The directional bank uses a pedal damping piston
that restricts sudden heading changes. The auxiliary
servo cylinder operates at 1,500-psi hydraulic
pressure supplied by the auxiliary hydraulic system.
The mixing unit consists of a system of bell
cranks and linkage.
The unit coordinates and
transfers independent movements of the lateral,
forward, aft, and directional controls. Movement is
sent to the primary servo cylinders and the rotary
rudder. The mixing unit also integrates collective
pitch control movements with those of the lateral,
fore-and-aft, and directional systems. It causes the
controls to move the three primary servo cylinders
simultaneously in the same direction. It changes the
pitch on the rotary rudder blades to compensate for
the change in pitch of the rotary-wing blades.
Primary Servo Cylinders
These three servo cylinders send flight control
movements to the stationary swashplate of the
rotary-wing head. If the primary hydraulic system is
operating, the servo cylinders hydraulically aid flight
control movement. If the power fails, they function
only as control rods. See figure 10-8. This is
accomplished by the spring-loaded bypass valve,
which prevents hydraulic lock and a sloppy link pilot
valve connection. The pilot valve and the lower
clevis of the power piston connect to the flight control
linkage by the same bolt. There is a very close
tolerance in the pilot valve connection. This tolerance
causes the pilot valve to operate before the power
piston clevis. The power piston is then mechanically
Fluid under pressure entering the servo cylinder
upper port closes the bypass valve and enters the
upper chamber. With the pilot valve in neutral, fluid
cannot escape from the lower chamber, and the piston
remains motionless. If the pilot valve moves upward,
fluid flows into the lower chamber. The piston will
rise because of a pressure area differential. If the pilot
valve moves down, the fluid in the lower chamber
flows to return. The piston will be forced downward
by upper chamber pressure.
Figure 10-8.Primary servo cylinder.
When flight control movements stop, the piston
will continue to move until the ports of the pilot valve
close. The pilot valve clevis will be in the center of
the sloppy link.
When pressure is off, the bypass
valve will open, preventing hydraulic lock.