I know what a stall is but I don't understand why there are in some cases two local maximums on the $C_L$ vs AOA curve of some airfoils. What is the reason for the mini stall that occurs before the main stall?

C_L vs. AOA

  • 1
    $\begingroup$ Related Q&A: Does a Stalled Wing Exhibit No Circulation $\endgroup$
    – ymb1
    Aug 13 '21 at 1:55
  • $\begingroup$ Note that the plot is up to 90°, which corresponds to the aircraft flying with the nose pointing straight up—or the aircraft falling straight down with the nose horizontal. $\endgroup$
    – Jan Hudec
    Aug 13 '21 at 18:03

The mini stall is the stall that we normally talk about when considering the flight of an aeroplane, and is therefore the main stall. Below the AoA where the first stall occurs, the wing profile is reasonably aligned with the airflow, and flow at the upper wing surface can remain attached. When upper surface flow detaches:

  • upper surface suction force decreases, causing the "mini-stall";
  • air resistance rises dramatically due to the stalled upper surface flow;
  • lift is still generated by the lower surface deflecting the airflow downwards, like a flat plate does.

from https://aviation.stackexchange.com/questions/16193/how-do-wings-generate-lift/37401#37401

So beyond the normal range stall the wing still generates lift, and increasing AoA produces even more lift. The reason why aeroplanes cannot fly in the region beyond the first stall can be found from considering aerodynamic drag as function of AoA as well.

enter image description here

From this answer: $C_d$ of a NACA 0012 profile staling at 11º. The jump in drag is dramatic, and would usually be impossible to compensate for with engine power. Drag is the reason why we want to remain at low wing AoA.

  • $\begingroup$ Also, below the main stall the flow is behaving in a linear way. Above, nonlinear behavior, resulting in positive damping, can ruin your day. $\endgroup$ Aug 13 '21 at 8:34
  • $\begingroup$ Thank you so much! $\endgroup$
    – user217340
    Aug 13 '21 at 13:51
  • $\begingroup$ Welcome to the site @user217340: consider pressing the check mark (✓) if the answer worked for you. $\endgroup$
    – ymb1
    Aug 13 '21 at 16:30

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