Describe CP and AC and their relation to static longitudinal stability.

• A. CP moves with angle of attack; AC is the point where pitching moment is independent of AoA; static stability requires CG ahead of the neutral point, near the AC.
• B. CP is fixed, AC moves with AoA; stable condition requires CG behind the neutral point.
• C. CP and AC are the same point for all AoA; stability depends only on wing area.
• D. CP is unrelated to lift; AC is the center of wing mass.

Answer: (A)

The main idea is how lift’s line of action and the aircraft’s pitching moment respond to changes in angle of attack. The center of pressure is not fixed; as the angle of attack changes, the distribution of lift shifts along the wing, so the center of pressure moves. The aerodynamic center is the special point at which the pitching moment does not change with angle of attack, and for an entire aircraft this point is the neutral point. For static longitudinal stability, the center of gravity must lie ahead of the neutral point; this creates a restoring pitching moment when the aircraft is perturbed, bringing the nose down and reducing the angle of attack. Saying the center of pressure moves with angle of attack, the aerodynamic center is the point where the moment is independent of angle of attack (the neutral point), and stability requires the CG ahead of that neutral point (near the aerodynamic center) captures the correct relationship. The other statements mix up which point moves with AoA or how stability depends on the CG location.