
Raskin about how plane wings lift - acqq
http://karmak.org/archive/2003/02/coanda_effect.html
======
jfoucher
That article does not explain anything. It's author merely notes an efffect he
saw from experiment, but does not describe why it should be so...

Wings works the way they do because othey curve the path of the air that flows
around them. They deflect the air flow, thus creating a force.
<http://amasci.com/wing/airgif2.html>

The "coanda effect" breaks if you give your lifting surface too high an angle
of attack (stall), but a reduced quantity of lift is still produced.

Planes can fly upside down because the increase the angle of attack, and the
path the air takes IS longer on the top (flat surface) then it is at the
bottom (curved surface). A plane with assymetrical wing section flown upside
down at a zero angle of attack will go down. This is why aerobatic planes
always use symetrical wing sections.

Flat wings work very poorly and stall at very low angles of attack, beause of
flow separation, but they do create lift, because the point where the flow
separates between upper and lower surface is not at the leading, but somewhere
on the lower surface. See this image:
<http://www.arvelgentry.com/images/plate2.jpg>

This guy should read "Theory of Wing Sections", by Abbbott and Van Doenhoff
([http://www.amazon.co.uk/Theory-Sections-Dover-Books-
Physics/...](http://www.amazon.co.uk/Theory-Sections-Dover-Books-
Physics/dp/0486605868/ref=sr_1_1?s=books&ie=UTF8&qid=1303135213&sr=1-1))
before posting misleading essays.

Sorry about the rant, but I don't want anyone taking what this guy writes for
the absolute truth...

Obligatory xkcd reference: <http://xkcd.com/386/>

~~~
machrider
Unfortunately, "this guy" is dead. I don't think he'll be doing any more
reading.

------
tzs
There's a serious problem with the organization of the article. Early on, he
tells the story of being scolded in school for noticing that his teacher's
explanation for how planes work cannot be right, because if you turned the
plane upside down, the Bernoulli force would be toward the ground.

He then goes on to talk about the Coanda effect, and how that combined with
the shape of the wing makes the wing direct air toward the ground, and thus
provides lift.

Suppose back in sixth grade, his teacher had taught that explanation instead
of the incorrect Bernoulli effect explanation. He'd still have the same
problem! Turn a wing upside down and the Coanda effect and shape are now
deflecting air toward the sky, and the lift is toward the ground.

He should have had a little section at the end covering how planes actually
fly upside down.

There are at least four ways to force air downward in order to help provide
lift. There's the Bernoulli effect, which he points out doesn't provide very
much lift. There's the Coanda effect, which he covers. Then there's simple
brute force--point whatever you are using to provide thrust so that the thrust
vector aims up somewhat instead of level. Finally, put something in the air
flow at an angle so that it directly deflects the airflow down.

The first two work in the wrong direction when upside down, so it is the later
two that are responsible for upside down flight. Note that both of these come
with significant cost. If you are pointing your thrust vector up, you lose
some forward thrust, so it is going to take longer and cost more in fuel to
get where you are going. If you stick some flat surface in the air flow to
just directly direct air downward, you are also introducing considerable drag,
again making your trip take longer and use a lot more fuel.

------
tzs
A lot more here, <http://www.grc.nasa.gov/WWW/k-12/airplane/>, at NASA'a
Beginner's Guide to Aeronautics, including nice simulators that let you play
with the various factors that affect flight in simulated wind tunnels and see
the results.

WARNING: the above site can be a HUGE time sink.

------
kenkam
A read on Wikipedia will tell you that Coanda effect is not the entire reason
why wings generate lift:
<http://en.wikipedia.org/wiki/Lift_(force)#Coand.C4.83_effect>

Lift is generated due various reasons: I recommend a read here
[http://en.wikipedia.org/wiki/Lift_(force)#A_more_rigorous_ph...](http://en.wikipedia.org/wiki/Lift_\(force\)#A_more_rigorous_physical_description).

(Yes, they're both from Wikipedia and I feel a bit ashamed that I couldn't
find a better source but the description there is succinct and rather complete
-- I can vouch with my Aero Eng degree on the information there! :D).

------
shib71
Awesome quote: "It was a shock to realize that my teacher and even the library
books could be wrong. And it was a revelation that I could trust my own
thinking in the face of such concerted opposition."

~~~
Confusion
However, also a dangerous one, used by many a crackpot to justify their
theories. It only flies when you have sufficiently obvious experimental
results at your disposal to cast doubt on a theory.

~~~
run4yourlives
>It only flies when you have sufficiently obvious experimental results at your
disposal to cast doubt on a theory.

Nonsense. Everything is wrong until proven otherwise. The burden is on the
presenter to justify the statement. The moment doubt is raised then the
presenter must address it. Someone pointing out a logical error in the
presenter's assertion is not required to come up with some other explanation
in order to explain that error.

Of course if they do they are held to the same standard.

~~~
Confusion
I think you misunderstand me: I mean that you can only use this argument
against a _well established, probably experimentally well supported_ theory,
if you have at least some experimental results that are contradicted by the
theory (and preferably explained by your alternative).

There are many crackpots that 'have been shocked to realize that library books
and teachers _were_ wrong and that they _should_ trust their own thinking in
the face of concerted opposition'. Only slightly different from the original,
but the italicized words make a world of difference. Einstein's (special)
theory of relativity faces scores of opponents that 'trust their own
thinking', despite not having a single _fact_ that casts doubt on it.

------
bergie
When I was doing my pilot's license, the alternate explanation to "how planes
stay in the air" was _with money_.

But indeed, even the aerodynamics manuals we were given simplified this part
quite a bit.

------
jswanson
I found out that the 'Bernoulli Effect' explanation was pretty much wrong
through this TED Talk:

[http://www.ted.com/index.php/talks/jonathan_drori_on_what_we...](http://www.ted.com/index.php/talks/jonathan_drori_on_what_we_think_we_know.html)

Was completely blown away, and angry at my education. And at myself for
believing it so easily.

------
camperman
Raskin is wrong and John Denker is right - a wing is a pump:

<http://www.av8n.com/how/htm/airfoils.html>

~~~
divtxt
So... we're ditching Coanda and going back to Circulation Vortex?

A question I always had about the circulation vortex: where's the energy input
to power the vortex?! (or why does it not need one?)

~~~
camperman
It's the aircraft's throttle normally. Or if it's a glider, then the winch or
another aircraft's throttle if you're getting towed.

To be less facetious, the energy input is the moving airfoil at a particular
angle of attack.

[http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-
summa...](http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summary)

~~~
divtxt
So circulation forms and self-sustains naturally when you push the airfoil
through the air. From the article, it looks like this is because the air
behavior is asymmetric between the leading and trailing edges, in favor of
maintaining circulation.

My question: why/how does the air do this?

For example, I thought the Coanda effect was the macro result of van der Waals
attraction. Is there a similar explanation for the behavior of air which
results in asymmetric leading & trailing edge behavior?

~~~
camperman
Because it's a fluid that has pressure and velocity everywhere, like all
fluids. I don't know how you would describe circulation in terms of a macro
result of some other effect.

------
prodigal_erik
Reminds me of an awkward moment dad had early on. He'd been reading textbooks
with big thrust/drag/lift/weight arrows for so long that he was startled
during a takeoff that he couldn't see them in reality. Despite studying
engineering, he hadn't quite reconciled himself to being kept alive by
_invisible_ forces.

------
perlgeek
I don't see how the Coanda effect explains how the classical airfoil profile
allows planes to fly upside down - I'd expect them to be dragged down by it,
not lifted up.

Can anybody please explain that, or link to an explanation?

~~~
jbri
If you look at air show footage of planes flying inverted, you'll see that
they fly with a pretty big angle of attack. It's similar to how a flat-wing
plane can fly.

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bcardarella
A great site to learn more about this is <http://sailtheory.com>

I'm blown away by how many sailors believe in the equal transit theory.

~~~
arethuza
One of the great things about sailing small high performance dinghies is that
the feedback is so immediate you don't really need a model of how sails work -
just the ability to observe what is going on and what works (although I admit
training does help).

As Rodney Brooks said: "The world is its own best model"

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aidenn0
It's been a long time since I took physics, but isn't the Coanda effect only
for fluid jets (i.e. like the straw in his experiment), and not applicable to
airfoils in general (though see blown flaps for where it is applicable)

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kleiba
Speaking of cheap balls: when you banana kick a soccer ball with a counter-
clockwise spin, it will bend leftwards. If you do the same with a cheap, thin
beach ball, it will bend rightwards. What's up with that?

~~~
perlgeek
I play table tennis, and thus have quite a bit exposure to small, cheap balls.
I can't confirm your observation.

------
toddh
Isn't lift generated by Newton's Third Law? For every action there is an equal
and opposite reaction

~~~
Confusion
Indirectly, everything is 'described' by Newton's Third Law. However, it
doesn't provide a very satisfying answer: it's a description, not an
explanation[1]. Knowing the wing goes 'up' because it reacts to the action of
the air pushing against it from below, does not explain why that force is
larger than the reaction to the action of the air pushing it down from above.

Bernoulli's principle is basically a statement about what happens when you
apply Newton's laws to a macroscopic amount of fluid and allows you to
determine what happens when an asymmetrical object is placed in the flow of
such a fluid: when the speed goes up, the pressure goes down. Pressure is
nothing but the amount of Newtonian action exerted on a surface, which brings
us back to the third law.

Then you find that Bernoulli's principle doesn't quite cut it, because it
doesn't take into account that fluids don't actually behave like the naive
application of Newton's laws would lead you to believe. However, in the end,
the Coanda effect is again a description of the behavior of fluids under
certain circumstances, describing the macroscopic effect of Newton's laws
applied to all individual particles.

[1] The exact difference between the two is subject of much philosophical
debate, but most would agree that something like Bernoulli's principle has
more explanatory power than Newton's law, because, for instance, it allows you
to understand _why_ a wing would lift.

~~~
toddh
I'm not sure why it's not satisfying. The airplane exerts a downward force,
the air exerts an equal and opposite force upwards. If you don't change the
motion of the air than you can't fly. The typical tear drop wing picture
doesn't change the motion of the air so it wouldn't fly. A downward deflection
of air is required to provide lift in the form newton's 3rd law. Initially air
moves horizontally. The wing serves to deflect air downward. So the wing
exerts a downward force on the air. So by the 3rd law the air exerts and equal
and opposite force upward. Just talking it out in my head. That seems way more
satisfying to me.

~~~
khafra
The movement of a fluid according to Newton's laws is described by the Navier-
Stokes Equations, which have no closed-form solution[1]. So, if your intuition
gives you a picture of air moving downward from a wing, that's great. But if
it feels like a complete explanation, that is a fact about your mind; not a
fact about the world.

[1] <http://en.wikipedia.org/wiki/Navier-Stokes_equations>

~~~
toddh
An equation is a description which is surely an artifact of the mind. At least
I'm using forces. So you are saying lift isn't provided as I describe?

~~~
khafra
I'm saying "lift is just Newton's third law" is approximately as descriptive
as "the mind is just the interaction of weighted electrochemical links between
neurons."

The word "just" covers a lot of detail that can make the short explanation
less satisfying--especially if you're trying to use it to build something.

