In what could set the stage for a fundamental shift in commercial aviation, an MIT-led team has designed a green airplane that is estimated to use 70 percent less fuel than current planes while also reducing noise and emission of nitrogen oxides (NOx).
The design was one of two that the team, led by faculty from the Department of Aeronautics and Astronautics, presented to NASA last month as part of a $2.1 million research contract to develop environmental and performance concepts that will help guide the agency’s aeronautics research over the next 25 years. Known as “N+3” to denote three generations beyond today’s commercial transport fleet, the research program is aimed at identifying key technologies, such as advanced airframe configurations and propulsion systems, that will enable greener airplanes to take flight around 2035.
MIT was the only university to lead one of the six U.S. teams that won contracts from NASA in October 2008. Four teams — led by MIT, Boeing, GE Aviation and Northrop Grumman, respectively — studied concepts for subsonic (slower than the speed of sound) commercial planes, while teams led by Boeing and Lockheed-Martin studied concepts for supersonic (faster than the speed of sound) commercial aircraft. Led by AeroAstro faculty and students, including principal investigator Ed Greitzer, the H. Nelson Slater Professor of Aeronautics and Astronautics, the MIT team members include Aurora Flight Sciences Corporation and Pratt & Whitney.
Their objective was to develop concepts for, and evaluate the potential of, quieter subsonic commercial planes that would burn 70 percent less fuel and emit 75 percent less NOx than today’s commercial planes. NASA also wanted an aircraft that could take off from shorter runways. Designing an airplane that could meet NASA’s aggressive criteria while accounting for the changes in air travel in 2035 — when air traffic is expected to double — would require “a radical change,” according to Greitzer. Although automobiles have undergone extensive design changes over the last half-century, “aircraft silhouettes have basically remained the same over the past 50 years,” he said, describing the traditional, easily recognizable “tube-and-wing” structure of an aircraft’s wings and fuselage.
The MIT team met NASA’s challenge by developing two designs: the 180-passenger D “double bubble” series to replace the Boeing 737 class aircraft, currently used for domestic flights, and the 350 passenger H “hybrid wing body” series to replace the 777 class aircraft now used for international flights.
The engineers conceived of the D series by reconfiguring the tube-and-wing structure. Instead of using a single fuselage cylinder, they used two partial cylinders placed side by side to create a wider structure whose cross-section resembles two soap bubbles joined together. They also moved the engines from the usual wing-mounted locations to the rear of the fuselage. Unlike the engines on most transport aircraft that take in the high-speed, undisturbed air flow, the D-series engines take in slower moving air that is present in the wake of the fuselage. Known as the Boundary Layer Ingestion (BLI), this technique allows the engines to use less fuel for the same amount of thrust, although the design has several practical drawbacks, such as creating more engine stress.
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Photo: MIT/Aurora Flight Sciences
Caption: MIT’s D “double bubble” series design concept is based on a modified “tube-and-wing” structure that has a very wide fuselage to provide extra lift. The aircraft would be used for domestic flights to carry 180 passengers in a coach cabin roomier than that of a Boeing 737-800.