As airlines step up demands for more efficient aircraft, NASA is rekindling research into technology to reduce noise, emissions and fuel burn, including ultra-high bypass engines and hybrid laminar flow control - both investigated extensively during the 1990s.
Following the complete restructuring of NASA's aeronautics research, work is beginning under the agency's subsonic fixed-wing project, the largest of four "fundamental aeronautics" research areas that include rotorcraft, supersonics and hypersonics.
"The subsonic focus is on reducing noise, emission and fuel burn by improving not only the engine, but also the airframe," says Dennis Huff, project scientist for the subsonic fixed-wing project. NASA has set two targets: "N+1" for the next generation of narrowbody in 2014-15, and "N+2" for hybrid wing/body aircraft in 2018-20.
"N is the current capability of the [Airbus] A380 and [Boeing] 787. N+1 is the next generation and is focused primarily on small twinjets to replace the A320 and 737," says Huff. N+1 goals include a 42EPNdB cumulative noise reduction relative to Stage3 and a 70% reduction in nitrogen oxides emissions relative to CAEP 2 limits.
"We were looking for a 15% fuel burn reduction initially, but more recent studies indicate we can go beyond that," says Huff. "UHB [ultra-high bypass] gives a significant noise and fuel burn reduction. Add hybrid laminar flow on the wing and we can increase the fuel burn reduction. The technology is developed and waiting for application."
UHB encompasses bypass ratios in the 13-16 range, he says, compared to 8-10 for engines such as General Electric's GEnx, and includes concepts such as Pratt & Whitney's Geared Turbofan, which the manufacturer plans to begin demonstrating later this year.
NASA Glenn Research Center in Ohio has issued a request for information (RFI) intended to lead full-scale engine tests to validate UHB component technologies including a variable-area bypass-duct nozzle and low tip-speed fan, as well as methods of measuring particulate and gaseous emissions.
"Part of the RFI is to see what is out there," says Huff. Ground tests to confirm the benefits predicted using subscale models and design tools are planned for next year. "We need to do system-level validation tests in full-scale engines by 2008 if we are to meet the N+1 timeframe."
Huff says NASA is also looking at open-rotor engines for the N+1 aircraft because of the emphasis on reducing carbon dioxide emissions. The agency flight-tested open-rotor concepts in the 1980s. "There is an increase in studies, but we are not committed to going back," he says. "It is very difficult to get the noise level. NASA's old reports projected Stage 3 for an open rotor."
The N+2 goals "look beyond small aircraft to 2018-20 and the opportunities in hybrid wing-body configurations", says Huff. These include powered-lift short take-off and landing concepts and blended wing-body aircraft such as Boeing's BWB. "N+2 involves noise shielding with engines above or in the wing and higher-efficiency configurations."
N+2 goals are still being analysed, but will be in the order of a 52EPNdB reduction in noise, 80% in NOx and 25% in fuel burn, says Huff, adding: "CO2 reduction goes hand in hand with fuel burn."
Work towards the N+2 goals includes NASA's partnership with Boeing on the X-48B experimental BWB, "which should fly any day now" at NASA Dryden Flight Research Center in California, says Huff. The X-48B is a subscale unmanned demonstrator to test the low-speed flying characteristics of the flying-wing design.