Static directional stability is defined as the initial tendency of an aircraft to return to, or depart from, its equilibrium angle of sideslip (normally zero) when disturbed.

When an aircraft is placed in a sideslip, aerodynamic forces develop which create moments about all three axes. The moments created about the normal-axis tend to turn the nose of the aircraft into or away from the relative wind. The aircraft has positive directional stability if the moments created by a sideslip angle tend to align the nose of the aircraft with the relative wind.

If directional stability is excessive, high turn rate is provided by the moment. This can result in excessive back and forth (left – right) turning if the wind is coming from alternating left – right side. Thus, it will be difficult to control the aircraft in case of sideslip. The sideslip angle relates the displacement of the aeroplane centre line from the relative airflow