propulsion, airframe-propulsion integration, and alternative fuels.

Each subsystem concept, in turn, addresses multiple technical challenges. The advanced wing work, for instance, addresses both drag and weight. The efficient, small-engine study involves energy consumption, emissions and noise, while the airframe-propulsion integration incorporates drag, weight, fuel burn and noise reduction efforts.

To reduce drag, NASA is exploring methods for cutting fuselage skin friction by new surface treatments and flow control. The goal is to reduce fuselage turbulent boundary layer drag by 10%. High-aspect-ratio elastic wing studies, also aimed at the drag reduction challenge, include shaping to reduce interference drag of the external bracing identified in Boeing's Subsonic Ultra-Green Aircraft Research (Sugar) concept, as well as passive and active concepts to reduce wave drag.

Drag reduction work also includes control concepts for flight control of “elastic” aircraft, which can change the shape of the wing to lower cruise drag as part of the elastically shaped aircraft concept. Under this effort, NASA and Boeing are studying a variable-camber continuous trailing edge flap device for wing shaping control. The flap system, combining several movable sections connected by shape memory alloy rods, is aimed at reducing drag with minimal impact on weight.

Circulation control methods, in which high-pressure air is blown over wing and control surfaces to improve low-speed high-lift and transonic cruise, are also being studied under the FAST-MAC (fundamental aerodynamics subsonic transonic modular active control) program. Preliminary results from tests conducted at the NASA Langley National Transonic Facility, Va., showed the feasibility of pneumatic-based maneuver control and increased maximum lift coefficient at low speed by 40%.

Flow control systems are also being tested to enable high-performance, low-noise, high-lift leading- and trailing-edge slat and flap systems that will be lighter and simpler than current mechanisms. The feature could also be useful for enabling cruise-efficient short-takeoff-and-landing (Cestol) designs, which are Boeing 737-sized airliners that can operate from a short runway. To help build up aerodynamic and acoustic validation data for active flow control on a Cestol, NASA is testing a 10-ft.-span model in its 40 X 80-ft. wind tunnel at Ames Research Center, Calif., built by California Polytechnic State University, San Luis Obispo.

Other high-lift improvements are to be wind-tunnel tested by Northrop Grumman “any day now,” says Del Rosario. The multifunctional, or advanced high-lift leading edge, concept will be evaluated in low-speed tests at a Northrop transonic facility. Few details of the concept have emerged, though NASA says the design is aimed at producing a “smooth edge without the current standard slats.”

MIT and teammates Aurora Flight Sciences Corp. and Pratt & Whitney are continuing propulsion airframe integration studies of the D8 double-bubble concept in MIT's Wright Brothers Wind Tunnel. These studies are building toward the first of three planned campaigns in the Langley 14 X 22-ft. wind tunnel in early 2013. The engines are mounted on the tail so that they ingest the boundary layer over the fuselage, reducing drag.

However, the exact performance of such inlets and the fans buried behind them is unknown, and preparations are underway for wind tunnel tests of an integrated inlet and fan at the 8 X 6-ft. facility at NASA Glenn Research Center, Ohio. The experiment will use inlet and distortion-tolerant fan hardware designed and fabricated by United Technologies Research Center. “The hardware should be fabricated and delivered in the second quarter of fiscal 2013, and the test is planned for the third quarter of fiscal 2013,” says Del Rosario.

As part of plans to cut operating empty weight by up to 25%, Cessna will begin the first test of a scaled multifunctional fuselage skin and structure panel in March. The STAR-C2 (smoothing, thermal, absorbing, reflective, conductive, cosmetic) program is studying the potential weight benefits of segregating the composite primary structure and the external protective skin. In addition to enabling a lighter structure, researchers say the skin could be made smoother for laminar flow, would provide lightning-strike protection as well as acoustic and thermal insulation, and could be more easily produced and repaired.

Other weight-cutting efforts include work with Virginia Polytechnic Institute and State University (Virginia Tech) to develop design optimization tools which can tailor structural designs and combine them with engineered materials. The result would be stiffeners made from new alloys that—unlike present-day straight, uniform units—would be bent and curved to reflect the best shape for handling localized transverse, shear and in-plane loads. Similarly alloys would be tougher at the base of the stiffeners for better damage tolerance, and transition to metal matrix composites for increased stiffness and acoustic damping.

“The goal is to reduce the weight of the fuselage structure by 25%,” says Karen Taminger, lightweight airframe and propulsions systems technical lead. With the increasing trend toward the use of composite materials for primary structure, researchers are also looking at ways to tailor the direction and placement of fiber laminates. Instead of uniform layup of material with additional plies for local strengthening, as done today, the goal would be to “arrange the fibers to be aligned to the axis of the aircraft along the crown and keel, but aligned for shear loads around the fuselage sides,” says Taminger.

By steering the individual fiber tow, or bundle of continuous filaments, it is possible to make tighter radii curves and control distribution of the material to better suit the local load requirements. “We have the technology to do that but are looking for the design and analysis tools to help us tailor that, and to steer around the cutouts, windows and doors,” she adds.

A two-pronged effort is focused on reducing the weight of the fan in a turbofan by 15% while, at the same time, improving efficiency. Researchers are studying concepts for mission-adaptive fan blades made from polymer composites integrated with shape memory alloys. The aim is to get the blade to automatically alter its twist or camber to suit differing thrust needs. With the right shaping, researchers believe the blade could adapt to a coarse-pitch, low-noise configuration for takeoff and landing, and a fine-pitch, fuel-efficient shape for cruise.

Meanwhile, research is also underway into methods for designing and producing thin, hollow composite blades that are aeroelastically tailored to avoid flutter. The tasks “are not trivial,” says Taminger, who expects the two initiatives may eventually come together. NASA believes the technology for both areas could also be applicable for lighter fan containment cases.