Does wing really "feels" effective airflow or it is just theory to fits numbers with experimental results?

Question

Theory predict downwash that will cause reduction in airflow angle,this we call effective airflow. 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?

quoted Douglas Mclean book:

"Wing is flying through air that is already moving generally downward between wingtips.Thus the wing can be thought of as flying downdraft, or downwash, of its own making. 3D downwash can thus be seen as downward shift in apparent angle of attack of each airfoil section along wing,often called induced angle of attack ".

Question is refer to wing that dont have wing infront of it, do not analyze wing that is behind onther wing, for example tail wing that has main wing infront it, or main wing that has canard infront it.

Quote from SOURCE: "The vortex sheet also induces a smaller downward velocity in front of the airfoil and a larger one behind the airfoil (Figure 14.23)."

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.

Answer 1

Yes, velocity can be summed up just like any other vector and just like the example with the boat shows.

Anyway here there's both a misunderstanding in terms and kind of a "philosophical" question. I'll try to explain both.

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As I have already answered here to another question of yours, downwash is just ¼ of the whole process of generating lift on an airfoil. When approaching an airfoil air:

1. goes up in front of it (upwash);
2. accelerates on the upper part;
3. goes down behind it (downwash);
4. and decelerates under it.

So, if a wing somehow reingests its own downwash then this has definitely an effect on the whole process of generating lift.

But how can it happen that a wing reingests its own downwash? Easy if the wing is a rotating wing i.e. a blade!

As soon as the blade makes a complete round, it bumps into its own downwash. And if there is another blade in front of it then it just bumps into that downwash as well. Since the downwash goes... well down, than its effect is reducing the AoA of the blade.

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You quoted Douglas Mclean: "wing is flying through air that is already moving generally downward between wingtips. Thus the wing can be thought of as flying downdraft, or downwash, of its own making".

This is another effect which, unfortunately enough, has got the same name (downwash) of the effect at my previous point 3.

Again, as already explained in the second part of my answer here, when we go from an infinite wing (aka airfoil) to a finite wing, lift forcefully changes spanwise due to the fact that the wing now ends and therefore lift must go to zero towards the tips (one mm before the end of the wing we have lift, one mm outside the wing we have no lift). By a theoretical point of view this spanwise lift change generates a vortex sheet behind the wing which is responsible for the generation of an aerodynamic force parallel to the freestream termed induced drag. This vortex sheet is also called downwash even if it has nothing to do with the downwash at the previous point 3.

Now, can we see this theoretical effect on a wing? Yes. There's a lot of pictures from wind tunnel experiments showing how airflow bends on the surface of a wing due to this fact. This bending of the airflow is quite complex but it can be reproduced with a high fidelity by any modern Computed Fluid Dynamics (CFD) code.

But how could be this effect incorporated in the design of a wing back in the '50s when there was no computer around? It could be obviously done only in a quite simplified and practical manner supposing that locally the AoA seen by the airfoil is diminished aka the lift is tilted back (or any other explanation like that).

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Now the philosophical question: why do we use this practical explanation if it doesn't really match with the formal reality? Well partially because, as said, maybe that's the only way to possibly do things (no computer around). But that's not the only reason and an example can clarify this: an object with a mass $m_{object}$ is attracted by earth due to its gravity $g=9.81$ (SI system) with a force of $F=g \cdot m_{object}$, right?

Nope!

An object with a mass $m_{object}$ and earth attract each other with an equal and opposite force of magnitude $F=G\frac{m_{object}m_{earth}}{distance}$.

Does anybody actually use this formally correct equation to calculate the force of gravity? Or does everybody use the more practical $g=9.81m/s²$? I bet almost nobody even remember the value of $G$ or the radius of earth, not even at NASA :)

Answer 2

Wing dont feels effective airflow from diagram.

Indeed vane AoA indicator will show lower AoA at wing then airfoil. As "airfoil" we can use airfoil section from wall to wall in big wind tunnel, that is the closest approximation of 2D airfoil by definition.

But reason for reduction in AoA is not math that tell some voritces behind the wing induce vertical velocity somewhere in the field,because vorticity cant induced velocity, fluid dynamics is not electromagnetism.This is explain in this VIDEO, listen from 28:25-37:00.

Wing has stronger downward turning immediately above and below then airfoil. Reason is more rapid dying off the pressure above and below the wing means stronger vertical pressure gradient then airfoil. More rapid downward turning ,resulting in larger downwash by the time trailing edge is reached,is also consistent with the fact that wing requires higher AoA to achieve same lift. The wing pressure field dies out more rapidly ahead of wing, which resluts in lower upwash. Wing has also pressure gradient in spanwise direction.

Cause and effect is two-way street and that velocity changes,or accelaratio of the flow are both cause by pressure gradients and also serve to the pressure gradients.

Vane AoA indicator at wing will feel and show this reduction in upwash angle.

Below is diagram of pressure distribution for a) airfoil and b) wing, upstream, at airfoil/wing, downstream. Red arrow is downwash or vertical pressure gradient above wing. Picture source: D.Mclean Understanding Aerodynamics

Conclusion:

AoA at wing is reduced, but reason for reduction in AoA is not what theory tells. Reason is results of 3D flow freedom to accelarate spanwise,"tip effects" pressure equalisation around tips,something that dont exist in airfoil.

The wing (airfoil in the picture) is the cause of what happened to the airflow ahead, above, below and behind the wing. Upwash ahead of wing is indicator how pressure changes can travel upstream for subsonic speeds. Once when airflow leave the trailing edge it has no more influence on the wing, behavior of that airflow in wake is caused by the wing.

Answer 3

Along with the excellent answers it may also be important to consider air behavior as a compressible gas.

We already have evidence that ground effect reduces drag by pushing tip vorticies away from the wing tips.

It is then easy to understand how this "cushion effect" also affects leading and trailing edge flow patterns.

These are not mathematical abstractions. Airflow pattern in ground effect information is readily available from wind tunnel experiments and can be described as "moving more parallel to the ground"$^1$, hence less upwash in front increases effective AoA. There is also less downwash behind the wing.

The result is that proportionally to total lift created, the upper wing is doing less lifting (creating negative surface pressure) while the lower wing and the ground are doing more.

$^1$ reference part I, paragraph 2

Answer 4

The wing only Feels, the impact of individual fluid particles whose velocity, relative to the airfoil, causes them to impact the surface of the airfoil, and, in bouncing off, transfer some small amount of momentum from the particle to the wing and vice versa. This change in Momentum is an aerodynamic force. All other descriptions or definitions of any aerodynamic force, or portion or component of any aerodynamic force, are just abstract mathematical constructs defined and utilized to serve some specific function in a way that simplifies or eases the calculations or explanations for some specific higher-level observable phenomenon.

Your diagram and discussion of the sailboat is misleading. The "windex at the top of mast", that always indicates this apparent wind, is not indicating the forces that the sail feels, it is only indicating the wind that the Windex feels. The fact that this might be the same as the aggregate vector sum of all the tiny winds that each point on the sail feels, (and I'm not sure that it is), does not mean that the Windex is actually sensing all those individual tiny forces on all the points on the sail. It is only sensing/measuring the wind at the top of the mast. (that might also be the wind at the leading edge of the sail), but it is certainly not necessarily the wind hitting the middle of the sail, or the back trailing edge of the sail.