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Which graph is the correct representation of the trailing edge flaps' effect on the critical angle of attack? Does their extension increase (as depicted on the first graph) or decrease (as depicted on the second graph) the critical AoA?

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    $\begingroup$ @mins Increasing camber using TE flaps also increases the whole wings AoA. Which means critical AoA is reached earlier. Reverse example is extending only LE slats. Result is a reduced wings AoA, which means critical AoA increases. The fact that tips stall last when flaps are down comes from their small AoA, relative to increased AoA, flaps extended wing roots. $\endgroup$ – qq jkztd Sep 29 '17 at 21:59
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    $\begingroup$ @min Then, what about the effect of the leading edge flaps on the critical AoA? You said the increased camber decreases the critical AoA, but the second graph shows extension of the leading edge flap, which also increases the camber of the wing, increases the critical AoA. $\endgroup$ – lemonincider Sep 30 '17 at 14:58
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    $\begingroup$ @qqjkztd is right, the virtual chord line obtained with the extension of the devices changes the measured angle of attack $\endgroup$ – mins Sep 30 '17 at 17:54
  • $\begingroup$ I strongly suggest you read How it flies?. It provides insight about how airflow is modified by flaps. $\endgroup$ – Manu H Aug 25 at 16:45
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Both right and both wrong, both over simplifications based on unknown airfoils, and so on. Real aerodynamics is complicated and 3 dimensional.

Flaps usually increase the critical angle of attack a few degrees, but they also increase the effective angle of attack for a given pitch. So the net result is that the pitch of the aircraft will be similar at stall with or without trailing edge flaps, but the true AoA will increase.

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    $\begingroup$ Yes. But it's better not to involve pitch here: it can be confusing and is not directly relevant. Just AoA is sufficient. The convention for flapped wing is that AoA is always measured with the same reference as for the 'no flap' configuration. The 'effective AoA' concept is handy for explanation, but the end result is that the critical AoA may change either way. Typically it will reduce for simple flaps. $\endgroup$ – Zeus Aug 26 at 9:51

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