more efficient structure because the weight
along the wingspan is distributed. It is not
all the weight in one body and a super long
wing. The load is equally distributed. The
payload of the G4 is amazing; empty weight
is 1,200 pounds. It carries 1, 100 pounds of
batteries and 900 pounds of people. The airplane can lift more than twice its own
weight; you can’t do that with a conventional
airframe. In the CAFE competition we were
flying at 3 ,290 pounds.
Q: HOW ABOUT THE ELECTRIC PROPULSION
SYSTEM? WHAT LESSONS HAVE YOU LEARNED IN
TINE: Most of the things we already knew
The size of the G4’s center section is evident as it flies overhead, as is the distance of the cockpits from the centerline.
Other obstacles were transport logistics—it is not easy to transport an airplane
with a half-ton of batteries; customs and
insurance are a problem. It is not easy to
insure such a crazy-looking prototype. (EAA
Aircraft Insurance Plan did it.) Logistics was
a problem. We constructed the airplane here
in Slovenia, but because of the competition
rules, we had to test it and fly it in the
WHERE IS SLOVENIA?
Slovenia is a small country with only 2 million
inhabitants, located between the Balkan states
and Western Europe. After separation from
Yugoslavia, Slovenia became a member of the
European community. Slovenia’s landscape is
largely alpine, with excellent infrastructure, a
well-educated workforce, and Austria
on its northern border
and Italy to the west.
It is about the size of
New Jersey and can be
roughly described as a
from the Taurus Electro G2. The electric
system architecture is not much different; it
is just bigger and more powerful. We built a
new motor, controller, battery management
system. We purchased only the battery cells.
The big difference between the production
G2 and the racing G4 is the electromagnetic
interference; because of the bigger motor
and batteries there is more electrical disturbance. You have to take care which cables
you use, where you put them, otherwise
your radio and instruments don’t work. Most
of our efforts around the electrical system
were focused on controlling the electromagnetic interference. This was something that
was not entirely expected.
Q.: WHAT WERE THE MAJOR OBSTACLES
WITH THE G4?
TINE: A major obstacle was our state of mind;
we could not afford a single mistake with the
airplane. The timeframe from the beginning
of the construction to the race was so short
that if something went wrong and we could
not repair it, the project would be lost.
That’s why we designed it in a way so even if
we did not make it to the competition, we
could still use it as the test platform for the
United States with the consequence that the
whole team and tools had to follow the airplane overseas.
Q: WHAT WAS THE EASIEST PART OF THE PROJECT?
TINE: The G4 proved that it was much easier
to fly than expected, even having the cockpit
displaced 10 feet from the airplane centerline. On takeoff and landing, you have to aim
off the centerline and not on the runway centerline—that’s pretty much it. Even stronger
winds and crosswinds were not a problem.
Q: WHAT WAS THE BIGGEST UNEXPECTED
PROBLEM YOU ENCOUNTERED?
TINE: We did not expect that much electric
interference, so we had to redesign some of
the signal network of the airplane before the
first flight. After the maiden flight none of
the major systems were changed; we were
only tweaking motor parameters for better
response, tweaking the center of gravity, and
several flap settings. The way the prototype
was performing was simply amazing!
Marino Boric, EAA 1069644, is an aeronautical engineer
and holds a private pilot license in Germany with commercial and instrument ratings (CPL/IFR). He also flew as a
ABOUT THE NASA
GREEN FLIGHT CHALLENGE
The NASA/CAFE Green Flight Challenge was designed
to push technology and make passenger aircraft
more efficient. Airliners average around 50
passenger miles per gallon (pmpg), and the initial
intention of this competition was to push teams to
greater than 200 pmpg. Pipistrel’s result of nearly
400 pmpg doubled the NASA target! Contest rules
required the aircraft to cruise at least 100 mph for
200 miles using the equivalent of only 1 gallon of
fuel per passenger. For safety the FAA mandated
contestants carry an additional 30-minute energy
reserve. The total prize money for this competition
was $1.65 million. Pipistrel’s G4 garnered $1.35
million; the remaining monies were awarded
to other contestants. The Green Flight Challenge
was run by the CAFE Foundation on September
25-October 1, 2011, at Charles M. Schulz - Sonoma
County Airport in California.
Initially 14 custom-built aircraft were
developed with electric, bio-diesel, and other
bio-fuel engines. The aircraft used various
technologies to improve aerodynamics, propulsion,
and structural efficiency for maximizing the overall