Aerospace engineers are working on new designs that will dramatically change the future shape of aircraft. The goal is to create quieter commercial airplanes that burn less fuel and emit less nitrogen oxide (NOx) than today’s jetliners.

A team of engineers at the Massachusetts Institute of Technology (MIT) recently designed a green airplane that can burn 70 percent less fuel than current planes, while also reducing noise and emission of NOx. The design was one of two that the team, led by faculty from the Department of Aeronautics and Astronautics, presented to NASA 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, when air traffic is expected to double. The objective is to develop concepts for quieter subsonic commercial planes that would burn 70 percent less fuel and emit 75 percent less NOx than today’s aircraft. NASA also wants an aircraft that could take off from shorter runways.

Designing an airplane that could meet NASA’s aggressive criteria requires “a radical change,” claims Ed Greitzer, a professor of aeronautics and astronautics who served as principal investigator on the MIT project. 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 points out, describing the traditional “tube-and-wing” structure of an aircraft’s wings and fuselage.

Greitzer and his colleagues created two different designs: a 180-passenger D “double bubble” series to replace the Boeing 737 class aircraft currently used for domestic flights, and a 35- passenger H “hybrid wing body” series to replace the Boeing 777 class aircraft now used for international flights.

The engineers conceived 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. The cross-section resembles two soap bubbles joined together. The engines are positioned at the rear of the fuselage instead of the traditional wing-mounted locations.

“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,” explains Greitzer. 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.

“The design mitigates some of the drawbacks of the BLI technique by traveling about 10 percent slower than a 737,” adds Mark Drela, a fluid dynamics professor and lead designer of the D series. “To further reduce the drag and amount of fuel that the plane burns, the D series features longer, skinnier wings and a smaller tail.

“Independently, each tweak might not amount to much, but the little 5 percent changes add up to one big change,” adds Drela. “Although the plane would travel slightly slower than a 737, some of this time could be recovered because the plane’s wider size should allow for quicker loading and unloading.”

The MIT team designed a high-tech version with 70 percent fuel-burn reduction, and a version that could be built with conventional aluminum and current jet technology that would burn 50 percent less fuel. According to Drela, the less complex version might be more attractive as a lower risk, near-term alternative.

The H series aircraft utilizes much of the same technology as the D series, including BLI, but a larger design is needed for this plane to carry more passengers over longer distances. It features a triangular-shaped hybrid wing body that blends a wider fuselage with the wings for improved aerodyamics.

The large center body creates a forward lift that eliminates the need for a tail to balance the aircraft. It also allows engineers to explore different propulsion architectures for the plane, such as a distributed system of multiple smaller engines.