AOPA Turbine Pilot Magazine - September 2013 - 69

PROPULSION / ELECTRIC

WHERE DO WE GO
FROM HERE?

powering an airplane. To have the same
amount of energy as a pound of gasoline
(at 5,500 watts per pound, gasoline reigns
in energy density), you’d need to carry
about 500 pounds of lead-acid batteries!
So lithium batteries it must be.
Still, they are heavy. The lithium
polymer battery that powers the e430
weighs 184 pounds, but has a weight-topower advantage over lead-acid batteries
of about 1,400 percent. In the e430, the
battery can last as long as three hours in
flight, and requires one hour of charging
time for each hour flown.
Other airplanes have similar endurance and recharge times, depending on
how long you use higher power settings.
Some electric airplanes stretch battery

power by using it for takeoffs and initial
climbs; after that, the engine is shut down
and the airplane can be flown as a glider.
A DOWNSIDE

With all of their attributes, do lithium batteries have any disadvantages? Apart from
short endurances and long recharge times,
lithium batteries generate heat when they
charge and discharge. Control units are
needed to prevent overcharging, manage
battery condition, and maintain balanced
charges among the cells.
But this is a mild inconvenience when
you consider that lithium is flammable
if exposed to air. If there’s leakage, or
damage to the battery’s protective case,
the battery can catch fire. There have

been lithium battery fires in hybrid automobiles. A Cessna Citation CJ4 had a
lithium-ion battery fire while hooked up
to a ground power unit; an emergency
airworthiness directive was issued, and
as a result Cessna gave all 42 CJ4s in the
fleet replacement nickel-cadmium or
lead-acid batteries.
And then there were the lithium
battery fires on Boeing’s Dreamliners—
one in flight, and one on the ground.
Overcharging was suspected, but as yet
there has been no determination of cause.
The fires prompted a grounding of the
entire fleet.
Advocates say that lithium batteries are safe if kept properly sealed and
protected, monitored carefully, and

HOW IT WORKS

ILLUSTRATION: STEVE KARP

It’s electric!

Yuneec International is by
far the biggest player in the
aircraft electric-propulsion
game, but Pipistrel also
produces its own power
systems. Whatever the make,
the propulsion systems share
the same basic design
objectives. First of all, the
battery must have a protective case to prevent
damage and keep lithium
from coming into contact
with air. It’s also imperative
to monitor the battery and
motor for signs of abnormally high temperatures—and
potential thermal runaways.
Special circuitry keeps
battery cell voltages in line,
as well as charging and
discharging rates. On the
pilot’s instrument panel,
voltage is prominently
displayed because voltage
levels serve as measures of
the “fuel” remaining.

1. Yuneec power drive 20-kilowatt (27 hp)
motor weight, 20 lb; 2,000-hour TBO.

2. Yuneec power controller
—Motor control
—Battery monitoring
—Controller monitoring

3. Yuneec 70-amp lithium
polymer battery pack with
18 cells and temperature
balancing and monitoring.

1

3

2

5

4. Yuneec motormanagement system
shows voltage,
amperage, prop rpm,
time aloft, and
temperature warnings.

4
5. Rheostattype “throttle.”

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AOPA Turbine Pilot Magazine - September 2013

Table of Contents for the Digital Edition of AOPA Turbine Pilot Magazine - September 2013
