
Why Cant Commercial Airplanes Go Faster? (2012) - jasonwilk
http://www.wired.com/2012/10/can-we-build-a-more-efficient-airplane-not-really-says-physics/
======
ricardobeat
This is in direct contradiction to this article (also posted today) on new
developments in tech: [http://www.compositesworld.com/articles/aeroengine-
composite...](http://www.compositesworld.com/articles/aeroengine-composites-
part-1-the-cmc-invasion)

~~~
iofj
Just because you can't gain efficiency by changing the speed of the plane,
with no other changes allowed, doesn't mean you have no options :

1) reduce weight (fuselage, engine, wings, have the passengers diet first,
...) 2) increase engine fuel efficiency, displace more air for fewer
hydrocarbons 3) (in the article) reduce drag on surfaces that don't generate
lift 4) lower cross section (a longer, thinner tube), of course, still needs
structural soundness

The lower cross section would have the additional advantage that it would make
the plane fly faster.

So in a way, the article is "wrong". You can make a faster plane, assuming you
can solve all the issues that the obvious modifications bring : longer,
thinner and lighter fuselage and wings, combined with similar efficiency
engines (of course they'd have to have that similar efficiency at higher
speeds).

Another way would be to fly (much) higher. But then the cost of climbing high
would start to offset the gains from flying faster.

------
grecy
I really like the simplified explanation of how much energy to takes to move
the air out of the way (v)^3 - I've heard this stated for vehicles before
which is why roof racks are so bad for mileage.

Does anyone know of a similarly simple explanation of how much energy a
vehicle must use relative to it's weight?

~~~
jcranmer
The best way to think of simple back-of-the-envelope calculations is this:

To go from point a to point b at a cruising speed of v, you need to speed up
to v, maintain v, and brake to a complete stop. Analyze each separately:

1\. Speeding up. Since this takes so little distance (compared to the total
length), let's ignore resistance. This speedup takes KE = ½·m·v² of energy to
do.

2\. Maintain speed. This means spending energy to resist air friction (drag)
and ground friction. Drag is ½·ρ·v²·Cd·A [0]; friction is (at most) μ·m·g.
Energy is force × distance, and since the distance at top speed is effectively
constant, it's going to be O(m + v²) in this case. [1]

3\. Braking. You can theoretically recover -½·m·v² here, but most of the
energy is going to be lost to heat.

So, when you sum these up, the energy it takes to drive a vehicle a certain
distance is linear in mass and quadratic in speed.

[0] The drag coefficient, Cd, is really weird. At low speeds, it's actually
proportional to 1/v instead of a constant (i.e., drag is linear in velocity,
not quadratic).

[1] Yeah, the article says that energy is proportional to v³. It's actually
wrong--the power is proportional to v³. Whether or not the total energy is
proportional to v³ or v² depends on what you hold constant: if you hold total
time constant, it's v³, but if you hold distance, it's v².

------
markbnj
Ah, nothing like a good old-fashioned brouhaha over angle of attack vs. the
Bernoulli principle. This same argument was going on when my Dad was training
as a private pilot back in the 60's. Tastes great! Less filling!

Personally, I think they're facets of the same thing, but I don't know
anything about it. Beyond that the piece seemed a bit of a mess to me. The
content has not aged well. I usually expect Wired to do a better job of
conserving the state of what they publish.

------
dmishe
Well then, how about charging more for faster flights, to account for fuel? I
mean, domestic business class isn't that great, but people who pay others
people money for it would presumably go for a faster flight.

~~~
stevewilhelm
> Well then, how about charging more for faster flights...

They did with limited success.
[https://en.wikipedia.org/wiki/Concorde](https://en.wikipedia.org/wiki/Concorde)

------
MrBra
I remember how happy I was when I firstly made this consideration myself :)
--> as racing cars have a back "aileron" which forces air to push them down
(and stick to the road), airplanes have the same thing but inverted, which
pushes them up. This becomes exceptionally clear when thinking about how flaps
change the shape of the wing.

------
yellowapple
I'm not sure if I agree with the conclusion of the article; if the problem is
that the effect of drag increases with speed, then the answer here seems to be
to decrease drag (perhaps with a more efficient flight profile). Or would that
interfere with lift?

------
tzs
OK, so the plane shoves air out of the way, and that takes energy. But when
the plane passes, air will fill the air tube. Is there any way in theory to
recover from that air filling the air tube some of the energy that was
expended in making the tube?

------
ryanburk
the article includes the link to the previous HN discussion on this here:
[http://news.ycombinator.com/item?id=4644712](http://news.ycombinator.com/item?id=4644712)

------
tantalor
Please fix the title!

Should be something like "Can We Build a More Efficient Airplane? Not Really,
Says Physics", not "Why Cant Commercial Airplanes Go Faster?"

------
velox_io
"Planes fly by throwing air down."

This is wrong. Planes actually fly by stretching air over their wings
(creating low pressure) and the higher pressure air below pushes the plane up
(to fill in the void) - So planes fly by moving air up.

See Bernoulli Principle
([https://en.wikipedia.org/wiki/Bernoulli%27s_principle](https://en.wikipedia.org/wiki/Bernoulli%27s_principle))

PS: The reason most commercial jets cruise at ~585mph (or 85% of Mach one) is
because of the huge amount of energy required to break the speed of sound.

Edit: I've you're going to downvote, you can at least comment.

~~~
ori_b
F=MA still holds for airplanes. If there's no downwards acceleration of air,
then there's no force to push the airplane upwards.

The way that this happens comes from a number of effects, from vortexes at the
tips of the wings to the angle of attack, but in the end it all comes down to
F=MA. And the only thing that can be accelerated by a plane is air. Ergo, air
is moving downwards so that the plane will move upwards.

~~~
deciplex
It's better to just remember that energy is conserved. Bernoulli's principle
is a real thing and will add to the lift a wing generates without contributing
to any net motion of the air upward or downward. The work done to compress the
air below the wing and expand it above helps keep the plane in the air.

But, as everyone else responding to the parent has pointed out, the forces
generated in this way are not nearly enough to keep the plane in the air on
their own.

~~~
vilhelm_s
The way I understood this, it's impossible to generate lift without creating a
net motion of air. Pressure is force per area, so saying that there is a
pressure difference between the upper and lower side of the wing means that
the air is exerting a force on the wing. By Newton's third law, that also
means that the wing is exerting a force on the air, which has to move.

I guess you can also see it from conservation of momentum. Gravity acts on the
plane, so each second there is a certain amount of downwards momentum added to
the system. But the plane is not in fact moving downwards, so the all the
downwards momentum has to end up in the air, so it has to move downwards. The
net "lift" force is equal to the rate at which momentum is added to air.

If I understand it correctly, you can calculate the lift on the wing either in
terms of the pressure on the two sides, or in terms of the downwards
acceleration of air--it should be two equivalent views giving the same result.
Bernoulli's principle gives a way to calculate the pressures if you only know
the velocity of the air (this is a typical situation in a wind tunnel, where
you make movies of moving smoke puffs), but it's not a separate effect from
the air motion.

~~~
deciplex
Unless I'm mistaken, ori_b is asserting that you can't generate lift without
sending air off in another direction (i.e. downwash), and this is false.
Again, most lift _is_ generated this way - my posts here seem to be mistaken
for arguments that Bernoulli force contributes significantly to the lift. It
does not. However, the work done to reconfigure the air surrounding the wing
by the Bernoulli principle will contribute to the lift, and while it does
result in moving air around (not necessarily down though, actually), it does
not impart any net motion to the air after the wing has passed.

~~~
vilhelm_s
Yes, both ori_b and I are asserting that you can't generate lift without
sending air off in another direction. If you don't impart a net motion to the
air, then by conservation of momentum you also can't generate a net lift.

