# Does a wing really "feel" the effective airflow or it is just theory to fits numbers with experimental results?

• Jurgen M
Stall angle at finite wing is delayed because adverse pressure gradient is lower, it is lower because of spanwise flow/pressure loss.

#### Jurgen M

Question is indeed very short and clear, but answer will eliminate many doubts and misleading conclusions.

Theory predict downwash that will cause reduction in airflow angle,this we call effective airflow(airflow with lower angle compare at geometric AoA). Does wing of aircraft during flight really "feels" this effective airflow or this is just mathematical manipulation of reality, the way how fit numbers with experimental results?

I will take example with sailing, vectors sum of true wind and boat speed gives apparent wind. This apparent wind sail really "feels", it is not just theory, indeed windex at the top of mast will allways indicated this apparent wind.

Last edited by a moderator:
There really is a difference that is apparent to the wing. Your example of sailing is not a good one. That is a different subject of combining the vehicle velocity with the wind velocity. This subject is how the wing actually changes the airflow immediately ahead of the wing. It does.

russ_watters and Lnewqban
The moving wing induces a perturbation to the mass of air in repose that extends all around well beyond the geometric limits of it.
The magnitude of that perturbation is proportional to the magnitude of the lift force and associate drag, within a range of useful lift.

russ_watters
FactChecker said:
There really is a difference that is apparent to the wing. Your example of sailing is not a good one. That is a different subject of combining the vehicle velocity with the wind velocity. This subject is how the wing actually changes the airflow immediately ahead of the wing. It does.
In both case we sum of two vectors, in sailing true wind+ boat speed=apparent wind that "hit" the sail
at wing freestream airflow +induced airflow caused by downwash= effective airflow. that "hit" the wing
But never mind..

So you think this effective airflow really happend at real wing during flight like this circulation theory predict? It is not just abstract mathematical constructs which use to explain reduction in lift in 3D cases?
How can we prove this with experiment?

Last edited by a moderator:
Yes, there really is an additional velocity component that reduces the effective angle of attack of each wing section, and it's dependent on the wing shape and aspect ratio. One fairly easy way to show this is that not only is the lift slope of the wing reduced for a low aspect wing, but the stall is also delayed, which you would only expect if the wing is genuinely experiencing a lower actual angle of attack. You can also use pressure taps around a model or aircraft and measure where the stagnation point is, which also demonstrates the reduction in AoA.

russ_watters and Lnewqban
cjl said:
Yes, there really is an additional velocity component that reduces the effective angle of attack of each wing section, and it's dependent on the wing shape and aspect ratio. One fairly easy way to show this is that not only is the lift slope of the wing reduced for a low aspect wing, but the stall is also delayed, which you would only expect if the wing is genuinely experiencing a lower actual angle of attack. You can also use pressure taps around a model or aircraft and measure where the stagnation point is, which also demonstrates the reduction in AoA.
I think you are wrong,indeed there is reduction in upwash angle that hit the wing, but not because of "induced vertical velocity", it is because of pressure loss that happend just because wing has end.

Finite wing has pressure loss in spanwise direction,this pressure gradient cause spanwise flow. Reduction in low pressure above the leading edge reduce upwash angle.

Stall angle at finite wing is delayed because adverse pressure gradient is lower, it is lower because of spanwise flow/pressure loss.

Induced drag theory become from equations from electromagentism, but there is nothing physical in it, vorticity in the wing wake cant induce velocity somewhere else,(in our case infront of wing)...

Listen carefully from 28:25 - 37:00

Last edited by a moderator:
Jurgen M said:
I think you are wrong,indeed there is reduction in upwash angle that hit the wing, but not because of "induced vertical velocity", it is because of pressure loss that happend just because wing has end.

The induced downwash ahead of the wing is absolutely tied to the vorticity at the wingtips (though it's all part of a bunch of simultaneous physical principles interacting, so pointing causality just in one direction is hard to do), and the magnitude of it is exactly what we'd expect based on that formulation.

Jurgen M said:
Finite wing has pressure loss in spanwise direction,this pressure gradient cause spanwise flow. Reduction in low pressure above the leading edge reduce upwash angle.

Yes, there is a minor spanwise loss, but the majority of the pressure increase on the upper surface of a finite wing vs a 2d section is indeed because of the induced downward velocity along the wing caused by the tip vortices (well, "caused" is doing a lot of work here, it's more that there are a lot of intertwined effects that all happen because of the same overall flow physics and mathematical principles).

Jurgen M said:
Stall angle at finite wing is delayed because adverse pressure gradient is lower, it is lower because of spanwise flow/pressure loss.

Stall angle is delayed because the wing section is experiencing a locally lower angle of attack. You can see this because wing fences don't eliminate this effect.

Jurgen M said:
Induced drag theory become from equations from electromagentism, but there is nothing physical in it, vorticity in the wing wake cant induce velocity somewhere else,(in our case infront of wing)...

No, the fact that equations for flow fields have some similarities to electromagnetic field equations is just because both are continuous vector fields that follow physical laws, it's not because of trying to shoehorn in some imaginary electromagnetism here. In addition, yes, every part of the flow field absolutely can affect every other part of the flow field. That's literally the key difference between the study of subsonic and supersonic flows - in subsonic flows, disturbances can propagate both up and downstream because the disturbances can travel faster than the flow can. In supersonic flows, disturbances can only travel downstream, so nothing ahead of an object can be affected by the object.

Jurgen M said:
Listen carefully from 28:25 - 37:00

Yes, listen carefully to him around 31 minutes. Note that he shows the flowfield at a cross plane cut through the wing, and it *clearly shows* downwash between the wingtips. He doesn't disagree with me, you just don't understand what he's saying. He's not saying that the underlying math is wrong, he's saying that it's easy to have misconceptions if you don't understand how the underlying flow all works together.

Here's an exact quote from him at 32:20:

"This downwash across the span of the wing of course results in a tilting back of the lift vector, and results in what we call drag due to lift"

That's not disagreeing with me.

I honestly think he goes a bit too far in his debunking, but the core idea he has (and he's absolutely right) is that just trying to eliminate that compact vortex core at the tip is really not going to do squat to eliminate the downwash across the wing, and so there are certainly a lot of misconceptions people have, but that doesn't change that the vortices and associated downwash absolutely exist (and as I said, you can get in a bit of a quibble about what "causes" what, since it's all tied together in a bunch of simultaneous PDEs).

phinds, russ_watters and berkeman
I am confused about the question. I thought that the airflow in a wind tunnel showed that the AOA actually increased due to an upward flow just ahead of the wing. Everyone here is agreeing that it decreases. Can you clarify it for me?

FactChecker said:
I am confused about the question. I thought that the airflow in a wind tunnel showed that the AOA actually increased due to an upward flow just ahead of the wing. Everyone here is agreeing that it decreases. Can you clarify it for me?
View attachment 318422
You are correct ,downwash dont exist ahead of wing/airfoil, ahead of airfoil is always upwash.
Downwash is above(from tangent to surface is horizontal downstream),under and far long behind the wing
Downwash is above(from tangent to surface is horizontal downstream) ,under and very short behind the airfoil.
Upwash is always ahead wing or airfoil.

Also downwash is stronger above/under the wing compare to airfoil.

But wing has lower upwash angle compare to airofil, but not because some vortices behind the wing induce vertical velocity ahead of wing,like integral shows.

Doug Mclean explain in video this is physicaly impossible in aerodynamics.

Last edited by a moderator:
cjl said:
The induced downwash ahead of the wing is absolutely tied to the vorticity at the wingtips
Downwash ahead of wing 3D or airfoil 2D dont exist, ahead is always upwash.

Downwash is above(from tangent to surface is horizontal downstream),under and far long behind the wing
Downwash is above(from tangent to surface is horizontal downstream),under and very very short behind the airfoil.
Upwash is always ahead wing or airfoil.

Also downwash is stronger above/under the wing compare to airfoil.

But wing has lower upwash angle compare to airofil, but not because some vortices behind the wing induce vertical velocity ahead of wing,like integral shows.

https://aviation.stackexchange.com/...better-than-an-equal-span-extension/8579#8579

https://aviation.stackexchange.com/...winglets-to-counteract-tip-vortex/23485#23485

cjl said:
Stall angle is delayed because the wing section is experiencing a locally lower angle of attack. You can see this because wing fences don't eliminate this effect.
Local upwash angle is reduced, but not because some integral tell that some voritces behind the wing induced velocity ahead of wing.

Real physical reason is more rapid dying off pressure above and below the wing, means vertical pressure gradient stronger at wing then on airfoil. So downwash is stronger at the wing.

Reason for all this is 3D flow that dont exist in airfoil..
Wing has lower adverse pressure gradient, so it stall at higher geometrical AoA compare to airfoil.

I never said downwash dont exsit, I just said downwash dont exist ahead of wing and vortitces behind the wing cant induce vertical velocity infront of wing. "wet road dont cause rain"

Last edited by a moderator:
FactChecker said:
I am confused about the question. I thought that the airflow in a wind tunnel showed that the AOA actually increased due to an upward flow just ahead of the wing. Everyone here is agreeing that it decreases. Can you clarify it for me?
There is upwash ahead of the wing, but "angle of attack" is usually referencing the freestream so it's just considered part of the way the wing itself is affecting the flow.

What's being discussed here is specifically 3d flow behavior, and how, compared to a 2d airfoil section in a wind tunnel, you end up with a bit of downwash when looking at an inboard section of an actual 3d wing, and that effectively tilts the entire flow (as pictured in your image of streamlines) slightly downwards.

Please don't listen to Jurgen's posts above, as they clearly do not have an actual education in fluids and the flow around wings.

russ_watters and FactChecker
Jurgen M said:
Downwash ahead of wing 3D or airfoil 2D dont exist, ahead is always upwash.

No, once again (and your own video agrees with me), a real 3d wing experiences some downwash, and the shorter the aspect ratio, the more intense this is.

Jurgen M said:
Downwash is above(from tangent to surface is horizontal downstream),under and far long behind the wing
Downwash is above(from tangent to surface is horizontal downstream),under and very very short behind the airfoil.
Upwash is always ahead wing or airfoil.

Downwash happens both behind and ahead of the wing, depending on aspect ratio, wing loading, geometry, etc. There is also upwash directly in front of the wing, but these two phenomena are relatively disconnected from each other and arise from different origins.

Jurgen M said:
But wing has lower upwash angle compare to airofil, but not because some vortices behind the wing induce vertical velocity ahead of wing,like integral shows.

The wing has a lower upwash angle, and possibly even local downwash because it has a downwash superimposed over the basic 2-d flow.

Jurgen M said:

I don't have time to go over an entire stackexchange post right now, but looking very briefly at it, my suspicion is that you're misinterpreting it the same way you have been here, since they appear to know what they're talking about. Maybe I'll look in more detail later though.

Jurgen M said:
Local upwash angle is reduced, but not because some integral tell that some voritces behind the wing induced velocity ahead of wing.

The vortices aren't behind the wing, they affect the entire flow field. You seem not to understand that in a subsonic flow, anything happening at any point in the flow affects the entire flow, not just things behind it.

Jurgen M said:
Real physical reason is more rapid dying off pressure above and below the wing, means vertical pressure gradient stronger at wing then on airfoil. So downwash is stronger at the wing.

There is no one "real physical reason" for almost anything in subsonic flows, since it's highly complex and interconnected, but if you had to point to something, the existence of the tip vortices is certainly a better explanation than your handwaving here.

Jurgen M said:
Reason for all this is 3D flow that dont exist in airfoil..
Wing has lower adverse pressure gradient, so it stall at higher geometrical AoA compare to airfoil.

And it has the lower adverse pressure gradient because it's experiencing a lower local angle of attack, due to induced downwash.

Jurgen M said:
I never said downwash dont exsit, I just said downwash dont exist ahead of wing and vortitces behind the wing cant induce vertical velocity infront of wing. "wet road dont cause rain"

And that just makes you wrong. Once again, to sound like a broken record here, in a subsonic flow, the entire flowfield can be affected by something happening at any point in the flow.

russ_watters
cjl said:
Please don't listen to Jurgen's posts above, as they clearly do not have an actual education in fluids and the flow around wings.
I talk from book of Aerospace Engineering, Aerospace University and some experts is aerodynamics.

Doug Mclean- book Understading Aerodynamics, quote:
"A linerized version of incrompressible inviscid sometime is useful for illustrating trend and providing insight into behavior of 3D wing,though is suffers a significant loss in physical fidelity."

Quote from PeterKampf:
"Now for the theory that downwash reduces lift. This is true for the downwash of a wing flying ahead of the wing concerned (as in case of a horizontal tail) but not for the main wing of a standard configuration airplane. What happens past the wing is a consequence of the flow conditions ahead of and over the wing, not vice versa.

What is probably meant by such a theory is that a reduced aspect ratio reduces the vorticity when flow is described as potential flow."

https://aviation.stackexchange.com/...ct-the-stalling-angle-of-the-wing/80615#80615

3D flow freedom, tip-effects pressure equalisation,pressure loss, at tips is reason why wing has lower pressures fields then airfoil. Consequence of this reduced pressure field is lower local AoA at each wing section, not other way around.
Like D.Mclean state, vorticity cant induce somewhere upstream velocity, aerodynamics is not electromagnetism.

Why do you think Biot-Savart law is physical in aerodynamics?

@FactChecker

source: D.Mclean Understanding Aerodynamics

potential flow is abstract concept,it is not physical.

https://aviation.stackexchange.com/questions/21664/how-complete-is-our-understanding-of-lift

Last edited by a moderator:
weirdoguy