
Xkcd: Airfoil - So what's the right explanation? - mhw
http://xkcd.com/803/
======
wazoox
Well, actually the Bernouilli effect has its part, and so has the Coanda
effect; however most of these explanation concentrate on an artificial setup,
without taking in to account the actual angle that the wing makes with the air
stream. When you learn to fly, you're first told that the angle of attack is
the most important parameter to get lift. See the wikipedia page :
<http://en.wikipedia.org/wiki/Angle_of_attack>

The Bernouilli and Coanda effects, in most flight configurations are of
relatively minor importance, except on some particular aircrafts.

~~~
Isamu
Consider a steep angle of attack on takeoff. The lower side of the airfoil is
more exposed to the onrush of air, and clearly this pushes the airfoil
upwards. But the significance of the Coanda effect is that the air also wraps
around the top of the airfoil, adhering to the surface, so that it too exits
downward. If it didn't adhere to the surface (which happens in a stall) then
you would lose all the downward force of that half of the airflow.

~~~
Retric
Downward force on a wing _reduces_ lift.

A laminar flow over a wing can maintain a lower pressure above the wing than a
chaotic flow. The best simple way to think about this is pooring water from a
2 liter bottle. As you tip it over the pressure of the air inside the bottle
decreases and new air is pushed into the bottle filling the partial vacume.
However, if you tip it to far you get chaotic behavior and the average
pressure increases.

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keitmo
As a private pilot and Cessna 182 owner I can say, with absolute authority,
you're all wrong.

Planes don't fly because of the Bernouilli Effect, or the Coanda Effect, or
angle attack, or any of these other technical explanations.

Planes fly because of money. As soon as you stop throwing money at them, they
stop flying.

Ask any plane owner and I think they'll agree.

~~~
quux
Student pilot here, this man speaks the truth.

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araneae
"It's complicated" actually kind of describes it. From the NASA website: "The
real details of how an object generates lift are very complex and do not lend
themselves to simplification...To truly understand the details of the
generation of lift, one has to have a good working knowledge of the Euler
Equations." - <http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html>

That said, I like this handling of it:
<http://www.amasci.com/miscon/miscon4.html#wing>

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hristov
I have seen many examples of this incorrect explanation in textbooks and
posters. It boggles the mind how it has survived for so long. I actually
shockingly saw the same incorrect explanation in one episode of NOVA (yes it
has gone downhill).

The biggest problem with this diagram is that the air leaving the wing is
horizontal, which is (a) wrong and (b) really screws up a kid's understanding
of Newton's laws. If the diagram correctly showed the air being forced down by
the wing, then a kid could figure out that the wing creates a downward force
on the air and the air creates an equal and opposite upward force on the wing.

~~~
borism
It seems to me it's you who got things wrong. Explanation about wing forcing
air down is very VERY wrong.

~~~
hristov
How so? The wing has to exert a downward force on something if it wants to go
up? If not air then what ?

~~~
borism
the lift comes from pressure differential between "top" and "bottom" surface
of the wing - that's the definition used by professionals in the field.

Here's why definition about deflection of the air is generally not accepted:
[http://en.wikipedia.org/wiki/Lift_(force)#Criticisms_of_defl...](http://en.wikipedia.org/wiki/Lift_\(force\)#Criticisms_of_deflection.2Fturning)

~~~
hristov
The link you provided does not support what you are saying. The link you
provided actually agrees with me saying that the theory about air being
deflected down is correct. It merely says that while correct, this explanation
is not rigorous enough to do engineering. Which is totally right. But I was
not trying to do any engineering, I was trying to provide a simple explanation
about how a wing works and how that relates to Newton's third law of motion.

~~~
borism
I'm glad that you found yourself vindicated and will refrain from interfering
with your righteousness.

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vitovito
The Coanda effect. Here's Jef Raskin's essay explaining it:
[http://web.archive.org/web/20070928072421/http://jef.raskinc...](http://web.archive.org/web/20070928072421/http://jef.raskincenter.org/published/coanda_effect.html)

~~~
hristov
This cannot be the Coanada effect because the airflow leaving the wing is
completely horizontal. If the Coanada effect was present the air travelling
over the top surface of the wing would leave the wing at a downwards angle and
that would cause lift.

~~~
aufreak3
If you want to satisfactorily explain this to a school kid, it seems like the
two most significant factors are a) the angle of attack during take-off - like
packing the air under the wing while creating a vacuum above it and b) the
curvature taking over the job of "vacuum creation" during steady flight.

A flat-wing plane with its wings slicing the air won't have any lift in steady
flight. The only way for such a wing to get lift is to use some angle of
attack.

The lift on a curved wing is enough to support its weight when flying fast,
but too subtle to literally lift the plane way up starting from lower speeds
as on the ground.

Does that about capture the essence of it?

Invoking the Coanda effect seems somewhat unnecessary. For example, if you
have a wing fitted with zillions of tiny holes through which a small amount of
air is being constantly ejected (to counteract the Coanda effect), it still
seems (in my mind) possible for the wing to get lift using the angle of attack
and curvature.

~~~
aufreak3
.. and that upside down flight needs flaps on the wings.

------
jgrahamc
Just read the Wikipedia page on Lift:
<http://en.wikipedia.org/wiki/Lift_(force)>

~~~
brc
From that page, I now know that the mistake shown in the xkcd comic is called
the "Equal Transit-Time Fallacy". There is no requirement for an airflow, once
separated, to speed over the top of the wing to match the airflow underneath.
In fact the top-layer airflow moves faster than the bottom layer, and arrives
at the trailing edge sooner.

~~~
regularfry
..and that's _precisely_ the revelation that made me realise that all of my
previous physics teachers had been lying to me.

------
quux
The Fly With Me podcast had a great episode where they tried to get to the
bottom of how a wing really generates lift.

<http://joepodcaster.libsyn.com/fly-with-me-episode-25>

The host is an airline pilot and he brought in a science podcaster to help him
interview a professor of aeronautical engineering. Things went pretty smoothly
until the professor started talking about in ring integrals and circular flows
of air around the entire wing forcing the interviewers to give up and say
"Look, it's really complicated." They put the whole unedited explanation at
the end of the podcast for anyone crazy enough to listen to it. :)

The bottom line was that there are 3 things going on at the same time,
Bernouilli, Newtonian action/reaction, and the Conada effect (I think, this is
where he started talking about ring integrals and blowing everyone's minds).
The problem is that even the experts can't say which of those is the principle
reason for lift, and which ones are actually side effects from the "real
reason." Fortunately the math doesn't care and works anyway, so they can still
design and model airfoils very well.

~~~
wbeaty
Hilarious! This is the usual problem with rigorous physics/engineering: we
don't understand it ourselves, and rather than being honest about it, we quote
math at you. It's like doctors in 1700 falling back to Latin in order to hide
their embarrassing ignorance from the public. In reality, "If you can't
explain it to your grandmother, then you don't understand it yourself." (A.
Einstein.)

Similarly, if you asked me some eletronics questions about a piece of
complicated circuitry that I didn't understand, I could baffle you with BS.
It's simple, I just pull out the equations from the SPICE component models.
(As if owning the printout of the software can tell us anything about the
behavior of the simulated circuit!!)

So, whenever you see an expert pull this crap, give them the above Einstein
quote.

------
simonjoe
Well, if you're willing to accept that a wing–as described–produces lift in
it's 'natural' upward direction (see other replies for some links as this step
is a bit complicated), then there's only one way a plane can fly upside down.
That reason is that it can also create lift opposite that direction.

Wings are not static. Wings either use flaps or some mechanism that causes
them to bend that alters how much lift they produce at a given speed, angle of
attack, etc.. The thing is that the reference frame you choose is important.

If you have 0 lift, you're in free fall…a phenomenon that a lot of planes can
do. If you can push whatever that distortion in wing shape (flaps or bending)
a bit farther, then you can dive faster than freefall, that is producing lift
in the downward direction.

Now, just turn the plane upside down and do the same thing.

The freakier thing is that helicopters can theoretically fly upside-down. Same
thing: if they can drop faster than free fall, they can fly upside down. The
catch is that it's a VERY unstable equilibrium that is a huge stress on
basically all of the parts of the motor, steering mechanism, structure, etc..

Or, it's because your parents' are Santa Claus. That's really equivalent.

------
ozchrisb
There are microscopic pixies that hold onto the wing and beat their little
wings really fast.

------
senki
Here is a good (and long) overview from the NASA with wind tunnel simulator
Java applets: <http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html>

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mgarrity
Mary Shafer explained it best many years ago:

[http://groups.google.com/group/sci.aeronautics/browse_thread...](http://groups.google.com/group/sci.aeronautics/browse_thread/thread/7716ab8ce98106de/f9588208eadf6bb4)

It's lift demons.

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alan-crowe
Isn't is basically the same deal as with a rocket, except that instead of the
reaction mass being in a tank, the airplane takes the air in front of it and
bats in downwards with the wing?

~~~
metageek
I think that's angle of attack.

------
acqq
Science Misconceptions in Textbooks and Popular culture

<http://news.ycombinator.com/item?id=1771101>

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lhorie
<http://www.terrycolon.com/1features/fly.html>

It's basically the same mechanism as helicopters

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shawndumas
<http://www.youtube.com/watch?v=6UlsArvbTeo>

------
83457
More than anything doesn't it come down to the forcing of air downward results
in lift upwards?

~~~
dedward
I believe so yes - all the other details are just about stability and
performance.

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meatsock
a plane can fly just fine upside-down for the same reason you can steer
downwards while right-side up.

~~~
aufreak3
.. while correct, the question is "what is that reason?"

