NASA believes flying-wing designs could deliver dramatic fuel savings and noise reductions in airliners entering service as early as 2020, but significant investment in large-scale demonstrations will be needed to realize that promise.

Boeing has been working on the blended wing/body (BWB) for years, and is flying the subscale unmanned X-48B with NASA (see pp. 52-53), but designers believe a large-scale structural test and manned flight demonstrator are needed to achieve an all-wing freighter in a decade or so, followed within 10 years by a passenger version. Europe is also researching BWBs (see article below), but sees challenges to their commercial acceptance.

NASA has not consulted the airlines to gauge their interest in the hybrid wing/body (HWB) - its generic term for the BWB. Instead its research is driven by internal studies that indicate the unconventional configuration is the only way to meet the aggressive environmental targets it has set for "N+2 generation" aircraft to be available around 2020.

NASA's generational goals, which are used to set its research priorities, are derived from objectives outlined in the U.S. national aeronautics R&D plan approved by President George W. Bush in 2007. For subsonic N+2 aircraft they include reducing noise to 42dB. below current Stage 4 limits, and fuel burn by 40% from today's GE90-powered Boeing 777. "A cargo version of the hybrid wing/body is now the focus of our N+2 research," says Faye Collier, principal investigator for NASA's subsonic fixed-wing program.

Boeing is confident that it has answers to the perennial questions about the BWB's passenger acceptance, evacuation safety and airport suitability, so NASA's limited aeronautics research budget is focused on unproven aspects of the configuration's aerodynamic performance, flying qualities and structural design. "Hybrid wing/body analysis is very challenging," says NASA engineer Craig Nickol. "It is such an integrated configuration and there is no full-scale data to support it."

While the X-48B is exploring the BWB's low-speed flying characteristics and flight control system, there is little real data to answer concerns about the configuration's transonic performance. More data is also needed on the cost and weight of building the design's unconventional airframe from composites. An advanced structural design is required and has only been demonstrated at small scale.

So far, the focus of research has been on the potential to reduce fuel burn because of the hybrid wing/body's aerodynamic and structural efficiency. Boeing's studies show the BWB could burn 20-30% less fuel than an equivalent 777-style tube-and-wing design because of the lower drag from its smaller surface area and lighter weight of its span-loaded structure.

Now the focus is shifting to low noise and NASA is funding Boeing to build and test aeroacoustic wind tunnel models of two HWB cargo aircraft concepts. Because of NASA's need for non-proprietary data, these are based not on Boeing's BWB design but on the SAX-40 concept developed jointly in 2006 by Cambridge University in the U.K. and Massachusetts Institute of Technology in the U.S. under the Silent Aircraft Initiative.

The initial N2A configuration has podded high-bypass engines mounted above the aft centerbody, and is aimed at achieving the N+2 target of a cumulative 42-dB. noise reduction with a fuel saving of at least 25%. Compared with Boeing's BWB-450 configuration, used as the basis for the X-48B, the N2A's vertical surfaces are moved inboard and the engines forward to increase airframe shielding of jet noise.

The more advanced N2B has embedded engines with serpentine inlets. Each engine has a core driving multiple fans. These reduce drag and weight by ingesting the boundary layer over the centerbody and eliminating the nacelle support structure, respectively. This configuration has the potential to achieve the sought 40% fuel-burn reduction, but with an in-service date beyond 2020 because the propulsion technology is not ready.