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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#31

Post by diffeomorphism »

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.

Speaking as a mathematician, no. We have the Navier-Stokes equations, which we are fairly certain are a good model and which work well in practice/numerically. So an engineer might agree with your view.

However, mathematically these equations are difficult, we have very few explicit solutions and the question of whether solutions to these equations are well-behaved in all circumstances is a famous open problem. Indeed, it is one of the Millenium problems and solving it will make you very famous.

Also, your "I'll let the article speak for itself" is rude and naive.

Re: The mysteries of aerodynamic lift

#32

Post by zkms »

Doug McLean (Boeing Technical Fellow) has a quite decent talk about the question of intuitive explanations for aerodynamic lift: https://www.youtube.com/watch?v=QKCK4lJLQHU

Also Philippe Spalart's quote is right on the money: "It's easy to explain how a rocket works, but explaining how a wing works takes a rocket scientist".

Re: The mysteries of aerodynamic lift

#33

Post by [deleted] »

ethn wrote: post #7

During my plane flights I passed the time rediscovering the equations of lift. The physical intuition I’ve found, is for plane flight, the plane must move faster in the direction parallel to the ground than the time it takes for those air molecules under the plane to move away and deform around the wing at some average velocity. In this manner, the plane can push off those air molecules as they’re unable to move out…

[deleted]

Re: The mysteries of aerodynamic lift

#34

Post by umvi »

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…

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.

Re: The mysteries of aerodynamic lift

#35

Post by Retric »

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

The billiard ball model works fine when you include the impact of other billboard balls on each other resulting in vortexes etc. It’s simply computationally expensive to do so.

Anyway, absolutely flat wings generate lift as long as the angle of attack is non zero. But, by changing the wings shape they get more efficient. The reasons for that are complex differential equations that don’t really have simple plain English explanations, which is why people do so much testing and simulation. Aka simple 2d diagrams don’t result in: https://en.wikipedia.org/wiki/Wingtip_device

What’s even more confusing is most wing diagrams put the center line in the wrong location. If you take a symmetrical tear drop shape with zero angle of attack the bottom will be sloped up at the same angle the top slopes down. Thus if you instead lay that teardrop so the bottom is flat that represents a positive angle of attack. Granted, wings generally don’t have a symmetric shape, but similar principles apply.

Re: The mysteries of aerodynamic lift

#36

Post by pmoriarty »

FabHK wrote: post #17
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 ups…

"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.

Re: The mysteries of aerodynamic lift

#37

Post by pdonis »

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…

> 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.

You don't have to suspect. Many aerobatic aircraft (not to mention many high performance military aircraft) have wings that are symmetrical top to bottom (they aren't exactly flat but the top-bottom symmetry is enough to prove the point). They still fly. (The article mentions this.)

Re: The mysteries of aerodynamic lift

#38

Post by WalterBright »

simonebrunozzi wrote: post #27
munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

> can also sail faster than the wind at times. Really? Can you elaborate?

Take a drafting triangle that has 30-60-90 degree corners. Place it between two objects, and squeeze the triangle between them. It'll move to the side faster than the two objects move together.

Or you can just think of it like squirting toothpaste.

The wind pressure on the sail and the water pressure on the keel form the two "objects" being pushed together and the sailboat "squirts" out the side.

Edit: The angle between the sail and the keel is like the angle on the triangle. The keel really was a great invention.

Re: The mysteries of aerodynamic lift

#39

Post by H8crilA »

I'm surprised people don't start with the basics on this confusing topic.

The third law of Newton's mechanics tells us that for the plane to get an up force to counteract the gravity, the air must receive and equal amount of down force. Therefore what planes must be doing is deflect air masses down. A plane must be applying a downward force to air masses, with total force value of "mass * g", i.e. supply "mass * g * 1second" worth of downward momentum per second. As simple as that.

Re: The mysteries of aerodynamic lift

#40

Post by opwieurposiu »

A wing is a device that pumps air downward, which in turn pushes the wing upward, by newton's third law. For a large plane, the wing will be pumping many tons of air per second.

Start with a cube of still air, with zero mean velocity. Fly a plane through it, and that cube will have a mean downward velocity.

http://www.aviation-history.com/theory/lift.htm

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