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Most importantly, modern flight control systems have contributed to improved
dynamical behaviour. Certain military aircraft cannot be flown without a stability
augmentation system; they use the open-loop instability, which is related to the
1.2. The FCS design cycle 3
agility of the aircraft, to obtain better performance and improved manoeuvrability
of the closed-loop system. For civil aircraft, active flight control systems can provide
gust suppression and auto-trimming, in order to achieve improved ride quality.
A disadvantage of these techniques is that the costs and efforts involved to develop
an advanced FCS have escalated over the years. The danger exists that the
economical benefits described above are nullified by higher design and maintenance
costs, while the increasing complexity of modern flight control systems can potentially
have a negative effect on safety. Clever use of modern FCS design techniques
and optimisation of the design tools will be necessary to counteract these disadvantages
[24].
1.2 The FCS design cycle
Regardless of the level of complexity, it is always possible to divide the control system
design process into a number of distinct phases. A practical division is given in
ref.[26] (a reference, which, albeit somewhat outdated with respect to the available
computer hardware and software tools, is still valuable for this discussion):
1. Establish the system purpose and overall system requirements. System purpose
can be equated with mission or task definitions, while the system requirements
can be separated in (i) operational requirements, derived from the functions
needed to accomplish the mission phases and (ii) implied requirements, derived
from the characteristics of the interconnected components of the control
system and the environment in which they operate.
2. Determine the characteristics of unalterable elements, command-signals, and
external disturbances. The characteristics of some parts of the system cannot
easily be changed by the designer. Often the vehicle itself, its control surface
actuators and sometimes also its sensors are ‘unalterable’.1 Moreover, the structure
of the commands and disturbances is a direct consequence of the mission
requirements and the environment in which the control system has to operate.
3. Evolve competing feasible systems, i.e. determine the basic block diagrams.
Usually, there are more ways to achieve the requirements, e.g. with different
sensed motion quantities and/or the application of different control theories.
Then it is possible to evolve competing candidate systems for selection on the
basis of certain desirable properties.
4. Select the ‘best’ system. The competing designs can be compared on the basis
of (i) design qualities, which include dynamic performance (speed of response,
bandwidth, etc.) and physical characteristics (volume, weight, power consumption,
etc.), and (ii) design quantities, which include safety, reliability, maintainability,
cost, etc. An optimum system is one that has some ‘best’ combination
of these features.
1If the FCS is designed for an all-new aircraft, the selection of the hardware (sensors, actuators,
computers, etc.) must be included in the FCS design and analysis instead of taking the hardware as
being unalterable. In this report, all hardware is considered to be given, so we will concentrate on issue
of developing appropriate control laws to make a given aircraft fly a certain mission.
4 Chapter 1. Flight control system development
5. Study the selected system in detail. The selected system must be evaluated for
all normal and abnormal operating conditions. At each step in the FCS validation
the assumptions made earlier in the FCS design process must be checked
for validity. If necessary, a new iteration of the design should be started from
the point where the wrong assumption was made.
This scheme reflects the FCS design process within a manufacturing environment.
In a research context, the design process may have to be modified somewhat, as the
research and manufacturing tasks are clearly different: whereas the task of research
is to determine what is required and to produce a clear and comprehensive definition
of the requirements, the manufacturing task is to make and deliver a reliable
and effective product [35]. Consequently, the first FCS design phase in particular
will often be different in a research environment, because the system requirements
are often poorly understood or may even be the objective of the research itself. In
addition, the design tools may still be immature, and their development may itself
be an objective of the research. See for instance the development of the FDC toolbox
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FDC 1.4 – A SIMULINK Toolbox for Flight Dynamics and Contro(6)
