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If I'm reading this post right: What is the effect of airfoil thickness on aerodynamic lift?

  • Airfoil thickness increases the lift coefficient.

And from here: What are the effects of the Boeing 787's very flexible wings?

  • Wing flex reduces wave drag.

I'm wondering about the how. My theory is that the air sees a thicker airfoil in a flexed wing, without actually building one. Is that assumption correct? Or is it related to the end-plate effect?

enter image description here
Front view of a flexed wing. Orange indicates that perceived increase in airfoil thickness.

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  • $\begingroup$ I'm having trouble with the air sees a thicker airfoil bit... $\endgroup$
    – fooot
    Commented May 5, 2017 at 17:26
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    $\begingroup$ Right but I'm having a hard time seeing why the direction is relevant. Lift will still be perpendicular to the wing, right? $\endgroup$
    – fooot
    Commented May 5, 2017 at 17:34
  • $\begingroup$ There's a whole 2D plane that's perpendicular to the airflow, you need another reference to get a vector. $\endgroup$
    – Steve
    Commented May 5, 2017 at 17:55
  • $\begingroup$ Right, but like Steve said air flow is one dimension, you have to have a reference plane to define something perpendicular. Seems like lift would be perpendicular to the surface that is generating it. $\endgroup$
    – fooot
    Commented May 5, 2017 at 17:56
  • $\begingroup$ But what's the rationale for taking the orange slice not perpendicular to the wing? $\endgroup$
    – fooot
    Commented May 5, 2017 at 18:27

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No, it is torsion which reduces wave drag.

The sweptback wing will twist as well as bend, and this twist will reduce the local angle of attack, and with it the suction peak on the top side of the wing. The same mechanism twists the outer part of a forward swept wing to higher angles of attack, necessitating a stiffer structure to avoid aeroelastic instability.

Air will not "see" any difference in wing thickness, regardless of the dihedral or bending. For air, there is no difference between a wing and a vertical surface (which would have almost infinite thickness following your assumption, but obviously doesn't).

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    $\begingroup$ Thanks, so the twist is TE up in the sweptback, and the opposite in forward-swept? $\endgroup$
    – user14897
    Commented May 5, 2017 at 19:47
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    $\begingroup$ @ymb1: Exactly right! To be precise, this is right when the wing bends upwards. $\endgroup$
    – Peter Kämpf
    Commented May 5, 2017 at 19:58
  • $\begingroup$ I am not understanding your statement that torsion reduces wave drag. Wave drag is a function of the longitudinal distribution of the total cross-sectional area. Are you saying that torsion changes that cross-sectional area? $\endgroup$
    – mongo
    Commented May 8, 2017 at 17:05
  • $\begingroup$ @mongo: No, that longitudinal distribution is only part of it. Transsonic wave drag is caused by shocks following low-pressure areas, and a higher angle of attack intensifies those low pressure areas on the top side of the wing. If that explanation is not sufficient, go ahead and ask. $\endgroup$
    – Peter Kämpf
    Commented May 8, 2017 at 18:07

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