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  • Sparking Advances in Electric Aircraft
    Posted by Graham Warwick 4:03 PM on Oct 26, 2011

    It takes a leap of faith to project forward from a modified motor glider able to exceed the equivalent of 400 passenger-miles per gallon to a future electric-powered airliner, but that looks to be the way technology is headed.

    And not just in propulsion - as Aviation Week & Space Technology's Imagining the Future special shows, advances in areas from materials to photonics and plasma aerodynamics are paving the way for ever-lighter, more-efficient and potentially all-electric aircraft.

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    Pipistrel Taurus G4 at GFC (Photo: NASA)

    Earlier this month, NASA's Green Flight Challenge (GFC) showcased the potential of electric propulsion when Pipistrel Aircraft's dual-fuselage Taurus G4 (above) won the $1.35 million prize by achieving an equivalent passenger-mpg (pmpg) of 403.5 while averaging 107.5mph over a distance on almost 200 miles.

    Stuttgart University's electric-powered e-Genius (below) came second with an equivalent pmpg of 375.7. That is a staggering vehicle-level equivalent mpg of 188 - which compares with 101 mpg for the Taurus G4, but because it had four seats to the e-Genius' two, Pipistrel won on passenger-mpg. The e-Genius used less energy, but the Taurus did more work, explains NASA Langley engineer Mark Moore.

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    Stuttgart University e-Genius (Photo: NASA)

    The scale of their achievement is illustrated by the fact that the third Green Flight competitor, Phoenix Air's conventionally fueled, piston-powered Pheonix motor glider, achieved a vehicle mpg of 47 - or a passenger-mpg of 94.3 for the two-seat aircraft (full results here).

    Green Flight showed what can be achieved using available battery and motor technology and optimizing the aircraft for efficiency. According to Moore, the Taurus G4 had an empty-weight fraction of only 39%, leaving a 61% useful load for batteries and payload. "The spanloading multi-body approach they used definitely provided an incredible structural efficiency," he says.

    "They achieved a 96% motor and controller efficiency at take-off, and 93% at cruise. The propeller had an installed efficiency of 85% at cruise," Moore says. "Jack Langelaan [Penn State professor and team lead] did a great job of optimizing the trajectory for in-flight winds aloft to fly the course as efficiently as possible, and the pilots executed the 28 GPS waypoints flawlessly."

    While Slovenia-based Pipistrel is applying the lessons learned (and the same 140kW electric motor) to its four-seat Panthera, planned to be in production within two years, application of electric propulsion to larger aircraft is still a long way off. But Moore points to the rapid advances being made in the energy densities of batteries and the power-to-weight ratios and efficiencies of electric motors.

    He also cites advances in microturbine alternators as a compact and powerful means of generating electricity from liquid fuel, and a potential alternative to batteries in aircraft. Moore highlights DARPA-supported work by Metis Design and ThinGap on a turbo-alternator (below) that can produce 30-110kW with a fuel consumption of 1lb/hr/kW and a power-to-weight ratio of 1.7kW/lb. This is six times the specific power, or 12-15 times the energy density, of lithium polymer batteries.

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    Graphic: Metis/ThinGap

    So potential pathways to all-electric aircraft are becoming clearer, and one of those is distributed propulsion - but that's another blog....

    Tags: awt, awimagine, propulsion

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