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2.0 Flight control systems
Introduction
- Characteristics vary greatly depending on the type of aircraft
flown.
- The most basic flight control system designs are mechanical
and date back to early aircraft.
- They operate with a collection of mechanical parts such as
rods, cables, pulleys, and sometimes chains to transmit the
forces of the flight deck controls to the control surfaces.
- Mechanical flight control systems are still used today in small
general and sport category aircraft where the aerodynamic
forces are not excessive. [Figure 1]
Flight control Systems
Figure 1:mechanical Flight control system
2.1 Flight control systems (FCS)
❑ Introduction to automatic flight control
- An aircraft autopilot with many features and various
autopilot related systems integrated into a single system
is called an automatic flight control system (AFCS)
- In the early days of flying, the FCS was mechanical. By means
of cables and pulleys, the control surfaces of the aircraft were
given the necessary deflections to control the aircraft.
- Currently, a modern technique, fly-by-wire FCS is used. In
this system electrical signals are sent to the control surfaces.
The signals are sent by the flight (control) computer
(FC/FCC).
Set-up of the flight control system
- The FCS of an aircraft generally consists of three
important parts.
- The stability augmentation system (SAS)
augments to the stability of the aircraft. It mostly does this
by using the control surfaces to make the aircraft more stable.
A good example of a part of the SAS is the phugoid damper (or
similarly, the yaw damper).
A phugoid damper uses the elevator to reduce the effects of
the phugoid: it damps it.
The SAS is always on when the aircraft is flying. Without it, the
aircraft is less stable or possibly even unstable.
Set-up of the flight control system
- The automatic control system
➢ It automatically controls the aircraft. It does
this by calculating (for example) the roll angles
of the aircraft that are required to stay on a
given flight path.
➢ It then makes sure that these roll angles
are achieved.
Set-up of the flight control system
❑ There are important differences between the above three systems.
- First of all, the SAS is always on, while the other two systems are
only on when the pilot needs them.
- Second, there is the matter of reversibility.
✓In the CAS and automatic control, the pilot feels the actions that
are performed by the computer. i.e when the computer decides to
move a control panel, also the stick/pedals of the pilot move
along. This makes these systems reversible.
✓ The SAS, on the other hand, is not reversible: the pilot doesn’t
receive feedback. The reason for this is simple. If the pilot would
receive feedback, the only things he would feel are annoying
vibrations.
Single axis flight control
- Simplest mode of control also known as
a "wing leveler“.
- Controls roll
- Used in light weight aircrafts
Two-axis flight control
- Controls roll and yaw axes
- Keeps the wings level and controls
the direction of flight.
- Found in small to relatively large aircrafts
eg. jets
Flight control systems
• An autopilot is an example of a
control system.
• Control systems apply an action based on
a measurement to give the desired results.
Autopilot { roll autopilot, yaw autopilot, pitch
autopilot
Single- axis flight control
Figure 3: single- axis flight control
Single- axis flight control
Figure 4. single axis flight control block diagram
Single- axis flight control
- The pilot sets a control mode to maintain the wings in a level
position. However, even in the smoothest air, a wing will
eventually dip.
- Gyroscopes (or other position sensors) on the wing detect
this deflection and send a signal to the autopilot
computer.
- The autopilot computer processes the input data and
determines that the wings are no longer level.
- The autopilot computer sends a signal to the servos that
control the aircraft’s ailerons. The signal is a very
specific command telling the servo to make a precise
adjustment.
Single- axis flight control
- This loop, shown in figure 2,works continuously, many
times, much more quickly and smoothly than a human pilot
could.
- Two- and three-axis autopilots obey the same principles,
employing multiple processors that control multiple surfaces.
- Some airplanes even have auto thrust computers to control
engine thrust.
- Autopilot and auto thrust systems can work together to
perform very complex maneuvers.
2.2 Servo motors
- Unit that control ailerons, horizontal stabilizer and rudder.
- A servomechanism (servo) is a device that provides mechanical
control.
- One servo exists for each control surface included in the
autopilot system.
- The servos take the computer’s instructions and use motors or
hydraulics to move the craft’s control surfaces, making sure the
plane maintains its proper course and attitude.
- They are connected to surface by clutches.
- If AFCS fail, the aircraft returns to manual control.
- In advanced aircrafts, control surfaces are manipulated by side
stick where surfaces are only moved by servo.
