Say you have a wing, no sweep or anything to simplify things. One side of it, let's just say the left, magically doesn't have a tip vortex, therefore no induced downwash from the tip vortex. Would the wing (assuming the left side was still was making lift), roll to the right?

I assumed it would because the induced downwash on the right side of the wing would 'pull' the right wingtip down. Is that correct?


1 Answer 1


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 smple.

We can talk about where the force imbalance comes from -- but you won't have a moment without a force imbalance.

  • $\begingroup$ Ah okay, I think I see what you mean. The downwash changes the local AoA and therefore the wing with the induced downwash would be the down going wing in the roll? I guess I'm basically asking if the wing side with the tip vortex would still roll downward, knowing this logic you provided. $\endgroup$
    – Wyatt
    Commented Mar 23 at 18:05
  • $\begingroup$ Lets start with the time when the wing is not rolling, but with one wake magically eliminated. The wing that sees greater downwash effects (the right wing) would see a decrease in the local angle of attack. It would therefore make less lift. There would be a rolling moment to the right -- which would initiate a roll. $\endgroup$ Commented Mar 23 at 18:52
  • $\begingroup$ Ah, that's what I was thinking, thanks! $\endgroup$
    – Wyatt
    Commented Mar 23 at 19:34
  • $\begingroup$ Isn't the downwash (the flow of air deflected down) induced by the airfoil profile, as a consequence of air acceleration by the top side of the airfoil? $\endgroup$
    – mins
    Commented Mar 23 at 19:47
  • $\begingroup$ @mins We often model a wing as a horse shoe vortex. One bound vortex at the quarter-chord of the wing. Two trailing vortices representing the wake. All of these contribute to downwash. The bound vortex is what you're talking about -- it is equivalent to replacing a 2D airfoil with a vortex at the quarter chord. In 3D, the trailing vortices also contribute to the downwash -- they are what cause the lift to go to zero at wingtips, they are the driver of 3D effects. $\endgroup$ Commented Mar 23 at 19:51

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