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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#111

Post by tathougies »

H8crilA wrote: post #39

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 *…

Certainly, but law is guided by empiricism. Newton's third law is not scientific fact prima facie. It was developed via observation.

In this case, you say simply that 'oh it's the equal and opposite reaction', and that seems theoretically possible. But then you do the experiment, and all of a sudden, you see not only a force due to the third law, but also a pressure differential. Based on the laws of pressure, a high pressure beneath the wing and a lower pressure on top must also exert a force. These pressure laws are as fundamental (and are actually simply a consequence of the third law, since pressure is just a statistical approximation of the third law applied to invisible microscopic particles).

Thus, while almost certainly true the third law does contribute, it's also true that it is not a full explanation. The third law could explain lift, but it does not explain why the pressure differential would exist, and thus cannot account for the extra lift due to that.

The third law would additionally predict that the pressure on top should be greater than that on the bottom. This is an easy experiment to do. Get a piston in a closed cylinder (the piston does not need to partition the cylinder). Place two pressure gauges on each side. Now pull the piston up. The side being compressed has higher pressure. This suggests that, if a flat board moves towards a side of a closed system, that side of the system ought to experience higher pressure. But it doesn't in the case of wings, so that's weird

Re: The mysteries of aerodynamic lift

#112

Post by lutorm »

H8crilA wrote: post #93
lutorm wrote: post #85

Earlier quoted context omitted.

Sure, that's just momentum conservation. But how do you know how much downward momentum the column of air has?

Airplane/helicopter mass * g * time Assuming the plane/helicopter didn't accelerate up or down and assuming there's no wind.

Sure, that has to be true if the helicopter can hover. But how do you know that's possible?

You can't postulate that something flies as part of an explanation why it can fly...

Re: The mysteries of aerodynamic lift

#113

Post by mike_ivanov »

phdp wrote: post #98
mike_ivanov wrote: post #95

Earlier quoted context omitted.

> the fundamental reason airfoils in boats and places work the way they do requires viscosity There is no viscosity in solar wind, yet the solar sail is expected to work. EDIT: s/solar wind/solar radiation/

A solar sail doesn't work via aerodynamic lift. It works on conservation of momentum. Also, the biggest contributor to force on a solar sail is not solar wind, but radiation.

Still, how exactly viscosity helps to create the lift force?

Re: The mysteries of aerodynamic lift

#114

Post by QuantumGood »

AceyMan wrote: post #66
simonebrunozzi wrote: post #27

Earlier quoted context omitted.

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

my analogy: if you have ever launched a pumpkin or watermelon seed by squeezing it between your fingertips just so then you can understand how the craft can move faster than whatever provides the impulse. \\ wing is to wind as seed is to fingers \\

That's how we were taught in sailboat racing school.

Re: The mysteries of aerodynamic lift

#115

Post by lutorm »

Sharlin wrote: post #80
o_nate wrote: post #72

Earlier quoted context omitted.

The obvious follow-up question is : why does a wing deflect the air down?

Why it deflects air below the wing downward is trivial: it is an inclined plane (the angle of attack is nonzero) Why it also deflects air above the wing downward – pulling the air down so to speak – is more complicated.

It may appear trivial, but thinking about it like a bunch of bullets bouncing off the wing on the bottom (the "particle regime") does not explain why it deflects air (the "fluid regime").

A fluid is governed by fundamentally different physics than a bunch of particles, so just because a bunch of particles hitting an inclined plane also would generate lift does not explain why air hitting the same plane does.

For example, in the particle analog, the upper surface of the wing is totally immaterial. You could make it look however you want without affecting the generated lift. In reality, the amount of lift generated is much more sensitive to what happens on the upper surface than on the bottom.

Re: The mysteries of aerodynamic lift

#116

Post by [deleted] »

mannykannot wrote: post #108
nimish wrote: post #103

Earlier quoted context omitted.

> The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards You need bernoulli to explain why the flow field is changed beyond just the area in contact with the flow. This induces the measured pressure differential, explaining part of lift along with the reaction effects of deflected flow for momentum conservation (NS, Newton's 2nd law). It's simply not _enough_ to say that…

No, you do not need Bernoulli, it is merely convenient (but only if compressibility is not an issue, which it is, of course, for cruising airliners as well as supersonic aircraft.) Bernoulli does not give you the velocity field. If you are looking for just one thing that is sufficient, it is Newton's laws applied to viscous fluids - i.e. Navier-Stokes. I may have made one mistake in that the the separation at a sharp…

[deleted]

Re: The mysteries of aerodynamic lift

#117

Post by lutorm »

namirez wrote: post #101

This is a confusing take on lift. To explain lift intuitively we need two ingredients: the Laplace equation and the Kutta condition. Most people have an intuitive understanding of the Laplace equation. For example lightning usually hits the peak of mountains. The reason is that in solutions of Laplace equations, field gradient is proportional to curvature. In fluid dynamics, this field is called the stream function.…

A flat plate generates lift, though, so the curvature is incidental.

Re: The mysteries of aerodynamic lift

#118

Post by jb775 »

Recently saw this crazy video of planes being lifted into the air during a microburst. At the time I wondered what caused the lift...food for thought for this discussion.

https://www.youtube.com/watch?v=b_WmjWAGkLI

A microburst is a rare weather event where a cloud basically shoots air downwards up to 100mph:

https://www.weather.gov/bmx/outreach_microbursts

Re: The mysteries of aerodynamic lift

#119

Post by lutorm »

tathougies wrote: post #111
H8crilA wrote: post #39

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 *…

Certainly, but law is guided by empiricism. Newton's third law is not scientific fact prima facie. It was developed via observation. In this case, you say simply that 'oh it's the equal and opposite reaction', and that seems theoretically possible. But then you do the experiment, and all of a sudden, you see not only a force due to the third law, but also a pressure differential. Based on the laws of pressure, a high…

The third law could explain lift, but it does not explain why the pressure differential would exist, and thus cannot account for the extra lift due to that.

There is no "extra lift" due to the pressure differential. The pressure differential IS the net force that changes the motion of the air. You can't change the motion of the air without a pressure differential, and you can't have a pressure differential without changing the motion of the air.

Re: The mysteries of aerodynamic lift

#120

Post by lutorm »

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…

Paper airplanes usually have flat wings and they fly fine, no need to suspect.
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