I'm asking this because I tested a NACA 2412 wing with an adjustable flap in a subsonic wind tunnel and I'm getting the lowest Cl values for the highest flap deflection angle.. which to my knowledge is theoretically wrong.

The inlet velocity is 15m/s. What is worrying me is that for a flap angle of 0 degrees, my cl value is a maximum. For 22.5 degrees, it gets less, and then for 45 degrees, it is the least. So the relationship is basically the inverse.
Note: the wing flap deflection is just to change the deflection angle of the flap; the wing area remains constant.

I'm guessing this could be due to increased turbulent flow after a certain flap deflection angle. That angle is what I am trying to determine.

  • 5
    $\begingroup$ Have you tried smaller defections such as 5, 10, 15? Maybe it's stalling prematurely due to excrescence/interference. $\endgroup$
    – JZYL
    Commented Nov 15, 2019 at 18:55
  • $\begingroup$ I'll try that and see how it goes, I guess I should get more realistic values $\endgroup$ Commented Nov 15, 2019 at 19:01
  • 1
    $\begingroup$ Are you measuring actual lift increase or are you comparing lift per angle of deflection? Because at low angles you get more lift per degree of flap but for higher flap angles you get more total lift, up to a point where the flow separates. Adding a slot helps delay that separation. $\endgroup$
    – Jan
    Commented Nov 16, 2019 at 17:35
  • $\begingroup$ I think what you are seeing is that you can't get anywhere directly in flow dynamics. Our intuition as pilots is that "adding flaps = more lift" and yet, it's actually much more complicated than that. Adding flaps may move the CP (center of pressure) of the wing to a different arm from the CG, causing a change to angle of attack, causing X, which then causes Y, etc etc. Extending the flaps changed some things that changed some other things and those second and third-order effects resulted in a higher C(L) and more lift. Nothing can be examined in any kind of isolation. $\endgroup$
    – Max R
    Commented Aug 29, 2022 at 22:53

1 Answer 1


Flaps shift the lift curve up and to the left causing a different very different cL at similar angles of attack. You cannot expect your results to make sense if you do not account for the shift in POSITION of the lift curve AS WELL AS the shift in SLOPE. I was surprised to realize that most airfoils STALL at a LOWER angle of attack when FLAPPED!


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