Hybrid-electric propulsion is the flavor of the month, and the FAA has picked the winners in a university competition to design a regional aircraft around the technology. Teams from Georgia Institute of Technology and the University of Virginia tied for first place with different approaches to the challenge.
Sustinere (Concept: University of Virginia)
UVa’s Sustinere design for a 50-seat jet eschews batteries in favor of a turboelectric distributed propulsion (TeDP) concept – two 2,500shp turboshaft engines under the wing generating electrical power to drive six 3,300lb-thrust fans arrayed in a duct that wraps around the upper fuselage.
Even with generous assumptions for improvements in specific energy and specific power by 2025, lithium-ion batteries would be too heavy, the team concluded, while the ability with TeDP to rapidly reroute generator power between fans would improve safety in case of an engine, motor or blade failure.
The underwing turboshafts drive superconducting generators that require cryogenic cooling. While other TeDP concepts use liquid hydrogen for fuel and cooling, the Sustinere burns jet fuel and carries hydrogen only for regenerative cooling, LH₂ circulating through the generators, motors and converters and a heat exchanger, with the small amount lost to evaporation being replaced between flights.
The unusual location for the 30in-diameter fans was chosen to ingest fuselage boundary layer and reduce drag, minimize asymmetric thrust and increase rudder effectiveness. The fans are mounted of a reinforced section of fuselage just aft of the pressure bulkhead for blade-out safety. The conjoined “letterbox” nacelle increases effective bypass ratio and outlet guide vanes improve fan efficiency.
The fans are driven directly and independently by hub motors at speeds that vary with flight phase to improve fuel economy. The UVa team calculates the Sustinere will burn 0.011gal per available seat mile, compared with 0.025 for an Embraer ERJ-145.
NXG-50 (Concept: GT Aircraft)
Georgia Tech’s NXG-50 concept, developed by GT Aircraft, a team of graduate and undergraduate students, combines batteries, turboelectric generators and electrically driven ducted fans (e-fans). There is no superconductivity. Mounted in the tail, the generators also recharge the 13,800lb of underfloor batteries and power all subsystems, eliminating hydraulics and engine bleed to save weight.
Based on the 50-seat Bombardier CRJ200, the design has two e-fans, each driven via a gearbox by a lightweight electric motor that can operate over a range of shaft speeds. Each e-fan requires 9,300shp at maximum power – 8,200shp from the turboelectric generator and 1,100shp from lithium-ion batteries. Fan noise is shielded by a U-shaped tail, and the NXG-50 is an estimated 13dB quieter than a CRJ.
The GT Aircraft team calculates hybrid-electric aircraft would have lifetime greenhouse-gas emissions 10-20% lower than the CRJ200, depending the sources of electrical power used to charge the batteries, and life-cycle energy consumption would be almost 15% lower. Reduced fuel burn and more reliable electric systems would reduce life-cycle costs by 4.2% over the regional jet, they calculate.
You can read the teams’ detailed presentations here for the Georgia Tech NXG-50 and here for the UVa Sustinere – and here for the runner-up, a concept from Arizona State University that replaces the turbine section of a turbofan with a magnetohydrodynamic power generator…