What determines the longitudinal stability of an airplane?

• A. The location of the CG with respect to the center of lift
• B. The wing area
• C. The tailplane size
• D. Engine power

Answer: (A)

Longitudinal stability comes from how the aircraft’s weight distribution interacts with the aerodynamic forces that act about the pitch axis. The crucial factor is the position of the center of gravity relative to the aerodynamic center (often called the center of lift). If the center of gravity sits ahead of the aerodynamic center, a small pitching disturbance creates a restoring moment that tends to push the nose back toward the trimmed attitude, giving positive static stability. If the center of gravity is behind the aerodynamic center, a small disturbance tends to be reinforced, leading toward instability.

This distance between CG and the aerodynamic center is called the static margin; a positive margin generally means stable flight, with larger margins offering more stability (though with some trade-offs in performance).

Wing area, tailplane size, and engine power influence how large the stabilizing or destabilizing moments are, but the fundamental determinant of whether the airplane is statically stable in pitch is the CG location relative to the aerodynamic center. Moving weight forward increases stability; moving it aft reduces stability.