Stability of Jack‐Type Aircraft Power Controls: An Analysis of the Characteristics of Hydraulic Jacks With Particular Reference to Inherent Damping and Non‐Linearities in the Valve Flow Equation

The impedance of a power control is the complex ratio of force to extension when the applied force is sinusoidal and the input does not move. This definition can be extended if required, however, to cover harmonic perturbations from a constant velocity steady state. The impedance can be expressed in terms of a stiffness and a phase angle or in terms of an inphase component and a quadrature (damping) component.