What factor contributes to an airplane's directional stability?

Directional stability in an airplane is largely influenced by the placement of the center of gravity (CG) in relation to the aircraft's vertical stabilizer and other design features. Having more fuselage located aft of the CG helps to enhance directional stability due to the increased moment arm that it creates.

When the center of gravity is located forward of the vertical stabilizer, the aerodynamic forces acting on the tail and fuselage work to restore the aircraft to its original heading after a disturbance, such as a gust of wind. The longer aft fuselage acts as a lever arm, providing greater influencing forces that help maintain a steady flight path and reduce the tendency for yaw.

In contrast, if there is less fuselage behind the CG, directional stability may be compromised, as the moment arm is shorter and restores the aircraft to its original position less effectively. Thus, the amount of fuselage extending aft of the CG plays a crucial role in ensuring that the airplane can self-correct its direction efficiently.