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

[doglover9754]

Sort of. When the plane is going fast enough, you want a more streamlined wing. You have planty of lift at high speeds so you can make the wing more streamlined. Otherwise, it would either slow you down or require more fuel to keep the speed up. Both are bad.

So in short, moving the extension of the wing would help the plane save fuel and keep the speed up? Interesting...

 

[sophiecentaur]

Hmmm. I see. So they move it to go fast because it would just make the airplane go slow?

And cost a lot more in fuel, too.

 

[Shadow89]

To make it even simpler - a wing has an upward force because it pushes air down. (I don't think the drawing is accurate in that respect)

To be slightly more precise: The blades on a helicopter "propeller" push air down. The wings on an airplane don't actually push air down. They generate lift.

What does this even mean? If you are talking about the "flaps" such as ailerons, elevators, and the rudder, then yes, those are used to get the plane pointed in the correct direction. That isn't what @doglover9754 was asking, though. They were asking about the flaps/slats that extend from the leading and trailing edges of the wing during takeoff and landing.

The answer to why these exist has to do with something we call camber. There are a few terms I will define here to explain what I mean.
Chord line: The chord line is a straight line drawn between the leading edge and trailing edge of an airfoil.
Camber line: The camber line is a line (possibly curved) drawn from the leading edge to the trailing edge that has exactly as much airfoil above it as it does below it at a given point.
Symmetric airfoil: A symmetric airfoil is one whose camber line and chord line are the same. In other words, it is symmetric about the chord line.
Cambered airfoil: A cambered airfoil is one whose camber line is curved. Positive camber means the camber line is above the chord line (the cupped part pointing down) and negative camber is the opposite.

So, flaps and slats... flaps and slats increase the camber of an airfoil. This does a few things to the wing.

• It allows the wing to generate more lift at low speeds, which is quite important for takeoff and landing.
• It allows the wing to generate more lift at a smaller angle of attack, allowing the plane to land in a slightly more horizontal orientation than it otherwise would (and also take off when it must start out horizontal).
• It typically greatly increases the drag on the airfoil.

So, in short, extending those flaps and slats when taking off and landing gives the airfoil greater lifting performance at low speeds at the expense of extra drag. This is an acceptable trade-off because the pilot wants to slow the plane down anyway, so he has some engine power to spare. You wouldn't want these implements extended during cruise, however, because that increase in lift is not necessary at those speeds and the increase in drag is going to hurt your fuel economy pretty dramatically.

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.

 

[rootone]

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)

 

[FactChecker]

The drawing is accurate. It's pressure pushing up, not air pushing down, at least that's always the way it was explained to me.

There is a combination of pushing air down and drawing air down into a low pressure area created by the wing. The force from the wing actually impacting air and pushing it down is not the whole thing, and maybe not the largest part. The combined result is that the air flow is diverted downward and the reaction is a force lifting the wing. Since the drawing does not show the downward direction of the airflow, it is not correct in that respect. The drawing is trying to emphasize the pressure differential and is not completely accurate.

 

[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.