Stability around the vertical axis is known as which stability?

• A. Lateral stability
• B. Directional stability
• C. Longitudinal stability
• D. Vertical stability

Answer: (B)

Directional stability is the aircraft’s tendency to maintain or return to straight flight about the vertical axis. When the nose yaws due to a gust or a sideslip, forces on the tail and the overall shape of the airplane create restoring moments that oppose the yaw and help bring the flight path back in line with the original direction. This is the axis controlled by the vertical stabilizer and is what keeps the airplane from undesired side-to-side rotation.

To place it in context with the other axes: stability about the longitudinal axis (roll) is lateral stability, and stability about the lateral axis (pitch) is longitudinal stability. The term vertical stability isn’t used to describe this aspect of flight dynamics. A design with a suitably sized vertical stabilizer and appropriate airflow over the tail enhances directional stability.