*3.4. Dynamic Stiffness of EHA*

Dynamic stiffness is also an important indicator for evaluating the dynamic performance of EHA. Low dynamic stiffness can severely degrade the control accuracy of the system. Hydraulic spring effects are a major part of the EHA stiffness. Therefore, the stiffness of the hydraulic spring is considered as an objective in this study. The dynamic stiffness is evaluated by the root-mean square (RMS) of the angle of control surface offset from the given value, during 12–15 s in the simulation. When the EHA holds the control surface stalled at a constant position under wind gust disturbance, more stiffness will result in a smaller RMS value.
