ORLANDO, Fla. - NASA is planning wind tunnel tests of low-noise hybrid wing/body configurations as it pushes to enable the introduction of a new generation of highly fuel-efficient large aircraft as early as 2020.

Boeing has been studying blended wing/body (BWB) aircraft for years, in the belief they could burn 20-30 percent less fuel than conventional tube-and-wing airliners because of the aerodynamic and structural efficiency of the flying-wing design (Aerospace DAILY, July 7).

But NASA is pushing for even greater savings under its Fundamental Aeronautics research program, and has set reducing fuel burn by 40 percent and noise to 42dB below Stage 4 as its goals for an "N+2-generation" aircraft entering service around 2020.

The research agency believes a hybrid wing/body (HWB) aircraft - its generic term for the BWB - is the only way to meet these goals, and is funding Boeing to study improvements to the configuration to further reduce noise and fuel burn.

"A cargo version of the HWB is now the focus of our subsonic N+2 research," says Faye Collier, principal investigator for NASA's subsonic fixed-wing program. A freighter could be available by 2020, he believes, with a passenger version following within 10 years.

Boeing, with funding from NASA and the U.S. Air Force, is to windtunnel-test two low-noise HWB configurations, called N2A and N2B, in 2011. N2A has podded engines mounted above the aft fuselage. N2B has embedded engines and S-duct inlets for lower drag. Both designs incorporate hybrid laminar flow control to further reduce drag.

Compared with Boeing's X-48B subscale BWB demonstrator, now flying at NASA Dryden, the N2A has more shielding of engine noise by the airframe. The twin engines are moved forward and, instead of winglets, the aircraft has vertical tails on either side of the engines to block the jet noise.

The N2B would have its engines embedded in the upper fuselage, so the serpentine inlets could ingest the turbulent boundary layer flowing over the airframe, reducing drag. This will require advances in inlet and engine design.

NASA and Boeing believe the podded-engine N2A has the potential to reduce noise to 42dB below Stage 4 and fuel burn by 25 percent below that for an advanced 777-configuration airliner, but there are aerodynamic issues to be overcome.

The low-drag embedded-engine N2B configuration would be needed to achieve NASA's 40 percent fuel-burn goal, but is higher risk and would not be available until some time after 2020, says Ron Kawai, BWB propulsion manager for Boeing Phantom Works.

"It looks like boundary-layer ingestion will slip to N+2 1/2 or N+3," Collier said, speaking at the American Institute of Aeronautics and Astronautics' aerosciences meeting in Orlando, Fla., last week. NASA's N+3 generation is targeted at around 2035.

X-48B photo: NASA