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

In summary, the plane flies because the pressure on the top of the wings is greater than the pressure on the bottom. The wings are moved back to the original position when landing in order to help brake.
  • #1
doglover9754
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I just went on a Japan trip. In the airplane, besides throwing up my ball and phone, I was doing a little bit of studying. While the plane was in takeoff, in the middle of flying, and landing, I noticed how the wing positions changed.

I already know how takeoff works. It works when the plane makes “cupped” wings to help push itself up higher using wind and air pressure? I haven’t watched a video about that in a while so I kind of forgot (maybe 6-12 months ago).

Anyways, what I was wondering was how does the landing and flying work? For flying, I noticed during the flight, the pilot puts the wings back to its original position? I’m not sure how and why this works. If you need the pressure on the wings to help it fly, then why change the position? Is it because the plane reaches a certain height to sustain that pressure to keep it flying? Now for landing, the pilot extends the wing flaps (if you sit next to the wing, you notice how the back of the wing extends and kind of flaps down) and moves these plates covering some wires and the joining part of the wing flaps. Is this to help brake as I know when planes fly, they fly fast to they keep up that speed even while on the runway (landing) until the pilot brakes? I figure that it helps brake because with the plates and fallen flaps, there has just got to be some air resistance. I’m honestly not totally sure.

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
 
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To make a very complex thing simple:

The wings lift the airplane because there is more pressure beneath them than over them.

This is because the wing is curved on the topside, so the air has to travel "further" to get from front to back.

The wing has adjustible flaps and such that help the pilot get the plane going in the right direction.

There are other aerodynamic pieces on the wing, such as winglets, which help increase the lift force on the wing.
 

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doglover9754 said:
I already know how takeoff works. It works when the plane makes “cupped” wings to help push itself up higher using wind and air pressure? I haven’t watched a video about that in a while so I kind of forgot (maybe 6-12 months ago).

Anyways, what I was wondering was how does the landing and flying work? For flying, I noticed during the flight, the pilot puts the wings back to its original position? I’m not sure how and why this works. If you need the pressure on the wings to help it fly, then why change the position? Is it because the plane reaches a certain height to sustain that pressure to keep it flying? Now for landing, the pilot extends the wing flaps (if you sit next to the wing, you notice how the back of the wing extends and kind of flaps down) and moves these plates covering some wires and the joining part of the wing flaps. Is this to help brake as I know when planes fly, they fly fast to they keep up that speed even while on the runway (landing) until the pilot brakes? I figure that it helps brake because with the plates and fallen flaps, there has just got to be some air resistance. I’m honestly not totally sure.
You aren't too far off. the main reason for flaps is they create more lift and allow the plane to fly slower for takeoff and especially landing. But at a cost of higher drag, so they are retracted during cruise when the plane is flying fast and generates plenty of lift without them.
 
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  • #4
Shadow89 said:
To make a very complex thing simple:

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)
 
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  • #5
doglover9754 said:
besides throwing up my ball and phone,
I'm sorry that you got sick on the flight. Maybe next time don't eat your ball or your phone before flying, and you will do better. :wink:
 
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  • #6
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.

Regards,

Frey
 
  • #7
berkeman said:
I'm sorry that you got sick on the flight. Maybe next time don't eat your ball or your phone before flying, and you will do better. :wink:
If the avatar was/is a kitten it could have been a hair ball.
Cats do that all the time, especially where you put your bare feet in the morning.
 
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  • #8
Vanadium 50 said:
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)
Frey Petermeier said:
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.

Regards,

Frey
Well, we have a very nice Insights Blog article on this very subject!

https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

:smile:
 
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  • #9
Vanadium 50 said:
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)
Frey Petermeier said:
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.
You're talking past each other. V50 said the *wing* is pushing air down, and Frey you said the air pressure is pushing the wing up. I agree with both...though 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.

And I think what V50 was saying about the diagram - which I agree with - is that it incorrectly implies the airstream returns to straight and level at the trailing edge. it should show (and even exaggerate) that the air leaving the trailing edge is traveling down.
 
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  • #10
Whatever jiggery pokery is going on between the wing and the air around it, the only way there can be an upwards force on the plane is if there is air being pushed downwards. Newton;s Third Law always applies, even if you need to dig a bit to identify where.
 
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  • #11
From the Insights article by @boneh3ad

Watch the airflow in the 2nd case where the wing is inclined to produce lift... :smile:

 
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  • #12
Whoa, someone deployed the bone signal!

Shadow89 said:
This is because the wing is curved on the topside, so the air has to travel "further" to get from front to back.

The distance traveled has absolutely no direct effect on the pressure distribution or on the lift.

Shadow89 said:
There are other aerodynamic pieces on the wing, such as winglets, which help increase the lift force on the wing.

Winglets are actually more important for drag than they are for lift. They reduce the effect of wingtip vortices, creating a bit of an increase in lift and a comparatively large decrease in drag.

Frey Petermeier said:
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.

It is both. These effects are two sides of the same coin. You wouldn't have one effect without the other. The drawing is, therefore, inaccurate assuming that wing is actually generating lift.

russ_watters said:
I agree with both...though 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.

This is incredibly misleading. First, I don't even know how you'd go on to quantify which surface "contributes more" to lift. Contributes how? This entire system is governed by elliptic equations. The upper and lower surface work effectively in tandem to generate the whole flow field. They don't operate independently. In no aerospace engineering curriculum with which I am familiar will you hear any talk of which surface "contributes more" (or at least not in those terms that are so undefined).

Shadow89 said:
The wing has adjustible flaps and such that help the pilot get the plane going in the right direction.

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.
 
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  • #13
doglover9754 said:
For flying, I noticed during the flight, the pilot puts the wings back to its original position? I’m not sure how and why this works. If you need the pressure on the wings to help it fly, then why change the position?
The flaps at the back edge of the (and on some aircraft similar things on the front edge, called "leading edge devices") change the shape of the wing. We do this because one shape produces lots of lift at low speeds but has too much drag to fly very fast, while the other shape has little drag but only produces lift at high speeds. For takeoff and landing when the plane is moving slowly you want the first shape; for high-speed cruise you need the other shape.

There are some other movable bits in the wings as well, but their motion is more subtle so they're probably not what you noticed.
 
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  • #14
boneh3ad said:
This is incredibly misleading. First, I don't even know how you'd go on to quantify which surface "contributes more" to lift. Contributes how?
We've discussed this before: By measuring/integrating static pressure along each surface of the wing it can be shown that for a flat-bottom wing at 0 AOA (like the illustration above) essentially all the pressure change that *is* lift happens on the top surface. Or by the Newton's 3rd law way, you can see that flow over the bottom is horizontal and flow over the top gets angled down.
They don't operate independently.
When we discussed this before I gave you several examples where there isn't even a bottom surface at all. Flow over a hill, a storm ripping a roof off a house, a car with an improperly created skirt (blocking flow without creating suction), etc.

Of course, there are situations where the bottom surface does essentially all the work or the top and bottom surfaces fight against each other. For a race car with ground effects, the top surface is trying to lift the car off the ground and the bottom surface pulling it down.
The upper and lower surface work effectively in tandem to generate the whole flow field.
While it is trivially true that 1+3=4, 3>1. And in the last example, 1+(-3)=-2.

Edit:
I know you like that NASA site's explanations of lift. See here:
https://www.grc.nasa.gov/www/k-12/airplane/presar.html
presar.jpg

The area *inside* the graph is the lift. Clearly, the larger piece of that is the contribution of the upper surface.
 

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boneh3ad said:
The distance traveled has absolutely no direct effect on the pressure distribution or on the lift.
This is missing the forest for the trees, stating a disagreement instead of with a minor tweak steering it toward agreement. The air flowing over the top surface of the wing *does* generally take a longer path (from the stagnation point to the trailing edge). It's true and it isn't a coincidence/accident that it is true; it is a consequence of the more direct cause: deflection. Many people prefer to use the word "deflection" -- deflecting something from its straight path makes it take a different and longer path. So rather than just throwing the baby out with the bath water, can we steer "distance traveled" to "deflection"?

Some other notes on verbiage for this:
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.
https://www.grc.nasa.gov/www/k-12/airplane/lift1.html
1. "Turning" ("deflect"ing) the air from the straight path it was on makes it take a longer path.
2. While not ranking the contributions, it says both make contributions, implying they can be treated separately. Then they confirm they can be treated separately:
3. "Any...surface": singular. They aren't talking about wings there, but generalizing to any surface that air flows past.
4. They say "neglecting the upper surface..." because when kids first learn about lift, they often think the lower surface is being impacted by air, creating lift and the upper surface does nothing. You can feel this by sticking your hand out the window of a car.
The shape of a typical airfoil is asymmetrical - its surface area is greater on the top than on the bottom. As the air flows over the airfoil, it is displaced more by the top surface than the bottom. According to the continuity law, this displacement, or loss of flow area, must lead to an increase in velocity. Consider an airfoil in a pipe with flowing water. Water will flow faster in a narrow section of the pipe. The large area of the top surface of the airfoil narrows the pipe more than the bottom surface does. Thus, water will flow faster on top than on bottom. The flow velocity is increased some by the bottom airfoil surface, but considerably less than the flow on top.
http://web.mit.edu/16.00/www/aec/flight.html

This repeats some of the above, but adds the Venturi twist that I like and you don't. They actually cite a Venturi tube to make the description.
 
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  • #16
russ_watters said:
The air flowing over the top surface of the wing *does* generally take a longer path (from the stagnation point to the trailing edge).
I'm wondering about the case of the oddball M2-F2 reentry prototype. Since it's a reentry prototype, the airfoil needed to be able to deal with hypersonic reentry speeds.

m2f2_1.jpg






or the rocket powered m2-f3

 

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rcgldr said:
I'm wondering about the case of the oddball M2-F2 reentry prototype. Since it's a reentry prototype, the airfoil needed to be able to deal with hypersonic reentry speeds.
It is difficult to tell with these wing shapes because they are so extreme. But I believe the angle of attack is higher than you may realize, which makes the stagnation point lower on the leading edge than you might realize.
 
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rcgldr said:
I'm wondering about the case of the oddball M2-F2 reentry prototype. Since it's a reentry prototype, the airfoil needed to be able to deal with hypersonic reentry speeds.

russ_watters said:
It is difficult to tell with these wing shapes because they are so extreme. But I believe the angle of attack is higher than you may realize, which makes the stagnation point lower on the leading edge than you might realize.
The angle of attack is high when landing (320 kph), but reasonable at higher speeds (600+ kph). The M2-F2 reached a gliding speed of 720 kph on it's maiden "flight".

https://en.wikipedia.org/wiki/Northrop_M2-F2
 
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russ_watters said:
We've discussed this before: By measuring/integrating static pressure along each surface of the wing it can be shown that for a flat-bottom wing at 0 AOA (like the illustration above) essentially all the pressure change that *is* lift happens on the top surface. Or by the Newton's 3rd law way, you can see that flow over the bottom is horizontal and flow over the top gets angled down.

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.

russ_watters said:
When we discussed this before I gave you several examples where there isn't even a bottom surface at all. Flow over a hill, a storm ripping a roof off a house, a car with an improperly created skirt (blocking flow without creating suction), etc.

Notably, hills, houses, and (most) cars don't fly and have entirely different flow fields than an airfoil.

russ_watters said:
Of course, there are situations where the bottom surface does essentially all the work or the top and bottom surfaces fight against each other. For a race car with ground effects, the top surface is trying to lift the car off the ground and the bottom surface pulling it down.

russ_watters said:
I know you like that NASA site's explanations of lift. See here:
https://www.grc.nasa.gov/www/k-12/airplane/presar.html
The area *inside* the graph is the lift. Clearly, the larger piece of that is the contribution of the upper surface.

A single surface alone cannot try to lift anything. If you only have one surface facing one direction, the net force on that surface is always in the direction away from the flow. That's how pressure works. Only once you include the other surface do you get a net force. Every top surface on every object is being pushed down by the local pressure and every bottom surface is being pushed up. It is the relative magnitudes that matter.

To illustrate this, look at the link you provided. Actually, the "area *inside* the graph", otherwise known as the integral, is not lift unless you are looking at the area between the two lines. In other words, only when you include both sides of the airfoil do you get lift out of that. Otherwise, you are getting simply the force on the individual surface.

russ_watters said:
While it is trivially true that 1+3=4, 3>1. And in the last example, 1+(-3)=-2.

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

russ_watters said:
This is missing the forest for the trees, stating a disagreement instead of with a minor tweak steering it toward agreement. The air flowing over the top surface of the wing *does* generally take a longer path (from the stagnation point to the trailing edge). It's true and it isn't a coincidence/accident that it is true; it is a consequence of the more direct cause: deflection. Many people prefer to use the word "deflection" -- deflecting something from its straight path makes it take a different and longer path. So rather than just throwing the baby out with the bath water, can we steer "distance traveled" to "deflection"?

The problem is that the overwhelming majority of people who start talking about "distance traveled" are equating it with the equal transit time fallacy. Further, you don't absolutely have to have a longer surface to have a lift-generating airfoil. That is usually true, but some flatback aifoils, for example, have very strange, long bottom surfaces and still generate lift.

russ_watters said:
Some other notes on verbiage for this:

https://www.grc.nasa.gov/www/k-12/airplane/lift1.html
1. "Turning" ("deflect"ing) the air from the straight path it was on makes it take a longer path.
2. While not ranking the contributions, it says both make contributions, implying they can be treated separately. Then they confirm they can be treated separately:
3. "Any...surface": singular. They aren't talking about wings there, but generalizing to any surface that air flows past.
4. They say "neglecting the upper surface..." because when kids first learn about lift, they often think the lower surface is being impacted by air, creating lift and the upper surface does nothing. You can feel this by sticking your hand out the window of a car.

1. Turning the flow does not requires a longer path. It often features that, but it is not a requirement.
2. Contributions aren't ranked because that concept is nonsensical.
3. Of course any surface can turn a flow, but we are talking about wings in this thread.
4. I don't know what that has to do with anything. I have been making the point this entire time that you cannot separate the effects of the upper and lower surfaces in discussing lift. That's sort of my core point. If you only look at the lower surface, you get a (wrong) positive lift. If you only look at the upper surface, you get a (wrong) negative lift. You always have to consider both.

russ_watters said:
http://web.mit.edu/16.00/www/aec/flight.html

This repeats some of the above, but adds the Venturi twist that I like and you don't. They actually cite a Venturi tube to make the description.

They also cite placing a wing into a pipe to make that argument. I don't like the Venturi analogy because it is flat out wrong. Only if you add a finite boundary is it correct, and to do that you either need to stick your airfoil in a pipe, which fundamentally changes the flow, or else you need to consider the streamlines as streamtubes and apply Venturi to them, not some arbitrary horizontal surface.

rcgldr said:
I'm wondering about the case of the oddball M2-F2 reentry prototype. Since it's a reentry prototype, the airfoil needed to be able to deal with hypersonic reentry speeds.

or the rocket powered m2-f3

These are good examples of how you don't need a longer top to generate lift.

russ_watters said:
It is difficult to tell with these wing shapes because they are so extreme. But I believe the angle of attack is higher than you may realize, which makes the stagnation point lower on the leading edge than you might realize.

Now you are just speculating. Yes, the stagnation point moves when the angle changes, but it is not a hard requirement that the top surface be longer.
 
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  • #20
russ_watters said:
You aren't too far off. the main reason for flaps is they create more lift and allow the plane to fly slower for takeoff and especially landing. But at a cost of higher drag, so they are retracted during cruise when the plane is flying fast and generates plenty of lift without them.
Hmmm. I see. So they move it to go fast because it would just make the airplane go slow?
 
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  • #21
berkeman said:
I'm sorry that you got sick on the flight. Maybe next time don't eat your ball or your phone before flying, and you will do better. :wink:
Good idea :biggrin:
 
  • #22
Glider pilots have a maneuver called "boxing the wake" It is performed while being towed by another plane. The glider descends in altitude until you are below the tow plane. At some point you will pass through the wake that is a result of the air hitting the bottom of the wing and descending. It is very noticeable as there is a lot of turbulence. That wake is so sharply defined that you can go through it to the calm air under it or you can find the left and right limits of the wake. To me this is a good demonstration of the lift being caused by air being directed downward by the angle of the wing. Google "boxing the wake" for more."
 
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  • #23
256bits said:
If the avatar was/is a kitten it could have been a hair ball.
Cats do that all the time, especially where you put your bare feet in the morning.
:cry: It’s a wolf puppy
 
  • #24
russ_watters said:
You're talking past each other. V50 said the *wing* is pushing air down, and Frey you said the air pressure is pushing the wing up. I agree with both...though 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.
Isn’t that the same thing?
 
  • #25
boneh3ad said:
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.
Notably, hills, houses, and (most) cars don't fly and have entirely different flow fields than an airfoil.A single surface alone cannot try to lift anything. If you only have one surface facing one direction, the net force on that surface is always in the direction away from the flow. That's how pressure works. Only once you include the other surface do you get a net force. Every top surface on every object is being pushed down by the local pressure and every bottom surface is being pushed up. It is the relative magnitudes that matter.

To illustrate this, look at the link you provided. Actually, the "area *inside* the graph", otherwise known as the integral, is not lift unless you are looking at the area between the two lines. In other words, only when you include both sides of the airfoil do you get lift out of that. Otherwise, you are getting simply the force on the individual surface.
I don't even know what you were trying to say with this.
The problem is that the overwhelming majority of people who start talking about "distance traveled" are equating it with the equal transit time fallacy. Further, you don't absolutely have to have a longer surface to have a lift-generating airfoil. That is usually true, but some flatback aifoils, for example, have very strange, long bottom surfaces and still generate lift.
1. Turning the flow does not requires a longer path. It often features that, but it is not a requirement.
2. Contributions aren't ranked because that concept is nonsensical.
3. Of course any surface can turn a flow, but we are talking about wings in this thread.
4. I don't know what that has to do with anything. I have been making the point this entire time that you cannot separate the effects of the upper and lower surfaces in discussing lift. That's sort of my core point. If you only look at the lower surface, you get a (wrong) positive lift. If you only look at the upper surface, you get a (wrong) negative lift. You always have to consider both.
They also cite placing a wing into a pipe to make that argument. I don't like the Venturi analogy because it is flat out wrong. Only if you add a finite boundary is it correct, and to do that you either need to stick your airfoil in a pipe, which fundamentally changes the flow, or else you need to consider the streamlines as streamtubes and apply Venturi to them, not some arbitrary horizontal surface.
These are good examples of how you don't need a longer top to generate lift.
Now you are just speculating. Yes, the stagnation point moves when the angle changes, but it is not a hard requirement that the top surface be longer.
That was kind of confusing...
 
  • #26
russ_watters said:
You aren't too far off. the main reason for flaps is they create more lift and allow the plane to fly slower for takeoff and especially landing. But at a cost of higher drag, so they are retracted during cruise when the plane is flying fast and generates plenty of lift without them.

doglover9754 said:
Hmmm. I see. So they move it to go fast because it would just make the airplane go slow?
The flaps increase camber (and angle of attack) for more lift at slower speeds, such as takeoffs and landings. Cruise speeds are much faster than this, so the wing can generate the same lift with much less of an angle of diversion of the relative (to the wing) air flow, so the flaps are retracted.
 
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  • #27
doglover9754 said:
That was kind of confusing...
The simple answer is: If air is diverted downward, there is an upward force on the wing. But there is no simple and general answer to how the air is diverted downward.
 
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  • #28
doglover9754 said:
Isn’t that the same thing?
"Pulling" usually refers to an attractive force, which doesn't happen between the wing and the air, so I would avoid this type of explanations. On both sides of the wing there is only "pushing" (repulsive force), but the pushing up at the wing's bottom side is greater than the pushing down at the wing's top side. So there is a net upward force.
 
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  • #29
doglover9754 said:
Hmmm. I see. So they move it to go fast because it would just make the airplane go slow?
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.
 
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  • #30
A.T. said:
The simple answer is: If air is diverted downward, there is an upward force on the wing. But there is no simple and general answer to how the air is diverted downward.
That makes sense. Thanks!
 
  • #31
FactChecker said:
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...
 
  • #32
doglover9754 said:
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.
 
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  • #33
Vanadium 50 said:
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.

boneh3ad said:
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.
 
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  • #34
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)
 
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  • #35
Frey Petermeier said:
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.
 
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