Fuel efficiency—and the economic and ecological benefits associated with it—continues to be the white rabbit of the global aviation industry. While engine builders look toward composites and electrification, and airframe designers toward lightweighting and aerodynamics, engineers at NASA’s Glenn Research Center (GRC) in Cleveland, OH, recently completed testing of a novel concept: the boundary layer ingesting (BLI) propulsor.

The BLI propulsor comprises a fan and inlet partially nested into the airframe—a departure from conventional subsonic fixed-wing aircraft arrangement where podded engines are positioned away from the fuselage. By embedding the inlet and fan, the BLI propulsor ingests the slow-moving boundary layer air that develops along aircraft surfaces during flight, the opposite goal of conventional engine placement.

The design is a cooperative effort between NASA and United Technologies Research Center, with research support from Virginia Polytechnic and State University.

The approach comes with challenges, as turbulent boundary layer air flow is distorted and affects fan performance and operation. A high-performance, distortion-tolerant fan capable of accelerating slow-moving boundary air needed to be developed.

“A key part of this research and testing was understanding the aeromechanics of how these fan blades react to a distorted flow and how to maintain their effective service lifespan,” said Jim Heidmann, manager of NASA’s Advanced Air Transport Technologies project.

Although the operational environment for the BLI propulsor is demanding, NASA engineers believe that the BLI propulsor is capable of achieving a 4-8% efficiency increase over current high-efficiency engines, such as the CFM International LEAP engine.

“Studies backed by more detailed analyses have shown that [BLI] propulsors have the potential to significantly improve aircraft fuel efficiency,” said David Arend, a BLI propulsion expert at NASA Glenn. “If this new design and its enabling technologies can be made to work, the BLI propulsor will produce the required thrust with less propulsive power input.”

The elimination of wake, drag, and weight of wing or pylon mounted engine nacelles can contribute to additional aircraft efficiency.

The BLI testing, which completed on December 9, was the first of its kind and required modifications to the NASA GRC 8’ x 6’ wind tunnel to accommodate and power the large propulsor model and boundary layer control system. NASA engineers varied wind speed, boundary layer thickness, and fan operation and monitored propulsor performance, operability, and structure. The experiment covered all phases of the flight envelope and simulated a wide range of operations including takeoff, max load, cruise, and descent.

Once all experiment data has been analyzed, the BLI propulsor fan and inlet arrangement will achieve technology readiness level (TRL) 4 status and the fully BLI propulsion-incorporated aircraft system will achieve TRL 3 status.

Planned applications for the BLI propulsor include “N+3” designs such as Aurora Flight Sciences’ D8 “Double Bubble” and NASA’s turboelectric STARC-ABL, both slated for 2030/2035.

According to Heidmann, a BLI demonstrator aircraft (or “X-plane”) may be possible within the next five to ten years.