AUTOMATIC FLIGHT CONTROL
of the desired flight path. You can again break
SYSTEMS (AFCS)
lift down into its vertical and horizontal
components. Look at figure 8-12, view B. To
Learning Objective: Relative to the auto-
maintain altitude, the vertical lift is increased until
matic flight control system, recognize
it is equal to gravity. With the cone at a flapping
angle, the helicopter accelerates in the desired
functions and operating principles and
modes, including air data and flap position
direction until drag is equal to horizontal
information and coordination inputs, and
lift.
identify AFCS components.
To accelerate the helicopter in a forward
direction, move the cyclic control stick forward.
In the human body, signals to move us from
place to place start with our five senses as they
You also must make a corresponding increase in
reference outside conditions. The brain processes
collective to maintain altitude. As the collective
these signals and sends them through the nerves
increases, torque on the main rotor blade
to the muscles. The body then does its required
increases. This makes the helicopter tend to rotate
movement by muscle power. Similarly, most
in the direction opposite to the rotor blade
rotation (nose right). A mechanical mixer
automatic flight control systems (AFCS) have
their component parts divided into three major
automatically changes the pitch of the tail
groups--sensors (information inputs), amplifier/
rotor to overcome the right turning tendency
computer, and output units.
(skid).
The sensors originate the signals as they are
To turn the helicopter (change heading), place
acted upon by outside references. They only sense
the cyclic control stick to the right or left.
changes and do not have sufficient power to make
Flapping action of the main rotor blades causes
corrections.
the cone to tilt in the direction of the desired turn.
The amplifiers and computers are the brains
As in the fixed-wing aircraft, the pilot must
for the AFCS. They receive the weak signals from
maintain coordination in a turn by using the
the sensors, which in most cases are synchros, and
rudder pedals to prevent skid or slip. Also, the
determine how much and in which direction
pilot must adjust collective to prevent a loss in
correction is necessary. The synchro signals are
altitude. In hovering flight, the pilot uses the
usually in millivolts, but the correct strength
rudder pedals only to turn the helicopter, thus
needed is in volts. Therefore, the amplifier
producing a skid.
increases the weak signal to a workable voltage.
The value of the synchro signal depends on the
amount of rotor displacement with respect to the
stator from the null position. The direction of
rotor displacement from the stator determines the
REVIEW SUBSET NUMBER 2
direction of the correction. Most amplifiers have
at least two stages of voltage amplification--one
Q1. On a fixed-wing aircraft, what control
stage of phase discrimination, and an amplifier
surface corrects for yaw?
where power amplification takes place, Other
types of amplifiers control the voltage to control
valves in hydraulic servos.
The output unit is the muscle of the AFCS.
Q2. As fuel is used, what must the pilot reduce
It consists of an electro/hydraulic booster
to lessen lift and prevent a gain in altitude?
package. There is a booster package for each
control surface--rudder, aileron, and elevator.
The boosters also assist the pilot in manual control
of the aircraft.
Q3. How does the pilot increase lift in a
Summing up the major groups, the sensors
helicopter with a constant speed rotor?
send a small signal to the amplifier/computer
when a displacement occurs. The amplifier/
computer amplifies the weak signal to a workable
voltage and sends it to the output unit. The output
Q4. When does the action called coning take
unit changes the electrical energy to mechanical
place?
