Aircraft Longitudinal Stability

The above discussed concepts are complemented by the figure below.

When the wing aerodynamics is reduced to the wing aerodynamic center, for fixed airspeed and altitude, a varying angle of attack determines only a variation of lift $L_\mathrm{W}$ while the arm $a$ and the moment $\mathcal{M}_\mathrm{ac,W}$ remain unchanged.
When the wing aerodynamics is reduced to the wing aerodynamic center, for fixed airspeed and altitude, a varying angle of attack determines only a variation of lift $L_\mathrm{W}$ while the arm $a$ and the moment $\mathcal{M}_\mathrm{ac,W}$ remain unchanged.