Bypass Valve. A bypass valve is provided to minimize the resistance of the servo piston to
movement when it must be moved manually. The valve opens automatically when there is no operating
pressure on the servo. This allows fluid to flow freely between the chambers on each side of the piston.
During normal aircraft operation, external forces from an aircraft's control surfaces, such as rotor blades
and ailerons, tend to move servo cylinders. This movement creates a pumplike action in the servo called
feedback. The irreversible valve prevents feedback through the servo to the control stick.
Figure 2-10 is a simplified schematic version of an irreversible valve. The broken-line block represents
the housing of the
Figure 2-10. Simplified Irreversible Valve.