Question

To achieve longitudinal static stability of a general aviation airplane, which one of the following conditions should be satisfied?

• A. The center of gravity of the airplane should be aft of the neutral point.
• B. The center of gravity of the airplane should be forward of the neutral point.
• C. The stability coefficient \( \frac{\partial C_m}{\partial \alpha} \) (where \( C_m \) is the airplane pitching moment coefficient and \( \alpha \) is the angle of attack) is positive.
• D. The static margin is negative.

Hint:

In general aviation airplanes, ensuring that the CG is forward of the neutral point is crucial for achieving positive longitudinal static stability.

Answer 1

The Correct Option is B

For longitudinal static stability, the center of gravity (CG) must be located forward of the neutral point. Here's why:

• The neutral point is the aerodynamic center where the pitching moment coefficient (\( C_m \)) does not change with angle of attack.
• If the CG is ahead of the neutral point, any disturbance causing a pitch will generate a restoring moment, helping the aircraft return to its original position — this is a sign of stability.
• If the CG is behind the neutral point, the aircraft becomes statically unstable and may diverge from equilibrium after a disturbance.
• A positive value of \( \frac{\partial C_m}{\partial \alpha} \) suggests that the pitching moment increases with angle of attack, which is actually unstable. For stability, we typically require \( \frac{\partial C_m}{\partial \alpha} < 0 \).
• A negative static margin (distance from CG to the neutral point) also indicates instability.

Conclusion: To ensure longitudinal static stability, the CG must be forward of the neutral point.