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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.

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    $\begingroup$ Have you tried smaller defections such as 5, 10, 15? Maybe it's stalling prematurely due to excrescence/interference. $\endgroup$ – JZYL Nov 15 '19 at 18:55
  • $\begingroup$ I'll try that and see how it goes, I guess I should get more realistic values $\endgroup$ – Ali Al Kayyali Nov 15 '19 at 19:01
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    $\begingroup$ 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 Nov 16 '19 at 17:35

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