Lateral and Directional Stability

On learning about Stability, I noticed that Lateral and Directional Stability always go together. May I ask why does this happen and what effect will it have?

If the lateral stability is disturbed, the aircraft will roll. At this point, the lift force will be tilted with the plane’s symmetry plane, while the plane’s gravity W will always be perpendicular to the ground. At this point, the lift force and the gravity force form a combined force F. This force acts as the centripetal force for the aircraft’s motion, and the direction of the aircraft’s motion is changed to produce a sideslip. After that, the aerodynamic force on the rudder disrupts the directional equilibrium of the aircraft.

If the directional stability of the aircraft is disturbed, the aircraft will deflect to form a sideslip, the effective wing component of the outer wing is larger, increasing the lift of the outer wing, the effective wing component of the inner wing is smaller, reducing the lift of the inner wing. The difference in lift between the inner and outer wings makes the aircraft roll towards the inner wing side and destroys the lateral stability of the aircraft.

To sum up, the directional stability and lateral stability of the aircraft are interrelated and interdependent, and the disruption of the directional stability will cause the disruption of the lateral stability if it is not corrected. Conversely, if the lateral stability is lost, the directional stability is lost as well.

Directly translated from my Flight Theory textbook (written in Chinese)


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