Which statement best describes the relationship between CG and longitudinal stability?

• A. The CG location relative to the center of lift determines longitudinal stability
• B. The CG location only affects vertical stability
• C. Longitudinal stability is determined by wing area
• D. Tail size determines longitudinal stability

Answer: (A)

Longitudinal stability is set by where the aircraft’s weight acts in relation to where the lift is produced. The relevant point where the lift acts is the center of lift (aerodynamic center). If the center of gravity sits ahead of that point, a small disturbance that raises the nose creates a restoring moment: the lift behind the CG increases in a way that tends to push the nose back down toward level flight. If the center of gravity is behind the center of lift, the same disturbance tends to amplify the pitch, making the airplane tend to diverge from its original attitude. The tail and wing design influence how strong that restoring moment is, but the fundamental determinant of whether the airplane is longitudinally stable is the CG’s position relative to the center of lift.