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How does the downwash behind a swept wing differ from a normal wing?

(I know you can’t trust everything you see on the internet, but this seemed legitimate)

Here it said:

The downwash pattern on a swept wing tends to increase the AoA towards the wingtip. We normally think of downwash as being caused by wingtip vortices, but really, we can visualize the entire wing as being constructed of an infinite number of vortices that get rolled up into one big one behind the airplane, so there is downwash created all along the wing. When you add the effect of all these vortices together, you get a different lift pattern along the wing than you do with a straight wing.

How does the downwash pattern differ for a swept wing? What along the wing does this?

Basically, why does a swept wing have different downwash amounts at different points, while a straight wing doesn’t?

Also, the same source said that this varying downwash makes the swept wing tip stall.


The only reason that I can think of is because the wing gets thinner as you get to the tip, so the downwash decreases.

Actually, this isn’t probably true. The downwash behind a wing is due to the wing making lift. Another way to put it is that the lift changes the downwash amount, not that the downwash amount changes the lift.

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We often approximate a wing with a bunch of horseshoe vortices located at the quarter chord. In the most simple approximation, we only use one such vortex.

Think of the trailing vortices as a whole bunch of vortex segments that lead off to infinity. Each one is just a short segment.

That vortex segment has strongest influence perpendicular to itself. Its influence drops off greatly before and after the segment.

When you apply these horseshoe vortices to a straight wing, the trailing vortices all start at the same location and all trail off together. So, their influence on the wing is all very similar.

When you apply these vortices to a swept wing, the inboard trailing vortices start first. They are in-line with the wingtips and have greater influence on the lift of the wing. The vortices that trail from the wing tips start later and do not have the same alignment with the center of the wing in order to have an effect.

This causes a swept wing to have a substantially different lift distribution than a straight wing.

Usually a swept wing will be twisted to adjust the lift distribution to avoid any serious problems with stall etc.

Edit: Add image of swept wing horseshoe vortex system.

enter image description here

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  • $\begingroup$ I see. So what effect does the bound vortex have? The bound vortex doesn’t actually exist if I’m correct. In other words, what flow changes direction because of the bound vortex? $\endgroup$
    – Wyatt
    Commented Jun 12 at 18:37
  • $\begingroup$ (From what I remember it's just the circulation around the wing, but I could be wrong) $\endgroup$
    – Wyatt
    Commented Jun 12 at 18:53
  • $\begingroup$ Everything effects everything. Yes the bound vortex is a primary driver of the downwash, but the bound vortex does not have uniform strength. The strength of the bound vortex is determined by the solution to the whole flow field. (This is a case where my answer is leaning heavily on the mathematical abstractions we would use to model this wing -- it is a case where you solving a homework problem in a textbook would go a long way). $\endgroup$ Commented Jun 12 at 20:13
  • $\begingroup$ I think I understand it a bit more now. So if the trailing vortices of a horseshoe vortex system are caused by the circulation around the wing, why would there be a trailing vortex near the wing root? There is no tip circulation or voticity there, right? $\endgroup$
    – Wyatt
    Commented Jun 12 at 21:13
  • $\begingroup$ Vorticity is shed anywhere lift is produced. So there certainly is vorticity at the root. I've added an image above -- the image isn't great, the 3D rotation provides a strange perspective. Each horseshoe vortex has three legs, the bound leg and two trailing legs. All three legs of a given vortex have the same strength. Where there are two adjacent trailing vortices, they have opposite direction and they partially cancel -- which goes to show that the local wake strength is related to the change in circulation across the wing. $\endgroup$ Commented Jun 12 at 21:21

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