[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.

[lutorm]

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.

It applies to fluid elements that have the same internal energy. In the case of the airplane wing, the air ahead of the wing is effectively unaffected by the wing's presence, so you can apply the Bernoulli effect to comparing different fluid elements.

Sure, if you compare the air coming out of a hair dryer with air that doesn't, you'll get in trouble, but that has nothing to do with an airplane wing.

[lutorm]

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".

Impossible, that would violate momentum conservation.

A pressure difference implies a force on the plate, so upward momentum is transferred to the plate. That momentum has to come from somewhere and it can only come from the air, which must move downward.

[RiderOfGiraffes]

If you mount the plate on a spring, and measure the steady state, the spring extends, showing that there is a force on the plate. There is no violation of conservation of momentum.

Look, this is pointless. Static states can isolate the individual effects, and then they all get combined into a dynamic state in varying amounts, and it becomes horribly complicated. People insist on trying to produce and explain overly simplistic models, and others insist on misunderstanding them. I've actually physically done these experiments and I know that what I say is true.

I'm not going to reproduce all the nitpicking tiny details in this forum because it's hard, inappropriate, and people will continually try to pick holes in it. It's the Monty Hall problem all over, and I'm just too tired to care.

There's every chance that your understanding is right in the cases you're considering, but I can't be bothered finding out where our experimental models differ.

I wish I hadn't bothered.

[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.