

Coanda Effect: Understanding Why Wings Work - razzaj
http://karmak.org/archive/2003/02/coanda_effect.html

======
theothermkn
One warning to the novice is that his experiments all take place at a very low
Reynolds number. It's very difficult for neophytes to visualize flow at these
scales, not least because the shape of the viscous boundary layer is on nearly
the same size scale as the wing itself. In fact, it was long believed that
"ideal" wings for aircraft would have a very thin cross section, primarily
because this is what worked so well in the tiny wind tunnels of the day. Just
look at the difference between an early WWI fighter and a late WWII bomber.
(IIRC, we actually lucked into this for structural reasons! Thicker wings are
easier to build!)

Over and above that, because the Coanda effect pertains to detached streams,
it doesn't actually apply to a baseball, nor to wings. The author seems bright
enough to handle potential flow calculations, and it would be a very
instructive exercise for him to model a 2D flowfield around an airfoil without
circulation, and then to input enough circulation to account for the Kutta
condition at the trailing edge. I would advise using a "typical" cross section
to avoid certain irregularities around the leading edge. _Fundamentals of
Aerodynamics_ , by Anderson, is a wonderful read, even if it is surprisingly
infuriating to learn how hopelessly wrong typical aerodynamic intuitions are.

My fluids prof used to comment that people feel perfectly confident making
pronouncements about aerodynamics where they'd be appalled to make the
equivalently technical statements about brain surgery.

~~~
onedognight
If you push an symmetric elongated body at a non-zero angle of attack in
potential flow it will generate lift without (applying the Kutta condition to
generate) circulation.

------
elteto
The Coanda effect is but one of several sources of lift, and _not_ the main
one. There are many, many contradictory and downright wrong explanations out
there. See this incredibly informative NASA website [0] and take the Theories
of Lift path. Quoting from the site, lift on a wing appears because "...the
integrated velocity variation around the object produces a net turning of the
gas flow. From Newton's third law of motion, a turning action of the flow will
result in a re-action (aerodynamic force) on the object". In other words,
wings move air "out of the way" in a specific manner, which causes a reaction
force on the wing, and hence lift. That is why planes flight, other effects
are secondary.

[0]
[http://www.grc.nasa.gov/WWW/k-12/airplane/lift1.html](http://www.grc.nasa.gov/WWW/k-12/airplane/lift1.html).

~~~
delinka
I've never quite understood all the "how do wings work?!?" business - you run
fast enough to make wind "blow" the bottom of the wing and _poof_ flying.

Edit: Instead of downvoting, how about an explanation as to why this is not a
layman's interpretation of the parent comment?

~~~
elteto
I didn't downvote you, I'm not part of that select club.

It is funny that you say "you run fast enough to make wind blow" and then you
fly, because at a very simplistic level, that is _exactly_ what is happening.

Making the wind "blow" (and not necessarily under the wing only, mind you)
implies a change in velocity, which can only be the result of a change in
momentum imparted by a force (the force of the wing on the air). By Newton's
third law this generates a reaction force and then you get your "poof flying"
moment :)

You really should read the content I mentioned, it kinda is explained all in
there.

~~~
delinka
Meta: wasn't blaming _you_ for the downvote - just speaking into the HN air to
whomever did downvote.

~~~
zokier
Continuing on meta: HN guideline explicitly advices against complaining about
downvotes.

------
aidenn0
This is wrong. The Coanda effect is about fluid jets; this is how blown flaps
work. However an airfoil is in a free-moving fluid not a jet.

[edit] A complete mathematical modeling of lift from first-principles is
essentially impossible, as the most straightforward method would likely be
Navier-Stokes. That, however, has serious issues with turbulent flow, which
will happen _somewhere_ IIRC it becomes quite inaccurate in stall conditions.

It's been a long while since I've done any physics, but my recollection is the
practical way to model it is to use Navier-Stokes along with empirically
determined approximations of turbulence.

[edit2] A quick reading of the wikipedia article tells me that was mostly
right, but NS doesn't have the stall issues, it's the Euler equations (which
is a simplified form of NS).

~~~
Cogito
It is interesting that the experiment he proposes actually does use a jet of
air!

I guess the best counterexample would be to have an airfoil with 0 angle of
attack - horizontal on the bottom and curved down on top. Can this wing divert
the stream of air downwards, generating enough to allow flight? Even if it
does, how much extra lift do you get when you increase the angle of attack?

~~~
lisper
For an asymmetric airfoil, zero angle of attack is _defined_ as the AoA that
produces zero lift.

~~~
Cogito
Yes, please excuse my ignorance of the technical use of the terms.

It would be interesting to see an experiment that isolated any divergence of
the airstream caused by Coanda-like effects. I don't know what that experiment
would look like, nor if any lift at all would be generated.

~~~
lisper
It's not a hard experiment to do: make an airfoil out of cardboard and stick
it in front of a fan.

~~~
Cogito
That would have no coanda effect though, right? I want an experiment that has
only coanda (or similar) effects in play.

~~~
lisper
> That would have no coanda effect though, right?

Why not?

------
jwr
I find it amazing that every time this topic comes back (and it does come back
regularly), there is a heated discussion with multiple contradictory
explanations and assertions. People point to multiple sources, each one saying
something different.

The net takeaway for me is that I _still_ can't be sure why airplanes fly and
there is no general agreement on an authoritative source that will explain
this.

~~~
jgable
I think that this topic is a classic example of people trying to answer the
problem at different levels of abstraction. There is a famous clip of an
interview with Richard Feynman where he cannot give a simple answer to "Why do
magnets repel each other?" [1]. He can't give a simple answer because there is
no simple answer -- any answer he could give could be followed by the question
"And why does THAT happen?", requiring ever more complex answers that quickly
descend into quantum mechanics that only top physicists can understand. He
makes a meta-point that a person who asks such a question needs to specify the
level of abstraction they expect, or barring that, the answerer needs to try
to anticipate the correct level of abstraction that will satisfy and/or
educate the questioner.

Now, some answers to why wings work are just plain wrong (like the original
wrong answer -- the Bernoulli effect). However, when one person says "Wings
work because they push air down, and the air pushes the wing up", and another
person yells "No, that's all wrong! It's because of <insert-fancy-effect-
here>", they can both be right. They are answering the question at different
levels of abstraction. They also can be right in different cases -- the source
of lift for wings, and the strength of different effects, can change with wing
and flow conditions (at low speeds, one effect dominates, and at high speeds,
another does. At supersonic speeds, a totally new effect takes over. and so
on...)

Here's my attempt at an answer aimed at an appropriate level of abstraction,
though of course it is doomed to failure: \- Air striking a wing is divided by
the leading edge into two streams, one that flows over the top, and one that
flows under the bottom.

\- Assuming the wing has some positive angle-of-attack, the stream under the
bottom of the wing will be deflected downward, and therefore pushes back
against the bottom surface of the wing. This part is reasonably
uncontroversial.

\- The stream flowing over the top of the wing tends to follow the surface of
the wing, even when the surface is curving down and away from the stream. Why
this happens is subject to the multiple levels of abstraction problem that I
mention above. If the curvature of the top surface is too severe, the flow
cannot follow the surface, and it separates. This is "stall". Again, why this
happens is complicated, and there are multiple effects and levels of
abstraction at work, and I only understand the basic levels, so I won't try to
go any further. The net result for a typical wing in typical flight conditions
is that the flow over the top surface is also deflected downward.

\- You can get the amount of lift on the wing by integrating the pressures
over the surface of the wing or by examining the curvature introduced to the
flow by the wing -- both methods will give you the same answer (and they damn
well better!). This is if you have modeled the airfoil and flow in a CFD
software package with a reasonably tight mesh so that you know the flow
conditions at every point in space near the airfoil. Or, you can pick a
standard airfoil whose properties have been determined experimentally! There
are exhaustive tables of NACA airfoils to pick from. [2]

Still confused? Yeah, so am I. This is about as deep as I'm prepared to learn
this topic, considering that I've given up my former life as a thermo/controls
specialist in mechanical engineering, and am now trying to stuff as much
understanding of software engineering and computer science into my tired brain
as I can. :-)

[1] [https://www.quora.com/Why-couldnt-Feynman-answer-the-
questio...](https://www.quora.com/Why-couldnt-Feynman-answer-the-question-
about-why-magnets-repel-each-other) [2]
[https://en.wikipedia.org/wiki/NACA_airfoil](https://en.wikipedia.org/wiki/NACA_airfoil)

* edit for typos

~~~
zokier
> Now, some answers to why wings work are just plain wrong (like the original
> wrong answer -- the Bernoulli effect). However, when one person says "Wings
> work because they push air down, and the air pushes the wing up", and
> another person yells "No, that's all wrong! It's because of <insert-fancy-
> effect-here>", they can both be right

But arguably "Wings work because they push air down, and the air pushes the
wing up" doesn't really answer the question because air being pushed down is
as much an _effect_ of wing working as is airplane flying, but the _cause_ why
wing works would remain mystery.

~~~
jgable
Which is exactly my point.

You: "But that doesn't answer the question. WHY does the air get pushed down?"
They: "Because of such-and-such effect..." You: "But that doesn't answer the
question. WHY does such-and-such effect happen?" They: "Because of fluid
viscosity and boundary layers and navier-stokes blah blah blah..." You: "But
that doesn't answer the question. WHY does fluid have viscosity?" They:
"Because a such-and-such bonds between the molecules of the fluid..." You:
"But WHY..."

See how it goes? Turtles all the way down.

------
GotAnyMegadeth
> When I pressed my 6th grade science teacher on this question, he just got
> mad, denied that planes could fly inverted and tried to continue his
> lecture.

During my time at school I had two teachers that when I started asking
questions they didn't know the answer to they'd say something like "Wow,
that's an interesting question, I'll try and find out the answer". All the
rest were terrible teachers.

------
lisper
This is the definitive source for how airplanes fly:

[http://www.av8n.com/how/](http://www.av8n.com/how/)

Particularly:

[http://www.av8n.com/how/htm/airfoils.html](http://www.av8n.com/how/htm/airfoils.html)

~~~
theothermkn
I wish I could agree, but there are a few howlers in there. The significance
of compressibility at low speeds around typical airfoils (M < .3 and no slots
or blown flaps) is truly negligible, and the flowfield can be very finely
approximated with uncorrected potential flow methods.

Also, the author mentions "suction," which is incredibly problematic. Just as
you cannot push string, you cannot suck air. We can talk of negative "gauge"
pressure, but that's just complicating things. There is a region of _low_
pressure above the wing, but the wing isn't being "sucked" into that; It's
being pushed into that by the higher pressure on the lower side.

This "force" formulation is equivalent to the "mass x acceleration"
formulation we get when we keep track of the mass of air moving about the
wing. (Newton might remind us that F=m x a. Newton; So cheeky!)

He does finally get around to the Kutta-Jukowsky theorem, but it seems buried
under a bunch of other stuff.

Oh, well. To each his own.

~~~
lisper
> the author mentions "suction,"

Yes, and he defines it immediately: "suction, i.e. negative pressure relative
to ambient" which is exactly the same as 'negative gauge pressure'. So why is
this problematic?

> the wing isn't being "sucked" into that

And he never says it is. You're attacking a straw man.

------
lmg643
Ed Seykota, one of the first systematic futures traders, has a whole web page
up dedicated to "stopping Bernoulli abuse" and instead proposing the "Theory
of Radial Momentum" as a way to explain lift:

[http://www.seykota.com/rm/](http://www.seykota.com/rm/)

The material isn't explained all that well (at least, for a non-physicist like
myself) but it would appear that an object or structure which forces a fluid
to expand in multiple directions, will reduce pressure and induce lift.

The example is a playing card adhering to a thimble with air going through a
spool. Also works with water from a hose.

Interested to see what someone who understands this stuff well thinks...

------
gus_massa
> _The reasoning--though incomplete--is based on the Bernoulli effect, which
> correctly correlates the increased speed with which air moves over a surface
> and the lowered air pressure measured at that surface. [...]A few years
> later I carried out a calculation according to a naive interpretation of the
> common explanation of how a wing works. Using data from a model airplane I
> found that the calculated lift was only 2% of that needed to fly the model._

The calculation in the bottom is incorrect. It assumes that the top of the
wing is greater, so the speed of the air is greater, so the pressure on the
top of the wing is smaller. The problem is that then it multiplies the
difference of pressure by the surface of the wing, but it doesn't consider
that the top of the wing is greater. (There is another problem, the surface is
curved, so you must consider the direction of the forces in order to add
them.) When you take into account this, the "lift" you get from this
calculation is not the 2%, it's exactly 0% (as 0% because there is a
mathematical theorem that says that it's 0%).

There is a more detailed and correct explanation in
[http://physics.stackexchange.com/questions/46131/does-a-
wing...](http://physics.stackexchange.com/questions/46131/does-a-wing-in-a-
potential-flow-have-lift) . The secret sauce that makes the planes fly is the
vortex around the wing.

But there is a detail that I don't like in that explanation. You need the
viscosity to get the vortex when the plane starts. You don't need the
viscosity to fly. You can fly without viscosity, but you can't "take of"
without viscosity. (Well, you need also the viscosity to change correct the
circulation of the vortex when you change the speed, o the correct misquotes
is "You can fly at constant velocity without viscosity.".)

------
pesenti
A much more visual, accurate and easier to understand explanation of lift:

[http://www.cam.ac.uk/research/news/how-wings-really-
work](http://www.cam.ac.uk/research/news/how-wings-really-work)

------
Fuzzwah
I always enjoy reading about this topic.

If you do too, then you might also enjoy thinking about how reverse swing
bowling in cricket works:

[http://www.espncricinfo.com/magazine/content/story/258645.ht...](http://www.espncricinfo.com/magazine/content/story/258645.html)

TL;DR normally a fast bowler will shine one half of the cricket ball to make
it swing in the direction of the rougher side of the ball. In certain
situations the opposite will happen and the ball will swing "the wrong way".

------
lordvon
It is helpful to realize the trigonometry of flight. Wings leverage a
horizontal force (thrust) to get a larger vertical force (lift). Freestream
momentum is deflected some net angle by a wing. If you take a horizontal
vector and tilt it, the length is reduced, and the height is increased. The
equal and opposite reactions of the freestream momentum deflection are lift
and drag on the wing. Turns out, the change in height is larger than the
change in length. The following equation expresses the lift to drag ratio:
sin(phi) / (1-cos(phi)+f). The phi is not angle of attack, but the net tilt
angle of the freestream induced by the wing. There is some correlation between
the two of course. f is a non-conservative skin friction factor.

I view aerodynamic phenomena such as stall and Coanda effect as mechanisms
that interfere or enable the deflection capability of wings.

------
nether
Yeah lift has nothing to do with air following a "longer path" on the upper
surface. A flat plate is symmetric, but makes a pretty decent wing for small
angle of attack (unfortunately it's structurally untenable). The article
confuses the Coanda effect with the Kutta condition, though both arise from
air viscosity.

------
dameyawn
I often see incorrect explanations of how wings works, this included. There's
really only one experiment you need to do that demonstrates lift at all
scales. Stick your hand out of a moving vehicle or get in water and spin with
your hand sticking out. You angle your hand up - your arm goes up. Down, and
your arm goes down.

Your hand/arm will go in the direction opposite of the directed flow because
the flow is pushing it to go that way. This is the same way a wing works.
There are complicated ways to calculate it all, but the general concept is
basic.

------
mcguire
"Using the Coanda effect to explain the operation of a normal wing makes about
as much sense as using bowling to explain walking. To be sure, bowling and
walking use some of the same muscle groups, and both at some level depend on
Newton’s laws, but if you don’t already know how to walk you won’t learn much
by considering the additional complexity of the bowling situation. Key
elements of the bowling scenario are not present during ordinary walking."

[http://www.av8n.com/how/htm/spins.html#sec-
coanda](http://www.av8n.com/how/htm/spins.html#sec-coanda)

------
awor
the McDonnel-Douglas 520 helicopter has no tail rotor and instead has a fan in
the tail which utilizes the coanda effect to counter act main rotor torque

[http://en.wikipedia.org/wiki/MD_Helicopters_MD_500#MD_520N](http://en.wikipedia.org/wiki/MD_Helicopters_MD_500#MD_520N)

[http://en.wikipedia.org/wiki/NOTAR](http://en.wikipedia.org/wiki/NOTAR)

------
Zariel
The coanda effect was used to great effect in f1 recently to redirect the
exhaust flow from the upward pointing exhaust to the floor with no interfering
aerodynamics. It's also very well explained, with a very good practical
demonstration of it here [0].

[0]
[http://www.youtube.com/watch?v=gryojy2cHnI](http://www.youtube.com/watch?v=gryojy2cHnI)

------
Ygg2
I've read in a physics journalist about Coanda effect, and why Bernouli effect
couldn't the be only reason, if it was, then it would be possible to make
levitating boxes just stuff a fan and a wing in a box and supply enough
electricity for the Bernouli effect to raise the box.

------
picomancer
I'm thinking the baseball spinning counterclockwise when viewed from above
will curve to the pitcher's right (the same direction shown by Trefil).

Basically friction will cause the ball to push air molecules near its surface
in its direction of spin. So air molecules in front will fly off to the left.
By conservation of momentum, the ball will be pushed to the right.

The same effect at the back of the ball will push the ball to the left. But
there would be fewer air molecules behind the ball, because that's the space
which has just been vacated by the ball. Air molecules haven't yet had time to
rush in to fill the space behind the ball at the same density as they fill the
space in front of the ball.

This makes the rightward push at the front stronger than the leftward push at
the back, causing the ball to move to the right.

Now I'm going to finish reading the article and see if my hypothesis is
correct.

~~~
picomancer
Crud. I assumed the left and right side were symmetric and would cancel, but
they're not -- the air's moving at different relative velocity on each side.

------
thatswrong0
Isn't angle of attack also pretty important to how wings work?

~~~
Cogito
That's covered in the article, at least briefly -

 _We then have to ask how a flat wing like that of a paper airplane, with no
curves anywhere, can generate lift. Note that the flat wing has been drawn at
a tilt, this tilt is called "angle of attack" and is necessary for the flat
wing to generate lift. The topic of angle of attack will be returned to
presently._

later

 _It is easy, based on the Coanda effect, to visualize why angle of attack
(the fore-and-aft tilt of the wing, as illustrated earlier) is crucially
important to a symmetrical airfoil, why planes can fly inverted, why flat and
thin wings work, and why Experiment 1 with its convex and concave strips of
paper works as it does._

and then in the footnotes

 _7 - In the 1930 's the Romanian aerodynamicist Henri-Marie Coanda(1885-1972)
observed that a stream of air (or other fluid) emerging from a nozzle tends to
follow a nearby curved or flat surface, if the curvature of the surface or
angle the surface makes with the stream is not too sharp._

The essential action of the wing is to divert a stream of air downwards,
generating lift and drag in the process. The Coanda effect describes how
fluids 'stick' close to surfaces they flow over. The magnitude of this effect
is driven by the radius of curvature, angle of incidence etc.

What isn't really covered that well is that the Coanda effect is not essential
for a wing to work. Simply having a wing at an angle of attack will divert air
downwards and cause lift. The Coanda effect can be used to generate greater
amounts of lift [1]

[1]
[https://en.wikipedia.org/wiki/Coand%C4%83_effect](https://en.wikipedia.org/wiki/Coand%C4%83_effect)

~~~
lisper
> the Coanda effect is not essential for a wing to work

Indeed. The easiest way to see that is to take a piece of paper about the size
of a wallet size check and drop it while giving it a little bit of spin around
the long axis. The paper will fly, and the reason it flies is _exactly_ the
same as the reason a non-spinning wing flies. See:

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

------
Animats
Mandatory XKCD:

[http://xkcd.com/803/](http://xkcd.com/803/)

