When representing a wing airfoil, often:
- The lift is positive
- The upper surface is the one with low pressure.
So the airfoil faces can just be called upper and lower surfaces and we easily understand their role in the lift generation.
Case of vertical airfoil
But things can get more ambiguous when the lift is not directed upward, e.g. for the rudder airfoil or, even more, for a blade of a compressor which rotates continuously while the lift remains constant related to the airfoil.
Are there names to identify the faces in relation with the direction of the lift, e.g. "the lift face" which could be used not only for wings but also when the airfoil is vertical?
Comparison with French wording
I'm asking this question because in French we have other names:
- Extrados (extra ~ out of) is the convex side of the camber curve, or the face in the direction of the lift vector for a symmetrical airfoil, and
- intrados (intra ~ within) is the concave side of the camber curve, or the face opposed to the lift vector for a symmetrical airfoil.
(We have some historical ambiguity between incidence and attack angles, depending on the author, as visible here.)