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

[doglover9754]

I understand that you know a lot about aerodynamics. Good for you. But I am just trying to give this ≈fifth-grader the general idea of how an airplane wing works, hopefully without scaring him away from academia forever.

Hope you can understand.

Fifth grader?

 

[doglover9754]

When airplanes are landing it's best to have a lot of drag (slows it down) while getting as much lift as possible (stops it hitting the ground).
That's what the flaps are for, (and also air brakes when the plane is on the ground)

Thanks for the clear up!

 

[boneh3ad]

I understand that you know a lot about aerodynamics. Good for you. But I am just trying to give this ≈fifth-grader the general idea of how an airplane wing works, hopefully without scaring him away from academia forever.

Hope you can understand.

It's certainly a good idea to try to explain something in terms that the audience can understand. That sort of a given. That is why I went out of my way to define terms. There is, however, a difference between giving an answer in simple terms and simply giving a wrong answer. You did the latter. What the OP was asking has nothing to do with navigating and orienting the plane and everything to do with lift performance during takeoff and landing.

 

[doglover9754]

It's certainly a good idea to try to explain something in terms that the audience can understand. That sort of a given. That is why I went out of my way to define terms.

And it sure was helpful :D I just had to think a little

 

[FactChecker]

Here is a simple experiment using only a sewing thread spool and paper card that anyone can do. It shows that the force from the Bernoulli pressure effects can be greater than the force from the impact of blowing air.

Applying this lesson to the case of a wing, it is clear that the entire airflow around the wing must be considered to really appreciate the forces involved.

PS. Here is a similar experiment using a funnel and pingpong ball

PPS. As @berkeman said, @boneh3ad 's insight article (https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/) is very well thought out and well worth reading.

 

[sophiecentaur]

The wings on an airplane don't actually push air down. They generate lift.

So an aircraft doesn't follow N3?
"Lift" and "reaction force" are not mutually exclusive concepts.

 

[jrmichler]

You could go to your local flight school and ask for an introductory flying lesson. Ask the instructor to demonstrate cruise speed with the flaps both up and down, and note the difference in speed at the same power. Then demonstrate slow flight with the flaps both up and down, and note how much slower it can fly with the flaps down.

There is no minimum age for a flying lesson.

 

[Shadow89]

So an aircraft doesn't follow N3?
"Lift" and "reaction force" are not mutually exclusive concepts.

Obviously there is a force/counterforce between the wing and surrounding air. I just think it may be confusing to say "wings push air down". There are many other inaccuracies in my posts on this thread, and it will be fairly easy for the majority of users to point them out. I have allowed myself these inaccuracies in an attempt to keep things as simple and understandible as possible; Because if I where to include every miniscule detail, the OP might as well go and read any university level textbook on the matter. I am confident that he/she has gained some understanding of the subjet matter from my somewhat limited explanation.

Even the simplest things can be made ininitely complex if studied at length.

Please note that I am a middle schooler and some “more educational” answers (answers with words that I have no idea what they mean) are hard for me to understand so it’d be great if any answers are put in the simplest way possible. Also, I have watched a YouTube video about how a plane works mainly focusing on how the wings, tail wings, and other parts of the airplane have an effect on how a plane flies. Any answers for any of my questions would be greatly appreciated as this may be a bunch of confusing stuff coming out of my brain right now and that was probably a lot to read

 

[jbriggs444]

Obviously there is a force/counterforce between the wing and surrounding air.

You tried to make a distinction between a helicopter rotor and an airplane wing. Both are airfoils. Both produce lift. Both result in a downward deflection of air. It is difficult to discern the distinction you were drawing.

 

[russ_watters]

[trying to take this a piece at a time]

That's not how this works, though. That pressure change doesn't happen solely because of the top surface. That pressure change happens because of the overall shape of the airfoil. If you change the bottom surface, the upper flow field changes as well.

...I don't even know what you were trying to say with this.

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.

 

[A.T.]

I just think it may be confusing to say "wings push air down".

Then just say "divert down" or "accelerate down".

 

[sophiecentaur]

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.

Whether you use a flat plate or even a brick at the correct angle you will also achieve some lift because of the downward deflected air. The aerofoil shape tends to be used because ti achieves the same lift with minimal drag.
Also, the sail on a sailing boat has the same profile for 'upper and lower' faces of the sail, yet you get the same sort of effect. I wonder why all sails are not made with a different upper and lower profile (a bag construction). Perhaps it's because a boat sail is required to work at all possible angles and (along with greatest convenience) the single skin is least worst at all angles.

 

[russ_watters]

Whether you use a flat plate or even a brick at the correct angle you will also achieve some lift because of the downward deflected air.

Yes; the contribution of the bottom surface generally comes from the angle of attack and higher pressure due to the deflection (inside the curve=high pressure, outside the curve=low pressure).

Also, the sail on a sailing boat has the same profile for 'upper and lower' faces of the sail, yet you get the same sort of effect. I wonder why all rails are not made with a different upper and lower profile (a bag construction). Perhaps it's because a boat sail is required to work at all possible angles and (along with greatest convenience) the single skin is least worst at all angles.

Complexity and bang-for-the-buck I suspect. You probably could use a ram-air system like on a parachute to gain some efficiency, but I'm not sure if it would work for all points of sailing. Some saiboats do use actual wings sometimes though.

 

[FactChecker]

Whether you use a flat plate or even a brick at the correct angle you will also achieve some lift because of the downward deflected air.

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.

 

[doglover9754]

You could go to your local flight school and ask for an introductory flying lesson. Ask the instructor to demonstrate cruise speed with the flaps both up and down, and note the difference in speed at the same power. Then demonstrate slow flight with the flaps both up and down, and note how much slower it can fly with the flaps down.

There is no minimum age for a flying lesson.

I don’t really know of one in my area. I’ll search it up. Thanks for the idea!

 

[sophiecentaur]

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]

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]

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]

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]

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.

That would be helpful.

 

[doglover9754]

PPS. As @berkeman said, @boneh3ad 's insight article (https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/) is very well thought out and well worth reading.

I read through it and the beginning is kind of confusing since I haven’t learned it yet. Hopefully I will learn it sometime soon! It sound interesting!

 

[boneh3ad]

[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.

 

[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]

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]

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

 

[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?