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I recently came across this question in a principles of flight exam:

When the angle of attack of an airfoil increases

  1. the high pressure below the airfoil increases, the induced drag increases
  2. the high pressure below the airfoil decreases
  3. the induced drag decreases
  4. can't remember this option

I chose 1. as the correct answer and it looks like I got it wrong. Although, I cannot tell for sure.

I know the induced drag will increase, but will the high pressure underneath the airfoil increase or decrease with an increase in angle of attack?

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    $\begingroup$ What makes you think you were wrong? $\endgroup$ – Robin Bennett Nov 26 '19 at 14:25
  • $\begingroup$ What is held constant? Airspeed, or lift force? Some the answers below make an assumption that airspeed is held constant and lift force is not, which may not be justified. But I suspect the answer would be (a) in either case. $\endgroup$ – quiet flyer Nov 26 '19 at 19:37
  • $\begingroup$ We need to find out what 4. was, 1. seems correct. 2. and 3. are incorrect. $\endgroup$ – Robert DiGiovanni Nov 27 '19 at 9:26
  • $\begingroup$ @RobinBennett when I got my report back, that question number was marked incorrect. The description on the report was "lift and airfoils" and it was one of only 2 Principles of Flight questions. $\endgroup$ – Skydemon Nov 27 '19 at 16:00
  • $\begingroup$ @quiet flyer - good point, if you wanted to continue in level flight with an increased AOA, you'd have to reduce speed. The pressure would stay the same, and I'm not sure what the induced drag would do... It seems like a big assumption to me though. $\endgroup$ – Robin Bennett Nov 27 '19 at 16:26
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Your answer is correct. The pressure distribution from the xfoil simulation on a NACA 0012 are shown below for AOA 0 and AOA 5. As you can see, the lower side pressure does increase. However, most of the lift is generated by the suction on the upper surface, which is typical of subsonic flight.

enter image description here

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    $\begingroup$ A third graph from 12 degrees AOA would be great. This may show the less efficient "bottom" lift, with its proportionally larger induced drag adding to the Coefficient of Lift (near stall). The polars of the NACA 0012 show max Clift to Cdrag at around 6-7 degrees AOA. $\endgroup$ – Robert DiGiovanni Nov 27 '19 at 9:32
  • $\begingroup$ Thanks for the explanation! $\endgroup$ – Skydemon Nov 27 '19 at 16:00
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I'm pretty sure you were right. More angle of attack means more lift. More lift is caused by higher pressure under the wing and lower pressure on top. More lift causes more induced drag.

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  • $\begingroup$ Yes, I agree there and that was my train of thought...there are many known mistakes in the exams database so I would not be surprised if it is a mistake. $\endgroup$ – Skydemon Nov 27 '19 at 15:58

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