IEEE Spectrum June, 2016 - 48

VApor ANd Some
riVAlS compAred

range
(Kilometers)

Speed
(Kilometers per hour)

Cirrus SR22
Diamond DA40
Cessna 182
Cessna TTx Corvalis
Piper PA 28 Arrow
Vapor
NASA

Minimum
Objective
0

500

1,000

1,500

I hesitated to decrease range too, so I
looked for other ways to trim energy
consumption that didn't involve millions
of dollars in technology investments.
Fortunately, electric propulsion offers
some flexibility that the engineers at
Cirrus did not enjoy. Unlike combustion
engines, electric motors are compact
and efficient. These small, light motors
can be placed in many more locations
on the aircraft than would be practical for a combustion engine. If applied
strategically, this tactic can distribute
the power production across more or
larger propellers. And the greater the
area swept by propellers, the more efficient and quieter they become.
I ran yet another analysis and found a
sweet spot in efficiency using two rather
large propellers attached to a pair of
motors. Instead of mounting them conventionally, on the wing or fuselage, I
put them in my design atop the plane's
V-shaped tail, where the airflow is cleaner.
This simple strategy not only improved
propulsive efficiency (from 85 to 92 percent), it also benefited the plane's aerodynamics. Now air could f low more
cleanly over both fuselage and wing.
And although the propellers were
large, putting them on the tail meant
that I didn't have to increase the height
(and therefore, weight) of the landing
gear. Having short gear made choosing
retractable wheels much more palatable, and this reduced drag even further.
When I ran the next analysis, I found
that this change, combined with some
48

JUN 2016

NORTh AMERICAN

2,000

150

250

350

more optimization, decreased the
plane's energy consumption by another
27 percent. Indeed, this design change
had lowered the power demand to
the point that it became feasible to fly
the plane on hydrogen-powered fuel
cells. That's when I dubbed my V-tailed,
hydrogen-powered design "Vapor."
Once the general parameters of the
design became clear in this way, I could
work on the details. One was to select the
best type and layout for the fuel cells. I
examined various fuel-cell systems that
have been used in the automotive and
aerospace applications and found that
the fuel-cell stacks and hydrogen tanks
of the type used in the 2015 Toyota Mirai
would create a lighter and more compact
system than was thought possible eight
years ago, when Boeing first flew its fuelcell powered HK36. What's more, all of
these components would fit nicely into
the Vapor's airframe.
The fuel-cell system I had designed
with available technolog y delivers
800 Wh/kg at 55 percent efficiency. That
was certainly better than 400 Wh/kg
for the best lithium-based batteries I
could expect or with 25 percent efficiency for modern gas engines. Combining fuel-cell power with the plane's
unconventional propeller placement,
I was able to arrive at a design that, if
mass-produced, could indeed compete
with the Cirrus SR22. It would weigh
and cost about the same, and its range
would be very similar: about 920 miles.
The plane's cruising speed would be

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somewhat less-173 as opposed to
212 mph. But that seems a reasonable bargain, given that the electric aircraft would consume about
a quarter of the energy per flight.
My goal was for Vapor to be
attractive to pilots-they are, after
all, the ones who buy, f ly, and
maintain small planes. And I think
the design meets that requirement.
The decrease in energy consumption and the elimination of the gasoline engine (and all the routine
maintenance it requires) will likely
reduce operating costs. What's
more, the reduction in noise level,
from 92 decibels to 76 dB, should
improve cabin comfort considerably.
And the very high reliability of electric
motors should give both pilots and passengers greater peace of mind.
iven recent advances in fuel
cells and electric motors,
Vapor, or something like it,
could well be built and
flown right now. The technology is certainly ripe for exploitation. But it's
unclear how regulatory authorities will
react to the advent of such all-electric
designs. That's important because
uncertainties in the certification process can doom an effort to develop an
aircraft for commercial production.
Another hurdle is that hydrogen has
not yet caught on as a fuel for automobiles, much less for airplanes. Both
applications suffer the chicken-and-egg
problem: Until it becomes a popular fuel
there will be little infrastructure to support the distribution of hydrogen, and
until there is infrastructure to support
its availability, it won't become popular.
Despite these hurdles, the proposition
of an all-electric aircraft flying seven
times as far and twice as fast as current
designs is exciting. With NASA's firstplace endorsement, it isn't a stretch to
say that the Vapor, or a plane based on
its design, could begin taking to the skies
by 2020, just as the competition organizers at NASA had intended. n
poSt your commeNtS at http://spectrum.
ieee.org/electricplane0616

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Table of Contents for the Digital Edition of IEEE Spectrum June, 2016