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Coanda Effect: Understanding Why Wings Work

128 points by razzaj 12y ago

thatswrong0 12y ago
Isn't angle of attack also pretty important to how wings work?
- Cogito 12y ago
  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 12y ago
    > 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
  - rosiusyves 12y ago
    I have the impression that the explanation of "Why wings work." is driven towards the Coanda effect more. However if my puny brains don't misunderstand, I would think planes can fly mainly because of drag. Question: What would for example happen if we move a imaginary wing, with a infinite surface (so that there is only a bottom) at an angle of attack.
aidenn0 12y ago
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 12y ago
  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 12y ago
    For an asymmetric airfoil, zero angle of attack is defined as the AoA that produces zero lift.
    - aidenn0 12y ago
      I've seen it defined as the chordline being parallel to the flow.
      - lisper 12y ago
        Yeah, you can define it that way too. But that's not a very useful definition when you're actually flying an airplane.
    - 12y ago
lisper 12y ago
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 12y ago
  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 12y ago
    > 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.
nether 12y ago
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.
theothermkn 12y ago
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 12y ago
  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.
- mannykannot 12y ago
  Perhaps your first paragraph explains why the Spitfire had thin wings. IIRC, one unintended consequence of this choice was that, as power and speed increased through WWII, the Spitfire avoided compressibility problems.
awor 12y ago
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
picomancer 12y ago
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 12y ago
  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.
Ygg2 12y ago
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.
pesenti 12y ago
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
GotAnyMegadeth 12y ago
> 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.
jwr 12y ago
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.
- gus_massa 12y ago
  The takeaway is that the correct explanation involves too much math, like complex analysis. If someone want to skip the math and use hand waving, the explanation is allays oversimplified and usually wrong. [I tried to find a link to a small easy to understand and correct explanation, but I couldn't.]
  - aidenn0 12y ago
    There really isn't much intermediate space between "An airfoil disturbs the air in such a way that the pressure on top is lower than the pressure on the bottom" and potential flow calculations... Note that potential flow calculations are significantly oversimplified; they work only for thin airfoils at low angles of attack (so they will correctly model basic flight, but not anything beyond that)[1]. So lets say you get an intuition of flow separation and turbulance on a 2D cross-section (which is already a bit of a stretch) you now still can't explain how a delta wing works. Really smart people who know a lot about how flight works and have expensive computers still need to test their ideas in wind tunnels. [1] Here's an example of something that actually happens, but isn't predicted correcty by potential flow: http://en.wikipedia.org/wiki/Lift_(force)#mediaviewer/File:Flow_separation.jpg http://en.wikipedia.org/wiki/Lift_(force)#mediaviewer/File:F...
- Retric 12y ago
  It's complex so simply analogies break down. If you want a simple explanation F=M * A. Wings work by pushing air down end of story. Note fan blades and propellers work the same way. With enough power you could use flat plates just fine which you occasionally see on metal fans or vary light aircraft like paper airplanes. And at speed the bottom of a wing acts like a flat plate pushing air down. The back of the wing get's complex and for efficiency you want a complex shape on propellers/fans/wings. Now, if you want to know why wings are snapped the way they are that's Flid Dynamics and you generally use a combination of simulation and wind tunnel testing to 'get it right'. PS: You can't pull a fluid. Straws work by having the air push down harder outside your mouth than inside. In much the same way the air above the wing is pushed down by the air above that.
gus_massa 12y ago
> 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-in-a-potential-flow-have-lift http://physics.stackexchange.com/questions/46131/does-a-wing... . 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.".)
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lmg643 12y ago
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...
Zariel 12y ago
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
elteto 12y ago
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 12y ago
  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 12y ago
    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 12y ago
      Meta: wasn't blaming you for the downvote - just speaking into the HN air to whomever did downvote.
Fuzzwah 12y ago
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.html http://www.espncricinfo.com/magazine/content/story/258645.ht... 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".
mcguire 12y ago
"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
lordvon 12y ago
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.
Animats 12y ago
Mandatory XKCD: http://xkcd.com/803/ http://xkcd.com/803/
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dameyawn 12y ago
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.

128 points by razzaj 12y ago

thatswrong0 12y ago

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

Cogito 12y ago
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 12y ago
  > 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
- rosiusyves 12y ago
  I have the impression that the explanation of "Why wings work." is driven towards the Coanda effect more. However if my puny brains don't misunderstand, I would think planes can fly mainly because of drag. Question: What would for example happen if we move a imaginary wing, with a infinite surface (so that there is only a bottom) at an angle of attack.

aidenn0 12y ago

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 12y ago
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 12y ago
  For an asymmetric airfoil, zero angle of attack is defined as the AoA that produces zero lift.
  - aidenn0 12y ago
    I've seen it defined as the chordline being parallel to the flow.
    - lisper 12y ago
      Yeah, you can define it that way too. But that's not a very useful definition when you're actually flying an airplane.
  - 12y ago

lisper 12y ago

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 12y ago
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 12y ago
  > 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.

nether 12y ago

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.

theothermkn 12y ago

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 12y ago
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.
mannykannot 12y ago
Perhaps your first paragraph explains why the Spitfire had thin wings. IIRC, one unintended consequence of this choice was that, as power and speed increased through WWII, the Spitfire avoided compressibility problems.

awor 12y ago

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

picomancer 12y ago

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 12y ago
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.

Ygg2 12y ago

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.

pesenti 12y ago

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

GotAnyMegadeth 12y ago

> 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.

jwr 12y ago

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.

gus_massa 12y ago
The takeaway is that the correct explanation involves too much math, like complex analysis. If someone want to skip the math and use hand waving, the explanation is allays oversimplified and usually wrong. [I tried to find a link to a small easy to understand and correct explanation, but I couldn't.]
- aidenn0 12y ago
  There really isn't much intermediate space between "An airfoil disturbs the air in such a way that the pressure on top is lower than the pressure on the bottom" and potential flow calculations... Note that potential flow calculations are significantly oversimplified; they work only for thin airfoils at low angles of attack (so they will correctly model basic flight, but not anything beyond that)[1]. So lets say you get an intuition of flow separation and turbulance on a 2D cross-section (which is already a bit of a stretch) you now still can't explain how a delta wing works. Really smart people who know a lot about how flight works and have expensive computers still need to test their ideas in wind tunnels. [1] Here's an example of something that actually happens, but isn't predicted correcty by potential flow: http://en.wikipedia.org/wiki/Lift_(force)#mediaviewer/File:Flow_separation.jpg http://en.wikipedia.org/wiki/Lift_(force)#mediaviewer/File:F...
Retric 12y ago
It's complex so simply analogies break down. If you want a simple explanation F=M * A. Wings work by pushing air down end of story. Note fan blades and propellers work the same way. With enough power you could use flat plates just fine which you occasionally see on metal fans or vary light aircraft like paper airplanes. And at speed the bottom of a wing acts like a flat plate pushing air down. The back of the wing get's complex and for efficiency you want a complex shape on propellers/fans/wings. Now, if you want to know why wings are snapped the way they are that's Flid Dynamics and you generally use a combination of simulation and wind tunnel testing to 'get it right'. PS: You can't pull a fluid. Straws work by having the air push down harder outside your mouth than inside. In much the same way the air above the wing is pushed down by the air above that.

gus_massa 12y ago

> 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-in-a-potential-flow-have-lift http://physics.stackexchange.com/questions/46131/does-a-wing... . 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.".)

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lmg643 12y ago

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...

Zariel 12y ago

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

elteto 12y ago

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 12y ago
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 12y ago
  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 12y ago
    Meta: wasn't blaming you for the downvote - just speaking into the HN air to whomever did downvote.

Fuzzwah 12y ago

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.html http://www.espncricinfo.com/magazine/content/story/258645.ht... 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".

mcguire 12y ago

"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

lordvon 12y ago

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.

Animats 12y ago

Mandatory XKCD: http://xkcd.com/803/ http://xkcd.com/803/

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dameyawn 12y ago

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.