One of the first things we learn about stalls is that they occur when a wing exceeds a certain critical angle of attack, which depends on the specific design of the wing. But if a given wing has a critical AoA of, say, 20 degrees, which definition of AoA are we using?
As shown in the image below, geometric angle of attack is measured between the wing chord and the undisturbed relative airflow.
Effective angle of attack takes into account the induced downwash caused by wingtip vortices and is therefore smaller than geometric angle of attack.
These definitions of AoA come up in explaining induced drag, but I never see them referenced in discussions of critical AoA. I am assuming that critical AoA is measured using geometric AoA, but is this correct?
As a follow up question / comprehension check: Induced drag occurs because lift is produced perpendicular to relative wind. When the airflow is deflected downwards, the lift vector is in turn deflected rearwards, contributing to drag. Would it also be correct to say that the magnitude of the lift vector is decreased, since the effective AoA is shrunk, compared with geometric AoA?