
Airplanes can't get much more efficient - milind
http://www.empiricalzeal.com/2012/10/12/can-we-build-a-more-efficient-airplane-not-really-says-physics/
======
mikeash
This doesn't make a whole lot of sense to me. Maybe I totally missed the point
somewhere, but as far as I can tell, it's basically saying that every airplane
has an optimal speed for best efficiency, and going slower makes it worse,
thus you can't make planes more efficient.

This totally ignores the possibility of using different airplane designs
altogether. Build something whose optimal speed is slower, and you can make
the whole thing more efficient.

Probably the best single-number measure of aerodynamic efficiency is the lift-
to-drag ratio. For most airliners, 787 most likely included, this ratio is in
the low 20s. The best machines out there are in the 70s. Of course, they fly
slow, which is a tradeoff airliners don't want to make, but it shows that
there's plenty of room for improvement.

Can we build a more efficient airplane? Of course! But we may not _want_ to.

~~~
callmebison
Reducing lift-to-drag ratio (for sailplanes for example) boils down to
increasing the aspect ratio (the ratio between the width of the wing and the
wingspan) and selecting an airfoil with a high coefficient of lift over
coefficient of drag (wings generate drag through flying, and the wings which
generate the most lift aren't necessarily efficient, which is why planes have
flaps).

See <http://en.wikipedia.org/wiki/File:Drag.jpg> for the different types of
drag on an aircraft (the 'pushing stuff through the air' drag is called
parasitic, and the 'side-effect of lift generation' drag is induced).

The problem is that by doing this, you increase the structural mass of the
wing for a given lift as you need a longer wing. There is a point on the
optimisation curve between increase in efficiency from AR & the increase in
structural weight from longer wings. Boeing and Airbus have teams of people
who do this stuff for a living.

There are also human factors involved, like airports being set up for a
maximum wingspan, and people being unlikely to choose an airline with slower
flights.

~~~
mikeash
I don't doubt that Boeing and Airbus have a lot of smart people. My whole
point is that there are things to optimize for besides brute efficiency. I'm
sure you could do a _lot_ better than a 787 in terms of gallons of fuel per
passenger-mile. I'm also sure that such an airliner would never sell, because
people don't want to take three days to cross the USA by airplane.

~~~
mcguire
Not especially; according to MacKay (the original article's reference) they're
roughly comparable now:

"To estimate the energy required to move freight by plane, per unit weight of
freight, ...then [a 747's] transport cost is 0.45 g, or roughly 1.2 kWh/ton-
km. This is just a little bigger than the transport cost of a truck, which is
1 kWh/ton-km....

"...This is a bit more efficient than a typical single-occupant car (12 km per
litre). So travelling by plane is more energy-efficient than car if there are
only one or two people in the car; and cars are more efficient if there are
three or more passengers in the vehicle."

[http://www.inference.phy.cam.ac.uk/withouthotair/cC/page_275...](http://www.inference.phy.cam.ac.uk/withouthotair/cC/page_275.shtml)

The original article is pretty bad; the answer to the title's question, "Can
we build a more efficient airplane?" is nothing but a link to MacKay. The rest
of the article entertaining lead-up fluff.

~~~
mikeash
You seem to be responding to me as if I'm comparing airliners to other forms
of transportation. I'm not. I'm comparing current airliners with imaginary
ones that are more efficient. The article claims that we've basically hit the
physical limit for airliner efficiency, while I maintain that this is
nonsense, and while we may have hit a limit for efficiency _while maintaining
all of the other attributes we want_ , we certainly haven't hit the limit of
efficiency period.

For example, an airplane with a 40:1 L/D (done in the 70s, easy today) able to
carry half its empty weight in payload (nothing too hard there) will require
about 0.3kWh/mile per ton of payload to maintain level flight. Even accounting
for propulsion inefficiencies, that's _way_ more efficient than the 747.

Why don't we have such airplanes? Not because it's somehow physically
impossible, nor even technologically impossible, but simply because it's not
worth the tradeoffs. Nobody cares about increasing the efficiency of air
travel if the result is an airplane that's ten times slower than a modern
airliner.

Can we build a more efficient airliner? Of course! To say "no" is to imply
that modern airliners are optimized for efficiency _above all other things_ ,
which is pretty much ridiculous on its face. Can we build a more efficient
airliner while still keeping all the _other_ stuff we want out of airliners,
like a mach 0.8 cruising speed and global range? Well, that's a bit harder.

------
rogerbinns
Note that the measurements are often cited per passenger. Part of the spat
between Boeing and Airbus over their next gen ~150 seat passenger planes
(737max & A320neo)[1] is over how many passengers are used in the calculations
(maximize for your own plane, minimize for the other guy). To get an idea of
how the argument goes see this article starting in the "war of words" section
which gives a good idea of all the factors making up the cost of flying:
[http://www.aspireaviation.com/2012/07/20/boeing-737-max-
ups-...](http://www.aspireaviation.com/2012/07/20/boeing-737-max-ups-the-ante-
in-dogfight-with-a320neo/)

The efficiency improvements come from all over the place including dealing
with wing tips (fences, winglets, "sharklets"), weight reductions (lighter
materials, redesigned components) and engines (weight, gearboxes, compression
ratios).

The plane manufacturers and engine manufacturers are constantly doing tweaks
to give improvements, usually managing around 1% per year. Search for
"performance improvement package" to see numerous press releases and articles.

In the future there are blended wing bodies and open rotor engines that have
another leap in efficiency.

[1] The tradeoffs are very similar to what we see in the IT industry. For
example the 737 is lighter and sits lower to the ground. That means it can't
use larger diameter more efficient engines. And makes it more efficient for
shorter routes due to less weight, but the heavier higher A320 can then be
more efficient over longer routes. Every change made involves retooling and
recertification so it isn't a simple decision to just do everything possible.
And newer better planes effect the residual values of older planes which makes
lessors potentially less likely to buy your newer planes due to economic
uncertainty. There was a lot of debate and speculation about re-engining the
existing models, re-winging too, fuselage material changes etc.

------
perlgeek
The central argument of the article seems to be that an air plane must spend
energy for drag an lift, and there's a limit to how good you can get at those.

That's all fine in the current model, but other modes of transport are
thinkable. For example an airship that doesn't need to spend energy on the
lift, or a ballistic transport model (think ICBM) that uses flight parabolas
around the curvature of the earth.

Ok, you probably wouldn't call their "airplanes" anymore, at least not by
traditional terminology.

Update: oh, and you can always try to improve the constant factors involved,
like the total aerodynamic drag and weight

~~~
callmebison
Given current technology, airships are slow, and suck at handling high winds.
Ballistic transport would cost a lot more fuel than current methods, as you
need huge amounts of energy to escape gravity.

If you are going for crazy, why not use a nuclear reactor?
<http://en.wikipedia.org/wiki/Convair_X-6>

------
frankus
One gripe: The OP goes on about how energy required goes up with the cube of
speed, but that's only true if the flights you're comparing take the same
amount of time.

Common sense says you should compare two flights covering the same distance.
The flight that's twice as fast takes half as long, so you end up with the
energy required being proportional to the square of velocity.

(The power required is still proportional to the cube of speed, which is why
e.g. a Bugatti Veyron costs so much.)

------
josephlord
This seems overly pessimistic to me. While doubling efficiency may be
impossible 10% is far too low as an estimate without a fairly short timeframe.

Weight reduction - Reduce the weight and you can reduce the air you need to
move.

There are also conceptual designs for integrated wing aircraft that if they
were easier to make and could be made to work with current airports might have
further real gains.

These added to the engines and other factors mentioned probably make at least
a 40-50% improvement possible (although not necessarily easy or quick to
achieve).

This doesn't break the concepts explained or mean that there is unlimited room
for improvement but 10% is not going to be the limit by a long way.

------
janus
The problem with this analysis is that it doesn't take into account the fact
that better wing profiles also improve performance by reducing drag, as we
have seen with sailplanes in the last thirty years.

A modern sailplane can fly at higher speeds than those with thicker wing
profiles with less drag, optimizing the lift. This is an approach that modern
airliners could follow, optimizing the lift and reduce the drag, to save on
fuel, not reducing the speed.

The engine is not the only thing that can be optimized.

<http://soaringcafe.com/2011/01/concordia-supersegler/>

------
zhoutong
Not only that. Because the air density is different at different heights,
there's actually an optimal speed for every possible height.

The optimal height of an aircraft, however, depends on the mass of itself and
the payload.

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cryptoz
Physics says no such thing. Flagged for ridiculous headline.

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jivatmanx
This Kickstarter project with a completely new, futuristic looking airplane
design seems fairly confident that there is.

[http://www.kickstarter.com/projects/launchsynergy/synergy-
ai...](http://www.kickstarter.com/projects/launchsynergy/synergy-aircraft-
project?ref=category)

(I'll have to defer commenting to someone with knowledge of physics than me)

~~~
callmebison
[http://www.fzt.haw-
hamburg.de/pers/Scholz/Airport2030/Airpor...](http://www.fzt.haw-
hamburg.de/pers/Scholz/Airport2030/Airport2030_PUB_DLRK_11-09-27.pdf) reckons
a ~10% efficiency increase, but the wing is twice as heavy!

------
jamesgriffiths
David MacKay's book "Sustainable Energy - without the hot air" goes into a
huge amount of detail about plane efficiency
[http://www.inference.phy.cam.ac.uk/withouthotair/cC/page_269...](http://www.inference.phy.cam.ac.uk/withouthotair/cC/page_269.shtml)

It's also a great book

------
milind
Correct, maybe that’s why such articles are interesting. They challenge an
assumption and or facts as known today and now it’s up to someone to counter
it. I really liked the way author explained the complex topic with simple
sketches.

------
leeoniya
flying wings like the B2 are much less stable without computer correction, but
significantly more efficient.

also, the SR-71 is very efficient considering it can cruise at mach 3.2, which
is a pretty insane feat.

------
mariusandreiana
Really cool explanations.

Quote: What you find is that it really just depends on a few factors: the
shape and surface of the plane, and the efficiency of its engine.

It would also depend on weight. New planes are also lighter, as article
mentions (eliminated aluminum sheets and fasteners). Some other weight could
be eliminated by replacing in-flight entertainment screens in every chair and
wires required for them with tablets + wireless.

~~~
kordless
The explanations are textbook, but they leave out an important detail. Wings
(mostly) generate lift from a difference in pressure above and below the wing
while in flight. The reduced pressure above the wing basically 'sucks' it
upwards. The result of this pressure difference creates a massive downward
force behind the wing, which is usually what's referenced when explaining
these things. Here's a paper that explains it a bit more:
[http://www.pilotfriend.com/training/flight_training/aero/lif...](http://www.pilotfriend.com/training/flight_training/aero/lift.htm)

~~~
JoeAltmaier
Urban legend. Wings deflect air downward. The OP even linked to the xkcd comic
debunking this.

------
mariusz79
It's funny but usually when I read article about something being close to it's
limits, in a day or two there is an article somewhere else on the web, that
describes potential solution and invalidates that limit.

------
pyrotechnick
Air travel is nearing capacity and will soon reach its peak.

High-speed rail and tubes are the future of transport.

~~~
jug6ernaut
For smaller high density countries i would agree yes. For larger more spread
out countries high-speed rail systems(assuming your referring to maglev) are
simply to expensive at the current time.

I have no idea what you are referring to by tubes though.

~~~
snogglethorpe
> _I have no idea what you are referring to by tubes though._

Probably a reference to Elon Musk's recent claim that he had come up with some
secret technology that could dramatically improve long distance
transportation, with much (much) higher speed, lower energy usage, and (much)
lower cost than conventional HSR.

Unfortunately the only detail Musk would give was a very hand-wavy reference
to "tubes," and a subsequent clarification that he didn't mean the usual sort
of "evacuated tube transport" people have talked about for ages and ages.

Based on all the discussion I've seen, though, it seems pretty likely it's
just a vague idea Musk had that he didn't bother to think through, and it will
fall apart when he tries to fill in the details...

