Page 50 - Mechatronics with Experiments

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is done by the trimmable horizontal stabilizer which changes the angle of attack
slightly, while allowing elevators to remain in their neutral position. Technically, the
role of trimmable horizontal stabilizer can be performed by the elevators. However,
that would result in increased drag.
3. The rudder is used to generate aerodynamic forces, hence torque about center of
mass, for the yaw motion.

Quite often, the orientation motions happen simultaneously, that is the airplane makes a
roll motion while at the same time making a change in its yaw and/or pitch orientation.
In modern large civilian aircrafts, the flight control surfaces are actuated by hydraulic
power, where the delivered hydraulic power is controlled by electrical signals. The total
hydraulic power generated by the hydraulic pumps, driven by the jet engines, on a Boeing
747, is about 300 kW, on a Boeing 777 it is about 400 kW, and on an Airbus 380 it is about
800 kW. The electrical control signals are delivered to hydraulic components by-wire, that
is an electrical current signal delivered to a servo valve. As a result, the name “fly-by-wire”
is used to describe flight control systems based on electro-hydraulic systems. Hydraulic
power is the most widely used power type for motion on large aircraft flight control surfaces.
In order to take advantage of digital computer control, the control of hydraulic power is
carried out electrically by digital computers. A closed loop fly-by-wire system means that
the control signal to the actuator is generated by the flight control computer based on the
error between a command signal (generated either by auto-pilot software or by the pilot
via the joystick) and the on-board sensors, just like any other closed loop control system.
If the control signal to the actuators is generated only based on the command signal, and
no feedback sensor is used, then it is an open loop fly-by-wire system.
The design of a fly-by-wire electrohydraulic flight control system is dictated by
requirements in

1. safety and reliability,
2. power,
3. weight,
4. operational and functional performance.
Safety and reliability are the utmost considerations in flight control systems. Failure
modes, such as loss of hydraulic pressure or control surface runaway, are classified in
different categories (minor, major, hazardous, catastrophic) by their effect on the function
of the plane. The highest requirement is for catastrophic failures, leading to possible loss
of the aircraft, for which it has to be proven that the catastrophic failure condition has a
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probability to occur in less than one in 10 per flight hour. Redundant systems are the key
for improved safety. That means designing double or triple redundancy for a given function
so that there are two or three more alternatives in actuating and controlling a surface if one
or two of them fails. The redundancy must be provided at the level of

1. the power source (i.e., engine driven pump, electric generator driven pump where
generator is driven by engine, ram air driven generator in case of all engine failure),
2. the power distribution and metering elements (redundant hydraulic pipe lines and
valves),
3. the power delivery (redundant hydraulic cylinders and motors),
4. the sensing (redundant position, orientation, pressure sensors), and
5. the control computer (redundant control signal lines to actuators as well as redundant
flight control computers).