AOPA Pilot Magazine - September 2013 - 67

flying since the mid-2000s, but a sighting would be rare in the United States.
Most have been on display in Europe, with
the biggest showcase being Germany’s
AERO Friedrichshafen exhibition.
However, Yuneec and Pipistrel
have made showings at EAA’s annual
AirVenture at Oshkosh. At this year’s
AirVenture, GreenWing International’s
eSpyder—an Experimental-category, amateur-built ultralight—was scheduled to
be showcased. GreenWing International
was formed by Yuneec to produce, sell,
and support the eSpyder and a two-seat
design, the e430.
UPS AND DOWNS

But if electrical motors are lightweight,
reliable, and stone-simple, the same
can’t be said for the batteries that power
them. And here we come to the central
issue affecting any future advances in the
performance and utility of electrically
powered aircraft.
The lithium-ion and lithium polymer
batteries used in these aircraft have benefitted from a number of improvements
in recent years. Most of them have come
from the burgeoning cellphone, consumer
electronics, and electric/hybrid automotive applications. Why lithium batteries?
Mainly because they are comparatively
light when compared to the batteries we are most familiar with, have very
low self-discharge rates, and require no
maintenance.
For example, the lead-acid battery in
your car is not only heavy, it also can selfdischarge 20 percent of its capacity per
month, is subject to eventual failure from
lead sulfate deposits, and its electrolyte
is highly corrosive. Lithium-ion batteries use a gel electrolyte; lithium polymer
batteries have layers of polymerized composite material that hold a lithium-salt
electrolyte, plus a polymer-laminate case
that can be shaped.
In terms of energy density (the amount
of energy produced per unit of weight),
lead-acid batteries put out about 11 watts
per pound. This gives them enough power
for short bursts at high output—perfect
for starting your car engine. But not for

SOLAR-ELECTRIC
HYBRIDOLOGY
BATTERY-POWERED ELECTRIC PROPULSION MAY BE THE FIRST ECO-FRIENDLY
technology to make it to market, but there’s another means of driving electric motors—and it’s been around longer. Solar-powered electric motors have
been powering light aircraft for years. Under this scheme, solar cells mounted on the upper wing surfaces collect energy.
Most recently, the Solar Impulse has been getting all the attention as it
crossed the United States. This technology demonstrator has made a huge
step forward in that its batteries have proven to last throughout the night.
Plans are for its successor to fly around the world in 2015, with one stop on
each continent.
But way back in 1981, famed aeronautical engineer Paul MacCready and
his team at AeroVironment, Inc. created the Solar Challenger, an airplane capable of flying several hours and hundreds of miles on the power of its solar
cells. The Solar Challenger even crossed the English Channel.
In 1990, designer Eric Raymond flew his Sunseeker solar-powered motorglider across the United States in 21 stages and 121 flying hours. Raymond
continues to fly his latest solar-electric Sunseeker II all over Europe. On one
flight he took off from Friedrichshafen, Germany; crossed the Alps; flew to
Sicily; and then to Slovenia. Raymond even helped to build Solar Impulse.
In 1996, a team from the University of Stuttgart in Germany flew its Icaré II
to win Germany’s Berblinger prize recognizing innovative designs featuring
environmental sustainability.
In 2001, MacCready’s AeroVironment built another solar-electric design,
the Helios. The 230-foot-wingspan Helios was remote-controlled and set
an altitude record of 90,000 feet. Two other electric drones, built in 2005
(by Alan Cocconi’s AC Propulsion) and 2010 (by Anglo-American company
QinetiQ), also have demonstrated impressive endurances.
While still in its infancy, solar power may yet augment future power systems
based on today’s battery-electric designs.
–TAH

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AOPA Pilot Magazine - September 2013

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