Bernoulli's Principle, Busted?

PhilosopherPilot said:
You're all circling around the same thing. The reason an airplane flies it that it puts downward pressure on air. The reason it puts downward pressure on air is Bernoulli. I'm not sure why people want to separate the laws of motion from fluid dynamics. They are all related and intertwined.

The notion that an aircraft isn't pushing down on the air is just ludicrous. It clearly is, and you can even see that it is from the airflow behind the aircraft.

If the plane doesn't push air down, the air won't hold it up. Simple. Why is this hard?
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because pilots argue about everything. It's a requirement in the commercial pts somewhere.....
 
charlie1017 said:
The thing is aerodynamics is ALL Newtonian physics (with some thermodynamics/kinetic theory and mass conservation sprinkled there). The distinction created by people about Newton action-reaction vs Bernoulli is a ludicrous one, created from a misapplication of basic physics. The Bernoulli equation can be directly derived from the Navier-Stokes equations that come from Newtonian physics. In fact is in an energy conservation statement. If you think in terms of force (not energy ) then pressure differentials and shear stress distributions are the producers of lift and drag.
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THIS! ^^^

Not surprisingly, John D. Anderson's book opens with....... integrating pressure and shear stress distributions over an airfoil. Like this:

img00048.gif


Obtained using the three-dimensional Navier-Stokes equations and the Euler equations (both from the excellent NASA Glenn website).

The "equal transit time" Bernoulli explanation is bogus, because there's nothing in reality causing air molecules to try and "catch up" to their friends on the bottom of the wing. @Fly_Unity already linked to it but it's worth doing it twice: http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

Yet on the next page, NASA Glenn also debunks the "skipping stone theory", which says lift is solely produced by deflected air by the bottom of the airfoil producing downwash: http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

As @Jimflyfast hinted at, the circulation based theories (the ones that have been "taught to real aerodynamicists for the last 80 years" :D ) are what are used in practice. While I haven't had much exposure to Computational Fluid Dynamics (CFD) which relies heavily on numerical methods and computers doing the brunt of the work, the circulation theories I learned in Aerodynamics class were the 2D Kutta-Joukowski Theorum, and the 3D Prandtl Lifting-Line Theory. Despite the fancy name, Kutta-Joukowski isn't that complicated: It just says that Lift force is directly proportional to the counterclockwise circulation of air around the airfoil (when the 2D airfoil is facing left).

I don't claim to be an expert on this, in fact I had a hard time trying to intuitively understand circulation theory and "vortex sheet" approximations. But at the end of the day I like to think the Bernoulli explanation and the Newton's 3rd-law downwash explanations are both right to some degree, and both wrong. Neither provides the complete picture, and ultimately multiple factors and physical phenomena (namely circulation) collectively contribute to the pressure and shear stress distributions that ultimately produce lift.
 
PhilosopherPilot said:
Do you guys think that the low pressure above the wing holds the plane in the air as if it were tied to a ceiling? Yes. There's a low pressure above the wing. But when it's all said and done, the wing pushes down on air. The air holds it up. It's like that myth busters episode when they put the birds in the box and tried to see if the box got lighter when they were flying. It doesn't get lighter because they are pushing down in the air while in flight, and the air pushes down on the box.

It's not like Newtonian laws of physics don't apply to airplanes...
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It is not getting pulled, it is getting pushed by the higher relative pressure under the wing. The air moving downwards happens after the fact, and is a result of the lift, not the cause of it.
 
cencal83406 said:
Stumbled across this article on "cracked".... yes it's a "comedy" website but still... very interesting. The other busted "myths" are strange too, but I couldn't pass up putting this in front of a panel of experts. The second link, below the article, goes into a lot more in-depth of an explanation as to why Bernoulli's Principle is "BS".

http://www.cracked.com/article_20669_6-ridiculous-science-myths-you-learned-in-kindergarten_p2.html



http://www.allstar.fiu.edu/aero/airflylvl3.htm
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The article is wrong.
 
seagull said:
Do a search in the tech forum here for some very detailed threads on this topic in the past.
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I could, and would have posted this article on those, but I didn't want to necropost, and specifically wanted to see a discussion with reference to the article.
 
cencal83406 said:
@tgrayson gave up on JC because he got a lot of crap for his poor delivery. Something along the lines of "I'm right, and you are wrong because you are an idiot".
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That's not even close to what happened. He left because people here were arguing to him that knowing less can be beneficial compared to knowing more, a statement he rightly rejected. Some people here got all butt hurt when he called them out for basically being anti-intellectual. So, he decided why try to teach people that feel that knowledge is detrimental. That's how I saw it anyway.
 
seagull said:
It is not getting pulled, it is getting pushed by the higher relative pressure under the wing. The air moving downwards happens after the fact, and is a result of the lift, not the cause of it.
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The fact remains that an aircraft cannot be held up in the air if it isn't pushing down on the air. Whether the force is applied to the top or the bottom of the wing is irrelevant to my point. Nothing can be held up without the thing that's holding it up applying force to it.

Sometimes when I read people's posts regarding their understanding of aerodynamics, it seems like people don't understand that the air is holding the plane up.
 
cencal83406 said:
People don't learn by being called idiots.
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Agreed I have never told a student he was dumb
But some really truly are wading around in the shallow end of the gene pool.
I once had a student try to convince me that magnetos produce lift.
He was so convinced of this, that to tell him otherwise was insulting his intelligence.

There is nothing more frustrating than an alpha personality attached to a complete moron.

Accept maybe explaining lift to one ;)
 
cencal83406 said:
@tgrayson gave up on JC because he got a lot of crap for his poor delivery. Something along the lines of "I'm right, and you are wrong because you are an idiot".
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I kind of preferred his direct approach to many others' frequent passive-aggressive approaches that are attempts to achieve the same thing. The internet would convey many of the same points but with much less words that way.
 
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