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Usual case

When representing a wing airfoil, often:

  • The lift is positive
  • The upper surface is the one with low pressure.

enter image description here

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.

enter image description here
From Wikipedia.

(We have some historical ambiguity between incidence and attack angles, depending on the author, as visible here.)

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    $\begingroup$ In Italian you would have Estradosso and Intradosso like in French, but I never encountered similar terms in English. Also, I don't know if French is different, but in Italian "Estra-" and "Intra-" are w.r.t. the camber curvature, not the lift vector (or they would have to switch during inverted flight, for example) $\endgroup$ – Federico Feb 7 '17 at 9:52
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    $\begingroup$ @Federico: Yes, you're correct, this is related to the camber curve, updating. $\endgroup$ – mins Feb 7 '17 at 10:08
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You can call the two opposite sides suction side (for face on the convex side of the camber curve) and pressure side (for face on the concave side of the camber curve).

This has been used in a number of cases- for example in NASA Report CR-1767 Section Data for Thin, Highly Cambered Airfoils in Incompressible Flow:

.. flow separation on the pressure side at negative incidence angles and on the suction side at positive incidence angles.

for airfoils and in National Energy Technology Laboratory Handbook:

The airfoils are curved, convex on one side and concave on the other, with the rotor rotating toward the concave side. The concave side is called the pressure side of the blade, and the convex side is called the suction side of the blade.

for compressor blades.

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