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While I was studying my PPL today, I've encountered a question right below and I wasn't really satisfied with the answer.

Which statement about the airflow around an aerofoil is correct if the angle of attack increases?

  • The stagnation point moves up
  • The stagnation point moves down (correct answer)
  • The center of pressure moves down
  • The center of pressure moves up

enter image description here

Up here, α is the degree between the chord line and the oncoming airflow.

enter image description here

1) Stagnation point

2) Transition point

3) Transition point

4) Separation point

So, knowing all of these facts I just can't understand the question above. Why is the stagnation point moves down instead of going up while increasing the angle of attack?

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  • $\begingroup$ If camber goes up? Camber is the curvature of the wing. It goes up when you extend slats or flaps. Angle of attack is the α in the first picture, and goes up by simply rotating the wing upwards. $\endgroup$
    – Jan Hudec
    May 24, 2019 at 23:09
  • $\begingroup$ That's what I said in a simple way or whatever I'll just edit that out thanks for the notification. $\endgroup$
    – FullCircle
    May 24, 2019 at 23:22

2 Answers 2

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From this site:

enter image description here

Notice that at low AoA the stagnation point is smack on the nose of the wing profile, while at higher AoA it is situated on the lower wing surface.

Hence: stagnation point location lowers with increasing AoA.

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Leading-edge stagnation point moving down with increasing angle of attack is true for inviscid flows, however the same does not always hold for viscous (or more realistic) flows, especially around stall. For a given airfoil, if angle of attack increases, the leading-edge stagnation point could move up or down from the leading-edge depending on the flow state on the airfoil upper and lower surface.

As an empirical example, Gregory and O’Reilly mapped the pressure distribution around the leading-edge of a NACA 0012 airfoil for a wide range of angles through stall and captured the motion of the leading edge stagnation point 1 with maximum pressure coefficient. Notice how the leading edge stagnation point moves back towards the leading edge after 17 degrees (after stall).

Stagnation point motion with angle of attack through stall

  1. N. Gregory and C. O’Reilly, “Low-Speed Aerodynamic Characteristics of NACA 0012 Aerofoil Section, including the Effects of Upper-Surface Roughness Simulating Hoar Frost,” R&M 3726, Aeronautical Research Council, 1970.
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