
World-record electric motor for aircraft - jonbaer
http://phys.org/news/2015-04-world-record-electric-motor-aircraft.html
======
paulsutter
While weight is very important in electric aircraft, I was hoping to see an
efficiency advantage as well (batteries weigh more than motors).

The top advantage of electric motors for aircraft is that they can fly at much
higher altitude, therefore much lower air resistance, reducing energy
requirements. Fuel-burning jet engines require a certain oxygen density to
operate[1]. Electric motors don't need oxygen, and propellers/fans can operate
even with very low air pressure.

[1]
[http://en.wikipedia.org/wiki/Jet_engine](http://en.wikipedia.org/wiki/Jet_engine):

"The limit on maximum altitude for engines is set by flammability- at very
high altitudes the air becomes too thin to burn, or after compression, too
hot. For turbojet engines altitudes of about 40 km appear to be possible,
whereas for ramjet engines 55 km may be achievable. Scramjets may
theoretically manage 75 km."

~~~
baddox
> and propellers can operate even with very low air pressure

How does that work exactly? Of course, my intuition is that lower air pressure
means less to "push off of" but also less drag. And you've got certain
constant overhead, like internal friction. Is there some sweet spot of air
pressure where you'll get the best efficiency?

~~~
krschultz
You change the pitch on the propellor. The steeper the pitch, the more air is
grabbed on each revolution.

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beloch
Let's do a quick comparison.

A candidate plane for this engine would be the Cesna Turbo Stationair.

-It is a six passenger plane with a base operating weight of about a ton.

-It's engine delivers 231 kW, which is slightly less than Siemens electric engine. It's mass is approx. 200 kg, so the Siemens engine frees up 150 kg of weight.

-It's fuel capacity is 237 kg of 100LL avgas. With an energy density of 44.0 MJ/kg, this gives the plane a stored energy of 10.4 GJ. The plane's max range is 1300 km.

-Tesla's best battery (the Smart) has an energy density of 132 WH/kg, which is 475.2 kJ/kg. That means you'd need 21.9 thousand kilograms of battery to store the same energy that's in the Cesna's tank when full.

-If we assume the plane's range is linearly related to it's stored fuel (an approximation not really accounting for take-offs), 237 kg plus the 150 kg saved in engine weight gives 0.051 GJ of stored energy, which would reduce the plane's range to 5% of normal, or about 63 km.

Based on this, don't expect an EV passenger plane anytime soon. Battery
technology just isn't good enough yet. A hybrid with a gas-turbine generator
is much more likely in the near future. This engine may also have applications
where range is not important, such as training, stunt and racing planes.

~~~
tlb
Piston engines are only about 30% efficient in converting gasoline to
mechanical energy. While battery/electric can be around 90%. So you may be off
by a factor of 3.

~~~
beloch
My intention was just to get ballpark close. 15% of normal range is still not
very good. Battery's may get there eventually, but not yet. As stated in the
article, they're going after hybrid applications and this is probably why.

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glabifrons
I don't see how hybrid-aircraft make any sense.

The whole point of hybrid cars are the fact that you're only on the throttle
periodically. You start, stop, speed up, slow down all the time (especially in
S. Florida!). Normally when you're slowing down, you're burning off the energy
as heat (friction from the brakes), etc. Adding an electric motor/generator to
this allows you to recapture energy that is normally wasted slowing back down,
so it can be reused to re-accelerate back up to speed. This is why hybrids
(unlike non-hybrid cars) get better fuel mileage in town than on the freeway.

Aircraft, on the other hand, are at full-throttle on takeoff, and close to 3/4
throttle during the vast majority of the flight. There is no slowdown to
recoup the energy until coming in for a landing. @beloch has already done the
math showing the electric range would be downright miniscule, so there's very,
very little to be gained here.

Until we have some utterly astounding advances in battery (or more likely,
supercapacitor) tech (probably via graphene), this won't really be a cost-
effective solution.

However, this would be an absolutely incredible motor for electric car
conversions. Imagine a pair of these (each powering an axle)... it would be
considerably more powerful than even the Tesla Model S P85D.

~~~
nether
Electric aircraft can recharge in flight from the sun. This will mostly be
long endurance, high altitude utility aircraft, like Facebook/Titan
Aerospace's wifi UAV or surveillance drones that can stay airborne
indefinitely. These aircraft can be pseudo satellites without requiring a
rocket launch. They must be extremely low in drag and weight efficient, with
powerplant weight being a major issue because previously large electric motors
have not had application in aerospace and have not been weight optimized as
gas turbines have. Siemens has seen the growing market (QinetiQ Zephyr for
another example) and is responding to it. We definitely won't be seeing any
passenger aircraft electrically propelled anytime soon.

~~~
glabifrons
The article specifically mentions using this motor for hybrid passenger
aircraft (which is what my post addressed). I mentioned nothing about UAVs or
solar power (which is only possible with an enormous wing/fuselage ratio).

FTA: "This innovation will make it possible to build series hybrid-electric
aircraft with four or more seats," said Frank Anton, Head of eAircraft at
Siemens Corporate Technology

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staunch
Elon Musk has joked that one day he will merge the work of Tesla and SpaceX
into an electric jet. I'm hoping we'll end up with electric-powered Star Trek
shuttlpods with Falcon-derrived takeoff-landing thrusters.

~~~
pchristensen
That's what he said in his cameo in Iron Man 2 - "I've got an idea for an
electric jet"

[https://www.youtube.com/watch?v=EuG2AVFB-g0](https://www.youtube.com/watch?v=EuG2AVFB-g0)

~~~
staunch
He has also discussed it on stage at least a few times.

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avdempsey
If you're interested in this you should check out the Electric Aircraft
Symposium (taking place an hour North of San Francisco) on May 1-2 [1].

A quieter, more efficient aircraft with full auto-pilot could make general
aviation attainable for far more people.

[1] [http://eas2015.cafefoundation.org](http://eas2015.cafefoundation.org)

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gsinkin
What are the benefits of having an electric motor? I imagine that fewer moving
parts means less maintenance and lower odds of failure, but what are the other
advantages? Quieter perhaps? Are these aircraft going to be loaded up with
battery packs? How is that going to work?

~~~
bhayden
The quiet aspect probably has a lot of value in military applications. And
it'd be useful in the solar powered unmanned planes we see, but maybe the
energy requirements for this are too high.

~~~
euyyn
Even if the motor is quiet you still have the propeller making a lot of noise.
I (honestly) don't know if you can really get it quiet enough for the motor's
noise to be important.

~~~
davepage
> The propeller, even at 500 feet over the maintenance area, made only a light
> flutter, heard just as it approached.

[http://en.wikipedia.org/wiki/Lockheed_YO-3](http://en.wikipedia.org/wiki/Lockheed_YO-3)

The largest source of propeller noise is transonic flow at the tips, so
turning the propeller slowly helps significantly. The tradeoff is a larger
propeller and more expensive engine installation due to the required gearbox.

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DigitalSea
Whatever reduces our reliance on fossil fuels and leads to reduced emissions I
am down with. I see some people getting confused, this is not about a solely
electric plane. The applications here would be for a hybrid plane that
utilises both existing jet engine technology and electric tech. Like a hybrid
vehicle, which hopefully leads to reduce fuel costs, greater distances
possible to travel in a plane.

Imagine if for take off and landing the pilot can fire up the existing jet
engines, then whilst cruising fire up the electric engines and wind back the
output from the jet engines (leading to a slower burn rate). You would still
be using both, but you would have some additional boost from the electric
engines. Put some lightweight solar panels on the roof of the plane, charging
up the batteries while the plane sits on the tarmac and readies for taxi. Take
the output of the existing engines and use it to charge the batteries in
flight (like an alternator in a vehicle, but on steroids).

Of course, weight is the limiting factor of all of this. I am more thinking
out aloud here, when it comes to reducing our reliance on fossil fuels, I
think anything (even a marginally small improvement) is a great step forward.

The future is looking great.

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WatchDog
Would driving one of these engines concurrently with the power generated by a
turbine engine make sense?

Does the thrust generated by a turbine engine hit some point of diminishing
returns that converting the engine energy to electricity and powering this
engine be more efficient?

I know Boeing have made advancements in the efficiency of drawing electric
power from a turbine engine with the 787, the generators are now directly
connected to the transmission of the engine.

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ghostberry
Out of interest, I looked up the motor used in the new Formula E racing cars.
[https://en.wikipedia.org/wiki/Spark-
Renault_SRT_01E#Electric...](https://en.wikipedia.org/wiki/Spark-
Renault_SRT_01E#Electric_Motor)

I'm sure there are details other than power and weight which makes a direct
comparison unfair.

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mapt
Could be useful for airships, and for long-endurance solar drones, where the
weight is costly.

I really expected this to be about high-temperature superconductor motors.
Whatever happened to those?

~~~
lotsofmangos
They tend to stay on the ground, beside the tank of liquid nitrogen.

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nikolaj
passenger quadcopters anyone?

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bigiain
To be fair, I'd prefer a hex or octo - gimme a little bit of redundancy, at
least in the first few versions...

~~~
bhayden
Redundancy for what? Normal helicopters only have a single rotor and are
perfectly safe.

~~~
bigiain
Normal helicopters can autorotate[1] to a landing if the motor stops. A quad
is unflyable with one motor out[2]. An octo can self correct for a single
motor failure, and is still controllable with two motors out if you get lucky
about which of the motors fail second.

[1]
[http://en.wikipedia.org/wiki/Autorotation](http://en.wikipedia.org/wiki/Autorotation)

[2] it's probably theoretically possible that a quad with reversible motors
(or reverse pitchable blades) could be built to survive a single motor
failure, but so far as I can tell nobody, at least in the hobby size
quadcopter world, has done so (thoughI'd be surprised if the KMel Robotics and
Pennsylvania University research teams don't know exactly how to do it).

~~~
sjtrny
> but so far as I can tell nobody, at least in the hobby size quadcopter
> world, has done so

It's not the motors that are the problem. They can spin freely any direction.
It's programming the ESCs to be reversible.

Anyway it's already been done a hundred times over:

\- Fixed pitch props but reversible ESCs
[https://www.youtube.com/watch?v=UoReJULASkw](https://www.youtube.com/watch?v=UoReJULASkw)

\- Variable pitch prop styles exist such as HobbyKings Reaper
[https://www.youtube.com/watch?v=mAFxkHAiimY](https://www.youtube.com/watch?v=mAFxkHAiimY)
and Curtis Youngblood's StingRay
[http://curtisyoungblood.com/V2/products/quadcopters/stingray...](http://curtisyoungblood.com/V2/products/quadcopters/stingray-500).

Autorotation on quadcopters is possible with the variable pitch configuration.

~~~
bigiain
>> but so far as I can tell nobody, at least in the hobby size quadcopter
world, has done so

> It's not the motors that are the problem. They can spin freely any
> direction. It's programming the ESCs to be reversible.

True - poor wording on my part. And even the ESC programming is a "solved
problem" \- all the brushless RC cars run ESCs which understand how to run
brushless motors in both directions.

So actually reversing the thrust is, as you say, already happening. What I've
not seen (yet) is a controller board with software designed to use that
ability to stabilise and safely land a quad with one motor out.

