Why does a rectangular wing stall first at the root? as opposed to tapering of the wing which causes the stall at the tip and moving inboard?

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The tip vortex (caused by higher pressure air from below the tip moving into the low pressure region on top by rolling around the tip) reduces the effective angle of attack at the tip and keeps the boundary layer attached longer as the stall progresses. The tip vortex has this effect with any wing shape, but shapes that reduce drag by shrinking the tip vortex will gain less from it, while a wing with less chord will tend to stall at lower AoA -- causing highly tapered wings to stall at the tip first.

I say "gain" because a tip stall is usually considered bad -- it can cause a spin entry, where a root stall with the tips still flying leads to a straight-ahead stall, other factors equal.

  • $\begingroup$ I also think washout is related. $\endgroup$ – ymb1 Sep 19 '18 at 13:42
  • $\begingroup$ @ymb1 Washout has the same effect and purpose -- and works on wings with substantial taper or sweep. A "plank" style wing, zero taper and zero sweep, doesn't really need it (or needs a good bit less). $\endgroup$ – Zeiss Ikon Sep 19 '18 at 13:47
  • $\begingroup$ A rectangular wing is a cheap way to get washout. Twisting the wing so that the tip has lower AoA is difficult to manufacture and costs more. A rectangular wing gets washout by using the vortex however there is the cost of more induced drag. If you want your plane to be spin resistant you need washout. $\endgroup$ – DLH Sep 19 '18 at 16:45

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