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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#21

Post by exabrial »

codeduck wrote: post #18
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.

And have poor stall characteristics because they're more likely to introduce flow separations near the back of the wing.

Re: The mysteries of aerodynamic lift

#22

Post by Isamu »

So this is about intuitive explanations of flight.

The complaint about using Newton's law is that it doesn't readily explain the low-pressure region above the wing. Let me give it a try.

Intuitively, all the forces on the wing are manifest as air pressure measured the wing surface and expressed as a force normal to the surface at that point. There are no other forces from the air. You sum them over the wing and the net force upward is called lift, the net force backward is call drag.

If there is net lift, that means the net normal forces summed over the wing are upward, implying that the pressure is low above the wing relative to below the wing.

The Newtonian explanation is that a force must be exerted to change the direction of the air. The net force on the air is in the direction that the air turns, and the reaction force on the wing is in the opposite direction.

On the underside of the wing the air is diverted downward and the reaction force on the wing is upward, and this is measured by the air pressure on underside of the wing.

On the top of the wing the air is again diverted downward and the reaction force on the top of the wing is again upward, and this is measured by a lowered air pressure, giving a net upward force.

Re: The mysteries of aerodynamic lift

#23

Post by [deleted] »

falcolas wrote: post #16
pmoriarty wrote: post #14

Earlier quoted context omitted.

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.

[deleted]

Re: The mysteries of aerodynamic lift

#24

Post by munificent »

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 because they generate all their power from the wind and yet can also sail faster than the wind at times.

As far as I understand it, the fundamental reason airfoils in boats and places work the way they do requires viscosity. Without that, the math falls apart and the ship doesn't move. It is not enough to treat air as a bunch of tiny billiard balls bouncing off the bottom of the wing (the Newton's third law argument). Likewise, you can't pretend the air above and below the wing are each confined to their own perfectly frictionless tubes with different velocities (the Bernoulli argument), since the air moving over the wing is part of a continuous whole extending out arbitrarily far.

In order to explain how fabric-thin sails are able to generate lift, how planes with curved wings can fly upside down, and how the ground effect comes into play, you have to treat the surrounding fluid as a continuous medium with viscosity. That's how the math works. Unfortunately, we don't seem to have a good intuition for how that feels so it's hard to treat that as a satisfactory explanation.

Re: The mysteries of aerodynamic lift

#25

Post by NikolaeVarius »

FabHK wrote: post #15
NikolaeVarius wrote: post #12

Earlier quoted context omitted.

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 ap…

You are misunderstanding what a full solution means and what the article is saying.

There are a systems of equations that allow us to model flight. But they are NOT a full model of flight characteristics, as a consequence of, like mentioned, lack of a full solution to the Navier Stokes equations

> The solutions of those equations and the output of the CFD simulations yield pressure-distribution predictions, airflow patterns and quantitative results that are the basis for today’s highly advanced aircraft designs. Still, they do not by themselves give a physical, qualitative explanation of lift.

Re: The mysteries of aerodynamic lift

#27

Post by simonebrunozzi »

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?

Re: The mysteries of aerodynamic lift

#28

Post by jayrot »

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…

When you were a kid, did you not ever put your hand out the window on the freeway? You can actually feel the pressure differential lifting your hand up once you get the right angle.

Re: The mysteries of aerodynamic lift

#29

Post by trhway »

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?

you can look at kiters and windsurfers - even on a mild wind they glide very fast - orthogonal to the wind you get constant ( i.e. practically independent of your speed) force driving you forward, and it accelerates you until it matches the force of the water and air resistance to your movement which grows with your speed.

And of course here comes that:

https://newatlas.com/wind-powered-car-world-record/11346/

"March 29, 2009 With a wind speed of just 30mph (48kmh), British engineer Richard Jenkins has set a new land speed record for a wind-powered vehicle at blistering 126.1mph. "

Re: The mysteries of aerodynamic lift

#30

Post by meedimusic »

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?

apparent wind
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