stick in the cockpit. See figure 16-3. The assembly
controlled, hydraulically assisted system. Because
consists of a bobweight, a viscous damper, and a
these systems reduce pilot fatigue and improve system
push-pull tube. The push-pull tube is the interconnect
performance, they are now commonly used. Such
between the control stick and the bobweight. The
systems include automatic pilot, automatic landing
damper is located at the pivot point of the bobweight
systems, and stability augmentation systems.
and restricts fast movement of the bobweight.
Navy specifications require two separate hydraulic
The aft bobweight and damper assembly works
systems for operating the primary flight control
with the forward assembly to overcome the heavy pull
surfaces. Current specifications call for an
of gravity and retard the chance of overcontrol. See
independent hydraulic power source for emergency
figure 16-4. This assembly is installed in the fuselage,
operation of the primary flight control surfaces. Some
forward and below the horizontal stabilizer. It connects
manufacturers provide an emergency system powered
to the elevator control cables.
by a motor-driven hydraulic pump. Others use a
ram-air-driven turbine for operating the emergency
The aft assembly consists of a bobweight, a
system pump.
viscous damper, and a load spring. The bobweight
connects to the elevator control bell crank and the
LONGITUDINAL CONTROL SYSTEMS
damper. The load spring is between the elevator control
bell crank and the fin structure to balance the forward
Longitudinal control systems control pitch about
and aft bobweights when the elevator is in a neutral
the lateral axis of the aircraft. Many aircraft use a
position.
conventional elevator system for this purpose. Aircraft
The elevator power mechanism changes the
that operate in the higher speed ranges usually have a
mechanical movement of the control stick to the
movable horizontal stabilizer.
hydraulic operation of the elevator. See figure 16-5.
The mechanism is in the aft section of the aircraft
Elevator Control System
directly below the horizontal stabilizer. As in the
aileron power system, the mechanism consists of a
The elevator control system, shown in figure 16-2,
hydraulic power cylinder, control valves, linkage, and
is typical of many conventional elevator systems. It
hydraulic piping.
operates by the control stick in the cockpit and is
hydraulically powered.
When the elevator controls are operated, the
control valves port hydraulic pressure to the power
The operation of the elevator control system starts
cylinder. The hydraulic pressure extends or retracts the
when the control stick is moved fore or aft. The
cylinder piston to move the push-pull tubes. The
movement of the stick transfers through the control
push-pull tubes deflect the elevators. The control
cables to move the elevator control bell crank. The bell
valves are two separate valves connected in tandem by
crank transmits the movement to the hydraulic
linkage. One valve is supplied hydraulic pressure by
actuating cylinder through the control linkage. The
hydraulic actuating cylinder operates a push-pull tube,
which deflects the elevators up or down.
T h e e l eva t o r s y s t e m u s e s f o r wa r d a n d a f t
bobweights. The bobweights induce a load on the
control stick during pitching and vertical acceleration
and prevent pilot-induced oscillations through the
elevator controls. If the gravity force is increased on
the bobweights, the induced load tends to return the
control stick to the neutral position. Viscous dampers
on the bobweight assemblies retard control stick
movement to prevent overcontrol. Overcontrol could
cause airframe overstress.
The elevator forward bobweight serves to help
recenter the control stick when a heavy gravity load
pulls against the airframe. The forward bobweight and
Figure 16-3.--Elevator forward bobweight and damper
damper assembly is in a housing forward of the control
assembly.
16-3
