Commercial aircraft designs that depart from today's tube-and-wing shape demand equally innovative propulsion systems if they are to stand any chance of breaking the mold and providing the benefits anticipated by their creators.
With its engines flush-mounted beneath a pi-shaped tail, the Massachusetts Institute of Technology's (MIT) D8 concept with its “double-bubble” lifting fuselage is no exception. Sized to replace aircraft in the Airbus A320/Boeing 737 category around 2035—a time frame NASA calls “N+3 generation”—the D8 promises fuel burn more than 70% lower than the 737-800's. And while the wide, twin-aisle fuselage has more drag than the 737's, its overall shape enables a lighter wing and landing gear and a smaller tail.
The make-or-break aspect of the D8 design, however, is whether the embedded engine location is feasible, and whether smaller, lighter turbofans will be able to operate in the challenging flow conditions over the aft fuselage. To answer these questions, research is underway at NASA, MIT, Aurora Flight Sciences, Pratt & Whitney and the United Technologies Research Center (UTRC) into how closely the airframe and propulsion system can be integrated and what fuel benefits are possible from ingesting the fuselage boundary layer.
MIT has just completed four weeks of tests of the D8 in a 14 X 22-ft. wind tunnel at NASA Langley Research Center, Va., aimed a quantifying the benefit of boundary-layer ingestion (BLI) through back-to-back comparison of the same 1:11-scale, 13.4 ft.-span model with embedded and conventional podded engines. Initial results show a benefit close to that predicted, with a measured 5-8% reduction in the electrical power required to drive the 6-in.-dia. fans in the embedded engines at the same cruise condition, says Alejandra Uranga, technology lead for MIT.
BLI is not new a new idea; it is already being used in torpedo and ship propeller design. When a podded propulsor is in freestream airflow, like a turbofan in a wing-mounted nacelle, the excess kinetic energy in the jet is wasted. But when the propeller is immersed in the slower-moving boundary-layer flow there is no excess kinetic energy, and less energy needs to be added to achieve the same thrust. The benefit comes from the propulsor reenergizing the wake and reducing drag.
The question regarding the D8 is whether the same principle can be applied to an aircraft-engine fan operating under higher propulsive loads, and whether that fan can withstand such a turbulent environment without its efficiency being overly compromised.
“The primary question is, 'Can you get a fan that can operate in that distortion?' Secondly, by ingesting the boundary layer, are we reducing drag, as most people believe we are? It is drag versus efficiency and whether the summation of the two is still positive,” says NASA Fixed Wing project head Ruben del Rosario
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