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BTW the low pressure generating the lift thing is a myth. The lift is caused by the fact that the air is being deflected downward and good ol' Newton's Third L

BTW the low pressure generating the lift thing is a myth. The lift is caused by the fact that the air is being deflected downward and good ol' Newton's Third Law.

mannykannot 10y ago
The common explanation of lift for laypersons contains a number of non-sequiturs and even falsehoods, but this is not one of them. Newton's laws applied to the situation play out in complicated fluid dynamics, and result in a pressure difference over the wing, with a reduction above. That difference, summed over the wing, is the lift (and also the source of the induced drag, I believe, but that is a step beyond.)
- ashark 10y ago
  Fun fact: One of the weirdest things about heavier-than-air flight is that upon posting any explanation for it online, several people will tell you you're wrong. Each of them will also disagree with one another. All will have sources. [EDIT] This also happens with the Oberth Effect.
  - deleted 10y ago
    [deleted]
  - mannykannot 10y ago
    True, though I think it's more of a fact about the Internet than about flight.
snovv_crash 10y ago
Yes, but by making the air move you introduce a low pressure where it moved from, and a high pressure where it moved to. They are interconnected, just like how you can determine speeds through conservation of momentum or conservation of energy.
thisrod 10y ago
You're both right. When Newton's third law is applied to a wing, the equal and opposite forces are pressure. No one tends to think about the pressure of the wing on the air, but it happens.
Tloewald 10y ago
It's just another way of framing the same phenomenon (albeit one that makes much more sense to me than high school fluid dynamics). What gets left out of the pressure explanation is what pressure is, which is a statistical representation of Newton's third law applied over huge numbers of tiny things moving in random directions in a given frame of reference
conistonwater 10y ago
Looking at https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html it doesn't look like it's true that it's air being deflected that causes lift. There was a recent submission on HN explaining the theories of lift in sails: https://news.ycombinator.com/item?id=13829625 https://news.ycombinator.com/item?id=13829625. AFAIU air is not dense enough to make this deflection account for the magnitude of lift forces actually observed.
- mannykannot 10y ago
  The NASA article discusses a deliberately simplistic model that does not consider the deflection of airflow over the top of the wing, but it would be easy to overlook the caveat if you don't already know the answer. Further down the page there is a link to a correct Newtonian flow-turning model.
vpribish 10y ago
it's not the Bernoulli effect, but it is a pressure difference (from the 3rd law, as you say)
- mannykannot 10y ago
  The Bernoulli equation is a valid description of what happens to the airflow pressure and velocity around a wing (at least at speeds well below sonic), but the problem is that it is not much of an explanation - it leaves you with something else to explain, namely the change in velocities. Attempts to do so without invoking calculus have resulted in the propagation of fallacies, such as the 'equal transit time' one mentioned above by tzs. The "Newton's 3rd. law" explanation treats the wing and its immediate surrounding as a black box: air flows in, and exits deflected downwards, so there has been some downwards-directed acceleration. It does not address details of that process, such as why the above-wing airflow usually generates the larger part of the lift, or even how the deflection occurs, though it is fairly obvious that some sort of deflection will result from driving an inclined plane through the air.
tzs 10y ago
There are myths related to low pressure (such as the infamous "equal transit time" theory), but low pressure generating lift is not one of them. The pressure explanations and the Newton's Third Law explanations are not different theories of lift. They are just different ways of looking at the same thing underlying thing. The motion of a wing through a fluid has to conserve mass, energy, and momentum. If you analyze lift by focusing on conservation of momentum, you get the Newton's Third viewpoint. If you analyze by focusing on conservation of energy, you get a pressure viewpoint. See: https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html

BTW the low pressure generating the lift thing is a myth. The lift is caused by the fact that the air is being deflected downward and good ol' Newton's Third Law.

mannykannot 10y ago

The common explanation of lift for laypersons contains a number of non-sequiturs and even falsehoods, but this is not one of them. Newton's laws applied to the situation play out in complicated fluid dynamics, and result in a pressure difference over the wing, with a reduction above. That difference, summed over the wing, is the lift (and also the source of the induced drag, I believe, but that is a step beyond.)

ashark 10y ago
Fun fact: One of the weirdest things about heavier-than-air flight is that upon posting any explanation for it online, several people will tell you you're wrong. Each of them will also disagree with one another. All will have sources. [EDIT] This also happens with the Oberth Effect.
- deleted 10y ago
  [deleted]
- mannykannot 10y ago
  True, though I think it's more of a fact about the Internet than about flight.

snovv_crash 10y ago

Yes, but by making the air move you introduce a low pressure where it moved from, and a high pressure where it moved to. They are interconnected, just like how you can determine speeds through conservation of momentum or conservation of energy.

thisrod 10y ago

You're both right. When Newton's third law is applied to a wing, the equal and opposite forces are pressure. No one tends to think about the pressure of the wing on the air, but it happens.

Tloewald 10y ago

It's just another way of framing the same phenomenon (albeit one that makes much more sense to me than high school fluid dynamics). What gets left out of the pressure explanation is what pressure is, which is a statistical representation of Newton's third law applied over huge numbers of tiny things moving in random directions in a given frame of reference

conistonwater 10y ago

Looking at https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html it doesn't look like it's true that it's air being deflected that causes lift. There was a recent submission on HN explaining the theories of lift in sails: https://news.ycombinator.com/item?id=13829625 https://news.ycombinator.com/item?id=13829625. AFAIU air is not dense enough to make this deflection account for the magnitude of lift forces actually observed.

mannykannot 10y ago
The NASA article discusses a deliberately simplistic model that does not consider the deflection of airflow over the top of the wing, but it would be easy to overlook the caveat if you don't already know the answer. Further down the page there is a link to a correct Newtonian flow-turning model.

vpribish 10y ago

it's not the Bernoulli effect, but it is a pressure difference (from the 3rd law, as you say)

mannykannot 10y ago
The Bernoulli equation is a valid description of what happens to the airflow pressure and velocity around a wing (at least at speeds well below sonic), but the problem is that it is not much of an explanation - it leaves you with something else to explain, namely the change in velocities. Attempts to do so without invoking calculus have resulted in the propagation of fallacies, such as the 'equal transit time' one mentioned above by tzs. The "Newton's 3rd. law" explanation treats the wing and its immediate surrounding as a black box: air flows in, and exits deflected downwards, so there has been some downwards-directed acceleration. It does not address details of that process, such as why the above-wing airflow usually generates the larger part of the lift, or even how the deflection occurs, though it is fairly obvious that some sort of deflection will result from driving an inclined plane through the air.

tzs 10y ago

There are myths related to low pressure (such as the infamous "equal transit time" theory), but low pressure generating lift is not one of them. The pressure explanations and the Newton's Third Law explanations are not different theories of lift. They are just different ways of looking at the same thing underlying thing. The motion of a wing through a fluid has to conserve mass, energy, and momentum. If you analyze lift by focusing on conservation of momentum, you get the Newton's Third viewpoint. If you analyze by focusing on conservation of energy, you get a pressure viewpoint. See: https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html