Why does the air flow faster over the top of an airfoil?

Question

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?

Answer 1

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:

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.

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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).

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)

Answer 6

The short answer: it doesn't.

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

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