Balance, or Equilibrium

An airplane can continue in steady unaccelerated flight only when the resultant external force and moment about the CG both vanish. In particular, this requires that the pitching moment be zero. This is the condition of longitudinal balance. If the pitching moment were not zero, the airplane would experience a rotational accelera­tion component in the direction of the unbalanced moment. Figure 2.3 shows a typi­cal graph of the pitching-moment coefficient about the CG1 versus the angle of attack for an airplane with a fixed elevator (curve a). The angle of attack is measured from the zero-lift line of the airplane. The graph is a straight line except near the stall. Since zero Cm is required for balance, the airplane can fly only at the angle of attack marked A, for the given elevator angle.

‘Unless otherwise specified, Cm always refers to moment about the CG.