Page 49 - Mechatronics with Experiments

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INTRODUCTION 35
The secondary (high-lift) flight control surfaces are used only during the take-off and
landing phases of the flight. As the name implies, these surfaces are designed to provide
increased lift for the airplane while it is at a relatively low speed. Secondary flight control
surfaces include the left and right flaps, left and right slats, and spoilers.The flaps and slats
are positioned to predefined positions during take-off and landing. Spoilers are primarily
used to reduce the lift and are generally used during landing, hence the name “spoilers”
which “spoil” the lift. Slats and flaps increase the lift coefficient of the airplane by providing
a larger lift surface area and by providing a guided airflow path for larger lift coefficient.
They are typically moved to a desired position and held at that position during the take-off
and landing phase of the flight until that phase is completed. Then these surfaces are moved
back to their original fold positions during flight. The secondary surfaces require a relatively
low bandwidth control system since they move to a predefined position and stay there. Slats
and flaps typically contribute to the lift by increasing the effective angle of attack and lift
surface area for a given airplane (Figure 1.25a). During take-off, the highest possible angle
of attack is desired, hence slats are fully deployed and flaps might be partially deployed
at a certain position. During landing, the lowest possible speed is desired, as opposed to
maximum attack angle during take-off, hence the flaps are fully deployed while slats might
only be partially deployed.
Primary flight control surfaces are used during the flight to maintain the orientation
of the airplane. The primary flight control surfaces include the left and right ailerons, right
and left elevators, and rudder. At any given time, the airplane has a commanded orientation
in terms of pitch, yaw, and roll angles. An on-board orientation measurement sensor (an
electromechanical gyroscope with a rotor spinning at a constant high speed by an electric
motor or laser based ring-laser gyroscope) is used to measure the actual orientation of
the airplane relative a fixed coordinate frame. Then the primary flight control surfaces
are actuated based on closed-loop controls in order to maintain the desired orientation.
The bandwidth of the closed loop control system for primary flight control surfaces must
be fast and well damped in order to maintain a very smooth flight condition. Whereas
the bandwidth requirements of the secondary flight control systems is much smaller. An
auto-pilot generates the desired orientation and engine thrust signals during the flight. A
pilot can over-ride these command signals from the auto-pilot and command them manually
with a joystick.
Although the motion of the primary control surfaces affects multiple orientation
variables, they are mainly related in a one-to-one relationship as follows:

1. Left and right ailerons are always actuated asymmetrically (in the opposite direction
and equal amount) to generate opposite aerodynamic forces, hence torque, for roll
motion. In some conditions, the roll motion is augmented by the motion of spoilers
if decreasing the lift, hence reducing the altitude of the plane, is desired during the
roll motion. This is typically needed during the approach for landing.
2. Left and right elevators are always operated symmetrically (in the same direction and
equal amount) to generate aerodynamic forces, hence torque about center of mass, for
the pitch motion. During flight, the center of mass of the airplane shifts as fuel is used
and passengers move around. In order to balance the forces and maintain the pitch
angle of the plane, trimmable horizontal stabilizer (THS) is used. The trimmable
horizontal stabilizer pivots up and down, hence moving the whole tail section up and
down. For instance, if during flight a change in cruise altitude is desired, the elevators
are used to induce a pitch moment and change the angle of attack of the airplane. On
the other hand, during a constant altitude cruising flight condition, the pitch moment
about the center of gravity of the airplane has to be balanced. As the fuel is used during
the flight and passengers move around, the center of gravity changes. This balancing