Green Flight Challenge Final Results

Dean Sigler Electric Powerplants, GFC, Sustainable Aviation 15 Comments

Steve Williams, CAFE Foundation board member and e-totalizer guru, released the final results for the NASA Green Flight Challenge sponsored by Google, held at the Charles M. Schulz Sonoma County Airport from September 25 through October 1, 2011.  Figures show a profound difference between the two electric winners and the two gas-powered and hybrid runners-up.

All competitors flew extremely clean motorgliders with demonstrated lift-to-drag ratios between 25:1 and 35:1.  Possible explanations for the large differences in energy use include low cooling drag for electric aircraft and the efficiency of electric motors – but the differences are still surprising.

Note that a little over 11 US gallons of gasoline (energy equivalent) were used to fly seven people (Embry Riddle’s Eco-Eagle flew with only one pilot) over a total of 725.5 miles (Embry Riddle flew a shorter total distance on both “runs”).  This is an enormous achievement for all concerned and a significant increase in efficiency over even the best general aviation craft available today.

Congratulations are in order for everyone who participated in making this event a grand success.

Efficiency Competition
  Pipistrel e-Genius Phoenix Eco-Eagle  
Fuel used 3.82 3.82 gallons 100LL
Energy used 65.4 34.7 3.8 kWh
Equivalent fuel used 1.94 1.03 3.98 4.10 gallons (auto fuel)
Flight time (for speed) 1:47:16 1:48:27 2:25:01 2:00:48
Flight time (for mileage) 1:49:37 1:50:23 2:25:43 2:04:07
Distance (for speed) 192.0 191.0 186.7 142.5
Distance (for mileage) 195.9 193.7 187.8 148.1
Mileage 403.5 375.7 94.3 72.2 ePMPG
Speed 107.4 105.7 77.3 70.7 MPH
Speed Competition
Fuel used 6.61 4.19 gallons 100LL
Energy used 68.3 37.5 3.0 kWh
Equivalent fuel used 2.03 1.11 6.90 4.47 gallons (auto fuel)
Flight time (for speed ) 1:41:55 1:47:45 1:22:11 1:43:21
Flight time (for mileage) 1:44:10 1:50:24 1:22:57 1:44:53
Distance (for speed) 193.0 192.7 188.4 143.9
Distance (for mileage) 196.8 196.2 189.5 146.2
Mileage 388.4 352.4 55.0 65.5 ePMPG
Speed 113.6 107.3 137.5 83.5 MPH
Score 72.7 68.3 35.1 25.2  
Score = 1/( 1/Speed + 2/Mileage )

Comments 15

  1. I am an infrequent follower, so apologies if this has been answered, but how was fuel equivalent calculated? Was it based on the total chemical energy released when burning a gallon of 100LL or was it a measure of useful energy based on an ideal thermal cycle, or some other measure?

    (Editor’s Note: The methods used to measure fuel or energy use are detailed in this white paper,

    See the FAQs for the competition for how energy equivalents for different “fuels” were calculated.

    Contest rules included this statement, which gives the basis for energy calculations used in the competition.

    “BTU: British Thermal Unit, a measure of energy based upon the amount of heat required
    to raise the temperature of one pound of liquid water by one degree from 60° to
    61°Fahrenheit at a constant pressure of one atmosphere. 1000 BTU equal
    0.29307107 kWh, making the 115,000 BTU energy content of 1 gallon of 87
    Octane unleaded auto gasoline be equivalent to: 33.7 kWh.”)

  2. My son and I attended the closing events at Moffett Field and were very impressed. Thank you for helping the advancement of (aviation) technology, and congratulations to everyone who competed.

  3. Congratulations to all contestants in the Green Flight Challenge! Thank you to NASA, Google, CAFE Foundation for making it possible. Note that the Phoenix motorglider is a stock production Rotax powered aircraft you can buy today achieving 94.3 MPG in the Efficiency competition and 55 mpg at 137.5 mph in the Speed competition!

  4. I bet the most energy-efficient possible design would have included pedals for extra power and solar panels coating everything.

  5. Looking at the table above released for the NASA Green Flight Challenge, it appears as though the e-Genius went about the same distance and speed, only used approximately 1/2 the fuel and energy as the Pipistrel, but came in second. Can anyone explain why?

    (Editor’s Note: The Pipistrel carried two pilots, but also enough concrete and sandbags to simulate two additional passengers. It doubled its passenger miles per gallon relative to the two-seaters by this expedient.)

  6. Just wait til the liquid battery gets worked out, then these e-planes will really soar. Check out the MIT web site.

  7. Re: the solar cells and pedals:
    Hah! The solar cells might provide enough energy to power small fans to help cool the occupants while they pedal. And then CAFE would have to figure out what a pedaling human is worth in equivalent gallons of 100LL. After all, humans aren’t particularly efficient, consume rather expensive fuel and put out stinky emissions.

    (Editor’s Note: CAFE’s rules on solar cells, etc. can be found in the Frequently-Asked-Questions part of the GFC explication,

    Solar cells, as they become more efficient, may become at least a supplemental form of energy harvesting for powering very light aircraft.

    Would the reader suggest that a street filled with cyclists would be overall more costly on a passenger mile basis – which might depend on whether riders are ingesting filet mignon rather than rice and beans, or that the street filled with automobiles would have a more pleasing aroma?)

  8. Congratulations to Pipistrel and thanks to NASA, CAFE and Google for advancing this research. While innovation is key, its not electric or hybrid propulsion systems alone. CAFE carefully crafted the competition to emphasize multifaceted technology improvements including propulsion, aerodynamics, systems and structures with a practical speed, distance and per seat success measurements. Well done to all!

  9. I am very excited about electric powered flight and appreciate all the hard work being done here by CAFE and the contestants. I also agree with Dave on appreciating what can be done NOW with market ready tech to reduce energy consumption while delivering great performance. I am also surprised that CAFE doesn’t factor in some PRIMARY fuel source coefficient. Calling electricity Energy seems inaccurate when totaling it with 100LL fuel to arrive at a total equivalent fuel used amount. Seems like all the energies used to power the craft should be normalized to a primary energy source, or even multiple, to address the issue of the efficiency of converting it to electricity.
    I can’t wait to fly an electric powered aircraft someday, and I am grateful for the work being done by all of you. Congratulations to Pipestrel.

  10. On the idea of having the crew pedal, a good cyclist can put out about 3W /kg for a few hour endurance. So, figure about 70kg and 210 W (shaft power). Pipestrel and e-Genius both used about 9 kW (electric) per passenger, so pedaling would be under 3% of the power required. And you would NOT like the smell of the cockpit after a few flights!

    The Daedalus human powered planes did about 4x the ePMPG of these planes (about .25 equivalent electric kW for one passenger) but it was at 17 mph and very fragile.

    For solar, figure about 1kW/sq meter of incoming flux (clear day, sun normal to panels), so about half that with lower sun angles and other losses, then figure 20% efficient for a really good panel (better is possible but $$$$$), so to get 9 kW per person, you need 72 sq meters per person. Thats a lot of area and drag. So solar can make sense for charging batteries, but not for real time power if you want to go 100+ mph.

    Anyway, very impressive airplanes, all of them!

    Bob Parks (one of the Daedalus designers)

  11. Even after reading the relevant pdf’s, it is far from clear to me how CAFE computed “mileage” for the electric aircraft. The right way of doing it is clear enough: base the mileage on the fossil fuel used to charge the batteries. Electric motors and batteries are not subject to Carnot efficiency limitations. They can seem to be nearly 100% efficient until one looks at the power source that charged the batteries. That objection would disappear if CAFE used solar power to charge the batteries, but the rules appear to require diesel generators.

  12. The discrepancy in performance of gasoline and electric powered aircraft is due to an absurd conversion factor between kwh and gallons of gasoline. The conversion should be calculated from the number of gallons of gasoline consumed by the engine driving the generator used to charge the batteries.

  13. “Would the reader suggest that a street filled with cyclists would be overall more costly on a passenger mile basis – which might depend on whether riders are ingesting filet mignon rather than rice and beans, or that the street filled with automobiles would have a more pleasing aroma?”

    Well, I wasn’t trying to be too serious, but, yes, filet mignon washed down with Dom Pérignon could be quite expensive fuel considering the average male would consume between 1/2 and 1 pound per hour of steak (if my quick internet search wasn’t flawed — and ignoring the fact that you’d want a more varied diet). Also, yes, I’d rather follow behind electric automobiles than a bunch of cyclists that sustain themselves on rice and beans!

  14. Gentlemen
    I want to thank Dean Sigler for his prompt publishing of the results of the green flight challenge

    The performance of the two electric powered entries was truly impressive and I do not want to minimize their achievements

    However the conversion factor assumed between the energy in a gallon of gasoline and the energy available from a kilowatt hour of electrical energy assumes a TOTALLY UNREALISTIC ASSUMPTION OF 100 PERCENT EFFICIENCY IN THE GENERATION OF ELECTRIC POWER FROM THE GRID..

    It is obvious that 100 percent efficiency is not realistic and more important is an insult to any entry powered by an internal combustion engine, 50 percent EFFICIENCY might be a more a more realistic value.

    The Pipistrel team would have still won and indeed meet the requirements of 200 seat miles per gallon.They would have achieved 201.5 seat miles per gallon for a total of 50.4 mpg for the aircraft

    The e-Genius Team would have come close achieving an excellent 187.5 seat miles per gallon for a total of 93.8 mpg for the aircraft

    I strongly urge that future competitions be conducted with a more realistic comparison



  15. Human powered efficieny

    “The Daedalus human powered planes did about 4x the ePMPG of these planes (about .25 equivalent electric kW for one passenger) but it was at 17 mph and very fragile.”

    … and note the specific consumption of the engine (the pilot) : 4 liters of energyzing liquid (water, sugar, salt) in 4 hours at 250 W, that is 4 kg/kW/h : 20 times the SFC of a diesel engine (0.2 kg/kW/h).

Leave a Reply

Your email address will not be published. Required fields are marked *