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The Green Flight Challenge  September 25-October 3, 2011 Santa Rosa, California


We are here at the Santa Rosa airport in Northern California for the Green Flight Challenge sponsored by Google. Three teams out of the original 13 entered have made it to the competition, having fulfilled all of the preliminary requirements. A NASA prize of $1.35 Million US Dollars goes to the team that can successfully demonstrate a 200 mile flight while consuming energy at a rate equivalent to 200 statute passenger miles per US Gallon of gasoline, averaging 100 statute miles per hour. A variety of technical solutions is on display here on the ramp at the CAFÉ (Comparative Aircraft Flight Efficiency) foundation headquarters. Electric, gasoline-electric hybrid, and pure gasoline solutions will battle it out over the mountainous race course in warm autumn weather. The competition was originally scheduled for July, but only one team reported that they were ready to compete, so organizer Brien Seeley delayed the contest by three months. Even with the delay, a number of anticipated entries either chose not to, or were not able to, make the trek to California.

Only three teams met all of the criteria to compete. To win the competition, an aircraft must fly 200 statute miles in less than two hours and use less than one gallon of gasoline per occupant, or the equivalent in electricity. All of the three final competitors have chosen a variation of the motor-glider theme, with long elegant wings for best aerodynamic efficiency. As the Green Flight Challenge is a US Government sponsored contest, each team was required to name a US partner and pilot in order to qualify as an “American” team, although all of the engineering and fabrication work on the finalists was done in Europe. An odd touch, and one that most US taxpayers would probably not be happy to know about. Competition for the prize is keen, as the difference between first and second place is $1.35 Million USD versus $120,000 USD. 

The starting grid:

Pipistrel Taurus G4 Electric
University of Stuttgart E-Genius
Phoenix (ROTAX powered)

Front runner for the prize must certainly be the Slovenian Pipistrel crew with their Taurus G4, a unique 4 seat dual fuselage configuration with a 150 kW electric motor slung between the fuselages on a stub wing. Based on wings and fuselage(s) of the production Taurus Electro, the G4 is a one-off competition airplane that will help Pipistrel prove out electric propulsion technologies for their upcoming 4-seat Panther hybrid aircraft. Pipistrel has won several of the previous CAFÉ/NASA aircraft efficiency challenges, so they clearly know how to play this game!  Team chief and all-around genius Tine Tomazic has held back some information on the ultimate performance of the G-4, wanting to create a bit of mystery for the other competitors. What we do know is that the G4’s empty weight is 1065 kilograms (2,343 pounds) with 470 kilograms (1,034 pounds) of this being batteries. Maximum takeoff weight is 1500 kilograms (3,300 pounds). The real advantage for this large heavy aircraft is that the prize is partly based on passenger miles per gallon, a measure that tends to reward a configuration with more passengers. The G4 “the Batmobile”, flown by test pilot Dave Morss “Batman” and Robert Reid “Robin”, may use a bit more energy to complete a 200 mile flight than the other competitors, but the energy used is divided by two more people than in its competitors.

The University of Stuttgart entry, the E-Genius, flown by renowned electric aviator Eric Raymond and German Klaus Ohlmann, must also be considered a contender for the prize. The University team, led by Professor Rudolf Voit-Nitschmann, has previously won the Berblinger Prize in Germany with their solar powered Icare motor-glider. The E-Genius is based on the fuselage of the Pipistrel Taurus, modified to accommodate a 58 kW Sineton electric motor perched on the top of the vertical tail. Sponsored by Airbus, this is a serious effort and has turned in some impressive performance in flight testing. Rudolf told me at the outset of the competition that they had managed a 2 hour 17 minute flight during the initial test period and just exceeded the required 100 miles per gallon (equivalent)) per passenger, leaving a small margin.

The final competitor is the lovely Czech Phoenix motor-glider, powered by a Rotax 912 engine and flown by American importer Jim Lee. This team was originally intending to use the electric powered PhoEnix, a highly modified version of the Phoenix with a Discus sailplane wing, retractable landing gear, and a 44 Kilowatt electric motor. Flight testing proved that this design was not ready for the competition, and the production gasoline engine Phoenix was substituted. It will be nearly impossible for this airplane to achieve the required 200 miles at 100 miles per hour speed using no more than two gallons of fuel.

Embry-Riddle Aeronautical University students made the long drive from Florida with the Eco-Eagle, a modified Stemme S10 motor-glider. A hybrid electric/gasoline configuration has been chosen, with a Rotax 912 providing takeoff and climb power and a 40 Kilowatt electric motor providing cruise thrust. The distinctive retractable “scissor” propeller of the Stemme has been replaced with a non-retractable prop optimized for the required 100 miles per hour cruising speed. A clever computer-controlled clutch mechanism allows either electric or gasoline Rotax power to be channeled to the propeller. Unfortunately, the airplane was not able to pass the technical inspection on the first day of competition as the GFC-required ballistic parachute was not installed, and the required second pilot was not provided due to university insurance provisions. After suggestions from the other competitors, the GFC organizers agreed to allow the Eco-Eagle to compete as an “exhibition” entry in recognition of the many hours of effort put into this student-led project.

In the Did Not Start category are a hundred tales of frustration, unfulfilled expectations, and tragedy. Most notable is the sad story of the Yuneec E1000, an innovative four seat electric aircraft designed by German ace Martin Wezel for Yuneec chief Tian Yu.  The aircraft crashed in early flight testing, killing Wezel and dashing Yuneec’s hopes for the Green Flight Challenge. Team Feuling entered the Electric Quickie, a modernized carbon fiber rendition of the Burt Rutan/Gene Sheehan designed Quickie of the 1970s. The original industrial gasoline engine has been replaced by an electric motor, with the lithium-ion battery packs powering it neatly packaged just forward of the firewall. The choice of the short span tandem wing configuration is not intuitively obvious for this competition, and the small battery pack will be hard pressed to provide 200 miles worth of energy along with the FAA mandated 30 minute reserve. Feuling has a long history of innovation in fields other than aviation, mostly in the area of auto and motorbike engines. The airplane entered flight test but encountered several challenges including a hard landing that damaged the canard, and in the end did not make it to the contest site. The Elektra One, an airplane about which much has been written but little has been achieved, was a no-show in Santa Rosa but in any case seems incapable of the lofty performance claimed by the designer. A lack of funds to get the airplane to California was stated as the reason for not coming, but the airplane’s appearance at Oshkosh a month earlier makes one wonder why they shipped it back to Germany instead of staying to compete?  The much anticipated Synergy, an innovative four seat diesel effort, remains unflown in a hangar in remote Montana. Time simply ran out on this ambitious effort using an unusual joined-wing configuration. The promising French Greenelis, an effort by noted aircraft designer Paul Lucas was displayed in a completed state at the Paris Airshow, but there has been no confirmation that the airplane has flown yet. The Windward Goshawk, a design by sailplane designer Greg Cole, sits incomplete in Windward’s Oregon base. I was really looking forward to seeing this two seat design with extremely high aspect ratio wing, but it will have to wait for another year. Other entries, such as the Seraph and Artemis, seem to remain mostly paper studies, consigned to glossy PowerPoint presentations but not the light of day.(pictures of these)

The Competition

Preliminaries – Takeoff and noise trials on Monday. Under bright sunny skies, the first flight demonstrations consisted of a noise and takeoff test. In order to pass this hurdle, the airplanes must be able to take off and clear a measured 50 foot tall obstacle within 2000 feet while producing no more than 78 decibels of noise. While the takeoff distance should be no issue, the noise might be. Many assume that an electric aircraft would be nearly silent, but such is not always the case. There may be little engine noise but the propeller tips make the same type of noise as we are used to in gasoline powered aircraft. As the technology of electric flight matures, more attention to detailed prop design will be required. Having said that, all four aircraft passed the test. The clear winner was the Stuttgart E-Genius, its Sineton electric motor producing enough thrust to clear the 50 foot barrier (just) while producing close to 4 dB less noise than the next nearest competitor. The hushed whir was an impressive counterpoint to the snarl produced by other conventional aircraft operating from the Santa Rosa airport.  The Pipistrel G4 was also quiet, but the 150 kW motor and very large laminated prop produced a more pronounced hum than the nearly silent E-Genius.(photos)

The first big trial – Tuesday 200 mile flight. At the 8 AM pilot briefing, GFC chief Brien Seeley described the (slightly less than) 200 mile triangular course for the critical mileage test. A premium is placed on accurate navigation, as deviating more than 2 miles from the planned course line or missing the turnpoints by more than ¼ mile results in disqualification. Heading north from Santa Rosa, the contest aircraft have to climb to an altitude of 4000 feet as they climb towards the turnpoint near Geyser Peak. The energy used by the electric aircraft is measured by an on-board energy meter installed by GFC personnel. The fuel used by the Rotax powered craft is carefully measured before and after the flight, and the aircraft weight is carefully measured on the sensitive CAFÉ scales.

Thee green flags wave to start the competition

The racers were flagged off from Runway 19 shortly after 10 in the morning, departing at 10 minute intervals. Team E-Genius went first, taking off with less than full power to conserve every watt of energy. The Pipistrel G4 went next, lumbering down the runway looking like something from a science fiction movie. The two pilots were in the right hand fuselage, and cement sacks to simulate the weight of two passengers was strapped in the left side cockpit.  The Phoenix went last. As the flight progressed it began to look like a 2 horse race, with E-Genius and Pipistrel circling around the turnpoints in tandem. A check with the Pipistrel flight planning team showed that half way through the race they were within .01 kilowatt-hour of what had been planned for energy consumption up to that point. The Pipistrel flight plan was for 1 hour 41 minutes and 14 seconds, a time that would leave about a four percent margin over the required 100 miles per hour average speed.  The E-Genius team crossed the finish line first, an hour and 51 minutes after the green flag dropped. Shortly after, the Pipistrel crossed the finish line with an elapsed time of 1 hour 41 minutes and 41 seconds, only 27 seconds more than the computer flight plan predicted! Jack Langellan of Penn State University was providing the very precise flight planning and weather analysis support that typified the complete and professional approach of the Pipistrel team to this challenge. Although the GFC organization will withhold the final results until the final dinner banquet, it appears as though both Pipistrel and E-Genius have met the contest targets for energy used and speed with a small margin. The Phoenix lagged all of the way around the course and appeared to have made an extra lap, landing long after the other two competitors.

wheel up for the E-Genius at the Start of Competition
The Pipistrel Taurus G4 crew flew a precise mission profile

The second big trial – Thursday 200 mile flight. For this second flight, the emphasis is on speed. With data from the first competition flight, the teams can decide how much energy can be used to push the speed up while still conserving enough for the required 30 minutes of reserve power on landing. In the end, a mathematical formula will be used to decide the winner, with the energy use per passenger and speed taken into account.


The three airplanes were pulled out to the end of the runway by electric golf carts, so as to not use up valuable energy while taxiing. As the flight was about to start, there was a slight panic as the Taurus G4 had a flat tire that had to be quickly replaced so the team could make it to the start line. It is ironic that the only airplane failure the team suffered was the lowest tech part on the airplane!  The course was four laps around a series of turnpoints, and as the laps progressed the Pipistrel team found that they had slightly more energy in reserve than expected, and speeded up for lap number 4. The team was jubilant as the airplane received the checkered flag, feeling that they had an advantage over the E-Genius team. E-Genius had a second flight that mirrored the first one on Tuesday, as the 200 mile circuit left them with little extra battery capacity to use for increased speed. The Phoenix team, knowing that they had no chance at victory, blasted around the course at 120 knots, with little regard for the fuel burned. He landed long before the other two competitors and was towed into the hangar to be accurately weighed post-flight.

At the end of the flight, a ground test was conducted to confirm that enough reserve energy remained for the FAA mandated 30 minute reserve. The Pipistrel team went first, running the motor on the ground at a reduced power setting for almost 120 minutes to simulate the electrical energy used in 30 minutes at 100 miles per hour. As soon as they had done so, they raised the power level back to cruise for a while just to show off a little bit.

And the winner is -  CAFÉ foundation personnel spent the weekend crunching the data collected during the competition, and discussing any potential mistakes or violations that might cause a team disqualification. By agreement with NASA, the results would be held secret until the final awards banquet. GFC chief Brien Seeley did a noble job of keeping a poker face, revealing nothing about who might have triumphed. In the end, the Pipistrel team from Slovenia was declared the winner with their G4. They take home a check for $1.35 Million dollars, and preserve their perfect score in CAFÉ efficiency competitions. At this rate, NASA will have to call their innovation prize efforts the Slovenia Economic Stimulus Package! The real result of this competition is that electric light aircraft propulsion has to be taken seriously as something on the verge of becoming practical. Just think of it – the equivalent of 200 passenger miles per gallon at over 100 miles per hour! Just as electric cars are becoming a more common sight on the streets of the world, electric airplanes will someday in the near future be parked on the ramp of your local airport. And the world will be a better place for it.

Final Results

Fuel efficiency –

1. Place Pipistrel G4 403.5 passenger miles per gallon (equivalent)

2. Silve medal   E-Genius 375.8  passenger miles per gallon (equivalent)

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