To that end, NASA worked with MIT to put pressure rakes at the fan face on the D8 model in the 14 X 22-ft. wind tunnel. “At cruise, inflow conditions looked pretty much as expected, but not so at high angle of attack and off-design,” says Greitzer. Work with UTRC on a distortion-tolerant fan will lead to tests of the fan in an 8 X 6-ft. wind tunnel in 2015. “We are having preliminary design reviews before we go into production of the fan itself,” says del Rosario.
Michael Hathaway, technical lead for propulsion within NASA's Fixed Wing project, says UTRC “went through a vehicle systems study with different configurations and numbers of propulsors and fans ingesting the boundary layer. They optimized around five designs that showed the best potential for BLI benefit and fuel-burn reduction.” The final candidate will be scale tested on a 22-in.-dia. fan rig at NASA Glenn, previously used in early development of the General Electric GE90 and Pratt & Whitney PW1000G.
“We are trying to keep the efficiency loss below 2%, and our current aerodynamic analysis is telling us we can stay within 0.5% with the design we have. That would be amazing,” says del Rosario. The initial rig-testing will include a “false floor” to simulate the airframe in front of the inlet (see diagram). “It cannot be tilted that much [for angle-of-attack simulation], but by raising different floors we can simulate different shapes of boundary layer,” says Hathaway, who adds that the rig is able to test effectively for cruise efficiency. “We have different options for doing a lot of cross-flow or angle-of-attack tests,” he says.
As well as overall efficiency, another focus is on the aero-mechanical impact of “the fan [blades] being buffeted in and out of the clean and distorted flow,” Hathaway says. “Forward sweep might be less, but it will be operating in a lower Mach-number flow, so it won't look radically different from a conventional fan. We may also alleviate some of the distortion of the flow by modifying or tailoring the inlet and stationary structure. That's what we hope to get out of the tests at UTRC.”
MIT and NASA, meanwhile, plan three separate wind-tunnel campaigns for the D8, with progressively larger and more sophisticated models. These will culminate with the third model in 18 months. “About that time, we will be finishing the fan data, so we will have both external aerodynamic and internal fan data,” explains Del Rosario. The airframe and propulsion system will be integrated to see if the potential benefit of BLI is still there, he adds. Overall, this could result in an 8-10% fuel-burn reduction.
For the next tunnel entry, early next year, the 1:11 model will be fitted not only with a refined tail, but also custom-designed fans for the embedded propulsors. The first tests used off-the-shelf model-aircraft electric ducted fans. “They were the best we could find, but it's a question how efficient they were,” says Uranga. “The comparison of two fans will also help assess the role of fan design in this non-uniform flow. We will also use higher tunnel speeds for greater measurement accuracy,” Greitzer says.
Tunnel speed will be increased to 100 mph from 70 mph for the first tests. Low-speed tests are adequate for assessing BLI because “boundary-layer evolution is very weakly affected by Mach number,” says Drela. “And high-speed wind-tunnel tests cost a lot more money.” Greitzer praises the high flow quality in the 14 X 22-ft. tunnel and the “exemplary” force-and-moment balance provided by NASA, which contributed to “exceptional repeatability and confidence in the measured data,” he notes.
Beyond the BLI aspect, the NASA/MIT/Pratt team is also investigating advances in engine design that would be required to gain the most benefit from the D8 configuration. The substantial drag reduction promised by the D8 means significantly less thrust would be required, calling for less powerful engines than on today's single-aisle airliners. “The core of those engines is going to be extremely small because you are going to have very high bypass,” notes del Rosario. “Gas generators will have to get smaller, so to get the energy to drive the fan, you have to raise the overall pressure ratio, which raises the temperature aft of the compressor,” adds Hathaway.
As a result, researchers are looking to identify the efficiency limits of small compressors and whether they scale with size. “If the aft end of the high-pressure compressor is running at temperatures over 1,500F, can the materials still handle that?” Hathaway asks.