For a symmetrical airfoil, when the lift coefficient is zero, what is the pitching moment?

• A. Positive
• B. Negative
• C. Zero
• D. Undefined

Answer: (C)

A symmetric airfoil has no camber, so its pressure distribution is mirror-like about the chord. The aerodynamic center is the point where the pitching moment is independent of lift, and for a symmetric airfoil that moment about the aerodynamic center is zero for any lift coefficient. When the lift coefficient is zero, there is no resultant lift force to create a moment about that point, so the pitching moment is zero.

A positive or negative moment would imply a nose-up or nose-down tendency even with zero lift, which isn’t the case here. An undefined moment isn’t appropriate because moments are defined even when lift is zero.