In correspondence with prof Art Rizzi at KTH the question of the definition of the term  airfoil has come up. Rizzi argues that an airfoil by definition has a sharp trailing edge, so that a wing with a rounded trailing edge would not be an airfoil.

When I ask Rizzi why a wing is supposed to have sharp trailing edge, I get the answer that this is because an airfoil is designed to have lift and and an airfoil has a sharp trailing edge by definition, and moreover a sharp trailing edge gives small drag.  When I ask if wings with round trailing edge can have lift, the answer is, yes they can, but they are not airfoils.

When I ask if a wing with rounded trailing edge of diameter 1% of the chord length is to be viewed as sharp or not, I get no answer. This is logical, since classical theory does not give an answer to this question, which is the real question.

Rizzi thus does not want to answer my question why airfoils are supposed to have a sharp trailing edge and neither if wings in practice have sharp trailing edge. I am simply left with all my questions, which seems to be nature of the subject which has not changed much since the time of Kutta-Zhukovsky-Prandtl.

The literature is not as strict on terminology as Rizzi and airfoil is often used as a synonym of wing, which can have a sharp or non-sharp trailing edge. But the idea that a wing should have a sharp trailing edge to give lift is deeply rooted: The UIUC Airfoil Database list 1550 airfoils all with sharp trailing edge, while no real wings designed from the database have sharp trailing edge.

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