Consider the airfoil shown, with a co-ordinate system set up from the leading edge $O$:
I want to find the moment of force about point $O$ due to pressure on an infinitesimal piece of the airfoil at point Q.
A text book I'm referring to(Fundamentals of Aerodynamics by John D Anderson) says that the moment is: $$ dM= (p dS) cos \theta ~ x_Q $$ (where $x_Q$ is the x co-ordinate of Q)
Is this expression an approximation that assumes thickness of the airfoil to be negligible compared to the chord length?
Because, the moment should be length $OQ$ multiplied by component of $pdS$ perpendicular to $ \vec{OQ}$ ... or have I made a mistake with the physics?