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I am in the process of designing a small wing (Re ~ 50-100k) and was planning to "un-twist" the tip of the foil by -5°. This corresponds to the 5° aoa where the center part of the wing has the highest Cl/Cd.

I am aware that wash-out is mostly about keeping the wing tips out of stall longer than the rest of the wing, but intuitively having the wing tips at 0° aoa during normal operations should reduce the amount of "redirected air" at the wing tips. This in turn should reduce the strength of the wing tip vortices, which in turn should reduce drag?

edit for clarification: I intent to have the tip of the wing be at 0° aoa with an airfoil that does not generate any lift during normal working conditions.

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  • $\begingroup$ I've seen wingtips create downward lift due to excessive washout-- that did not reduce drag! $\endgroup$ Oct 11 at 14:41
  • $\begingroup$ @quietflyer I intent to keep the tip of the wing "perfectly" aligned at 0° aoa during normal operation. If that works out there should be no downward lift generated there. $\endgroup$
    – fho
    Oct 11 at 14:47
  • $\begingroup$ @fho - If they produce no lift then they are dead weight. Why extend the wing if it’s not going to do anything? $\endgroup$
    – Jim
    Oct 16 at 1:50
  • $\begingroup$ @Jim I was under the impression that reducing the lift along the wing span would have a similar effect as winglets. Both should reduce the impact of wing tip vortices. $\endgroup$
    – fho
    Oct 17 at 16:59
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The wake vortex is the shear between the air redirected downward by the wing—which balances the momentum given to the aircraft to counter gravity and keep it flying—and the unaffected air outside the span. By not producing lift near the tips you are effectively sacrificing some of your span, which means you need stronger downwash in the middle and that increases the drag, not decreases.

The most efficient configuration (for given span) is when the lift distribution is elliptical. The lift coefficient reduces towards the tips, but only gradually and never goes down to zero. There are many explanations for that around the site already. Try starting e.g. with For the elliptical wing, which property is actually elliptically distributed? (Peter Kämpf's answers are fairly well linked together so you'll find further references there).

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    $\begingroup$ Note that if you're concerned about stall, you should avoid a pure elliptical wing: when the wing stalls, the whole wing stalls at once. $\endgroup$
    – Mark
    Oct 11 at 23:09

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