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models. This chapter provides an overview of flight control systems in general, and
offers some insight in the FCS design process, in order to put the objectives behind
the FDC toolbox into the right perspective.
1.1 Automatic flight control systems
A major factor inWright brothers’ success in achieving the first powered flight in December
1903 has been the emphasis they placed on making their aircraft controllable
by the pilot, rather than inherently stable. However, the difficulties of controlling
early aircraft and the progress toward longer flight times, expanded flight-envelopes,
and bigger aircraft created a need for the development of power-driven aerodynamic
control surfaces, stability augmentation systems, and ‘automatic pilot systems’. This
evolution of flight control systems has been parallelled by the development of closedloop
control theories and major achievements in computer technologies [33].
Since the term ‘automatic flight control systems’ can be used to describe a wide
variety of controllers, categorizing them into different categories can be beneficial.
The first class of automatic controllers are the Stability Augmentation Systems (SAS).
2 Chapter 1. Flight control system development
These systems are used to damp and stabilize high-frequency rotational modes of
the aircraft, making it easier for pilots to control the aircraft. Common types of SAS
are roll dampers, pitch dampers, and yaw dampers.
If an augmentation system is intended to control a rotational mode and to provide
the pilot with a particular type of response to the control inputs, it is known as
a Control Augmentation System (CAS) [33]. Examples are controllers for the roll rate,
pitch rate, and normal acceleration of an aircraft. CAS systems that are coupled to
a conventional control system with a direct mechanical linkage between the control
actuators and the control column, are often called ‘control wheel steering systems’
(CWS). If the connection is achieved by means of electrical (or optical) wires, such
controllers are usually called ‘fly-by-wire systems’.
Although the slow modes of an aircraft (phugoid and spiral) are more easily controllable
by a pilot, it still is undesirable for a pilot to have to pay continuous attention
to controlling these modes, especially during longer flights. Therefore, an automatic
controller is often needed to provide ‘pilot relief’. An autopilot is an automatic
control system that provides both pilot relief functions and special functions such as
automatic landing. Typical autopilot functions are attitude hold, altitude hold, turncoordination,
heading select, automatic approach, and VOR navigation. Modern
aircraft also allow the autopilot to be coupled to a flight management system (FMS),
which offers flight path optimisation and wide-area navigation through the use of
inertial reference system and global positioning system guidance and navigation.
In summary, we can conclude that the main function of an FCS is to contribute
to the safe, comfortable, and economic operation of the aircraft, such that the intended
flight missions can be accomplished and unexpected events can be handled.
A modern FCS consists of three main components [24]: (i) sensors, which provide
the flight control computers information on air data, inertial data, and cockpit data,
(ii) the flight control computers themselves, which are used to evaluate the flight control
laws, and (iii) the actuation systems of the aircraft control surfaces and throttles.
Feedback control is used to provide tight pilot command tracking, to attenuate external
disturbances such as gusts and turbulence, and to provide robustness against
modelling errors.
Fly-by-wire systems allow the pilot to control aircraft states, in contrast to the conventional
direct control of aerodynamic control surfaces and engines. This offers
enhanced flexibility for the control laws, which provides new opportunities to increase
the overall safety level. For example, error-tolerant control laws can provide
flight envelope protection, and help the pilot to successfully achieve critical flight
manoeuvres and to recover from unusual attitudes. The use of a modern FCS can
also be beneficial from an economic point of view: fuel consumption can sometimes
be reduced by relaxing static stability, counteracted by the application of active control,
and the weight of fly-by-wire systems is often lower than that of conventional
control systems. Furthermore, fly-by-wire systems make it possible to give different
aircraft of different sizes almost the same ‘feel’, allowing the creation of a large
aircraft family that will significantly reduce pilot training costs.
　
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