If the CG is moved forward, how is longitudinal stability typically affected?

If the CG is moved forward, how is longitudinal stability typically affected?

• A. Increases stability
• B. Decreases stability
• C. Unchanged
• D. Fluctuates with speed

Answer: (A)

Longitudinal stability comes from how a small pitch change creates a restoring moment about the aircraft’s center of gravity. The key is the static margin, the distance between the center of gravity and the aerodynamic center (which is roughly fixed relative to the wing). When the CG is moved forward, the CG is closer to the front, increasing that margin, so a disturbance in pitch produces a stronger restoring moment about the CG. The tailplane supplies forces that generate this restoring moment, and with a forward CG the moment arm for those forces is larger, making the nose tend to return to level more readily. So the aircraft becomes more pitch-stable. Of course, too far forward can compromise elevator authority and overall handling, but in terms of inherent stability, forward CG increases it.

Longitudinal stability comes from how a small pitch change creates a restoring moment about the aircraft’s center of gravity. The key is the static margin, the distance between the center of gravity and the aerodynamic center (which is roughly fixed relative to the wing). When the CG is moved forward, the CG is closer to the front, increasing that margin, so a disturbance in pitch produces a stronger restoring moment about the CG. The tailplane supplies forces that generate this restoring moment, and with a forward CG the moment arm for those forces is larger, making the nose tend to return to level more readily. So the aircraft becomes more pitch-stable. Of course, too far forward can compromise elevator authority and overall handling, but in terms of inherent stability, forward CG increases it.