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I read in the internet that when an airplane stands at the airport (horizontal) it has zero degree aoa. The captain must pitch up the plane during cruise to generate an amount of lift and when the airplane pitches the relative wind also changes, and this will affect the angle of attack. This is really confusing me

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    $\begingroup$ The angle of attack is not defined when the aircraft is stationary on the ground, since there is (virtually) no relative wind $\endgroup$ – J. Hougaard Jul 4 '17 at 14:41
  • $\begingroup$ Yes. No angle to attack. $\endgroup$ – Koyovis Jul 4 '17 at 14:59
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    $\begingroup$ what about angle of incidence ? $\endgroup$ – D. Frederico Jul 4 '17 at 15:06
  • $\begingroup$ Possible duplicate of What is the reason of having an Angle of Incidence on an airplane? $\endgroup$ – Sanchises Jul 4 '17 at 15:59
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    $\begingroup$ @Sanchises I don't think this is a duplicate. This is distinctly asking what the AoA is, not why there is an angle of incident. It is however a nicely related question. $\endgroup$ – Notts90 Jul 4 '17 at 16:36
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You're right to be confused. The AoA can be almost anything, depending on wind direction. Normally, a wing has some camber so the lift it creates in flight is possible with an AoA of 0°. Normally, the wing is mounted at an incidence of a few degrees so the fuselage remains horizontal in flight. With this incidence and horizontal wind, the AoA at rest is just this incidence angle. But if the aircraft experiences tailwind, the AoA can be near 180°. If heated air below the wing rises up as a thermal, the AoA can be momentarily around 90°. In almost all cases, flow speed is much lower than that required for flight, but there are exceptions.

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  • $\begingroup$ George Guelis says, you do need a positive angle of attack. A flat plate (or a barn door, for that matter), or a symmetrical airfoil, will give zero lift with zero angle of attack $\endgroup$ – D. Frederico Jul 4 '17 at 22:57
  • $\begingroup$ @D.Frederico: Note that I made camber a condition for lift at zero AoA. See here for the difference between cambered and encumbered wings. $\endgroup$ – Peter Kämpf Jul 5 '17 at 7:09
  • $\begingroup$ you mean a cambered wing can generate lift even with zero degree angle of attack but flat plate or symmetrical airfoil can't $\endgroup$ – D. Frederico Jul 28 '17 at 12:14
  • $\begingroup$ @D.Frederico: Yes. $\endgroup$ – Peter Kämpf Jul 28 '17 at 12:22
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If an wing is stationary and there is no wind, then there would be no airflow and therefore no angle of attack. Assuming there was a wind, then there would be a relative airflow and so an angle at which it meets the chord of the wing.

If a stationary glider is pointing into a breeze with one wing on the ground, it is possible to lift the wing off the ground using the ailerons. This is accomplished by increasing the angle of attack of that wing, creating additional lift, despite the glider not moving forwards. Conversely, if the airflow was from behind, the ailerons would be used in reverse (opposite aileron) - a consideration for glider pilots trying to keep wings level as they slow down if they land in a tailwind.

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