Kevbo is correct on this one, Bernoulli works both ways. The (Newtonian)lower pressure over the wing results in a faster airflow as per Bernoulli. Airflow over the wing is thus accelerated before it is deflected downward. This gives it greater inertia; this is why air deflection over the wing is the primary source of lift.
rwj
:smack: I meant CurtCwas correct.
100% - Action-reaction.
Its either that or magic.
rwj
I hope I’m not picking a disagreement with you here (mainly because you’re always right), but a wing that doesn’t deflect air necessarily creates zero lift. So yes, air deflection accounts for 100% of the lift. You can also account for it all with Bernoulli if you know the speeds and pressures. You can also use the other methods that airplane designers actually use in practice.
The reason this subject creates controversy IMHO is that so many of us were taught how wings fly based on two incorrect ideas (I was taught the same thing in freshman physics and in flight school, both around 1979). First, they taught us that wings don’t fly by deflecting air (incorrect), they do it by this thing called the Bernoulli effect. What happens is that the air going over the top goes faster, so creates a low-pressure zone over the wing. How do we know it goes faster? It has to, because, of course, the air going over the top and the air going underneath have to arrive at the trailing edge at the same time (incorrect), so the longer path length means it has to go faster.
Curt, at least your ground school didn’t use the phrase, “Nature abhors a vacuum”, like mine did. I wanted to correct the instructor, but desire not to be seen as a smart-ass won out.
Nooooooooo! You cannot.
[repeat]Bernoulli only deals with airflow parallell to a surface, not deflected airflow.[/repeat]
Wings deflect airflow.
rwj
RM: Yes it is oversimplified; but unlike Bernoulli, it is completely accurate.
Peace
rwj
Well, thrust has something to do with it, sometimes. 
rwjefferson seems to disagree, but that seems to be in disagreement with aerodave’s staff report too.
On preview: peace
PS: in theory, would we say a stationary hotair balloon is deflecting air? What about a helium balloon?
Actually it doesn’t. If you read back through the (rather excellent) Staff Report, aerodave says:
Either theory is adequate, though a direct, force-based Newtonian explaination is both more intuitive and (for a non-controlled volume) easier to work with.
That’s an entirely different situation. A stationary (with respect to the wind) hot air or helium balloon is in quasistatic equilibrium (owing to average gross density equivilence with the surrounding atmosphere, a la Archimedies’ Principle) and doesn’t need to push against anything to stay afloat.
Stranger
SoaT: I do not find the Bernoulli explanation nor its “technical” interpretation adequate or accurate. If you please, I wish to know where my logic has failed.
Bernoulli speaks of force tangential to airflow velocity; Newton of airflow acceleration. I do not see a question as to which is the greater force. The smooth ball/rough ball experiment seems to support my understanding.
Which technicality am I missing?
Is it wrong to invoke the Bernoulli principle when explaining how an airfoil works? No, but only as a side bonus to Newton.
Peace
rwj
May be there is a Quantum Bernoulli Effect. Hey, it’s possible. 
Another salute. 
As you are speaking of a hot air balloon as opposed to a cold as h~ balloon, you will by definition have turbulence and air deflection.
Peace
Back at You
rwj
Yes!! You can.
I’ve said it a few times before, so why not one more time? If you know the entire velocity field around a wing, you can calculate the pressure at any point based on Bernoulli’s Principle. You can integrate these pressures to get the lift. And that lift will necessarily be correct. Period.
The problem? Knowing the velocity field is the tricky part, and is not usually information we readily have at our disposal. It’s difficult or impossible to arrive at analytically. And you wouldn’t generally run an experiment to map that velocity field, when an experiment to directly measure lift is incredibly easier. That’s why the process I described above is of little practical value.
Now, lest you think this supports the idea that “Newton” is far more correct than “Bernoulli” (bear in mind that I don’t consider them to be opposing or even separable ideas), I could say the same thing about the Newtonian method. If I knew the velocity field around the wing, I could calculate the momentum change through the control volume, and figure out the same value for lift. But, again, it’s pointless unless I know the velocity field. Predicting lift based on air deflection is correct, but of little practical use. But, similarly, predicting lift based on Bernoulli-derived pressures is equally accurate, but of equally little use.
Bernoulli’s equations (incompressible or compressible) don’t care what kind of surface is involved. They don’t even care if there is a surface. They only describe the trade-off between static and dynamic pressures along a streamline. Being parallel or being deflected has nothing…I repeat, nothing…to do with it.
If there’s one point I could impress on people, it’s the fact that Bernoulli isn’t the reason planes fly, but that doesn’t mean it’s incorrect. Gale Craig does a great job tackling this idea in his books (like the one I referenced in the article), but I realize most people won’t have the opportunity or desire to read them. The core of that argument is nicely documented on the first part of this web page of his. The problem isn’t that Bernoulli’s Principle is wrong (because it’s not). It’s that lift generation theories that invoke BP are generally predicated on the WRONG notion that air magically keeps pace over both wing surfaces. He calls it the “Hump Theory.” To me, it sounds derogatory…and if so, it’s for good reason.
(The latter part of Craig’s page introduces the idea of circulation, which is a concept I’m relieved has stayed out of this thread so far. It’s a creepy mathematical abstraction that just happens to provide somewhat accurate predictions. It’s NOT a concept grounded in good first-principle physics, but was actually formed by early aerodynamicists’ efforts to parallel fluid dynamics with Maxwell’s equations. To the dismay of aero students for the next 100 years, they make a workable model.
The circulation model isn’t totally off-base, but it’s a needlessly complex—and not physically meaningful—way of capturing the flow behavior. When you boil it down, all that circulation impleis is merely that the air goes faster over the top. Duh, we knew that. The trick is—that through nothing but sheer luck—the theory sometimes lets us figure out how much faster it goes.)
In other news:
That’s exactly right, and it’s something I mentioned this in the article. And I should add the interesting fact that the margin by which the “top” air outpaces the “bottom” air is *directly proportional * to the lift coefficient!! As a graduate research exercise, I ventured to demonstate this correlation with a set of numerical examples. I didn’t come up with this cool fact myself…I just stumbled upon it accidentally once in the course of a project and thought it would be interesting to validate it with some simple simulations.
Here (the bolded text) is where you are incorrect. Bernoulli’s Principle doesn’t speak of “force tangential” to anything; it establishes a relation between velocity and pressure; the latter, by definition, doesn’t act directionaly but anisentropicaly (without regard to direction). A fluid in which the velocity is reduce will act by increasing pressure in all directions, which results in a net upward force on the wing (and a corresponding downward force on the air below the wing).
Bernoulli’s Principle (and equation) behave exactly as explained by Newtonian mechanics (for a noncompressible, inviscid fluid). It’s no more a comparison between Bernoulli’s Priniple and Newton’s Laws than it is an argument between Kepler’s Laws of Planetary Motion and Newton’s (fundamental) Law of Mechanics. While it is true (to extend the analogy) that Kepler determined his Laws empiracally and in absence of knowledge about Newton’s revelations, in retrospect Kepler’s main discovery (that the line connecting a planet to the sun sweeps out equal areas in equal amounts of time) is simple a rendering of Newton’s Laws as applied to orbital motion.
Similarly, Bernoulli’s Principle, when expanded to included the compressability of air (the viscosity of air can be largely ignored for gross effects, though it does have to be taken into account to explain turbulence and the Coanda Effect) does adequately describe how the differential of pressure between the top and bottom sides of the wing provides for lift. In trying to make a distinction between Newton’s Laws of Mechanics and Bernoulli’s Principle you are, if you’ll excuse the analogy, arguing that the Pope can’t be a Catholic because he’s a Christian.
Stranger
Sorry, I wasn’t clear, I meant that rwjefferson was in disagreement with aerodave’s report. As you point out.
I dunno, depends on how you look at it.
A helium balloon is in equilibrium true, but that means that the pressure on the underside exceeds the pressure on the top by exactly the weight of the balloon. In the context of this thread, that means that air is getting deflected downwards–sure, if the balloon were not there, the air would still be deflected downwards by other air. But that’s the point, whether it’s balloon or it’s air, it has to be held aloft somehow. I think that this is just the sort of analysis that gets people into trouble when they start to look at the problems.
For another instance, look what happens when we drop a piece of paper (or suddenly puncture the balloon). Lift. Not enough to keep it in the air, but there’s enough to keep it from falling as fast as it would have. It seems to reach terminal velocity pretty quickly. It would only have to generate a bit more lift to fly–and if the line of thrust were not parallel to the ground, there’d be a vertical component, maybe enough to keep it in the air. I’m going to call that the Archimedien theory of flight.
Permit me to relay a quote of his that cleared some issues up for me (emphasis added) :
So the principle of “equal transit time” has no basis in theory or observation. OTOH, the air pressure below the wing is greater than the air pressure above, generating lift.
Weirdly enough, the “air-deflection” argument, which I had not encountered until reading aerodave’s article, seems simpler to me than Hump Theory.
CurtC, aerodave, RM(?): Please signal the next time you depart reason for calculation.
I understood the conversation as follows:
The question was “How do airplanes fly?”.
I then answered:” As you accelerate air downward, the air pushes back.”
RM responded to me: “Do you think that the air deflected by the wing … accounts for (causes) the lift?”
In turn CurtC’s responded with “Air deflection accounts (causes) 100% of the lift. You can also account (show cause) for it all with Bernoulli.”
I was mistaken. The answer meant: “You can also account for (calculate) it all with Bernoulli”.
That is much more difficult to communicate when ambiguous words are used contrary to the line of the conversation…
The hope we all agree that the cause is Newton, not Bernoulli,
Peace
rwj
The pressure in the direction of the airstream will decrease as the velocity decreases. This is what I was trying to convey with the word “tangential”.
Please, also remember that I am speaking of causes, not calculations.
Planets do not orbit because of Kepler’s Laws; they orbit because of Newton’s gravity.
Airplanes do not fly because of Bernoulli’s Principle; they fly because of Newton’s mass.
I understand that this may be a semantical distinction, but somehow it seems important and much less confusing to me.
Peace
rwj
While Newton’s Third Law is sacred ground for physicists, I don’t think it’s right to give it credit for causing lift. I’d call it the mechanism by which lift takes place…or rather, a description of just one way that mechanism manifests itself.
And even if you can consider a Newtonian model to supply cause, there’s no reason a theory directly derived from Newtonian principles (i.e. Bernoulli’s equations) should be any less able to show cause.
Of course, the question “why” necessarily drags us into the realm of philosophy. What is causation? Is any scientific theory or law able to provide reason? Or are they all just meant to describe and predict the things we see?
Can you provide a true reason for any phycial phenomena? We have sound theories and laws all over the place, but I’m not aware of any that provide an answer to the question “why?” Tackling questions like that can quickly turn scientists into philosophers.
I prefer to avoid that mess. Perhaps a better question to answer in the first place would have been “How do aiplanes fly?” Although I’m still not sure you can avoid the causation trap.
Maybe “What do airplanes do to the air when they fly?” I can answer that without ambiguity…but it’s not likely that someone would have phrased the question like that.
You’re thinking of dynamic pressure. It doesn’t have direction, either. But it does decrease with the square of velocity. Dynamic pressure also only applies force when it’s converted to static pressure as the air slows down. When we (fluids people) talk about pressure without specifying which component of total pressure we mean, we usually mean static pressure. That’s the pressure that actually applies force to things. And static pressure increases with velocity.
Hey!
Oh, OK, nevermind…
Newton 6 Bernoulli 6
OTOH, you guys are going to have to catch up. Newton’s laws being the cause of things is so last century–if not the century before that.
I’m still waiting for a relativistic lift theory.