The integration of small-core turbofans into a subscale model of an air transport concept – the D8, developed by the Massachusetts Institute of Technology (MIT) – was evaluated in the Langley Research Center’s 14-by-22 Subsonic Wind Tunnel in December 2014. The D8 concept features a flat-bottomed fuselage, 17.3 feet across, that contains two pressurized cabin cylinders – earning the plane the nickname “Double Bubble.” An upswept nose shifts the center of lift forward, easing the burden on the tail section, and the engines are mounted not under the wings but atop the fuselage’s downward-sloping rear end. In this position, the engine inlets are designed to capture the boundary-layer flow sweeping over the fuselage – a slower airflow into the engine intake than the “clean” flow that enters wing-mounted engines. A slightly slower cruise speed than contemporary airliners enables a longer wingspan with a lower sweep, which can decrease drag and boost efficiency. Engine noise is reduced by twin vertical tail sections on either side of the engine assembly.
“Those wind tunnel tests were really focused on that boundary-layer ingestion aspect of the MIT configuration,” said Rich Wahls, a scientist with the Advanced Air Transportation Technologies Project, “to see if you could measure a benefit, if you could really expect to reduce fuel burns.” Two different configurations were tested, one with boundary-layer ingestion, and the other with the engines mounted to the side of the fuselage in a more conventional, clean-flow position.
The wind tunnel test revealed that MIT’s boundary-layer ingestion configuration required 7 to 8 percent less power to fly. “So that kind of proved,” said Wahls, “that for a commercial transport vehicle, you could get a fuel burn reduction here. There is still a lot more devil in the details.”
Another concept, developed through Boeing’s Subsonic Ultra Green Aircraft Research (SUGAR) program, was evaluated in January 2014, when researchers mounted a 15-percent-scale “semi-span” model – a model cut in half longitudinally – in Langley’s Transonic Dynamics Tunnel (TDT). The SUGAR concept features a long, narrow wing designed to reduce weight and drag.
Longer wingspans are enabled by the use of a strut or truss, and Boeing’s optimized truss-braced wing has a span of more than 173 feet, compared to 113 feet for the Boeing 737. The longer trussed span introduces two complicating factors: Struts are notorious for producing drag in themselves, especially at their attachment points; and as a wing lengthens and narrows, it becomes more prone to flutter or oscillation, an effect known to engineers as aeroelasticity.
NASA researchers are working with industry and university partners to develop ideas for future airplanes that dramatically reduce noise, emissions, and fuel consumption. One idea comes from a research team led by the Massachusetts Institute of Technology (MIT) and funded by a NASA grant. MIT and NASA engineers recently tested a 1/11th scale model version of the D8 in the 14 feet by 22 feet Subsonic Wind Tunnel at NASA’s Langley Research Center. The test was designed to produce data, now being analyzed, that can determine whether incorporating the engines into the fuselage of the airplane actually reduces drag. Pictured (l to r): Alejandra Uranga, research engineer and aeronautics MIT technical lead, and Mark Drela, professor of aeronautics and astronautics at MIT. NASA Langley/Kathy Barnstorff
Boeing engineers carefully designed a strong truss that they believe will minimize drag, but many questions remained about aeroelastics – and Langley’s TDT was designed specifically to understand and solve aeroelasticity issues.
The knowledge gained through such evaluations is incremental: By focusing on isolated elements of a design, Wahls said, NASA and aircraft manufacturers are gathering the data they’ll need to validate designs for the aircraft of the future. Much about Boeing SUGAR’s truss-braced concept – including the exact nature of the drag experienced around the truss joints – remains to be investigated. “But we’re chipping away at it,” said Wahls, “and still seeing promise.”
It shouldn’t surprise anyone if, 20 years from now, planes resembling Boeing’s SUGAR and MIT’s D8 are carrying passengers in the sky. But neither should it be surprising, pointed out Dryer, if the planes of the future look very different from these concepts. “Our work isn’t about enabling a very specific configuration,” he said. “It’s about understanding some of the underlying technologies that might help enable multiple configurations, and opening up the trade space that allows industry to work. These projects have really brought out a lot of creativity, both externally and internally. It’s really been a nice combination of NASA’s own research and our coordination and collaboration with the external community, of both industry and universities, to help get us there.”
This article first appeared in the NACA/NASA Celebrating a Century of Innovation, Exploration, and Discovery in Flight and Space 1915-2015 publication.