Longitudinal Static Stability - Center of Gravity

Center of Gravity

The longitudinal static stability of an aircraft is significantly influenced by the position of the center of gravity of the aircraft. Such potential cg adjustments are typically imagined as increments forward or aft from the "aerodynamic center" of the wing which is a position, typically near 1/4 chord aft of the leading edge, where changes in angle of attack do not change pitching moment. The pitching moment is not typically zero there, just constant.

As the center of gravity moves forward the longitudinal static stability of the aircraft increases. The reason is twofold:

• the moment arm between the horizontal stabilizer increases and
• the contribution of the wing's lift to pitching moment is also stabilizing in consequence of ITS changing moment arm.

Similarly, if the center of gravity is moved aft, the longitudinal static stability of the aircraft decreases. Moving the cg aft far enough will destabilize the airplane, at a point when the wing's upsetting moment exceeds the restoring moment of the tail.

Stability boundaries for a particular aircraft include limitations on the most forward and aft locations permitted for the center of gravity. No attempt should be made to fly an aircraft if its center of gravity is outside the approved range, or will move outside the approved range during the flight.