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

#### sophiecentaur

Gold Member
That seems wrong. The smooth flow of the above-wing air being drawn downward provides a large part of the lift force. I think that the turbulance behind a brick would disrupt that. But I have to admit that it is not something I have studied.
Of course a brick would have a ridiculous amount of drag but that was not my point and my example was extreme. A sheet of plywood will fly off the top of a car if it's not strapped down and that has not aerofoil shape.

#### sophiecentaur

Gold Member
I don’t really know of one in my area. I’ll search it up. Thanks for the idea!
Flying lessons are hideously expensive in most parts of the world. In places where they're cheap, they are probably more risky. So you cannot win. Flying model aircraft is cheaper and you can learn all you need without actually being up there.

#### anorlunda

Mentor
Gold Member
The quality of the discussion in this thread is not high. I think it would be better if everyone cites their sources when asserting facts.

#### doglover9754

Gold Member
Flying lessons are hideously expensive in most parts of the world. In places where they're cheap, they are probably more risky. So you cannot win. Flying model aircraft is cheaper and you can learn all you need without actually being up there.
Come to think of it, I might have a model airplane in my room. My dad told me to build it going with the directions but I’m not so much on the directions... in any case, I can always go to the nearby hobby shop

#### doglover9754

Gold Member
The quality of the discussion in this thread is not high. I think it would be better if everyone cites their sources when asserting facts.

#### doglover9754

Gold Member

Gold Member
[trying to take this a piece at a time]

I'm saying that what you are saying is trivially true and while trying to disagree with what I said, it doesn't. A 1 and a 3 both contribute to 1+3=4, and you don't get 4 without both of them (your position), but that doesn't mean that the 3 doesn't contribute more than the 1 (my position).

And yes, I'm aware that if you make a change on one surface it makes a change to the flow on the other, but a change to the top surface geometry makes a greater change to the top surface airflow. Or,in other words, the airflow and pressure profile over the bottom surface in a flat bottom airfoil when the bottom is horizontal looks very much like freestream or a flat plate. Not exactly, but close. But the top surface looks very different from freestream/flat plate.
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'll state again that this is not a quantification that I've seen anywhere in any text on the subject [1-4], nor is it one that I can see being very useful in any sense. The two sides cannot be decoupled in any way without affecting the answer.

Also, the bottom side of a flat bottom akrfoil would not look simply like the free stream except in some very specific and impractical configurations.

[1] Anderson Jr, J. D. (2016). Fundamentals of aerodynamics. Tata McGraw-Hill Education.
[2] Katz, J., & Plotkin, A. (2001). Low-speed aerodynamics (Vol. 13). Cambridge university press.
[3] Karamcheti, K. (1980). Principles of ideal-fluid aerodynamics. Krieger Publishing Company.
[4] Abbott, I. H., & Von Doenhoff, A. E. (1959). Theory of wing sections, including a summary of airfoil data. Courier Corporation.

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#### A.T.

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?
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.

#### FactChecker

Gold Member
2018 Award
I think that it is important (and counter-intuitive) to realize that the Bernoulli effects of air flow around an object can be more forceful than the effect of the dynamic impact of air on the surface.

#### CWatters

Homework Helper
Gold Member
Please lets not get into the "is it Newton or Bernoulli" debate.

#### FactChecker

Gold Member
2018 Award
Please lets 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.

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#### sophiecentaur

Gold Member
Please lets 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

Mentor
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.

Gold Member
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

Mentor
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

Gold Member
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

Mentor
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

Gold Member
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

Mentor
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

Mentor
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.

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

Mentor
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

Mentor
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.

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