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I know that a thicker airfoil at root will strengthen the root spar but what is the reason behind using thinner airfoils at the tip?

Is it because thinner airfoils have higher stalling angles of attack? If so, why?

Comparing both thick and thin airfoils, who generates more lift and why?

Also, my final question is that in what kind of designs would such a change of airfoils be recommended? I'm currently working on designing a high lift wing (selig s1223 airfoil) for delivering a payload of about 15kgs. with the total aircraft weighing about 20-22kgs. Would changing the airfoils in my wing design be recommended?

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  • $\begingroup$ Thicker airfoils delay stalls - not thinner. The thicker the leading edge radius the higher angle of attack before stall. $\endgroup$
    – jwzumwalt
    Commented Jul 4, 2018 at 1:43

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There are two main reasons for thinning the wing towards the tips:

  1. Thinner airfoil sections cause less drag simply because they disturb airflow less.
  2. Thinner sections down to 10% have a higher maximum lift coefficient, which helps both near stall and during maneuvering: A section with a higher maximum lift coefficient will show more aileron effectiveness, too.
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You hit it -- the thinner airfoil has the same effect as the wing twist known as "wash-out", that of delaying the stall at the tip. This is generally desired, because a tip stall tends to initiate a spin, and a spin at low altitude has a high fatality rate (if only for the airplane, in the example you give), where a staight-ahead stall leaves some slight chance to recover.

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  • $\begingroup$ It is much better and more effective to use more area at the tips in order to delay stall there. The best designs add tip sweep, too. $\endgroup$
    – Peter Kämpf
    Commented Jul 3, 2018 at 22:20

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