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I asked a similar question on this recently, (here), and the answer said this:

Downwash is not a force. It is a small increment in the velocity vector. Downwash can change the local angle of attack -- which can change the lift -- which is a force. However, A wing has a rolling moment because the force on the two sides is not equal. Period. Full Stop. If the lift force on the left wing is greater than on the right, the wing will roll to the right. It is that simple. We can talk about where the force imbalance comes from -- but you won't have a moment without a force imbalance.

This makes sense, but if the downwash changes the direction of the air above the wing, wouldn't that in a way push on the upper surface of the wing?

As seen here, the black arrow represents the velocity vector imparted on the flow because of the induced downwash (from tip vortices). Wouldn't that air then go on to 'hit' the wing, pushing it down? The answer above says downwash isn't a force, but wouldn't this count as a force from downwash? (apologies for the bad drawing of the arrow)

enter image description here

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NO.

First, this is a 2D image and represents a 2D flow. There are no tip vortices (trailing vortices) in a 2D flow. Instead, it is all about the bound vortex (at the quarter chord).

Notice the upwash in front of the airfoil. This is also because of the bound vortex.

The act of creating lift causes the streamline in front to come up -- and the streamlines behind to go down.

The upwash / downwash does not cause the force.

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  • $\begingroup$ I think I understand what you're saying. So the downward bending streamlines on the rear side of the airfoil isn't from the induced downwash, which makes sense. But would the induced downwash (on a 3d wing where there are tip vortices) create an extra 'bending' of those streamlines downward, which would intersect with the wing and push it down? See this photo to maybe see what I mean. (Sorry if I'm not picking up on something obvious) $\endgroup$
    – Wyatt
    Mar 27 at 19:56
  • $\begingroup$ You can't separate these effects the way you're trying. It all goes together. The downwash after the airfoil or wing is downwash, it is caused by the presence of the wing, it influences the flow on the wing. However, the solution (i.e. the forces and flowfield you see) is a coupled combination of these effects. You can't separate them out artificially. Everything effects everything -- and that has already been accounted for. $\endgroup$ Mar 27 at 20:05
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    $\begingroup$ Again, I emplore you. Stop using crappy internet resources. Read the engineering textbooks you have. $\endgroup$ Mar 27 at 20:06
  • $\begingroup$ Yeah that’s probably a good idea; anyone can post anything on the internet. Even if someone is trying to post useful resources, they themselves might be misinformed. $\endgroup$
    – Wyatt
    Mar 29 at 3:38
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It might help for you to draw an arrow up over the wing and down after the wing to represent the balance of forces involved with lift.

It is intuitively correct that the arrow hitting the top of the wing is bad. This can happen when angle of attack exceeds the critical stall limit.

It is important to understand the wing must have sufficient forward velocity to pass by a given parcel of air before the downwash hits the back of it, collapsing the low pressure area that creates lift.

Key to understanding this is the concept that air molecules have mass, therefor inertia. Get them moving up and a downward force can only return them to their original position in a given amount of time. By that time the wing has passed.

Now on to wing tip vorticies. There one would try to design a wing tip to keep the vortex as far away from the lift producing area of the wing as possible, so it's downward rotation cannot strike the top of the wing. Hoerner wingtips push the outflow from the bottom of the wing further away from the wingtip. Swept wings can use winglets to both minimize and move the vortex further away. This increases the lifting efficiency of the wing, allowing for fuel saving lowering of AoA to create the same amount of lift.

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