Which two factors affect directional stability?

• A. Keel effect; sweepback
• B. Dihedral angle
• C. CG location
• D. Tailplane size

Answer: (A)

Directional stability is about the aircraft’s tendency to resist sideslip and return its nose to align with the flight path when yaw occurs. Two main contributors to that stability are the keel effect and wing sweepback.

Keel effect comes from vertical surfaces like the vertical tail and the fuselage acting like a keel. When the airplane yaws, these vertical surfaces catch the airflow and produce a restoring moment that tends to realign the aircraft with the relative wind. The larger the vertical stabilizing surface and the further its force acts from the center of gravity, the stronger this restoring tendency.

Wing sweepback also enhances directional stability. With sideslip, the geometry of swept wings creates a differential aerodynamic effect that resists the yaw and helps dampen the sideslip, producing a stabilizing yawing moment.

Dihedral mainly affects roll stability, not yaw. The CG location influences longitudinal and some lateral stability in specific configurations but is not a primary driver of directional stability. Tailplane size affects pitch stability more than yaw.