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.
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.
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.
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.
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.
Not a downvoter, and I haven't looked at the parent comment's reference recently, but any explanation that doesn't involve forcing a parcel of air down in order to generate a force on the aircraft up probably isn't useful.
The NASA site on lift is phenomenal. Particularly the Java applet which does a simple simulation of of an airfoil with velocity and pressure plots. I've completed a specilization in Aerospace engineering, and this simulation basically sums up the fundamentals of the curriculum. The intuitive take away is the most important part, not the equations.
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.
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).
The Navier-Stokes have no problem with stall. The problem is that the Navier-Stokes equations are hard to solve (to use them predictively). The Euler equations do not account for turbulence at all. The Euler equations are good at modeling pressure, which is good for lift and shocks, but does not predict drag, and does a bad job when stall occurs. The RANS equations (a simplified form of the Navier-Stokes equations), do account for tubulence, but one needs a good turbulence model. No turbulence model handles stall well on a broad spectrum of cases, but the RANS equations do do pretty well for attached boundary layers (loosely, when shapes are "aerodynamic" -- like an airplane rather than like a truck or sphere).
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?
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.
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.
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.
"Wings work by pushing air down end of story". Nope, that is hardly the end of the story on such a complex matter. And you contradict yourself by saying that due to complexity simple analogies break down, and then go ahead and provide a very simplistic and incomplete explanation, kinda ironic isn't it?
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?
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.
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.
I believe this is because the angle of attack shifts as well. A plane that is generating lift away from the force of gravity, if turned 180 degrees on it's forward facing axis, will generate lift facing towards the force of gravity. The angle of attack needs to change so that the top side (when the plane is upright) of the wing is again at an appropriate angle of attack to generate lift. So the nose ends up pointing more skyward to get the appropriate angle.
I mentioned it in another comment but I take this site as the authority on the subject [0] (for the layman, of course). And if anyone knows about why things fly, these are the guys.
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.]
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.
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. :-)
> 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.
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..."
I have no background in any field that could help me understand the basics of this discussion. However your post makes me wonder if many internet discussions are victim to this same phenomenon.
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".
> 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.
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:
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...
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.
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.
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 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%).
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.".)
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.
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.
"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."
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.
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
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].
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.
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.
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.
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]
> 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:
#mediaviewer/File:Flow_separation.jpg){kind=link}
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.