
Electrifying flight: very different aircraft may end up taking to the sky - prostoalex
http://economist.com/news/science-and-technology/21664944-using-electric-and-hybrid-forms-propulsion-very-different-looking-aircraft?cid1=cust/ednew/n/n/n/20150917n/owned/n/n/nwl/n/n/NA/email
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Animats
A related possibility is multi-rotor helicopters where the main power source
is a jet engine, but the power distribution is electrical. One of the huge
headaches of the Osprey is the mechanical linkage by which one engine can
power both props. There are flexible shafts, U-joints gears, and a big clutch.
This might provide a way to get to VTOL without insane mechanical complexity.

NASA has built a big model tiltrotor craft with 10 props to test this.[1]
There's also the Latitude VTOL drone.[2] This simply has electric quadrotors
for VTOL, plus a gas engine, wings, and prop for sustained flight. So it has
the range of a winged drone, without the need for a runway.

[1] [http://www.extremetech.com/extreme/188338-nasas-electric-
ver...](http://www.extremetech.com/extreme/188338-nasas-electric-vertical-
takeoff-airplane-takes-first-flight-aims-to-eventually-replace-the-helicopter)
[2]
[https://latitudeengineering.com/products/hq/](https://latitudeengineering.com/products/hq/)

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erikpukinskis
Elon Musk has said he would just put one big engine on a gimbal and just ditch
the ailerons and everything. No clutch necessary.

~~~
Animats
One-engine VTOLs need an ejection seat.

NASA built several "one big engine on a gimbal" lunar landing trainers for
Apollo. Over the short life of those special-purpose craft, three pilots had
to eject - two astronauts and a test pilot.

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nkoren
The real breakthrough in electric aviation will come when it is combined with
beamed energy. Rather than storing power onboard the vehicle, you have ground-
based stations which power the vehicles with tracking microwave tightbeams.
That way, you only need enough onboard battery power to take off, land, and
bypass one broken ground station -- maybe 15-20 minutes of total flight time.
This would allow electric aircraft to be considerably lighter than hydrocarbon
vehicles, even considering the current energy-density of batteries.

Beamed-energy flight systems are being developed for space launch[1], but I
don't know of anybody yet doing this for plain old aviation. It's a gap
waiting to be filled.

1: [http://escapedynamics.com/](http://escapedynamics.com/)

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mrfusion
What about the beams killing birds? Maybe have three beams that don't converge
until the plane?

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debacle
Forget the birds, lets not microwave the atmosphere.

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nkoren
If you think about it (or run the numbers), you'll realise that our present
system of jet engines actually dumps far more heat into the atmosphere than
this would.

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BetaCygni
Non-paywall link:
[https://www.google.nl/url?sa=t&rct=j&q=&esrc=s&source=web&cd...](https://www.google.nl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAAahUKEwi_tYqN55HIAhVIbxQKHXGIA_w&url=http%3A%2F%2Fwww.economist.com%2Fnews%2Fscience-
and-technology%2F21664944-using-electric-and-hybrid-forms-propulsion-very-
different-looking-aircraft&usg=AFQjCNFpotPOlevrJ1nEQYgreE7hj1I_5A)

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peteretep
I'm a paying customer, and I'm using your link because logging in on my
computer will log me out on my iPhone, along with a security error. This, for
content I've paid for. I hate the future.

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gambiting
The batteries have also another problem not mentioned in the article - you
have to carry their weight for the entire duration of the flight, while with
regular fuel you burn it up during the flight, making the aircraft lighter and
more efficient. Not to mention, that many aircraft can't land with their tanks
full - but electric aircraft will have to be able to land with the same weight
they took off with.

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Sanddancer
I don't think that's an issue, especially with the projected use cases. The
article says that they're aiming for battery storage of about an hour of
flight time, which is plenty for takeoff, final approach, and other situations
where you'd want a lot of power in a hurry. Most of the flight, unless it is a
very short commuter-type flight where you can conceivably charge with shore
power between flights, is going to be running much like a hybrid car, with the
jet engine powering a generator working at the best point on its
performance/economy curve. So it'll still use fuel, and still get lighter as
the trip goes on. Also, those batteries mean you don't need a set of engines
that need to handle the power needed at take off, so those can be lighter as
well. So I imagine that a 1:1 replacement capacity-wise would probably have at
worse the same empty weight, taking into consideration the lighter engines and
the smaller fuel tank that would be needed.

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ams6110
Li ion batteries still have a LONG way to go in energy density compared to Jet
A kerosene.

Jet A, about 43 MJ/kg. Lithium ion, <1 MJ/kg.

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alkonaut
You have to consider the system as a whole. The kerosene tank with kerosene,
plus the motor and gearboxes = X kg. The battery + The electrical motors and
driveline = Y kg.

Example take a small aircraft as an example:

\- Combustion: Motor 150kg (1kW per kilo) fuel 100kg (43MJ/kg) = 250 kg,
4300MJ

\- Electrical: Motor 20kg (5kW per kilo) batteries 230kg (1MJ/kg) = 250kg,
230MJ

The amount of batteries carried is more than twice the weight of the fuel, so
the difference in carried energy between those planes is now no longer 43x,
its < 20x. Still a big gap, but the point is that it's much smaller when all
things are considered.

Now consider any additional weight we didn't count here (gearboxes or other
drive line components), the complexity of those planes, e.g. the number of
moving parts. Also consider noise level, fuel economy, environment,
reliability. For multi-engine craft it becomes even more pronounced. I don't
think batteries will ever _need_ to come close to the energy density of
combustible fuels for electric flight to be viable.

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thedrbrian
On the other hand batteries don't get that much lighter as you use the energy
stored within. Also you're going to have to design quick capacity change
battery packs or always fly with the maximum weight penalty.

~~~
alkonaut
That's true, the average weight of the fuel is probably 60-70% of the takeoff
weight or something, varying wildly depending on aircraft type and flight.
Takeoff weight is probably the limiting design factor, so there you can
compare the batteries to the full tank, which is what I have above.

On the other hand jet airliners are designed to be lighter when they land,
landing gear and brakes don't have to manage a landing at full weight (which
is why they have to dump fuel before making an emergency landing immediately
after takeoff). An electrical plane would have to have brakes and landing gear
designed for the heavy batteries, as it will be as heavy at landing as it was
at takeoff. This might add quite a lot of weight.

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joekim29
So basically, once we get technology that doesn't exist yet, we may get
vehicles that take advantage of that technology.

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m4x
Well, sure, but you could have said the same thing about modern airliners when
Louis Blériot flew across the channel for the first time in 1909.

Electric aircraft are a technology that does exist, already. Two different
aircraft have been built and flown over the channel already. Electric
airliners will require significant additional tech development, but the fact
that planes have already crossed the channel multiple times is a pretty good
start.

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rflrob
I would be curious whether electric planes significantly reduce the carbon
footprint of flying. My intuition is that you wouldn't be dumping CO2 and H2O
(both fairly potent greenhouse gases) directly into the upper atmosphere, so
it would be a huge win, but I am neither a climate scientist nor an aerospace
engineer, so maybe I'm missing something.

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Shadow6363
Water is a greenhouse gas?

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credit_guy
Not only water (vapor) is a greenhouse gas, it's by far the most important.
Per wikipedia, its contribution to the greenhouse effect is 36-72%, while the
next contribution, of CO2, is 9-26%.

[https://en.m.wikipedia.org/wiki/Greenhouse_gas](https://en.m.wikipedia.org/wiki/Greenhouse_gas)

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tajen
The article raises all hopes, then finishes with the necessity of developing
superconductivity.

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leoedin
We _have_ developed superconductivity. MRI scanners wouldn't work without it.
The problem is that it needs to be cooled to very low temperatures, which
requires complicated plumbing and expensive cryogenic gases. Not really
something viable on an aircraft. Superconductors have been slowly improving in
operating temperature though - perhaps the temperatures found at aircraft
cruising altitude are within reach?

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amalag
What about hydrogen fuel cells? I thought that is the high density energy
approach electrical planes will take.

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mmariani
For anyone who would like to watch a cool video of the MVP being built, here
it is... [https://youtu.be/qfBfZJBQH_I](https://youtu.be/qfBfZJBQH_I)

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Havoc
Suddenly Musk's cameo line in Iron Man 2 seems a touch prophetic despite being
intended as ludicrous at the time.

