Coanda Effect: Understanding Why Wings Work
karmak.org
karmak.org
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.
Edit: Instead of downvoting, how about an explanation as to why this is not a layman's interpretation of the parent comment?
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.
Fact is, objects in a moving fluid generate lift based on angle of attack. Varying the object and the fluid characteristics yields different results. Since it is a very complex system to model from first principles, it makes sense to look at real world data and simulations to develop intuition.
[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).
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?
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.
Why not?
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.
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:F...
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.
What you propose as a solution is the skipping stone theory which is not entirely correct and is mentioned as one of the incomplete/incorrect theories on the NASA site.
Edit: I forgot but a typical counterargument to the skipping theory is the question of how can planes fly inverted then, if the wing in the standard configuration pushes the air down?
Further, you will note they use F=MA on their correct page: http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html
The issue is modeling individual particles is several orders of magnitude beyond our best computers so even the most accurate models used by supper computers are still approximations. Further simply measuring pressure on various places on a wing. However, that is somewhat pedantic it might be true that's why they fly, but it's not a useful model for building an aircraft. People talk about flow, boundary separation layers, turbulence, and vertices but there all just analogy's that break down.
Anyway, wings don't have a fixed shape. Aileron's allow a plain to rotate. http://en.wikipedia.org/wiki/Aileron But, Flaps and slats are used to adjust angle of attack. http://en.wikipedia.org/wiki/Flap_(aeronautics).
While inverted the flaps are pushed down which changes the angle of attack. This of course ignores things like thrust vectoring etc. And most aircraft are much more efficient flying normally, with inverted flight basically just a brute force solution.
PS: So yes. F=MA (and enough processing power to handle 10^30th particles) is really all you need, but we just don't have enough processing power to physically model what's actually going on.
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... [2] https://en.wikipedia.org/wiki/NACA_airfoil
* edit for typos
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.
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.
Different levels of abstraction are presented to explain "why", they're understood differently by different people (due to varying expertise), which causes discussions to eventually spiral into what appears to be disagreement, but is actually different layers of detail. The layers may seem to be contradictory but are often related. Hence the term "arguing in circles".
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.
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...
Particularly:
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.
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.
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...
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".
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... . 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.".)
I view aerodynamic phenomena such as stall and Coanda effect as mechanisms that interfere or enable the deflection capability of wings.
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.
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.
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]
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:
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.