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this article is so confused and unscientific i have a hard time forming a coherent response. > although bernoulli's theorem is largely correct ... the theorem

this article is so confused and unscientific i have a hard time forming a coherent response. > although bernoulli's theorem is largely correct ... the theorem alone does not explain why this is so or why the higher velocity atop the wing brings lower pressure along with it this blurb is accompanied by an upside down plane with the caption "doesnt explain why planes can fly inverted". this is a "tide goes in, tide goes out, cant explain that" statement [1]. im speechless. some planes can fly upside down, and some cant. its not a mystery we discover through trial and error - theyre designed that way by people who use bernoulli's equations, et al. equations are not explanations. calling a theorem partially correct because you dont know why it works is totally unrelated. either the theorem accurately represents and predicts reality, or it doesnt. no equation says why itself works, thats nonsense talk. [1] https://www.youtube.com/watch?v=_fSlJaZrUhs https://www.youtube.com/watch?v=_fSlJaZrUhs

FabHK 7y ago
I find the article perfectly reasonable. We can model flight quite well, and predict the behaviour of wings quite well, yet it is hard to give an accurate account of it at a layman's level. The article goes on to discuss two accounts that have been given historically, and outlines why they are insufficient. A plane flying upside down is a perfectly fine refutation to the naive Bernoulli "the wing is curved on the upside" explanation. Which parts do you have a problem with? > equations are not explanations. As the article makes painstakingly clear.
- pmoriarty 7y ago
  "We can model flight quite well, and predict the behaviour of wings quite well, yet it is hard to give an accurate account of it at a layman's level." The problem in that statement is an implied equivalence between having a good model and having a good understanding of the phenomena in question. It's not so much a layman's understanding that's at issue, but having what's often called an "intuitive understanding" that doesn't resort to the model. Sure, it's easy to plug in numbers in to a computerized model, and get answers back about the behavior, as from a black box, but it's quite a different thing to oneself have an intuitive understanding of what's going on. It's often useful to have visualizations of mathematical models and equations precisely because complex models and equations are so difficult to understand, and this difficulty in understanding them highlights that models and equations are not the understanding itself.
- Rury 7y ago
  Perhaps this: >In inverted flight, the curved wing surface becomes the bottom surface, and according to Bernoulli’s theorem, it then generates reduced pressure below the wing. That lower pressure, added to the force of gravity, should have the overall effect of pulling the plane downward rather than holding it up. This is not what Bournoulli's theorem states. Bournoulli's theorem doesn't say anything as to why fluid flows the way it flows. It just says for a fluid in a steady state flow, increases in speed are associated in decreased pressure. Hence the principle suggests, if you find a plane is able to fly steady upside down, then expect to find the air flowing faster over the wing on the opposite side of gravity under those conditions - just likewise when a plane is normally flying right side up. Again why fluid flows the way it does around airfoils is separate question -and really the crux of the mystery here. Answers to this is probably hidden in the solutions to the Navier–Stokes existence and smoothness problem: https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_existence_and_smoothness https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_existenc...
umvi 7y ago
This is like "we can't explain gravity" articles. If you drill down and keep asking "but why", you'll eventually encounter phenomenon you can't explain.
pdonis 7y ago
The main thing I see that the article leaves out is circulation. A wing with a positive angle of attack doesn't just push air down; it creates circulation.
caconym_ 7y ago
> equations are not explanations. calling a theorem partially correct because you dont know why it works is totally unrelated. either the theorem accurately represents and predicts reality, or it doesnt. no equation says why itself works, thats nonsense talk. Bernoulli's principle and e.g. the equal-travel-time Bernoulli explanation of lift are totally different things. The point the article is making is that common explanations of lift like the latter are typically incomplete or misleading, if not outright incorrect, regardless of whether they invoke uncontroversial scientific results. The article also discusses what an actual correct and complete explanation might look like, but (IIRC) outright states that the formal theoretical side of things is uncontroversial as it stands today.

FabHK 7y ago

I find the article perfectly reasonable. We can model flight quite well, and predict the behaviour of wings quite well, yet it is hard to give an accurate account of it at a layman's level. The article goes on to discuss two accounts that have been given historically, and outlines why they are insufficient. A plane flying upside down is a perfectly fine refutation to the naive Bernoulli "the wing is curved on the upside" explanation. Which parts do you have a problem with? > equations are not explanations. As the article makes painstakingly clear.

pmoriarty 7y ago
"We can model flight quite well, and predict the behaviour of wings quite well, yet it is hard to give an accurate account of it at a layman's level." The problem in that statement is an implied equivalence between having a good model and having a good understanding of the phenomena in question. It's not so much a layman's understanding that's at issue, but having what's often called an "intuitive understanding" that doesn't resort to the model. Sure, it's easy to plug in numbers in to a computerized model, and get answers back about the behavior, as from a black box, but it's quite a different thing to oneself have an intuitive understanding of what's going on. It's often useful to have visualizations of mathematical models and equations precisely because complex models and equations are so difficult to understand, and this difficulty in understanding them highlights that models and equations are not the understanding itself.
Rury 7y ago
Perhaps this: >In inverted flight, the curved wing surface becomes the bottom surface, and according to Bernoulli’s theorem, it then generates reduced pressure below the wing. That lower pressure, added to the force of gravity, should have the overall effect of pulling the plane downward rather than holding it up. This is not what Bournoulli's theorem states. Bournoulli's theorem doesn't say anything as to why fluid flows the way it flows. It just says for a fluid in a steady state flow, increases in speed are associated in decreased pressure. Hence the principle suggests, if you find a plane is able to fly steady upside down, then expect to find the air flowing faster over the wing on the opposite side of gravity under those conditions - just likewise when a plane is normally flying right side up. Again why fluid flows the way it does around airfoils is separate question -and really the crux of the mystery here. Answers to this is probably hidden in the solutions to the Navier–Stokes existence and smoothness problem: https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_existence_and_smoothness https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_existenc...

umvi 7y ago

This is like "we can't explain gravity" articles. If you drill down and keep asking "but why", you'll eventually encounter phenomenon you can't explain.

pdonis 7y ago

The main thing I see that the article leaves out is circulation. A wing with a positive angle of attack doesn't just push air down; it creates circulation.

caconym_ 7y ago

> equations are not explanations. calling a theorem partially correct because you dont know why it works is totally unrelated. either the theorem accurately represents and predicts reality, or it doesnt. no equation says why itself works, thats nonsense talk. Bernoulli's principle and e.g. the equal-travel-time Bernoulli explanation of lift are totally different things. The point the article is making is that common explanations of lift like the latter are typically incomplete or misleading, if not outright incorrect, regardless of whether they invoke uncontroversial scientific results. The article also discusses what an actual correct and complete explanation might look like, but (IIRC) outright states that the formal theoretical side of things is uncontroversial as it stands today.