Where should the center of gravity (CG) be located for positive longitudinal stability?

For positive longitudinal stability, the center of gravity (CG) must be located ahead of the aerodynamic center. This positioning is crucial because it creates a restoring moment that acts to return the aircraft to its original flight path when it experiences a disturbance in pitch attitude.

When the CG is forward of the aerodynamic center, any upward pitch causes the wings to generate more lift, which increases the nose's tendency to return to level flight. Conversely, if the aircraft pitches down, the reduction in lift leads to a moment that effectively causes the nose to come back up toward the desired attitude. This inherent behavior promotes stability and helps the pilot maintain control of the aircraft.

In contrast, placing the CG behind the aerodynamic center can lead to an unstable configuration, where pitching motions are amplified instead of dampened. Being at the center can result in neutral stability, which may not provide the desirable response characteristics. Being above the aerodynamic center is irrelevant in the context of stability, as vertical positioning does not contribute to longitudinal stability. Thus, positioning the CG ahead of the aerodynamic center is essential for ensuring that the aircraft responds predictably and safely to any pitch changes.