[arakyd]

It's true that the usual explanation with the Bernoulli effect is wrong, but the idea that the Coanda effect is important for normal flight is also wrong. The relationship between wing camber, angle of attack, and lift is a little more complicated than the post, or the presentation it references, implies. See http://av8n.com/how/htm/airfoils.html#sec-other-fallacies for more details.

A few planes have been built with the Coanda effect in mind (http://en.wikipedia.org/wiki/Boeing_YC-14), but in general it isn't very important.

[kurtosis]

For anyone interested in this I recommend the really great "What makes airplanes fly" by Peter P Wegener.

http://www.amazon.com/What-Makes-Airplanes-Peter-Wegener/dp/...

This book was part of a course he taught for non aerospace engineers. His explanations of tough topics like boundary layer theory and airfoils are clear

[adamc]

Kind of expensive for anything short of consuming interest, though.

[kurtosis]

Here's more info on the author

http://opa.yale.edu/news/article.aspx?id=6066

Apparently he worked on the V2 rocket. But there are lots of scientists that can write technical books targeted to experts. This book is worth it because this guy is devoted a lot of his teaching career to explaining difficult topics to non-experts. I think he did a marvelous job, of mixing technical material and history. I think this book was part of a course he taught at yale univ.

It is an expensive book, but I'll go in and say it's worth it. Check your univ library.

[aneesh]

The used copies of the older first edition are reasonably cheap.

http://www.amazon.com/gp/offer-listing/0387975136/

[netsp]

$23.48 shipped with some of the sale going towards literacy charities.

http://www.betterworldbooks.com/What-Makes-Airplanes-Fly-id-...

[lutorm]

Another good discussion about this is in the online pilot textbook "See How It Flies" at http://www.av8n.com/how/ (section 3). That author also disagrees with the Coanda effect conclusion.

The discussion in there is the best I've seen, it centers on the concept or circulation, the vorticity of the flow field around the wing. One example that's used is the fact that if you just take a wing by itself (try it with a long, flat piece of cardboard) it will "fly" while rotating in the air. It doesn't fly very well, admittedly, but it certainly exhibits a glide ratio a lot better than something with the same cross-sectional area that's not a wing. In that case, there's no top or bottom of the wing at all.

[arakyd]

Actually the whole av8n.com site is a great resource for anyone interested in learning more about physics. One of the reasons why I linked it ;).

[raquo]

Hm. a hundred years after first plane took off, we still argue how exactly do they fly...

[pg]

Not because there's something especially mysterious about it; it's just an instance of the more general rule that (a) people misunderstand anything complicated and (b) this doesn't stop them from expressing opinions about it.

There is one thing that's slightly different in this case: the misleading coolness of diagrams of the Bernoulli effect. One can easily see how in the hands of a popular writer, that could get transformed from an interesting optimization for wing cross-sections to "how planes stay up." I was fooled by this till a later age than I'd like to admit. It never occurred to me to ask how planes fly upside down, if the shape of the airfoil is how they generate lift.

[lutorm]

Well, but it is complicated. Hydrodynamics are not intuitive, and the fact is that none of these rules that are being thrown around are sufficient to solve the problem. They all just express different aspects of it.

You can't say, is it the Bernoulli effect or is it the fact that the wing makes the air behind it go down. Those are both different ways of expressing the same thing. You can't have one without the other. This is why I like the "See How it Flies" discussion.

[RiderOfGiraffes]

> You can't say, is it the Bernoulli effect or is it the fact that the wing makes the air behind it go down. Those are both different ways of expressing the same thing. You can't have one without the other.

Actually, you can. It's fairly straight-forward to construct an example where you do get the Bernoulli effect and there is no downwash. What is true is that when you fly you have both, and both contribute.

What's more true is that there are many effects, each of which contributes, many of which are inter-dependent, and all of which are simple in isolation, and complex in interaction and action.

[lutorm]

Well, yeah, I meant in the explanation of why an airplane flies. Sure, in a venturi you have Bernoulli effect with no downwash, but the fluid is instead changing speed. The effect is still there, it's just operating in a different direction. Any time you have a pressure change, you must have an acceleration.

In the venturi, the pressure is lower where the speed is higher, that's a fact. But is the pressure lower because the speed is higher, or is the speed higher because the pressure is lower? That question makes no sense, because it depends on how you think about it.

On the one hand, you can say: mass conservation dictates that the fluid must go faster in the narrow part of the tube. If the fluid is to go faster, it must accelerate, so there must be a pressure gradient. Hence, the pressure in the narrow part must be lower.

On the other hand, you can say: Since the fluid goes faster in the narrow part of the tube, the pressure is lower there. Since the pressure is lower, there's a pressure gradient, and that's what causes the fluid to speed up.

Neither of these explanations make sense, because there is no cause and effect in the problem, it's just that one state globally obeys all constraints on the fluid and that's the state with higher speed and lower pressure in the narrow part.

[jacquesm]

http://www.cabrillo.edu/~jwelch/bernoulli-coanda.html

[RiderOfGiraffes]

An interesting video, that reinforces everything I saw when I explain these effects - I've taken a copy and must get in touch with the owner to talk about using it.

The point about Bernoulli is that it only applies in its naive form in fluid flow where it's effectively a closed system. If you introduce airstreams of varying speeds then all bets are off.

Effectively Bernoulli works because the velocity changes are being caused by the pressure differences. Take a very, very long plate with a hump in small part of it:

    ____________XXXX____________

Laminar flow requires that the "streamlines" are closer over the top becuase there is, effectively, less space to get through. The fluid has mass, so tries to go in a straight line. Considerably above the plate the fluid will move effectively in a straight line, so the fluid between that straight line and the plate has to move faster.

As the fluid approaches that faster flowing area, it must accelerate, and the only thing to accelerate it is a pressure gradient. In the video he is using other means to accelerate the fluid flow, so it's different.

At the end of the plate there is no downwash, so the only effect is Bernoulli, and in this experiment you do get a pressure difference between the sides of the plate, and hence "lift".

In the case of the flow around the "elbow" there is no acceleration of the air, hence no Bernoulli effect. Quite the opposite, I would expect a Bernoulli effect to push the "elbow" to the left. However, the air is being sucked around the plate, so the "downwash" effect dominates.

There are some really, really bad explanations in the literature and on the net, often written by people who do one experiment without separating the effects. They go on to teach, and unsurprisingly people get confused.

[JabavuAdams]

I'm ashamed to admit I didn't think about inverted flight until I read this article. Shit.

Once I realize something like this it makes me want to go back and do an integration / cleaning pass on my web of knowledge. Not sure how to do that in practice, though.

[Herring]

The only way to know that you know is to test yourself. Find a college aerospace text & solve some problems. Personally I don't care about finding gaps in knowledge that I haven't tested.

[netsp]

René Descartes did that once. I don't think it turned out well.

[marvin]

For upside-down flight, though, the shape of the wing isn't irrelevant. Many aerobatic aircraft have symmetric wing profiles, since the curvature of the wing reduces performance when inverted. (Higher speeds are required to maintain the required lifting force, and drag is increased).

Some of the popular explanations of aerodynamic lift are outright wrong, but most of them are correct about part of the story. Our CFD equations are obviously correct, otherwise our simulation software would give bad results. Maybe it's just hard to explain the results of these equations in an unambiguous way.

It is true that "air goes down, wing goes up", but this explanation is only one "why" closer to the heart of the matter.

[luckyland]

There's that, and the fact that both Bernoulli and Newton's laws offer a thorough explanation of flight.

[baddox]

From everything I've read, it seems there is debate on exactly how to describe/explain what generates lift. Most of the explanations are correct, but insufficient by themselves.