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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#81

Post by o_nate »

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.

I guess it works as a very high level explanation. However, as a practical guide to designing an actual working airplane wing, it's probably too high level to be helpful. For instance, if Newton was a complete explanation, then a flat wing at a 45 degree angle should work well. A more complete explanation would also explain why airplane wings typically have the shape they do.

Re: The mysteries of aerodynamic lift

#82

Post by frank2 »

o_nate wrote: post #81
Sharlin wrote: post #80

Earlier quoted context omitted.

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.

I guess it works as a very high level explanation. However, as a practical guide to designing an actual working airplane wing, it's probably too high level to be helpful. For instance, if Newton was a complete explanation, then a flat wing at a 45 degree angle should work well. A more complete explanation would also explain why airplane wings typically have the shape they do.

A flat wing at 45 degrees produces massive turbulence above and behind the wing, which in turn greatly increases the drag-to-lift ratio compared to a standard airplane wing. Also I suspect that ailerons and other control surfaces become much less useful in turbulent flow.

Re: The mysteries of aerodynamic lift

#83

Post by o_nate »

baq wrote: post #69
o_nate wrote: post #64

Earlier quoted context omitted.

A scientific explanation though would explain why those regions of high and low pressure get created.

Depends. Newton’s third law is scientific enough to explain what’s going on. Bernoulli and Navier-Stokes and all the hard equations are needed when you need to optimize the system or understand the micro scale, but they’ll simplify into conservation of energy or momentum in the macro limit.

Newton's law doesn't do a good job of explaining why airplane wings have the shape they do. Why not just use a flat wing at a 45 degree angle?

Re: The mysteries of aerodynamic lift

#84

Post by hinkley »

bloomer wrote: post #74
munificent wrote: post #58

Earlier quoted context omitted.

Imagine a sailboat pointed 90° relative to the wind so that the wind is coming right at its side. The sail is curved so that it takes that wind and redirects it towards the rear of the boat, giving it forward thrust. The boat starts moving forward. But, because the wind is perpendicular to the boat, even when its moving the wind is still coming in at the same velocity, so it's still producing thrust. If you can get t…

https://en.m.wikipedia.org/wiki/Blackbird_(land_yacht) Was built to demonstrate that you can sail directly downwind faster than the wind as well.

That is not a conventional land yacht, but I'm sure there are land yachts that can go faster than the wind, by exactly the same principle as sailboats.

The motive force on that vehicle is a gear and chain between the prop and the wheels. And prop blades are set at an angle to the wind.

Re: The mysteries of aerodynamic lift

#85

Post by lutorm »

jvanderbot wrote: post #49
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 *…

Yes, and in rotating-wing configurations (e.g., helicopters), the lift is basically calculated using the momentum of the column of air being forced downward + the momentum of the chassis. By accelerating air downward in a column below the rotating wingspan, the column of air gains a net negative momentum (downward), necessitating a net positive (upward) momentum to the chassis to keep the system's momentum conserved.…

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

Re: The mysteries of aerodynamic lift

#86

Post by renox »

o_nate wrote: post #81
Sharlin wrote: post #80

Earlier quoted context omitted.

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.

I guess it works as a very high level explanation. However, as a practical guide to designing an actual working airplane wing, it's probably too high level to be helpful. For instance, if Newton was a complete explanation, then a flat wing at a 45 degree angle should work well. A more complete explanation would also explain why airplane wings typically have the shape they do.

I don't know if 45° wings provide good lift or not, but what I know is that they would induce a lot of 'drag'. And that the goal is to maximize lift while minimising drag.. So it isn't that complicated why the wings are the shape they are: if there is a 'gentle slope' the air will be redirected downwards without inducing too much drag.

Re: The mysteries of aerodynamic lift

#87

Post by caconym_ »

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…

> 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.

Re: The mysteries of aerodynamic lift

#88

Post by thrustmaster »

zarmin wrote: post #3
thrustmaster wrote: post #2

I thought it was a combination of newtons third law and Bernoulli's principle, the wings are hitting the air and the air exerts equal force in the opposite direction, Bernoulli's principle allows this equal opposite force to provide easier lift.

Wright you are.

Really? I got a few down-votes... I also know the angle of attack of the wing is important, no angle of attack, no lift?

Re: The mysteries of aerodynamic lift

#89

Post by o_nate »

frank2 wrote: post #82
o_nate wrote: post #81

Earlier quoted context omitted.

I guess it works as a very high level explanation. However, as a practical guide to designing an actual working airplane wing, it's probably too high level to be helpful. For instance, if Newton was a complete explanation, then a flat wing at a 45 degree angle should work well. A more complete explanation would also explain why airplane wings typically have the shape they do.

A flat wing at 45 degrees produces massive turbulence above and behind the wing, which in turn greatly increases the drag-to-lift ratio compared to a standard airplane wing. Also I suspect that ailerons and other control surfaces become much less useful in turbulent flow.

Sure, but now explain drag and turbulence using Newton's laws.

Re: The mysteries of aerodynamic lift

#90

Post by mannykannot »

nimish wrote: post #75
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 *…

This doesn't explain all of the induced pressure differences. You need Bernoulli (Conservation of Energy, more or less) as well. They interact in a complicated way. You really can't "just" explain lift simply.

It is more like that there is a hierarchy of explanation. There is nothing going on that cannot be explained by Newton's laws, conservation of energy does not need to be introduced as an extra constraint, and Bernoulli's law is itself explained by Newton's laws. The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards, you still don't know how the air moves around the wing, so you can't calculate which streamlines are being accelerated by how much and in which direction.

For a general solution to that question, you need to solve the appropriate Navier-Stokes equations, which take into account both the inertia and the viscosity of the air, and are also explained by Newton (up to the point where Newton does not explain the origin of viscosity.) Once you have the velocity field, you can, if and only if the velocity is low enough that the air behaves as an incompressible fluid, calculate the pressure on the wing using Bernoulli.

One significant issue is that if you do this without taking into account friction at the surface of the wing, and the boundary layer that results, you will find that there is no lift at all! Your solution will show the air that passes under the wing turning around the trailing edge, and flowing forward for some distance over the upper surface. In practice, the presence of a boundary layer causes the flow to separate at the trailing edge (if not before).

Modelling at this level of detail is computationally very costly, and the Kutta–Joukowski theorem can be used instead, for a wide range of practical airfoil profiles.

So yes, it is complicated, once you go beyond the barest hand-waving.

       https://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem
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