Static stability derivatives
Static stability requires that a restoring action to any motions caused by forces and moments is auto-matically produced along and about the same axes. Note: Static stability requires Yβ and N′β positive because β is defined in the opposite sense to both the motion ψ and moment N associated with the Zaxis (see Linear Lateral Dynamics). The various forms of static stability of the difierent axes are thereby determined by the signs of the stability derivatives for the associated forces and moments of these axes. A positive sign indicates that the restoring action acts in the direction of a positive motion for the axis. This is except for lateral motion which is more conveniently referred to the sideslip β rather than lateral translational velocity vand so a needs positive sign for static stability.