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r/explainlikeimfive•
DraggerLP

ELI5: why does airflow speed up when flowing over a wing?

Physics

I mean, I get that in order to get back next to the airmolecule that took the bottom path under the wing, and the path on top beeing longer results in a faster speed and therefore a lower pressure (because the energy for that needs to come from somewhere), but why does it do that in the first place?

What's causing the air to speed up suddenly instead of building pressure above the wing, due to it deflecting air up?

Posted Nov 9, 2023, 8:56 PM UTC

1225
SoulWager•

It speeds up because it's flowing into a lower pressure area, not the other way around. The most important factor that causes that change in pressure is the angle of attack. The energy is coming from the wing's movement through the air.

Length of the side has nothing to do with it, as the air at the leading edge of the wing doesn't know how long the sides are, and the air on the top of the wing doesn't have to meet back up with the air on the bottom of the wing.

20
DraggerLPOP•

Ohhhhh, the wing pushes the air away, causing a low pressure just behind/above it. Totally overlooked that

6
csl512•

in order to get back next to the airmolecule that took the bottom path under the wing

There are a lot of right and wrong explanations of lift, but equal transit theory is extra wrong.

No offense, but it's easier to just start over on understanding lift and (as the cliché goes) forget everything you think you know about it.

https://ocw.mit.edu/courses/16-687-private-pilot-ground-school-january-iap-2019/resources/lecture-2-airplane-aerodynamics/

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/aerodynamic-forces/

======

I forgot to write additional explanation sufficient for rule 3, so here's what I linked to. I'm not sure exactly where your misunderstanding lies, but all should be explained through these.

15
ResilientBiscuit•

the air on the top of the wing doesn't have to meet back up with the air on the bottom of the wing.

To some extent it does though.

If an airfoil is just sitting in the air, air molecules are happily bouncing around in their own little areas.

As it starts to move forward the the fluid some of the fluid goes over the top, some goes under the wing. Lets imagine that the fluid going over the top of the wing with the 'longer' path, didn't speed up. So its 'partner' molecule on the bottom of the wing got to the trailing edge first.

That means there is now a gap that needs to be filled. That creates the low pressure area you describe that speeds up the air.

If the air on the top were going faster and getting ahead of the the air on the bottom, you would have a high pressure area that would form and slow it down.

So it doesn't have to match up exactly, but to maintain a steady state, its going to end up doing roughly that.

-6
X7123M3-256•

So it doesn't have to match up exactly, but to maintain a steady state, its going to end up doing roughly that.

It doesn't. The air flowing over the top of the wing will arrive at the trailing edge well ahead of the air flowing under the wing.

Moreover, as a consequence of the Kutta Joukowski theorem, it can be shown that if the air does meet up again at the trailing edge then the lift on the wing is zero.

12
TwentyninthDigitOfPi•

As others have pointed out, this isn't the case. But since seeing is believing, here's a short video (1m15s) where you can actually see the top air beating the bottom air to the finish line: https://youtu.be/UqBmdZ-BNig?si=rXjzcueDv8obSVXg

3
ResilientBiscuit•

You can see the vortices formed at trailing edge, particularly evident because it is at such a high angle of attack. That vortex is caused by the difference in velocities and is your induced drag. An efficient wing doesn't have nearly as much of that.

In a more efficient wing at a lower angle of attack, you are going to get a lot less of a vortex because it will be less disruptive to the steady state of the air right?

1
TwentyninthDigitOfPi•

You still get a lot of vortex. You can find pictures online of passenger jets flying through clouds and such, and see the vortices. (I'm too mentally zonked right now to look them up myself, sorry!)

1
X7123M3-256•

That vortex is caused by the difference in velocities and is your induced drag

No, this isn't right. You're mixing up a few different concepts. Those vortices you see in that clip are ultimately caused by viscous friction between the air and the wing - they do contribute to drag, but not *induced drag.

In an ideal flow without friction, the upper and lower pressures are equal at the trailing edge (this is known as the Kutta condition), and therefore the velocities are also the same. Air flowing over the top surface of the wing will speed up as it flows into the low pressure region above the wing, then slow down again as it approaches the trailing edge, while the air flowing under the wing does the opposite. The air flowing over the upper surface takes significantly less time to flow over the wing, but when the two flows meet at the trailing edge they have the same speed.

What is happening in the video above is that friction with the wings surface slows down the air close to the surface of the wing. This region of slowed air is called the boundary layer. This creates a difference in velocity between the air close to the wing and the air further away, and that is what causes the vortices you are seeing. This turbulence is highly undesirable because it does indeed increase drag, but it is also unavoidable - at the Reynolds numbers typical of any full size aircraft, your boundary layer will be turbulent. The drag created is called "parasitic drag".

Induced drag is something different. Lift induced drag is the drag created as a direct result of lift production - i.e as a result of the deflection of the airflow by the wing which gives rise to lift. A perfectly frictionless wing would have no parasitic drag, but would still have induced drag, and induced drag increases the more lift is being produced. It is induced drag that is minimized by having a high aspect ratio wing.

Induced drag is related to the formation of vortices, but those vortices rotate around an axis parallel to the direction of travel of the wing. So they wouldn't be visible in the above video looking side on - you would see them in a view from behind the wing. In fact, for a wing which spans the entire width of the wind tunnel, which is probably what you are seeing in the above video, the induced drag will be zero as the aspect ratio is effectively infinite. This is often done in order to study the properties of an airfoil profile independent of the 3D shape of the wing.

1
SoulWager•

Lets say you have two hoses, with the same mass of air being pumped into one end every second. The throughput at the outlet has to match, but the amount of time a bit of air takes to get through each hose can be dramatically different, as can the speed it travels inside the hose. Even if the hoses are the same length and diameter, if one is twice the pressure, then a packet of air will take twice as long to get through it.

2
ResilientBiscuit•

If the hoses are the same length and diameter, but the rate one is being pumped causes it to be twisted the pressure then your original claim that the same mass is being pumped into them every second is false.

The only way to achieve a higher pressure with two hoses of the same length and diameter is to pump more mass.

1
SoulWager•

If the hoses are the same length and diameter, but the rate one is being pumped causes it to be twice the pressure then your original claim that the same mass is being pumped into them every second is false.

I didn't say the exit was unrestricted.

2
Consistent_Bee3478•

Nah air going over top reaches their faster.

No reason you wouldn’t be able to have a tiny temporary high pressure area

1
csl512•

in order to get back next to the airmolecule that took the bottom path under the wing

There are a lot of right and wrong explanations of lift, but equal transit theory is extra wrong.

No offense, but it's easier to just start over on understanding lift and (as the cliché goes) forget everything you think you know about it.

https://ocw.mit.edu/courses/16-687-private-pilot-ground-school-january-iap-2019/resources/lecture-2-airplane-aerodynamics/

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/aerodynamic-forces/

10
explainlikeimfive-ModTeamMOD•

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2
FuckDaQueenSloot•

I remember learning that when I was a kid too, but it's not really an accurate explanation. An airfoil that has upper and lower surfaces of equal length still produces lift (with an appropriate angle of attack), but the stereotypical airfoil design is more efficient.

Wings generate lift (or downforce) by creating a pressure differential between the two sides. The air on the low pressure side is moving faster than the air on the high pressure side, but it's not so much the air on the low pressure side speeding up as it is the air on the high pressure side slowing down. The air hits the surface of the wing, pushes against it and is deflected downward (or upward for something like an F1 car). This force creates a high pressure zone (pressure = force / area) relative to the opposite side of the wing where the air is not pushing against the wing, but traveling smoothly along its surface until it reaches the other end.

It's a common belief that the air has to travel across both sides of the wing in an equal amount of time. Since the low pressure side of the wing is usually longer than the high pressure side, that means the air has to be traveling faster on the low pressure side than it is on the high pressure side. People assume that the air on the low pressure side speeds up, resulting in lower pressure, and since that's the basic idea of Bernoulli's principle, that's the explanation that gets tossed around. However, air pressure doesn't drop just because it's moving. Air blown from your mouth is moving but it's the same atmospheric pressure as all the air around it. Bernoulli's principle also only applies within a single flow field. Once the airflow is split between the two sides of the wing, you're looking at two separate flow fields.

Think of air pushing against the wing in the same way that a jet of water from a hose can hit an empty soda can and push it. At high enough speeds the air pushes against the bottom side of an airplane wing hard enough to push the entire aircraft into the air.

5
X7123M3-256•

Bernoulli's principle also only applies within a single flow field. Once the airflow is split between the two sides of the wing, you're looking at two separate flow fields.

Strictly speaking, Bernoulli's principle applies only along a streamline - but since the air in the far field is all moving with the same velocity, Bernoulli's principle in this case can be applied to the whole flow field both above and below the wing. Indeed, Bernoulli's principle is the answer to OP's question - the air is flowing faster on the upper surface because there's lower pressure there.

Bernoulli's principle is just conservation of energy as it applies fluids. If you roll a ball down a (frictionless) hill, you can write a nearly identical law relating the height of the ball to its speed. It'll speed up when rolling downhill and slow down when rolling uphill. People get it backwards and thing that making the air move faster causes the pressure to drop, when it's really the other way around.

Bernoulli's principle in fact does apply to blowing air out of your mouth - the pressure inside your mouth is higher than atmospheric pressure, as the air flows out of your mouth, the pressure drops, and you can use Bernoulli's principle to calculate the resulting velocity with which the air will leave your mouth.

However, once the air leaves your mouth, it mixes with the surrounding air and the flow becomes turbulent. Then Bernoulli's principle no longer applies - the kinetic energy is lost to heat, not converted back into pressure.

Bernoulli's principle is exact only for an inviscid fluid. Calculating the lift on a wing using Bernoulli's principle is a valid approximation for small angles of attack, but ignores the effect of the boundary layer close to the wing where friction is important, and cannot predict stall.

2
MansfromDaVinci•

Normally the air isn't moving over the wing the wing is moving through the air, showing it the other way around is a convention because it's easier to represent and test that way.

The air molecules don't have to 'get back next to the air molecule that took the bottom path'. Why would they? They are air molecules, they don't have friends.

The air above the wing is stretched and so lowers in pressure, the air below stays the same, but most importantly the wing deflects air downwards which by Newton's third law pushes the wing up, thus lift, it is also forced to accelerate in the direction of travel of the wing, thus drag.

If you were to look at the air after a wing passed through it on a windless day it would be flowing down and a little in the direction of the wing's travel because that's the way the wing pushes it. It would also be developing turbulent vortices because that's what fluids do when you push them around.

you could create lift with just a flat piece of metal going through the air at an angle but a curved wing is much more stable and creates much less drag and way less turbulence.

3
Unlikely-Star4213•

The air molecules don't have to 'get back next to the air molecule that took the bottom path'. Why would they? They are air molecules, they don't have friends.

This is just what I was going to say, but he's right that I've always heard it explained that way as if it made total sense.

1
amatulic•

Air speeds up because the flow over the wing is being constricted. Any fluid speeds up through a constriction.

A venturi tube works this way. The flow is constricted and the air flows faster through the narrow region to maintain the volumetric flow rate of all the air behind it. And in that constriction, pressure is reduced, allowing fuel to be sucked in through a small opening in the side of the constriction.

In the case of a wing, the constriction is formed by the airfoil on one side and the atmosphere on the other side.

-1
pulp_hateful•

normal airflow just goes whoosh but when it hits an aircraft’s wings it makes it go WOOSH WOOSH creating lift

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