
How I designed a practical electric plane for a NASA competition - sohkamyung
http://spectrum.ieee.org/aerospace/aviation/how-i-designed-a-practical-electric-plane-for-nasa
======
thearn4
I work at NASA primarily supporting our aeronautics programs, and I'm excited
about all-electric and hybrid-electric civil aircraft conceptual design
analysis and optimization (for both fixed-wing and rotary-wing). It's one of
the big "what-if" areas that is being looked at by NASA aeronautics in the
next decade, and an area that has a surprising diversity of intuitive
reactions within the industry at large.

Meaning, there is a camp if industry experts that feels that it is a dead end,
and another that sees it as a practical way forward. My opinion is that there
are gains to be made there, but practical systems will have to be designed and
optimized in a far more integrated manner than how most civil aviation systems
have been in the past (integrating, for instance, the design optimization of
the propulsion, airframe, and mission profiles together at mid to high
fidelity early in the conceptual design process).

~~~
walrus01
> feels that it is a dead end, and another that sees it as a practical way
> forward

Until the Wh/kg ratio of batteries improves significantly, it will be a huge
problem to have electric planes. Look at the energy contents of 1 litre of
Jet-A or avgas compared to the same weight in batteries. Hydrogen + fuel cell
systems to achieve the same energy density require very costly extremely high
pressure tanks and fueling systems, and that is before you get into the
argument of whether hydrogen is a terrible use of energy.

If your hydrogen supply is coming from an electrolysis process an IMMENSE
amount of kWh has been consumed to fill your hydrogen tank, which should be
considered for the total environmental cost. So unless your electrolysis plant
is hooked up to a massive PV array or a 4GW nuclear power plant, the economics
are questionable.

for comparison:

lithium ion batteries 0.36–0.875 MJ/kg

Jet-A: 42.80 MJ/kg

~~~
kbenson
What efficiency is the engine converting those 42.80 MJ/kg into power at? I
know it's still far above batteries, but what's the actual level batteries
have to reach to be competitive in the power to weight ratio? Even that's not
the full picture though, as for a system like this you would presumably
include the engine weight for an engine that could produce comparable output,
but that makes it more complicated.

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6stringmerc
A very enlightening read and quite excellent in breaking down the approach and
technologies researched and employed in the concept! I'm an amateur in the
arena[1], albeit with some first-hand flight experience, but study this kind
of development every chance I can get. As an academic concept, this is superb
and the aerodynamic advantage of the prop/tail design deserves high praise -
that's some innovative thinking right there!

From an implementation standpoint I'm curious to how practical using a fuel
cell will be from an aviation safety and regulatory standpoint. From what I've
read regarding their implementation in automobiles, the testing is rigorous,
so that's a plus! The author does point out that the plane needs to be
accepted by the target markets, and honestly, I think the plane could be built
and working by 2020 but there'd be a total of six mechanics qualified in the
world to work on the thing. It might be an exaggeration, but give it a little
consideration.

We're talking a significant overhaul - an advancement, sure! - to the existing
notion of flight. Hey I'm cool with it if it works! I've got no love for
burning tons of fuel in the upper atmosphere. Yet also I've been quite
skeptical of all the excitement around electric battery tech - e.g. graphene,
super-capacitors - until it starts getting used and tested in real world
applications. Advancements are underway, sure; time will tell if they live up
to the hype though. See also: Moeller Air Car. There's something to be said
for the punch that Jet A still packs - the new FlyBoard air debuted recently
and I'm not sure an electric version is possible yet, or within the near
future.

[1] I participated in the LocalMotors LITECAR challenge and still am sore that
an "exoskeleton" and "body panels made out of this lighter metal" entries were
deemed more revolutionary than an organic composite sedan powered by
microturbines and electric motors because what was _said_ was the purpose of
the contest did not match what was _chosen_ in the long run. Grr.

~~~
pjc50
_There 's something to be said for the punch that Jet A still packs_

Personally I suspect that the US Navy nuclear-powered CO2-to-JetA system, or
something like it, will be the long term solution when fossil JetA becomes
prohibitively expensive. Electric prop planes are feasible but electric jets
really aren't.

~~~
Torkel
When I read this article and also when I see quadcopter endurance tests (just
over two hours) I come to the same conclusion as you do. Electric jets aren't
happening.

So it is so strange and fascinating that Elon Must seems to think that it
could be done.[1] Like: if we take todays lightest and most aerodynamic sail
plane, add batteries and an electric motor, and it's still worse than a cessna
[2] - how exactly does he reach the conclusion that electric jets, _vertically
starting and landing_ _jets_ , are feasible? I really wish for something like
the hyperloop design-docs to explain the thinking behind it :)

[1] e.g. [http://bgr.com/2016/02/07/elon-musk-electric-jet-great-
idea/](http://bgr.com/2016/02/07/elon-musk-electric-jet-great-idea/) \- “Well
I have been thinking about the vertical takeoff and landing of an electric jet
a bit more,” Musk said. “And I think I have something that might be close. I’m
quite tempted to do something about it.”

[2] MEA:
[https://www.iei.liu.se/machine/applications_re?l=en](https://www.iei.liu.se/machine/applications_re?l=en)

------
elmar
I have practical market experience on building a two passenger electrical
aircraft and trying to market it, what I found is that "range anxiety" is a
real issue for pilots and the market today for electrical planes is really
limited to trainers, so we decided to modify our aircraft to an hybrid
configuration.

~~~
paulmd
As a private pilot: yes, you absolutely do not want to be landing on empty.
Range is the same thing as endurance, and endurance means you have options
when things don't go your way. Endurance is already a concern even on
conventional motors - your average Cessna 150 only has a useful payload of 344
lbs with full fuel tanks. If you want to put two people in it, you're not
taking off with full tanks, especially once you account for possibly thinner
air above sea level or due to warm temperatures.

You need to keep a minimum (!) 30 minute reserve of fuel for VFR. The 150 has
a 4-hour max endurance with full tanks. Usually you get less, because that
doesn't include full-throttle time for takeoffs/climb. So if you take out half
the gas (buying yourself another 76 lbs of payload) you've actually removed
about 3/4 of the useful range of the aircraft. The same problem applies to
battery-driven electric aircraft - extra weight kills endurance, and
compensating for this causes a death spiral.

Twin-engine aircraft are really the way to fix this, but they're more
expensive to get certified for, more expensive to run, more expensive to
insure, etc. I'm a big fan of the Diamond DA42, twin diesels are cool as hell
and it's totally nuts how slowly they sip fuel.

The other thing not mentioned in the article is that with batteries, as you
drain them the output voltage also drops. That translates into reduced
torque/power, and possibly an aircraft with different handling characteristics
in stall situations. Conventional engines will deliver 100% power on-demand
right up until they're out of gas.

~~~
vvanders
The voltage droop isn't as bad as you'd think, I don't get limited power until
I'm below 10% in our Tesla 85D. Even "full throttle" for most electric motors
isn't near the max C rating(because amps = heat = power loss).

The rest of the stuff thought, yeah that's an issue and I feel like one of the
areas where Fuel Cells might actually be applicable(you don't have the long-
term leakage/storage/fueling issues as with cars).

~~~
paulmd
An aircraft can't afford to be hauling around 2-3x as much battery as is
actually necessary to fly, as is common in electric cars. I can assure you
than when an electric airplane is at the limit of its range, its performance
characteristics will change in a negative way.

Electric planes are already at a 100x disadvantage in terms of Specific Energy
- liquid fuels (JP-1 jet fuel or gasoline) contain about 46 MJ/kg while Li-ion
batteries contain about 0.36-0.875 MJ/kg. They cannot afford to make that a
300x disadvantage. It doesn't work in terms of lift.

As someone else posted in a thread, electric power already takes you from a
900 mile range to a 150 mile range. 50 miles is not acceptable because you're
talking <30 minutes of endurance including takeoff, and you have zero safety
margin if something should go wrong.

------
goodcanadian
This article has me thinking about hybrid electric aircraft (is anyone working
on that?). A small internal combustion engine could generate electricity to
charge the batteries to run electric motors. With this set up, you could still
get the efficiency advantage of the alternate propeller placement. The engine
could be much smaller and lighter than it is conventionally because the
additional power needed for take-off and climb thrust could be temporarily
drawn from batteries. Further, in the event of engine failure, you would still
have battery power available to the propellers for a short time in order to
make an emergency landing.

~~~
mikeash
It's definitely being worked on. Google "hybrid electric aircraft" and you'll
find some discussion of the efforts.

It's less advantageous than it is for a car. For one, the difference between
maximum power and cruise power is far lower in a typical light airplane than
it is in a typical car. A Prius might max out at 130hp but cruise at 25hp. A
light airplane often cruises at around 75% of maximum power. Another problem
is that the hybrid powertrain adds significant weight, which hurts efficiency
much more in airplanes than it does in cars. I don't think these kill the
advantages entirely, but it's probably why it's not as far along in airplanes
as it is in cars.

~~~
drostie
Right, the first thing to always remember when you're doing a hybrid is that
you're stating up-front, "I want to have two engines in this machine -- that's
a weight and space cost that I want to pay up-front." As mentioned by the
article, with airplanes when you increase weight everything has to get bigger
to accommodate it. That doesn't make it impossible, but does make it a bit
tricky.

Another interesting question: the fuel cell design in the original post is
probably not reversible, is it? It would be really cool if given 10-20 more
years of work we then used variable-pitch propellers with reversible fuel
cells allowing for a plane which, during descent, could regenerate most of the
energy used to make its ascent. It might also make pilots less worried about
limited range, because they'd know that in a pinch they could get some more
energy to the battery.

~~~
matheweis
> allowing for a plane which, during descent, could regenerate most of the
> energy used to make its ascent

The Pipistrel aircraft do almost exactly this. They claim about 10-15% energy
recovery rate for thier regenerative systems. No need for variable pitch
either; they simply windmill the main prop.

------
andrewtbham
Fuel cells are energy inefficient. The future of electric air craft is elon
musk's vtol super sonic design.

[https://medium.com/@andrewt3000/what-do-we-know-about-
elon-m...](https://medium.com/@andrewt3000/what-do-we-know-about-elon-musk-s-
electric-plane-design-981e704f59a3#.n5s0jmadr)

------
eloff
Why would people would consider electric for airplanes? You want maximum
energy density for weight. Jet fuel is pretty damn good when scored like that.
We can make synthetic jet fuel from green electricity - if the process uses
carbon from the atmosphere it's carbon neutral. Granted it would be expensive,
but that's neither here nor there until fossil fuels surpass that price point.

I suspect well-meaning people like the idea of an electric airplane because
it's greener, but FYI we live in a market economy. We will continue to extract
and burn fossil fuels until it is uneconomic to do so. No matter how many
people shout, no matter how much damage it does to the Earth, even if it kills
us all. Markets don't care. If you have a problem with that, as I do, the only
practical solution is to make fossil fuels not a cost effective alternative.
Electric airplanes could be part of a solution - but only if they can compete
on cost. I personally like the idea of taxing fossil fuels to factor in the
negative externalities - but given the political makeup of the world as a
bunch of competing nation-states, it seems unlikely. Game theory tells us
there is too much incentive to be the nation state that doesn't support the
agreement. Instead what I think will happen is that we'll innovate our way out
of the mess by making green technologies more cost effective than fossil
fuels.

~~~
sp332
"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."

"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."

------
rbf
Interesting read. I don't have high hopes for hydrogen cars as batteries seems
to solve the range problem adequately for most scenarios, and it's easier to
build charging stations than hydrogen filling infrastructure. However, for
planes it seems that the advantage of low weight is a necessity to
electrification. I wonder if the future will be (mostly) electric road
transport and hydrogen air transport.

~~~
6stringmerc
Agreed on the perspective regarding hydrogen cars - I never quite figured out
the thought process behind trying to make an experimental platform of that
type into a mass-consumer product. I mean, I guess it makes environmental
sense but the R&D path seemed so long and trying that EVs & hybrids really had
a lot of growing up time in parallel.

For me I was always more interested in a turbine system for cars that would
use an advanced biofuel (ex: eventual algae-based fuel) which would power
electric systems (wheel motors, some batteries, KERS) and sort of convert they
existing infrastructure into a more modern one - tanks, pumps, etc.

~~~
mikeash
Even from an environmental perspective, hydrogen cars make little sense.
Currently, hydrogen is a petroleum product. It's typically made from natural
gas, and this process emits plenty of CO2 (plus all the pollution inherent in
oil extraction). You can make hydrogen cleanly by using renewable electricity
to power electrolysis, but nobody is doing that currently and the efficiency
is terrible. You'd need three times as much generating capacity to power a
fleet of hydrogen vehicles as you would to power an equivalent fleet of EVs.

I really can't figure out why Toyota is pushing them so hard.

------
nxzero
>> "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."

Is this plane going to be in production?

~~~
6stringmerc
A reasonable question! From my experience with general and professional
aviators, the answer is a tentative "No" for quite a while. It would likely
start out as an "Experimental" certification by the FAA[1]. Following that,
I'm not sure what the course would be to official production for purchase
(though Experimental planes can be purchasd on the free market to my
knowledge).

[1]
[https://www.faa.gov/aircraft/air_cert/airworthiness_certific...](https://www.faa.gov/aircraft/air_cert/airworthiness_certification/sp_awcert/experiment/)

~~~
elmar
The easiest option is to sell it as a KIT where the buyer assembles it and
requests certification as an experimental Light Sport Aircraft.

[http://www.kitplanes.com/issues/29_3/builder_spotlight/Under...](http://www.kitplanes.com/issues/29_3/builder_spotlight/Understanding_Experimental_Light_Sport_Aircraft_20376-1.html)

------
XorNot
This is a fantastic read. But I'm now really curious what things look like at
the big end of the spectrum: does the solution scale (or do similar benefits
emerge) for 747-scale electric aircraft?

~~~
goodcanadian
I am not an aerospace engineer, so take my comments with a grain of salt.

This will not really scale to the commercial airliner market. The author got
most of his improvements out of propeller efficiency, and propellers are very
efficient. However, they don't scale to jet speeds. I am not at all certain
that an efficient electric powered jet engine could be produced (I suspect the
physics just won't support it). If you are willing to take a step back and use
hydrogen, I am very willing to believe that an efficient hydrogen powered jet
engine could be built, but that would be completely new technology and I would
bet it would take decades before it were available even if someone were to
start working on it immediately. Besides, it is not clear to me what the
advantage of using hydrogen over jet fuel would be. By weight, the energy
density is higher, but by volume it is much lower.

~~~
rjsw
Hydrogen has problems too. There is a chapter in the Ben Rich book "Skunk
Works" on trying to build a hydrogen powered plane and the supporting
infrastructure before they did the A12 & SR-71.

------
Gravityloss
Having the electric motors at the top of the V tail planes indeed enables a
short landing gear. But they increase the tail plane loads, requiring a
heavier structure there.

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fit2rule
I honestly cannot wait until these plane designs become more available ..
electric flight would be my preferred way of getting around.

------
Glyptodon
So I'm curious -- could a non-fuel cell electric generator (gas, turbine,
etc.) make this design perform even better?

------
mrfusion
I was under he impression that fuel cells were very expensive compared to
batteries. Is that not the case?

~~~
gvb
The contest required a cost analysis, but had no cost _requirement._

[http://aero.larc.nasa.gov/files/2014/09/2020-E-GA-design-
con...](http://aero.larc.nasa.gov/files/2014/09/2020-E-GA-design-contest-
ARMD-1.pdf)

~~~
kbenson
Yet the author thinks the cost is comparable, so there is that.

 _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._

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fvjwiur
why not store many small batteries in the wings ?

~~~
hcho
As the article states a few times, weight is the main issue.

~~~
fvjwiur
what part of lithium-ion batteries is heavy ? Lithium is light

~~~
ozzy6009
The rest of them. Lithium is only the electrolyte. They also need cathode and
anode materials.

Specific energy of gasoline: 46.4 MJ/kg

Specific energy of high end Li-ion batteries: 1.3 MJ/kg

~~~
fvjwiur
it amazes me that we've achieved so many hard things (going to the moon, to
Mars, to Saturn, making computers) and yet batteries are so primitive that we
can't even beat gasoline

