Longitudinal Stability: Which stability is controlled by the center of gravity and horizontal stabilizer?

Longitudinal Stability: Which stability is controlled by the center of gravity and horizontal stabilizer?

• A. Lateral Stability
• B. Directional Stability
• C. Vertical Stability
• D. Longitudinal Stability

Answer: (D)

Pitch stability, or longitudinal stability, is how the aircraft resists nose-up or nose-down movement about the lateral axis. The center of gravity sets the pivot point for these pitching moments, and the horizontal stabilizer provides the restoring force that returns the airplane to its trimmed pitch. If the CG sits ahead of the aircraft’s aerodynamic center, a disturbance that pitches the nose up creates a restoring moment that tends to push the nose back down. The horizontal stabilizer adds a balancing force (often downward on conventional tails) that increases this restoring tendency as the angle of attack changes. Together, the forward or aft position of the CG and the tail’s aerodynamic effectiveness establish a positive static margin, yielding stable pitch behavior. Move the CG aft, and the restoring tendency weakens, making the aircraft less longitudinally stable. Lateral stability deals with roll and is influenced by wing dihedral, sweep, and weight distribution; directional stability (yaw) is governed mainly by the vertical stabilizer.

Pitch stability, or longitudinal stability, is how the aircraft resists nose-up or nose-down movement about the lateral axis. The center of gravity sets the pivot point for these pitching moments, and the horizontal stabilizer provides the restoring force that returns the airplane to its trimmed pitch.

If the CG sits ahead of the aircraft’s aerodynamic center, a disturbance that pitches the nose up creates a restoring moment that tends to push the nose back down. The horizontal stabilizer adds a balancing force (often downward on conventional tails) that increases this restoring tendency as the angle of attack changes. Together, the forward or aft position of the CG and the tail’s aerodynamic effectiveness establish a positive static margin, yielding stable pitch behavior. Move the CG aft, and the restoring tendency weakens, making the aircraft less longitudinally stable.

Lateral stability deals with roll and is influenced by wing dihedral, sweep, and weight distribution; directional stability (yaw) is governed mainly by the vertical stabilizer.