Coefficient
The pitching moment coefficient is important in the study of the longitudinal static stability of aircraft and missiles.
The pitching moment coefficient is defined as follows
where M is the pitching moment, q is the dynamic pressure, S is the planform area, and c is the length of the chord of the airfoil. is a dimensionless coefficient so consistent units must be used for M, q, S and c.
Pitching moment coefficient is fundamental to the definition of aerodynamic center of an airfoil. The aerodynamic center is defined to be the point on the chord line of the airfoil at which the pitching moment coefficient does not vary with angle of attack, or at least does not vary significantly over the operating range of angle of attack of the airfoil.
In the case of a symmetric airfoil, the lift force acts through one point for all angles of attack, and the center of pressure does not move as it does in a cambered airfoil. Consequently the pitching moment coefficient for a symmetric airfoil is zero.
Pitching moment is, by convention, considered to be positive when it acts to pitch the airfoil in the nose-up direction. Conventional cambered airfoils supported at the aerodynamic center pitch nose-down so the pitching moment coefficient of these airfoils is negative.
Read more about this topic: Pitching Moment