Theoretically, it's known that Cm is independent or doesn't vary with angle of attack if the airfoil is symmetric and steady because the aerodynamic force will tends to act at the pitching center which is a quarter of chord length. From experimental results and simulation it's not so as there are some variation in the values of cm against angle of attack of airfoil in a steady state.
What factor(s) contributes to that? Why and how?
Hi,
While it is true that it is generally considered that the pitching moment coefficient is constant with the angle of attack, this is only valid for small angles of attack (smaller than, e.g., aprox 10º) and only valid when we take moments with respect to a specific point in the airfoil: the so-called aerodynamic center (AC). For the sake of simplicity, the AC is usually assumed to be placed in a constant position located at 25% of the chord, for subsonic flight, and 50% of the chord, for supersonic flight. But that is, as I said, an idealization. In reality, for starters, the AC usually moves forward (toward the airfoil leading edge) as the angle of attack increases, so its real position is not really constant.
So, if you are evaluating the pitching moment coefficient with respect to any other point other than the AC, then that pitching moment coefficient will depend on the angle of attack. In addition, the pitching moment coefficient will depend on the location of the point with respect to which we are evaluating the moments. On the other side, the values of such aerodynamic coefficients depend always obviously on the geometric shape of the airfoil. Finally, these coefficients also depend on the values of the Reynolds and Mach numbers, although this influence is of second order.
Hope this is useful,
Jose
@Subhankar it's believe theoretically that for a steady airfoil with symmetric geometry cm is independent of AoA because the pitching center is usually taking as the reference point (a quarter of the chord length).
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