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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#11

Post by kryogen1c »

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

Re: The mysteries of aerodynamic lift

#12

Post by NikolaeVarius »

FabHK wrote: post #10
NikolaeVarius wrote: post #6

Needlessly click-baity article title. TLDR: We don't have a full solution that models flight. We also don't have a full understanding of how bicycles work.

> TLDR: We don't have a full solution that models flight. We do, actually. It's just hard to explain.

There is a Millennium Challenge problem to provide a solution to the Navier Stokes Equations which are a fundamental component of modeling flight.

The entire article about how none of the current models fully explain how a plane stays in the air.

So what is this full solution you are talking about because my Aero professors must have done a bad job teaching the topic to me

Re: The mysteries of aerodynamic lift

#13

Post by falcolas »

falcolas wrote: post #4

So, since the title is a bit baity: On a mathematical level we understand and have consensus; but the "dirtiness" of practical applications of mathematics makes it a bit more complicated to explain fully.

I'm not sure if it's because they're trying to break it down for laymen, but there's a lot missing from these explanations and their supposed problems. There's no discussion of boundary layers, turbulent vs laminar flow, the effect of wing textures, nothing.

There's also no discussion about what happens when you try and tilt a wing too far upwards - a stall - or why the simple buildup of ice on a wing can cause an otherwise fine wing to lose its ability to produce lift.

Re: The mysteries of aerodynamic lift

#14

Post by pmoriarty »

falcolas wrote: post #4

So, since the title is a bit baity: On a mathematical level we understand and have consensus; but the "dirtiness" of practical applications of mathematics makes it a bit more complicated to explain fully.

Well, the quibble here is whether having equations that are solvable and useful for making predictions is equivalent to having an understanding.

If you answer "yes", then the problem with that view is most evident in physics where equations and successfully tested predictions result in counter-intuitive "quantum weirdness" which is hard to reconcile with claims of understanding. As Feynman was supposed to have said, "If you think you understand quantum physics, you don't."

Another problem with such a view is it results in descriptions which sound absurd on their face, such as claims that when people look they see photons. But people look they see objects, fields of color, or have other experiences which are irreconcilable with the abstractions or equations created by physicists.

This is related to what is known in analytic philosophy circles and cognitive science as "the hard problem of consciousness", which is how one reconciles "scientific" explanations of how the mind works with people's actual experiences.

Re: The mysteries of aerodynamic lift

#15

Post by FabHK »

NikolaeVarius wrote: post #12
FabHK wrote: post #10

Earlier quoted context omitted.

> TLDR: We don't have a full solution that models flight. We do, actually. It's just hard to explain.

There is a Millennium Challenge problem to provide a solution to the Navier Stokes Equations which are a fundamental component of modeling flight. The entire article about how none of the current models fully explain how a plane stays in the air. So what is this full solution you are talking about because my Aero professors must have done a bad job teaching the topic to me

I'll let the article (which we are discussing here) speak for itself (my highlighting though):

> accounts of lift exist on two separate levels of abstraction: the technical and the nontechnical. [... The former] exists as a strictly mathematical theory, a realm in which the analysis medium consists of equations, symbols, computer simulations and numbers. There is little, if any, serious disagreement as to what the appropriate equations or their solutions are.

Re: The mysteries of aerodynamic lift

#16

Post by falcolas »

pmoriarty wrote: post #14
falcolas wrote: post #4

So, since the title is a bit baity: On a mathematical level we understand and have consensus; but the "dirtiness" of practical applications of mathematics makes it a bit more complicated to explain fully.

Well, the quibble here is whether having equations that are solvable and useful for making predictions is equivalent to having an understanding. If you answer "yes", then the problem with that view is most evident in physics where equations and successfully tested predictions result in counter-intuitive "quantum weirdness" which is hard to reconcile with claims of understanding. As Feynman was supposed to have said,…

It can be explained, though. I took aerodynamics courses over a decade ago, and they had a decent explanation which covered all of the points brought up in the article, incorporating both Bernoulli's principle and Newton's third principle.

The big challenge seems to be making it explainable to someone with no domain knowledge.

Re: The mysteries of aerodynamic lift

#17

Post by FabHK »

kryogen1c wrote: post #11

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 goe…

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.

Re: The mysteries of aerodynamic lift

#18

Post by codeduck »

mech1234 wrote: post #8

I suspect if you made a wing out of a flat piece of material, tilted at the appropriate angle of attack, it would be sufficient to fly a plane. It just wouldn't be optimized at all. Really, you need full Navier-Stokes behavior to explain all the forces acting on the wing. Bernoulli doesn't generalize to a full vector field, it's a simplified version of Navier-Stokes. Calling in the big guns doesn't make for an easy d…

Flat wings work perfectly fine, they just need a higher angle of attack to generate similar lift, and therefore have higher drag.

Re: The mysteries of aerodynamic lift

#19

Post by FabHK »

jermaustin1 wrote: post #5

I'm tired of this headline, its an oversimplification. We absolutely DO understand how and why they stay in the air, we just don't have the hard numbers behind it.

> We absolutely DO understand how and why they stay in the air, we just don't have the hard numbers behind it.

The article claims the opposite: we do have the hard numbers behind it (we can predict how a wing behaves), but we don't have a good conceptual explanation for it at a layman's level (the how and why, if you so will).

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