# Why is the pitching moment of this symmetric aerofoil non-zero?

As I understand it, the pitching moment coefficient of a symmetric aerofoil should be zero, under potential flow assumptions. Obviously, approaching stall and post-stall, these assumptions are invalid and the pitching moment coefficient would no longer be zero.

However, looking at this graph of pitching moment coefficient against angle of attack, for a NACA0009 aerofoil at Re = 1e+6, it seems like Cm does actually vary considerably at low and moderate angle of attack.

Why?

• <<the pitching moment coefficient of a symmetric aerofoil should be zero>> and it is, correctly, in the figure you show, for zero alpha. why do you think it would be constantly zero? Apr 1, 2020 at 10:00
• The centre of pressure does not vary with (moderate) alpha for a symmetric aerofoil, so never changes difference from the reference point, so if zero pitching moment at zero alpha should be zero pitching moment at any (moderate) alpha - F*d is always zero if d is always zero. Apr 1, 2020 at 10:24
• The definition of the aerodynamic centre is the point around which pitching moment does not vary with angle of attack - therefore, if it is zero at zero alpha, and does not vary with alpha, it should still be zero for a moderate range of alpha. Apr 1, 2020 at 10:26
• From NASA: "For symmetric airfoils, the aerodynamic moment about the ac is zero for all angles of attack." - grc.nasa.gov/WWW/K-12/airplane/ac.html Apr 1, 2020 at 10:29
• Also, for a textbook reference - page 354 of Introduction to Aerodynamics, John Anderson. Apr 1, 2020 at 10:53