How i designed a practical electric plane for nasa ieee spectrum

1. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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24 May 2016 | 15:00 GMT
How I Designed a Practical Electric Plane for NASA
To win a competition, a Georgia Tech student devised a fuel-cell plane to
rival today’s best-selling small aircraft
By Tom Neuman
Illustration: John MacNeill
Submitted to a NASA competition for students, the design for this electric aircraft had
to meet certain requirements, the most important of which was that it could be
manufactured within five years.
piston-powered planes, you will burn
many gallons of fuel per hour and
suffer through the noise equivalent of
a ride on a power mower. But unlike
what you face while cutting your
grass, if the engine quits, it means an
immediate emergency landing at best
and a crash at worst. Fortunately, it’s
now possible to envision replacing
those noisy gas guzzlers of the sky
with electric airplanes, which would
be considerably quieter, cleaner,
safer, and more efficient than today’s
aircraft. Indeed, electrification could
transform the current small-airplane experience into something vastly more attractive to both pilots and
the communities over which they fly.
If you fly one of today’s small
Recognizing the possibilities—and hoping to spur innovation—
to design a four-seat,
all-electric aircraft capable of entering service by 2020. After I read an announcement describing the
contest, I came up with a design that ultimately won first place in the graduate division of NASA’s
competition. My design relies on fuel cells for propulsion and uses an unusual motor placement to
maximize efficiency. For me, working on its design was a window into the possibilities and also a vivid
lesson about the significant challenges ahead for electric flight.
NASA recently challenged students
(http://aero.larc.nasa.gov/files/2014/09/2020-E-GA-design-contest-ARMD-1.pdf)
this challenge in 2014, I was a graduate student in aerospace engineering at the
, in Atlanta. But this contest wasn’t my first
foray into electric flight. In 2008, when I was 17 years old, about six months after the first
hit the road, I worked on another electric
plane—one with a 9-foot (3-meter) wingspan designed for a remote-controlled aircraft competition.
When NASA issued
Georgia Institute of Technology (http://www.gatech.edu/)
Tesla
Roadsters (https://en.wikipedia.org/wiki/Tesla_Roadster)

2. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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At the time, electric flight was becoming an increasingly reasonable proposition as lithium-ion batteries
became lighter and cheaper. Flying with electric motors made the planes cleaner, quieter, and easier to
operate. Still, the technology available in 2008 would have limited pilots to flights of a few minutes.
Similar considerations applied to Tesla’s first car: It offered some compelling advantages, but it couldn’t
go nearly as far as its gasoline-powered counterparts.
Since then, battery technology has improved to the point where all-electric cars have become an
established niche: Nissan introduced the mass-market
at the end of 2010, and Tesla has increased the size of its flagship automobile from the two-
seat Roadster to the five-seat . Electric planes have also
benefited from these battery advances. In the past two years, both ,
the Slovenian light-aircraft manufacturer, and , based in France,
have introduced electric two-seat trainers.
Leaf (http://www.nissanusa.com/electric-
cars/leaf/)
Model S (https://www.teslamotors.com/models)
Pipistrel (http://www.pipistrel.si/)
Airbus SAS (http://www.airbus.com/)
While the range of electric aircraft has been growing, limited flight time remains their main weakness.
The reason is that batteries are heavy relative to the propulsive energy they provide—by a factor of 10 or
more compared with that of gasoline-powered internal combustion. For ground vehicles, designers can
compensate somewhat for this shortcoming by adding more or bigger batteries. But aircraft are
extremely sensitive to extra weight: Just about every component of a plane’s structure must grow in size
for each added kilogram. The requirement for beefier components in turn leads to a heavier aircraft, one
that requires still more energy and therefore larger batteries to fly. This vicious circle means that for
electric-plane design, adding batteries to boost range isn’t a viable strategy.

3. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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(https://en.wikipedia.org/wiki/Gaston_Tissandier#/media/File:Elktroluftschiff.jpg)
(https://en.wikipedia.org/wiki/Hydrogen-
powered_aircraft#/media/File:Boeing_Fuel_Cell_Demonstrator_AB1.JPG)
Nevertheless, NASA has repeatedly encouraged innovators to try to build electric aircraft that could
compete on both range and size with gasoline-powered airplanes. In 2011, the top two aircraft in NASA’s
were completely battery-powered and flew for nearly 2 hours at an average speed exceeding 100 miles
(161 kilometers) per hour. In an unusual twist, the designers of the second-place winner, the
, built by researchers at the
, positioned the electric motor atop the tail. Its distance from the
center of the fuselage allowed the plane’s makers to install a large and highly efficient propeller while
keeping landing gear short and light.
Green Flight Challenge
(http://www.nasa.gov/offices/oct/early_stage_innovation/centennial_challenges/general_aviation/)
e-Genius
(http://www.nasa.gov/offices/oct/early_stage_innovation/centennial_challenges/general_aviation/e-
Genius_asset_index.html) University of Stuttgart (https://www.uni-
stuttgart.de/home/index.en.html)
1/9
(https://en.wikipedia.org/wiki/Gaston_Tissandier#/media/File:Elktroluftschiff.jpg)
The brothers Albert and Gaston Tissandier developed
, first flown in 1883.
the first electrically propelled dirigible
(http://www.thisdayinaviation.com/tag/tissandier-electric-airship/) Image: Library of
Congress/Wikipedia (https://en.wikipedia.org/wiki/Gaston_Tissandier#/media/File:Elktroluftschiff.jpg)


4. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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In 2014, NASA and two industry partners launched a project called
(short for leading-edge
asynchronous-propeller technology). They’ve constructed a special carbon-fiber wing equipped with 18
electric motors and propellers, which they used for ground testing. They plan to place a similar wing on
an Italian-built light aircraft. The
multitude of propellers, each potentially adjustable independently, will allow this plane to use a smaller
wing than normal, reducing the amount of energy consumed in flight.
LEAPTech
(http://www.nasa.gov/centers/armstrong/Features/leaptech.html)
Tecnam P2006T (http://www.tecnam.com/aircraft/p2006t/)
Photo: Susan &
Allan Parker/Alamy
The author’s
design could
compete with the
Cirrus SR22,
today’s best-
selling four-
seater.
One To Beat:
In September of that same year, NASA announced a design competition for
students that it hoped would push the limits of electric-airplane technology far
beyond what previous projects had accomplished. Always looking for the
opportunity to tackle a unique design challenge, I signed up without hesitation
and ended up as the only one-person team in the contest.
had in its sights seemed impossibly distant, and yet it
required the designs to be flight-ready in five years (by 2020). NASA’s design
objective for the competition was a four-seat airplane with a range of 800
nautical miles (921 statute miles, or 1,482 km) and a cruising speed of 175 knots
(equal to 201 mph, or 323 km/h). And while most designers might scoff at such
a challenge, seeing these specs as completely normal, the goals were more than
seven times as far and twice as fast as the brand new Pipistrel Alpha Electro
electric airplane could go. Quick math shows that the NASA performance targets
demand about 30 times as much energy as the Pipistrel is capable of providing.
What could change so drastically in five years so as to allow that?
The target NASA
Some proponents of electric flight have argued that matching the ranges of gasoline-powered planes is
not really necessary. They point out that pilots, like drivers, rarely make use of the full range of their
vehicles, so little utility is lost in designing for shorter flights. Think of the Nissan Leaf, which has only
one-third the range of a typical car.
That’s all true. But it’s also true that range anxiety is a scarier proposition in a plane than in a car. And
certifying and selling a fully electric plane would be enough of a battle, I reasoned, without it having a
limited range. So I resolved to meet as many of NASA’s performance objectives as possible. That way, if
some manufacturer should decide to build it, the plane could compete head-to-head with existing
gasoline-powered models, including the US $500,000 four-seat
, the best-selling single-engine aircraft of the past decade. That,
I knew, was really the plane to beat.
Cirrus SR22
(http://cirrusaircraft.com/aircraft/sr22/)
I started my design project by determining whether converting the power plant of an existing gasoline-
fueled plane to electric, using either batteries or fuel cells, would work. I knew that batteries lack energy
and fuel cells lack power, but I didn’t know which hurdle would be more difficult to overcome.

5. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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I started with batteries. They were familiar and flexible—you can go from one cell to 6,831, as the
designers of the Roadster did, if needed. I discovered that if I swapped out all 1,020 pounds (462
kilograms) of engine and fuel from a Cirrus SR22 and replaced them with a lithium-ion battery and an
electric motor, the plane could fly for about half an hour. That would allow it to travel only about 100
miles (some 160 km). Boosting the range to anything like the SR22’s by adding more batteries proved
futile, because of the vicious weight circle I described earlier.
The Vapor Aircraft
This design for a four-seat electric airplane borrows heavily from the fuel-cell-powered
Toyota Mirai automobile.
(/image/Mjc1NjcxOQ)
Illustration: John MacNeill
This exercise showed why Pipistrel and Airbus are making trainers and not practical airplanes—there’s
not enough energy. Despite this, I gave batteries one more chance, because they should improve in the
next few years. The Airbus battery cells provide 200 watt-hours of energy per kilogram. Projections
suggest that with substantial technology investment, advanced lithium cells may reach 400 Wh/kg by
2020. Doubling the energy means doubling the range to 200 miles, but that would still be far short of
NASA’s 921-mile goal.

6. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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As I confronted this reality, I began to think carefully about energy storage. I knew that fuel cells can
offer substantially more energy per unit mass than batteries. The trade-off is that to get this energy, fuel
cells have low specific power—the number of watts you can get per kilogram. Still, it was reasonable to
consider using fuel cells to power an aircraft: In 2008, for example, Boeing
, a two-seat
motor-glider, to electric power using a proton-exchange-membrane fuel cell in addition to batteries.
converted a Diamond HK36
Super Dimona (http://www.boeing.com/news/frontiers/archive/2008/may/ts_sf04.pdf)
So I considered the possibility of retrofitting a Cirrus SR22 with fuel cells. The SR22’s normal power
plant is a Continental IO-550-N, a six-cylinder, horizontally opposed, air-cooled engine that weighs 187
kg and provides 310 horsepower (231 kilowatts). By removing the engine and fuel and replacing them
with a fuel cell of the same weight, I could possibly produce a similar amount of power. But to do that,
the fuel cell would have to provide 500 W/kg of specific power. And at that level of specific power, the
specific energy of the fuel cell would be about 400 Wh/ kg, roughly as good as the best batteries I could
expect to use, which I already knew wouldn’t let my plane fly very far. To provide a specific energy of 800
Wh/kg, the fuel cell’s specific power drops to 200 W/ kg, well below the power needed to fly at 200 mph.
My options seemed to be narrowing the more I investigated the limits of electric energy storage. The only
solution was to reduce the energy and power demands of the aircraft. But I knew that the SR22 was
already a nearly optimal aircraft made of lightweight composites. Design tweaks wouldn’t cut it.
When an aircraft designer gets stuck between a rock and hard place, there are two ways out, and neither
one of them will make the boss happy: Compromise performance, or invest in technology development. I
decided to take a closer look at the performance before risking my design on unproven or expensive
technologies. But which performance requirement should be sacrificed, I wondered—speed or range?
Some of my earlier analyses had shown that the sensitivity to range was similar to the sensitivity to
speed, so my choice of the two had to rely on what it takes to compete in the four-seat general-aviation
market. I examined six different aircraft being sold and found that their ranges were all near NASA’s
objective of 921 miles, while their cruising speeds differed substantially. This variance led me to aim for
the longest possible range and worry less about speed. After a few more calculations, I decided to shoot
for 150 knots (173 mph, or 278 km/h) instead of the desired 175 knots.
Vapor and Some Rivals Compared
Reducing the cruise speed to this level diminished energy consumption by 30 percent, and it also slashed
the amount of power needed for propulsion. But it still didn’t put my design in any position to meet
NASA’s requirements. 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.

7. 4/16/2018 How I Designed a Practical Electric Plane for NASA - IEEE Spectrum
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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 flow 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 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 would create a lighter
and more compact system than was thought possible eight years ago, when Boeing first flew its fuel-cell
powered HK36. What’s more, all of these components would fit nicely into the Vapor’s airframe.
Toyota Mirai (https://ssl.toyota.com/mirai/fcv.html)
The fuel-cell system I had designed with available technology 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 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, fly, 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.
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.
Given recent advances

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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 first-place 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.
This article appears in the June 2016 print issue as “Fly the Electric Skies.”
About the Author
Tom Neuman is an engineer at
in
Michigan.
Toyota’s Technical Center
(http://www.toyota.com/usa/operations/engineering-manufacturing/toyota-technical-center)