Flight: Bernoulli or Coanda ?!

It is my understanding that there is some controversy over what gives a wing its lift. Some say Bernoulli, others say Coanda. (Here is Wikipedia’s entry on the Bernoulli principle; there is a Coanda link on the page.)

Can anybody tell me if one really is the reason? Are they both workable models that explain all the evidence such that neither can really be eliminated? What’s the Straight Dope?

I wouldn’t exactly describe it as “controversy” about Bernoulli vs. Coanda. One can explain lift just fine without stating that the flow must “stick” to the surface, but no one knows why. That’s almost as bad as “a duck’s quack doesn’t echo, and no one knows why.”

I cut & paste one of my favorite links:
See How it Flies.
Basically the Coanda effect has nothing to do with the production of lift. As the author writes elsewhere:

In fact, lift is quite simple, the airfoil directs the airflow downwards, thus, the mass of air moving down, by the action-reaction priciple, pushes the wing up.
Think of a helicopter, the rotor blades (long slender wings actually) blow air down, chopper goes up, if it would be the case that wings are sucked up by low pressure there wouldn´t be a rotor downwash, at least not quite as strong.
Yes, there´s a little Bernoulli in there; but the net contribution to the overall lift is minimal.

Ale, did you read my link or even the excerpt I quoted?

It’s not “a little bit of Bernoulli plus a bit of Newton plus some other stuff equals lift.” All of the effects produce lift; it is not a cumulative effect. The moving fluid generates a low-pressure area on the top of the airfoil and vortices coming off the trailing edge of the airfoil (or rotor blade, if you prefer) generates a downwash. Simply stating that because there is a downwash, the low-pressure area has no effect is simply not correct. Or provide a cite for those assertions.

Well, your first link says that the Coanda descriptions are “fairy tales” that are “worse than useless”; whereas this professor of aeronautics says that the Bernoulli description is useful only because it is simple to explain and that using it to actually calculate lift we find that a Cessna 172 has to travel at over 400 mph to get off the ground.

So, I’m guessing that I don’t understand what you were saying.

As an aerospace engineer, I get confronted with this question disturbingly often. And my personal way of explaining it is that Bernoulli is a perfectly accurate way of describing the effects of lift generation, but is not an explanation of its cause. in other words, you can use Bernoulli to calculate what the air is doing, but it doesn’t explain the mechanism responsible.

It’s a bit like general relativity. It’s a great tool for using to describle how things behave, but trying to apply it as a tool for understanding why is a huge mistake.

I got into an interesting debate about this subject in another thread. Look there, and you’ll see some of the conclusions drawn, and I won’t have to repeat myself.

You´re wrong, I´m affraid. I said that Newton plus a little bit of Bernoulli explains a wing´s lift; we have already dicussed all the theory and empirical data a few months ago on a thread about How do airplanes fly upside down? bbeaty in particular put forth a good deal of information and cites.
While it´s true that there are regions of different pressures along the chord of an airfoil, in the end the bulk of the lift is produced (simplifying) by the downwash at the trailing edge.

Bingo! :smiley:

I went to a lecture at the Royal Institution manyt years ago given by an engineer who had harnessed the Coanda effect to effect the efficient burning of waste gasses.

My brother and I reached closure on this a few months ago.

A body must deflect air downwards to generate lift. A wing is an air deflector: That’s Newton’s third law. Which means barn doors fly just fine. A good wing, however, is a low drag air deflector.

The only means for a mass of air to transmit forces to a body moving through it is via shear frictional forces and normal pressure forces.

Thus, for us, a wing generates lift if it deflects air downward. The net pressure differential on a body which deflects air downwards is upwards. There has to be that pressure differential if you’re deflecting air.

While I agree that “blowing air down” is the simplest, most intuitive way to explain lift, you could also explain lift in a computational sense by integrating the pressure on all surfaces of the airplane - if it’s in level flight, that pressure times the area for each little parcel of surface has to add up to the weight of the plane. So the “regions of different pressures” could completely account for the lift, if you just had a practical way of figuring out what they all are.

I’d suggest reading the thread previously linked about planes flying upside down. bbeaty provides many good explanations (some taken from my www.av8n.com link). What, I, bbeaty, Ale, audilover, and everyone else seem to be saying is that among Newton, Bernoulli, Coanda, or Navier-Stokes, none alone are sufficient to explain where lift comes from, or to calculate lift forces. One needs to include all the effects for an explanation and apply more complicated equations to calculate forces.

I agree with you, av8rmike, with the exception that Navier-Stokes would be the ultimate way to explain lift.

If the full Navier-Stokes equations could be completely solved for any reasonably complex geometry, it would comprehensively describe all the forces of interest, including lift. However, they cannot be solved analytically, and even state-of-the-art numerical methods (the best CFD we’ve got) can’t do the math to get an answer with any reliable fidelity.

The problem is, while a true solution of the NS equations would explain lift, I doubt the explanation would make any more intuitive sense than it does now.

I also believe that downwash is a necessary byproduct of lift generation (Newton’s 3rd law), and the integrated pressure distribution over a wing can give the right amount of lift.

According to the pages by NASA that you linked to in the other thread you linked to, the cause of lift is “turning a fluid.” They explained why a spinning cylinder creates lift; they explained why stone skipping, equal transit times, and the Venturi effect don’t give a wing its lift; but, I had a little trouble coming up with a clear, simple statement as to why a wing does create lift. In spite of their own protestations to the contrary, they seemed to insist that the bottom of the wing causes a fluid to turn, and they seemed to assert without explanation that the top of a wing causes a fluid to turn.

It would have been incredibly easy to just say that as a wing cuts through the air, the wing “turns” the air, i.e. forces the air down. Because of equal & opposite reactions and all that, the wing is forced up. Even though NASA seemed quite unhappy with the bottom of the wing being an explanation, it evidently is invovled in causing the air to turn down and is at least partially responsible for the upward force. Now, can you tell me why the air going over the top of the wing is turned down? If so, will you? If so, then please do.

p.s. MonkeyMensch, thank you for being simple and intuitive, not to mention straight forward. Can you tell me why the top of the wing turns the fluid? As it turns out, The Navier-Stokes equations are the foundation of fluid mechanics and, strangely enough, are rarely recorded in their entirety isn’t really what I was looking for. After all, saying that a transitive and complete preference relation is the basic assumption of consumer choice theory and is therefore the ultimate way to explain why popcorn is so expensive at the movies really isn’t answering the question “why is popcorn so expensive at the movies?”, is it?

There certainly is a controversy. The above aeronautics professor and the owner of “See How It Flys” violently disagreed on several points, and some really disgusting politics took place behind the scenes. It eventually resulted in the material now appearing on the NASA GRC site.

Be warned that the stuff about “fairy tales” on the “See How It Flys” is based on the strange idea that attachment of flowing air to a wing surface is NOT called Coanda effect …and that Coanda Effect only involves thin layers of moving fluid.

Yes, if Coanda Effect only involves narrow fluid jets sticking to a surface by adhesive forces, then wings do not employ the Coanda Effect. The attached flow on a wing surface is obviously not a narrow jet. Does there seem to be something wrong with this answer? I thought so too. Declaring that flow attachment on wings is not called “Coanda Effect” …is screwy. There is very weird psychology mixed in with these websites and all through this subject area. (My own site is not immune of course :slight_smile: , but at least I acknowledge some of it: The Airfoil Misconception in K-6 Textbooks)