Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I understand the common explanation of lift, which describes the airflow over the top of the wing as moving faster than the air below the wing. However, I don't quite understand why the air moves faster.

I've read some explanations citing a circulation effect among others, but haven't found a good, clear explanation for the exact cause. Can someone help me out?

share|improve this question
4  
In fact, the equal-time explanation is not fully correct, and a full description of lift generation in a airplane wing is complicated. Nor do I understand it well enough to leave something I would call an answer. Hopefully someone else can. –  dmckee Aug 1 '11 at 0:35
6  
Here's a related (borderline duplicate) question: physics.stackexchange.com/q/290 –  David Z Aug 1 '11 at 1:28
1  
The air particles in front of the separation point make an appointment to meet after the travel around the airfoil: "When will we two meet us again, behind airfoil, flaps and engaine?" As You can see in the animation below, the upper stream realizes after passing half the way, that he is late and hurries up :=) –  Georg Aug 1 '11 at 9:11
1  
This website is probably the best I've found on the internets yet: av8n.com/how/htm/airfoils.html –  genneth Aug 1 '11 at 9:13
add comment

8 Answers 8

The common explanation given is that it flows faster over the top of the wing because the top is more curved than the bottom of the wing. However, I understand why you would find this explanation unsatisfactory.

To start with, I think we need to identify the point at which the flow separates. Looking at Wikipedia, I'll post two images:

airfoil

enter image description here

The argument that the wind flows faster over the top is mostly a consequence of geometry. First identify the point at which the flow separates, meaning the point above which the fluid goes over and below which the fluid goes under, this is slightly below the front-most point of the airfoil, due to the fact that it's angled slightly upward. If both paths take roughly the same time to pass over the wing, then the average velocity of the fluid from the point of separation to the tail where the flow rejoins will be roughly proportional to the distance from those two points.

Now, you may say, "but it will flow faster over the top even if the top isn't curved more!" You would be correct. A plane can function with no additional curve on the top of the wing, as the famous xkcd comic points out. Such a mode of flying, however, will still see the fluid passing over the top of the wing faster. A simple argument for this is that the point of separation is lower than the front of the wing, again, since the wing is angled up. A plane can fly upside down, but I don't know of a plane that can maintain altitude with the wings not angled up. The curved top, however, increases efficiency by intensifying that natural effect.

I hope that helps some, this is intentionally not a rigorous answer, and I want to recognize that I am not addressing the more hairy details of the actual fluid equations associated with this, which are required for a full explanation. In short, the fluid velocity over a surface isn't completely proportional to the distance traveled. Even without getting into that, however, I think your question is mostly answered.

Another Attempt

I realize that my answer up to this point may not only be incomplete, but might not answer the question. The question is why the flow on the top is moving faster than on the bottom. Let me post another image.

Wiki gif

There are 2 things I want to note here.

  1. The fluid above the wing does speed up and the fluid below the wing does slow down. I just want to confirm this is still the case.
  2. The fluid going above and below does not have the same travel time over the wing.

Number 2 is particularly important because it is simply not correct to say that the speed is proportional to the distance between the separation and rejoin point. That way of looking at it may still have some usefulness. But I digress.

At this point I'm repeating Wikipedia's explanation, but refer to the 2nd image in this answer. Under some assumptions the fluid does not cross the blue streamlines. That means that when the channel size between 2 blue streamlines narrows or widens, the fluid speed changes correspondingly. I'm still hand waving away plenty of technical detail, but please let me offer this as basic level answer.

The fluid on top of the wing is accelerated and the fluid on the bottom of the wind is slowed down compared to velocity of the aircraft itself because the wing geometry and angle narrows the flow area above the wing and widens the flow area below the wing

This is the absolute best explanation I have. If you assume that the fluid is incompressible it works great, if not, it works less great but still works. There are also some other assumptions, I hope that the general point is still the same with all those included. The bold text is the best answer I have and I think it's a good one.

share|improve this answer
    
Note that while these things are often diagram with symmetric or nearly symmetric shapes wings work with a wide range of actual shapes many of which are not at all symmetric. –  dmckee Aug 1 '11 at 1:11
1  
"If we don't consider separation of flow, then the average velocity of the fluid from the point of separation to the tail where the flow rejoins will be roughly inversely proportional to the distance from those two points." I've read this five times and I still don't get it. Why does the flow need to take roughly the same time over top and bottom? What do you mean by "If we don't consider separation of flow..."? Everything before that was describing separation of flow, and now we ignore it? Why does the angle of attack tell you where the flow separates? –  Mark Eichenlaub Aug 1 '11 at 2:32
1  
@Mark that sentence didn't make any sense, yes. I was thinking "laminar" when I wrote that, which isn't correct anyway. The inversely proportional statement must assume the travel time is the same for top and bottom. You are correct, the travel time is very much not the same for both. I'll add some more, here is a useful addition en.wikipedia.org/wiki/File:Karman_trefftz.gif –  AlanSE Aug 1 '11 at 2:55
    
@Zass Thanks for the clarification. One other point: I think all the instances of "inversely proportional to the distance" should just say "proportional to the distance". –  Mark Eichenlaub Aug 1 '11 at 4:15
    
@Mark Well that was an embarrassing error. Thanks. –  AlanSE Aug 1 '11 at 4:17
add comment

A fluid only transmits forces through pressure. If an airfoil generates lift, this must be because the top is at a lower pressure than the bottom in the steady-state flow. The amount of lift can be understood by the rate of deflection of air downward, but the mechanism of lift is always high-pressure on the bottom and low pressure on the top. Low pressure regions speed up a fluid, and high pressure regions slow it down, just because the fluid is doing work to enter the high pressure region and has work done upon it to enter the low pressure region. Therefore the velocity at the top is higher than at the bottom.

The speed at the top and the bottom do not guarantee that the transit time is the same, however, so the explanation that the airfoil lifts because the air above is moving faster is incorrect. But the low-pressure and high-pressure regions exist. Note that if you make the angle of attack of an airfoil such that the air going past it is deflected upward, the pressure at the bottom is less and at the top is more, so the air at the top is moving slower.

share|improve this answer
add comment

The essential feature of a wing is that it redirects air flow. Oncoming air moves horizontally (relative to the plane), and behind the wing the air mass has a downward velocity component.

In effect the air mass that is moving relative to the wing is turning a corner. It follows that the air mass on the "outside lane" is covering a longer distance than the air mass that follows the "inside lane".

The lift arises from action-reaction dynamics. For comparison: a helicopter rotor produces lift by exerting a force upon the air. This force accelerates the air mass downwards, and the reaction carries the helicopter.

The primary phenomenon is air mass being redirected (downward) by the wing. This has two consequences:
* The wing produces lift
* Air mass flows faster over the top of the wing

It's common to see the suggestion that the air mass flowing faster on top is what produces the lift. However, that is not the actual rundown from cause to effect. Primary is that air mass is being redirected.

share|improve this answer
1  
"It follows that the air mass on the 'outside lane' is covering a longer distance than the air mass that follows the 'inside lane'." Yes, but it does not follow from this that the air above the wing will be moving more quickly. –  nibot Aug 3 '11 at 22:07
add comment

I remember distantly about comparing it with pressure-flow relation in pipes: due to Bernoulli's principle the top of the airfoil acts as a thin "pipe", wherein (if respecting volume/mass conservation) for more fluid to be transported, it flows faster, decreasing the pressure on the pipe (increasing the pressure in the direction of the flow).

thin-thik-pipes

That is the reason for reduced pressure above the topside of the airfoil and the uplift

I think the tilt is used for strearing reasons (amplification, but not the cause of the uplift), so the plain will fly with wings parallel to the ground as well.

@xkcd - flying on the back may imply much steering in the opposite direction (above)

share|improve this answer
    
This is the same conclusion I was coming to in my answer. The quoted mechanism is the correct one, and is the proper answer to the question. The way the wing diverts flow speeds up fluid and reduces pressure on the top, the opposite on the bottom (although there may be a problem with the fying with no angle). This is also my point about downvoting. Many downvotes on physics SE are coming from a position of not understanding the problem physics. –  AlanSE Aug 1 '11 at 19:27
    
I meant that in the context of this answer being downvoted, not mine. It was of physical merit, so I thought it was silly. –  AlanSE Aug 7 '11 at 19:49
add comment

Basically, the air flows faster over the top, but not because it has to obey some 'equal time' rule - it is really a totally made up rule which is not true.

The wing is angled, and behind the top of the wing there is area of lower pressure - very simply, wing has just moved out of that place - just like a pump piston - the air has to turn to meet up the wing. In front of the wing there is an area of higher pressure, because the air is being pushed by the wing. This pressure difference acts on wing and produces lift.

The often quoted Bernoulli's law needs some explaining.

It is really very simple. When air flows into area of low pressure, it accelerates akin to rollercoaster going down - it gets sucked into low pressure area! And when it enters high pressure, it slows down akin to rollercoaster going up. When there is no friction between flow lines, the only way flow velocity can change is via this pressure driven acceleration and deceleration. The air entering low pressure area on top of the wing speeds up. The air entering high pressure area on bottom slows down. That is why air on top moves faster.

That results in deflection of the air downwards, which is required for generation of lift due to conservation of momentum (which is a true law of physics). For the air to be deflected downwards, it is necessary that air on top goes faster, but this is nonetheless not an explanation of how we made air move faster on the top, it is merely an explanation why we would want to.

The common explanation of lift does it backwards - it assumes faster flow over the top of airfoil, using a made up, incorrect law of equal transit time to justify this, then uses Bernoulli's relation between speed and pressure to explain low pressure and lift.

In common introductory textbooks the approach is taken to anyhow reduce everything into things that the author can't be blamed for not explaining, ideally 'laws'. The goal is essentially psychological; to produce feeling of understanding in the mind of the typical reader.

The science works differently; instead of striving to achieve particular feeling in the head, it produces theories that should allow to predict things.

share|improve this answer
add comment

The front wedge on the airplane wing strikes at the onrushing wind forcefully,because of he geometry of the upperside of the wing, so the airmass at the collision point gets pushed upward.

So, the air molecules just behind the wing are thinner and get more thinner when the travelling plane shifts position forward. The dense down-the-wing airmass pushes up the plane into this cavity behind the wedge, which has low pressure inside because of fewer molecules inside.The creation of cavity is a continuously forward-travelling low-pressure region which fact maintains the lift continuously. As the cavity contains fewer molecules, the molecules of the airmass rushing backwards have freer route for travelling towards the tail, so has more velocity.

This explains the creation of both low pressure and higher velocity on the top. Higher velocity on topside and longer distance over the curved surface both together ensure that the topside-flowing airmass separated at the wedge joins the stream flowing through the downside of the wing at the same time when they pass over the tail. Here the Bernoulli equation is taking place while the lift earlier described is acting as per the newtonian laws. In fact, both laws mean the same with Bernoulli describing an after-effect of Newton's, using different wordings.

share|improve this answer
add comment

The short answer: it doesn't.

Link #1: http://xkcd.com/803/

Link #2: http://en.wikipedia.org/wiki/Airfoil

share|improve this answer
2  
It does. Again, the claim is: "air flows faster over the top of an airfoil." If my understanding is correct, this claim is correct for any airfoil generating lift. If someone can coherently argue the other way I would like to hear it. –  AlanSE Aug 1 '11 at 0:53
1  
@Zass: In the usual design air does flow faster over the top, but it does not flow enough faster to preserve the equal-time condition that is usually invoked in simple explanations. I am told that the way the flow separates from the trailing edge is also important, because it results in pushing the airstream down (and therefore a simple thrust). –  dmckee Aug 1 '11 at 1:10
1  
As the accepted answer (and the wikipedia animation) shows, it flows even faster than equal-time condition would suggest. –  Suma Aug 1 '11 at 8:15
add comment

First you have to stop and look at what is really happening when an aircraft flys. The air is not moving, the aircraft is, whether it is a rotor wing or fixed wing. The same principles apply. Bernoulie and Newton both are right because they look at the same thing. Air is fliud and airpressure is equal all around the wing, The wing smashes into the air seperating the airmolicules. The air on the top is pushed away at a high rate of speed. This causes the air molicules on top to bump more molicules higher up. Just aft of the airodinamic ridge is the lowest point of airpressure. This is why a long wing creates more lift. Since air is fluid it is pushing against the molicules twd the rear at the static air press. So they are forced back to the surface. That is Bernoulie!, the under side is Newton the air is compressed against the bottom of the wing depending on the angle of attack, the wing works like a wedge and slides up over the air. Wings with lower camber work better for jets because of the high turbulence at higher speed. This maybe different explanation than you will get most places. But I had the same question and had to figure it out due to bubble gum answers.

share|improve this answer
    
Several things disasterously wrong here. First, all motion is relative and a correct physical explanation can't depend of which of the fluid and the wing is moving. Second, the answer seems to suggest that air molecules are broken which is simply false. –  dmckee Nov 18 '13 at 1:23
add comment

protected by Qmechanic Aug 10 '13 at 15:31

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.