
An aircraft with a parallel hybrid engine has been successfully tested in the UK - lelf
http://phys.org/news/2014-12-airplanes-hybrid-electric.html
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falcolas
A few notes from a hobby pilot:

The aircraft they are using for this test looks like it has glider-like wings,
which means it requires a lot less thrust to keep aloft, making a low torque
and low power electric motor more able to work with it, at the compromise of
lower speeds. A more traditional 2-4 seater aircraft capable of 100+ knots
would require significantly more power.

The payload capacity of a general aviation aircraft fueled with power-rich
gasoline is typically around 500 lbs. Unfortunately, the potential 30% savings
in fuel weight (~65 lbs of fuel in a Piper Cherokee) would not be offset by
the batteries (using the Prius' 100 lbs of batteries as a reference).

More weight = more power necessary to stay aloft = more power consumption from
both the motor and engine.

I hope they can get the technology to work, but there's many more hurdles for
a plane than there is for a car; however there have been electrically assisted
gliders for some time, so the potential is certainly there.

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minthd
Hybrid electric flight is a perfect fit for military UAV's. It enables longer
flight times, with lighter loads due to higher efficiencies, maybe at reduced
vibration(which can be a problem for imaging and reliability) due to better
engine design(because of the smaller design range), and at critical times when
total silence is needed , the UAV can run only on electricity and be quiet and
almost without vibration.

And it won't be surprising to learn that some militaries already have such
systems in use.

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matt_morgan
This article claims that the plane is more efficient, and that the batteries
recharge in flight, but it doesn't say where that gain in efficiency comes
from.

Hybrid cars work mainly by recovering energy from braking or going downhill to
charge the batteries. Planes don't do that. Where do they recover energy?

~~~
dragonwriter
> Hybrid cars work mainly by recovering energy from braking or going downhill
> to charge the batteries.

While hybrid cars do those things, they aren't the main source of the
efficiency of parallel hybrids, parallel hybrids like the Prius work by
keeping the engine at the most efficient throttle setting (which is much
higher than what would normally be used for cruising) whenever it is running,
and using excess power from the engine to recharge the batteries.

The article describes this aircraft as doing the same thing.

~~~
matt_morgan
Thanks, that makes sense. One other question: what is "excess power from the
engine" and how is it possible (2nd law) that using it to recharge the
batteries is better than using it to move the plane directly?

~~~
brulez
Planes need extra power for takeoff and emergency scenarios like a twin on a
single engine.

It's possible that these requirements lead to engines which have a optimal
power output above what would be required in straight & level flight.

~~~
bdamm
Many planes have a "constant speed propeller", which allows the engine to be
kept at its most efficient load, while adjusting the pitch of the propeller.
Optimal use of power needed for climbing but otherwise left unused in straight
and level flight is a solved problem.

~~~
darkmighty
How can you say the this approach is more efficient than one which keeps the
engine at a constant load and varies propeller speed, for example? (it
involves complicated fluid mechanics to figure it out)

This is a reason mathematicians loathe the overuse of "optimal" by engineers:
in the real world there are often too much variables.

~~~
lutorm
You can't have constant load -- the power needed is dictated by the
conditions. But what _is_ known is that you want to operate at full throttle
to be the most efficient, so the way to vary power is to vary RPM. (Propellers
are also generally more efficient at low RPM, but I don't think that's what
was referred to here.)

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pdabbadabba
The article is a little light on details, and does not cover how the batteries
are charged in flight. Reading to the end, though, it appears that this was a
technology demonstration designed only to show that an electric motor can
power an aircraft engine (a prop, I assume? This would be more interesting if
it were a jet aircraft.), and that the weight of the batteries is not a deal-
breaker, as some had thought they would be. So I suppose the batteries _weren
't_ recharged.

So, in short, the term "hybrid" makes this a little misleading, since it
conjures imagined of the hybrid technology in cars. But this was a hybrid in
the sense that an aircraft engine was powered by both a fossil-fuel burning
four-stroke engine, and an electric motor in the same flight.

Perhaps one day, though, light enough batteries would allow a fully electric
commercial aircraft (though hampered by the lack of an equivalent to
regenerative braking), or technologies could be developed to partially charge
the batteries using, e.g., air brakes during landing, or solar power.

~~~
notahacker
The accompanying video is much better than the article in that respect: it
explains quite clearly the additional thrust from the electric power is most
useful when the aircraft is climbing, and the rest of the time it can be
recharged via the main engine.

Fully electric commercial aircraft are going to need to stretch electric
engines as well as battery technology well beyond what they're presently
capable of. The video is probably being kind when suggesting it's mere
"decades" away.

~~~
phkahler
>> Fully electric commercial aircraft are going to need to stretch electric
engines as well as battery technology well beyond what they're presently
capable of.

No, just the battery. Electric motors can reach 96 percent efficiency at the
mid to high power levels you need in aircraft. I hit 95 percent for a motor
and inverter combined with I was in EV development. It's all battery
improvements from here.

~~~
notahacker
How does that efficiency transfer to edge cases like accelerating 500,000lb of
mass to Mach 0.75 at 30000ft through air though? That's pretty standard
performance for a modern commercial aircraft with intercontinental capability.
Turbofan engines might be pretty inefficient for many purposes, but excel at
generating the thrust required for commercial aircraft, which are rather more
demanding than lightweight piston-powered testbeds. Serious question: are
there _any_ existing electric propulsion systems remotely capable of the same
performance characteristics as a turbofan even assuming a zero weight battery?

~~~
phkahler
>> Serious question: are there any existing electric propulsion systems
remotely capable of the same performance characteristics as a turbofan even
assuming a zero weight battery?

Sorry about the late response, but NO. When I wrote that I forgot about power
density. Electric motors are currently better than (or comparable to) ICE, but
probably not close to a turbine.

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superuser2
The airline industry is essentially beholden to the price of fuel, right? If
an airline figures out how to not need so much fuel, they could undercut and
make a larger profit than everyone else.

Is there work on hybrid or electric passenger flight? Will there be?

~~~
_delirium
Fuel cost does have a huge impact, but up-front capital costs and maintenance
costs are also a big part of the picture. Which is why there are still a lot
of planes in major airlines' fleets that are not as fuel-efficient as the
latest models. Airlines are slowly turning them over, but the capital costs
for a new plane are very high [1], and they also have to fit any new models
into their maintenance plans.

[1] Around $200-400m ea for a modern wide-body, according to Boeing's pricing
page:
[http://www.boeing.com/boeing/commercial/prices/](http://www.boeing.com/boeing/commercial/prices/)

~~~
notahacker
Aircraft finance is readily available though, and at current prices airlines'
annual capital cost per aircraft (amortised over a 20 year lifespan, and often
on a lease for 5 years or less) is about half the annual fuel bill if that
aircraft is in regular operation. Whilst there are certainly savings to be had
from sharing existing spares pools and pilot and engineering type-ratings
across a large fleet[1] which make some optimisations unprofitable, a drastic
improvement in fuel economy would see Western fleets improved as fast as the
new aircraft could be delivered in an era where 1% per annum is the
improvement rate the engine OEMs strive for. The real obstacle would be
actually delivering workable technology and getting it through the
certification process. By which time we might all be flying delta-winged
contraptions with open rotors. Or more likely, similarly-shaped aluminium
tubes with turbofan engines that have improved an awful lot over several
decades.

[1]which is why Northwest/Delta were able to get away with running thirsty but
long paid-for DC-9s that had been obsolete for nearly a couple of decades
until recently.

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joshfraser
I've been fascinated with the idea of electric airplanes after hearing Elon
Musk discuss the benefit of being able to fly at a much higher altitude and
speed.

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ck2
That website is a facinating mixture of horrifying and then interesting news.

Going to a page like this gives you all sorts of scary related stories:

[http://phys.org/news/2014-05-mexico-nuke-
threat.html](http://phys.org/news/2014-05-mexico-nuke-threat.html)

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qwerta
Piston engines were already replaced by something more efficient: jet engine.
I dont buy that batteries are 'light enough'. More likely we will see modern
jet-turboprops even in tiny aircrafts. Or perhaps some sort of catapult for
faster take-off.

~~~
sokoloff
Turbines are excellent for reliability and power-to-weight ratio
specifications. They are worse than pistons for specific fuel consumption
overall, way worse for specific fuel consumption at low altitudes, and capital
cost specifications.

Which is more important in a given application depends of course, but small
turbines are terribly inefficient compared to pistons (or even to large
turbines), so I wouldn't expect to see your prediction come to pass.

~~~
qwerta
I think turboprops have lot of space for improvements. They are limited by
materials and cost.

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hartator
I don't fully get it. How come an airplane can recharge its battery in flight?
I was thinking it will need to use breaking energy at some point but airplanes
rarely slow down compared to cars.

~~~
phkahler
>> I don't fully get it. How come an airplane can recharge its battery in
flight?

In a normal plane takeoff is at full power, while cruise is at 75 percent
power. So build a smaller engine that can achieve what the big one did at 80
percent and supplement takeoff with the electric motor. Now during cruise you
can use the extra 5 percent to recharge the battery. Smaller engine = possibly
better efficiency and weight.

