Practical
Electric Airplanes
Where we’re at, and what ‘practical’ really means
BY PETER S. LERT
THERE’S BEEN MORE AND more buzz—or perhaps we should say
hum—about electric airplanes lately. For the last few years, we’ve
been hearing that we’ll soon be able to fly, emission-free, for pennies
an hour. Now attention is turning to the NASA-funded CAFE Green
Flight Challenge, scheduled for July 11-17, 2011, in which the task is
to fly 200 statute miles in two hours or less, using no more than 1
gallon of fuel, or its equivalent in electrical energy, per occupant. Are
there electric-powered airplanes in our future, and will they ever be
practical for general use?
Well, the answers would be “yes” to the first one—after all, there
are electric airplanes flying right now—and “yes” to the second as
well, so long as we are careful to define “practical for general use.”
Will you be able to hop into your electric airplane and zip across the
country at speeds and ranges comparable to today’s gasoline (or diesel) burning ships? Probably not for quite a while. But will you be
able to hop into a small, efficient electric airplane for, say, an hour of
fun flying or flight training? Absolutely—and probably sooner than
you think.
BATTERIES
At present, and probably for the foreseeable future, the primary limiting issue for electric airplanes is energy storage—in layman’s
language. For all their shortcomings, liquid hydrocarbon fuels (
gasoline, diesel, or jet) are very efficient ways of storing energy and, more
important, hauling it around with us. Gasoline is a very energetic
substance—pound for pound, some six times more so than dynamite!
APPLES TO APPLES
To make a meaningful comparison between electric and internal
combustion power, we need to find a common unit to describe both.
A handy one is the kilowatt (which is already
how European carmakers specify power); 1
k W equals 1. 34 hp. For example, the popular
Rotax 912 found in many light-sport aircraft
(LSA) is rated at 80 hp, or just less than 60
k W. And to measure energy storage, we can
use the kilowatt/hour (k W/h)—the amount
of stored electricity that can deliver 1 kilowatt for an hour.
With today’s technology (admittedly, a
moving target in terms of rapid development), the best commonly available
lithium-ion batteries can store about 100
watt/hours per pound of weight. Gasoline,
by contrast, can store about 5. 75 kilowatt/
hours per pound—almost 60 times as much.
However, it’s not that simple: Gasoline
engines aren’t very efficient, with only about
25 to 30 percent of their energy input showing up at the propeller shaft; the rest is heat,
most of which goes out the exhaust. Electric
motors are much more efficient, ranging
from 90 to 95 percent, so if we compare
overall powerplant and storage efficiency
together, the ratio is only about 20-to- 1,
rather than the 60-to- 1 we saw at first glance.
We should also give the electric motor credit
for being much lighter than an internal combustion engine of the same power—typically,
only half to a third as heavy. Let’s say,
