Researchers work to uncover turboelectric propulsion’s unknown challenges
The addition of the BLI propulsor that drives the STARC-ABL from the aft results in an aircraft that needs less thrust to fly. The downsized wing-mounted engines supply 80 percent of the thrust required during takeoff, but at cruise, the conventional turbofans produce 55 percent of total thrust, while the tail-mounted, all-electric BLI turbofan can account for the remaining thrust. (image source: NASA)
 

Researchers work to uncover turboelectric propulsion’s unknown challenges

Turboelectric aircraft propulsion and boundary layer ingesting propulsors may be a breakthrough technology for more-efficient air travel. Researchers are currently uncovering unforeseen hurdles to deploying the technology, but that’s not a bad thing. It’s just part of development.
The NASA STARC-ABL – or “single-aisle turboelectric aircraft with an aft boundary-layer propulsor” – concept is under consideration as one of the agency’s future X-planes. For the most part, its design is similar to the proven tube-and-wing aircraft seen every day. However, unlike conventional airliners, STARC-ABL used turbo-electric propulsion and routes approximately three megawatts of electrical power from its two underwing podded turbofan engines to generators that run a boundary layer ingestion (BLI) propulsor partially nested into the tail cone of the fuselage.

By embedding a ducted fan into the fuselage, the BLI propulsor takes in the slow-moving boundary layer air that develops along aircraft surfaces during flight. The electrically driven propulsor then accelerates that slow boundary layer air to produce efficient thrust.
 

Read more: NASA BLI propulsor may leapfrog current high-efficiency designs


The benefit of this type of turboelectric propulsion is that the aircraft produces less wake and drag relative to a conventional aircraft that creates wasted energy through excess velocity and drag from jet exhaust.

The addition of the BLI propulsor that drives the STARC-ABL from the aft results in an aircraft that needs less thrust to fly. The downsized wing-mounted engines supply 80 percent of the thrust required during takeoff, but at cruise, the conventional turbofans produce 55 percent of total thrust, while the tail-mounted, all-electric BLI turbofan can account for the remaining thrust. NASA researchers predict a potential fuel consumption improvement of roughly 10 percent using this turboelectric arrangement and additional benefits in emissions and noise level reductions.



While NASA is preparing for initial ground tests of a subscale STARC-ABL concept later this fall at NASA’s Electric Aircraft Testbed (NEAT) at Plum Brook Station in Sandusky, Ohio, several vehicle-level development challenges remain: how to balance aerodynamic efficiency, appropriately optimize the engines and aft BLI fan, validate the BLI benefits, store energy, compensate for additional weight, and meet safety and operational requirements.

To further investigate the challenges surrounding the hybridization of commercial aircraft, NASA is working with industry and academic experts to develop preliminary single-aisle, 150-seat aircraft designs using promising electric-enhanced propulsion and vehicle configuration concepts.

The two teams currently working on designs include a Boeing and Georgia Institute of Technology partnership and a group from Rolls-Royce plc’s LibertyWorks division and Empirical Systems Aerospace.

“During the 12-month cycle, we’ll work with the teams to take a deep dive into their hybrid and turboelectric aircraft concepts,” says Amy Jankovsky, NASA’s Advanced Air Transport Technology subproject manager. “These concepts will provide in-depth, detailed analyses of the propulsion and electrical systems, and we will recommend technology development paths for their concepts.”
 

Read more: SAE International on aircraft engine technologies


The final reports from the year-long developmental study will arrive near the end of 2018 and provide insight on new approaches and unforeseen technological hurdles. One of those hurdles, discussed at the 2018 EnergyTech conference in Cleveland, Ohio, looked forward, past the STARC-ABL concept, to the challenge of managing the BLI while a turboelectric aircraft during non-cruising phases of flight.

“How do we know if we’re actually driving that [BLI] fan correctly, whether its pitch control or speed control? With STARC-ABL, we can monitor the fuel on both of the wing mounted engines and when those reach a minimum, we know we’re ingesting the boundary layer efficiently. But what happens when we put a BLI propulsor on the N-3X or a fully-distributed propulsion system?” said Dennis Culley, technical lead for Distributed Engine Control for the NASA Subsonic Fixed Wing Project.



The N3-X concept uses a number of superconducting electric motors to drive distributed fans to lower the fuel burn, emissions, and noise. The power to drive these electric fans is generated by two wing-tip mounted gas-turbine-driven superconducting electric generators. (Image source: NASA)


“When it goes into a turn, or any kind of maneuver, the way the fans need to operate is going to change. You can’t just treat the case of straight and level flight. If you’re then going to be able to achieve that same capability for ingesting the boundary layer, you also need to manage the control of all of those motors to achieve that,” continued Culley.

More technical issues will likely be uncovered; however, many researchers feel that this technology is only a few steps away from causing a major aviation revolution, and that a commercial aircraft using NASA-developed, hybrid-electric or turboelectric propulsion technology could be flying as soon as 2030.

And while those proposed industry concepts or future aircraft could look like STARC-ABL, the ultimate objective is to transform commercial aviation by using new propulsion technologies that meet NASA’s aircraft-level requirements of energy use, life-cycle carbon, landing-and-takeoff emissions and noise.


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William Kucinski is content editor at SAE International, Aerospace Products Group in Warrendale, Pa. Previously, he worked as a writer at the NASA Safety Center in Cleveland, Ohio and was responsible for writing the agency’s System Failure Case Studies. His interests include literally anything that has to do with space, past and present military aircraft, and propulsion technology.

Contact him regarding any article or collaboration ideas by e-mail at william.kucinski@sae.org.
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