Which of the following is a factor affecting directional stability?

• A. Keel effect and sweepback
• B. Forward CG during take-off
• C. Increase elevator effectiveness at high speed
• D. Propeller effect

Answer: (A)

Directional stability is the aircraft’s tendency to maintain or return to a straight path about the vertical axis when a yaw disturbance occurs. Two primary factors influence this stability: the keel effect and wing sweepback.

The keel effect comes from the fuselage itself acting like a keel. The side area of the fuselage in the direction of travel provides a stabilizing moment when the aircraft yaws, helping to resist the rotation and steer the nose back toward the flight path. The larger this fuselage side area and its placement relative to the center of gravity, the greater the directional stability.

Sweepback contributes as well because swept wings change how the lift and flow interact during a yaw. When the aircraft yaws, the airflow interacts with the swept geometry to produce a restoring yawing moment that tends to align the aircraft with the relative wind, enhancing directional stability at various speeds.

Forward CG during takeoff mainly affects longitudinal stability and pitch, not directional stability. Increasing elevator effectiveness at high speed influences pitch control rather than yaw stability directly. Propeller effects can introduce yaw moments in certain situations, but they’re not the primary factors determining directional stability like keel effect and sweepback are.