Airplane wings -- How do they work and why do they change shape?

[FactChecker]

Please let's not get into the "is it Newton or Bernoulli" debate.

I don't know if anyone is saying that. Both theories agree, assuming that the calculations are done correctly. The experiment in the link below shows a (IMO) surprising and counter-intuitive result that is important for appreciating the lift on a wing. I'll leave it for each individual to discribe the result as he wishes.

 

[sophiecentaur]

Please let's not get into the "is it Newton or Bernoulli" debate.

I thought we'd already been there - or is it a groundhog thread?

 

[russ_watters]

Of course $3>1$. Those are both easily-quantifiable objects. What you have failed to provide is the means of quantification of the "contributions" of the upper and lower surfaces of an airfoil. How are you proposing to do that? How are you defining "contribution" in this sense?

I *did* provide an explanation: by integrating the pressure distributions on each surface and comparing them.

It is just so bizarre to me that this is an issue to you. It's obvious and even trivially true. And more importantly, it follows from common beginner questions and explanations about what each surface is doing(which is why I brought it up...by response). No, it isn't the end-all. Yes, digging deeper provides more nuance. Yes each surface will affect the other. But that doesn't make this trivial/obvious fact untrue.

So please, I implore you to focus on what the beginner OP asks rather than arguing over the wording used to describe a particular fact.

 

[boneh3ad]

Again, integrating the pressure distributions indicates that the force provided by the upper surface is negative. It pushes downward. If anything, I'd call that "taking away from" lift, not "contributing to" lift. Talking about this the way you do implies to a beginner that the upper surface is somehow pulling up on the airfoil a bit more than the bottom surface is pushing up on it, which is patently false. Your way of describing this is at best misleading to a beginner.

 

[russ_watters]

We had this discussion before. The idea of "upper side contributing more to lift" is apparently based on the greater difference to ambient pressure on the upper side.

I don't like this argument at all, because the local forces on the wing are completely determined by the local absolute pressures at the wing (which are all positive). Expressing those pressures relative to some non-local ambient pressure doesn't change the result and doesn't add any new physics. It just creates the wrong idea that the air (with negative relative pressure) can "pull" the wing up, like there was some attractive force between them.

While I appreciate the preference for taking all pressures as absolute (it seems common here), differential pressure (deviation from freestream) is a common convention (per the source/graphs I provided).

Since either will work it is mostly a matter of preference, but I prefer differential in this and many cases for its simplicity/ease of use, both in calculation and in practice.

 

[sophiecentaur]

Again, integrating the pressure distributions indicates that the force provided by the upper surface is negative. It pushes downward. If anything, I'd call that "taking away from" lift, not "contributing to" lift. Talking about this the way you do implies to a beginner that the upper surface is somehow pulling up on the airfoil a bit more than the bottom surface is pushing up on it, which is patently false. Your way of describing this is at best misleading to a beginner.

What is that doing to help the poor OP? There is positive pressure all the way round the wing. There is just more pressure on the lower part than on the upper part. We call that net result "lift". This is how I read the @russ_watters post.
Yes. It is a groundhog thread.

 

[russ_watters]

Again, integrating the pressure distributions indicates that the force provided by the upper surface is negative. It pushes downward.

No it doesn't. Please have another look at the graphic and explanation I posted in post #14. It's not absolute pressure, it's differential. Students in aero classes use exactly the method I'm describing to measure lift in wind tunnels!

 

[sophiecentaur]

No it doesn't. Please have another look at the graphic and explanation I posted in post #14. It's not absolute pressure, it's differential. Students in aero classes use exactly the method I'm describing to measure lift in wind tunnels!

Aha - but do they ever measure the increased weight of the tunnel and equipment?

 

[A.T.]

Again, integrating the pressure distributions indicates that the force provided by the upper surface is negative.

No it doesn't.

Are you seriously claiming that the force of the air on the upper wing surface points upwards?

 

[russ_watters]

Are you seriously claiming that the force of the air on the upper wing surface points upwards?

Clearly I'm not. I can't imagine why you would think that...unless; are you saying you don't know what differential pressure is?

 

[russ_watters]

Guys. Please stop. This is not hard. I know you guys know what differential pressure is - please stop acting like you don't just so you can disagree about something.

I'm not making this method up. Here's an aero lab using the method I'm describing:

Because the velocity of the flow over the top of the airfoil is greater than the free-stream velocity, the pressure over the top is negative.

https://www.google.com/url?sa=t&sou...FjABegQIBxAB&usg=AOvVaw2PfTNUfEBfBfgg58XPkqBf

I know you guys are smart enough to know that's differential pressure being referred to. There is no need to argue over a convention everyone understands. You prefer absolute- fine! But differential is common here (probably more common) and integrating it to find lift is a common method.

 

[A.T.]

Clearly I'm not.

If you agree that the air is pushing down on the upper wing surface, and by Newton's 3rd, that the upper wing surface is pushing up on the air, then what do you mean by "pulling" in the quote below?

I would add/clarify that the top surface contributes more than the bottom and would call that the top surface *pulling* the air down. I would say a wing pulls and pushes air down.

Can you provide the physical definition of "pushing" vs "pulling" that you are using here?

 

[russ_watters]

If you agree that the air is pushing down on the upper wing surface, and by Newton's 3rd, that the upper wing surface is pushing up on the air, then what do you mean by "pulling" in the quote below?Can you provide the physical definition of "pushing" vs "pulling" that you are using here?

I'll hold to give you time to incorporate post #71...and acknowledge the concept of gauge/differential pressure.

 

[A.T.]

I'll hold to give you time to incorporate post #71...and acknowledge the concept of gauge/differential pressure.

Acknowledged. Can you now provide the physical definition of "pushing" vs "pulling" that you are using here?

 

[russ_watters]

Acknowledged. Can you now provide the physical definition of "pushing" vs "pulling" that you are using here?

Sure; when gauge pressure is negative, that's the surface pulling on the air. When positive, that's pushing.

 

[sophiecentaur]

Sure; when gauge pressure is negative, that's the surface pulling on the air. When positive, that's pushing.

That's a convention I would need to get used to but it's hardly rocket Science.

 

[boneh3ad]

Rather than reply to the last few posts individually, I will try to address these last few issues all at once since they are related.

It is correct to say that differential pressure (or more correctly, gauge pressure, since it is generally referenced to atmosphere) is used extremely frequently when dealing with the pressure distribution over an airfoil. It is definitely the most common method of doing things, typically in the form of the dimensionless pressure coefficient, $C_p$. This is done because using gauge pressure is more mathematically compact and lends itself to collapsing the data into relationships that work over a range of free-stream conditions.

That said, gauge pressure is also more physically misleading. If the person trying to learn about the process in the first place does not understand the distinction, then he or she will note that the upper surface shows a negative pressure, which implies that the air should "pull" on the wing on that side. While this still all works out mathematically, it leads to physically incorrect conclusions in the hands of a novice. So, circling back to the concerns about beginners, I still maintain that doing this in terms of gauge pressure right off the bat is misleading and the wrong approach.

On top of all of that, the fundamental concept of the top "contributing more" is still wrong. It is a given that the gauge pressure on the top is going to be negative. It is true that in some cases, the integrated force from that upper gauge pressure would be greater than on the lower surface. That is not a universally true fact, however. This is where gauge pressures can again be misleading. Consider the pressure coefficient, which varies across an airfoil surface:
C_p = \dfrac{p-p_{\infty}}{\frac{1}{2}\rho V_{\infty}^2} = \dfrac{p_{gauge}}{\frac{1}{2}\rho V_{\infty}^2}.
The value of $C_p$ is effectively constant for any velocity so long as the flow is incompressible[1]. So, let's rewrite this equation
p_{gauge} = \dfrac{1}{2}C_p\rho V_{\infty}^2.
Since $C_p$ and $\rho$ are constants, then increasing the velocity is going to increase $p_{gauge}$ on the lower surface (where $C_p$ is positive) and decrease the gauge pressure on the upper surface (where $C_p$) is negative. Here's the kicker, though: while gauge pressure can become arbitrarily high, it cannot be arbitrarily low (after all, you can't have a negative amount of gas in a volume). So, as you increase speed, eventually the magnitude of the gauge pressure on the bottom must become larger than that on the top surface, at which point your argument about the top surface "contributing more" becomes invalid.

[1] Anderson Jr, J. D. (2016). Fundamentals of aerodynamics. Chapter 3

 

[A.T.]

Sure; when gauge pressure is negative, that's the surface pulling on the air.

So by your definition, a surface exerting an upwards force on the air above it, is actually "pulling down on the air"?

 

[jbriggs444]

So by your definition, a surface exerting an upwards force on the air above it, is actually "pulling down on the air"?

It's a negative gauge pressure and might be visualized as a pull. Similar to the way a vacuum cleaner "pulls" dirt from a rug.

 

[russ_watters]

So by your definition, a surface exerting an upwards force on the air above it, is actually "pulling down on the air"?

By this convention, that is what it says, yes. That's what it means when the pressure and thus the force is negative.

 

[anorlunda]

@doglover9754 ,

I'm sorry. Sometimes adults get so excited about the best way to say something that they can't answer a middle school student's simple question. It's not like this all the time on PF. Usually, simple questions get simple answers.

Thread closed.

 

[russ_watters]

It is correct to say that differential pressure (or more correctly, gauge pressure, since it is generally referenced to atmosphere) is used extremely frequently when dealing with the pressure distribution over an airfoil. It is definitely the most common method of doing things, typically in the form of the dimensionless pressure coefficient, $C_p$. This is done because using gauge pressure is more mathematically compact and lends itself to collapsing the data into relationships that work over a range of free-stream conditions.

Thank you!

That said, gauge pressure is also more physically misleading.

If the person trying to learn about the process in the first place does not understand the distinction, then he or she will note that the upper surface shows a negative pressure, which implies that the air should "pull" on the wing on that side. While this still all works out mathematically, it leads to physically incorrect conclusions in the hands of a novice.

Fine. So in the future, i would hope that you will recognize the convention without it needing to be described to you, so we can avoid repeating much of this.

So, circling back to the concerns about beginners, I still maintain that doing this in terms of gauge pressure right off the bat is misleading and the wrong approach.

Conventions are personal choices and cannot be right or wrong if both work when applied correctly. You prefer your convention, and I prefer mine and both work and that's fine. Please be ok with that. Or at least pretend to be for the sake of the OP!

On top of all of that, the fundamental concept of the top "contributing more" is still wrong. It is a given that the gauge pressure on the top is going to be negative. It is true that in some cases, the integrated force from that upper gauge pressure would be greater than on the lower surface.

So again: something true cannot be wrong just because you prefer a different convention or verbiage. The word "contribution" is used in academic settings. You don't like that. That doesn't make it wrong, but you don't have to use it. But more importantly - again - please try to recognize differences in conventions/wording and not to be so quick to argue over them. It is not helpful to the OP.

That is not a universally true fact, however.

Agreed/understood. But it *is* true for the example first given to the OP in post #2, which is a common example because it is common in real life and simple for the very first thing someone learns about lift. This isn't something that should create a disagreement, but rather a stepping stone to the next, deeper point.