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in the context of the Beaver autopilot studies.
The design, simulation, and implementation of control laws within a research
context will be similar to the manufacturing task, although more flexibility of the
tools will sometimes be required. For instance, it should be possible to rapidly alter
the control laws within the flight control computers of the aircraft in order to evaluate
different solutions to typical control system design problems with a minimum of
programming efforts. In the research environment, step 4 does not necessarily need
to include the selection of a ‘best’ system since it may be useful to evaluate competing
control solutions, if necessary even all the way up to evaluation in real flight, just to
gain more knowledge about their advantages and disadvantages. A typical example
of this can be found in ref.[24], which treats a GARTEUR design challenge of robust
flight control systems.
Finally, the requirements with respect to fail-safety of the FCS may be less restrictive
in a research context than for manufacturing. For instance, during the autopilot
design project for the DHC-2 Beaver laboratory aircraft, a single flight control
computer (a ‘luggable’ 80286 Compaq PC, coupled to a 16-bit ROLM-1603 generalpurpose
computer that handled the I/O functions; see refs.[6], [28], [37], and [38])
was used, whereas production aircraft normally apply multiple FCCs that constantly
monitor eachother’s command signals.
1.3 Taking a closer look at the FCS design cycle
Let’s explore the individual design phases a little further. Figure 1.1 visualizes the
first step in the FCS design process: the definition of the mission, which imposes requirements
upon the shape of the flight-path and the velocity along this flight-path.
This translates into the control problem depicted in figure 1.2: how to generate appropriate
deflections of aerodynamic control surfaces or changes in engine power or
thrust such that this mission can be fulfilled.
The classical approach to the FCS design problem is to start with the complete set
of non-linear equations of motion, and then make assumptions which enable these
equations to be linearized about some local equilibrium point. Getting familiar with
1.3. Taking a closer look at the FCS design cycle 5
Mission
Flight-path ( t )
Control Surface
Deflections d ( t )
Figure 1.1: Definition of the aircraft’s mission
Specify Control
Problem
Design Control
Laws
Achieve Specified
Behaviour
Model
Real World
Figure 1.2: The general flight control problem
the dynamical behaviour by means of trimming, stability and control analysis, and
nonlinear open-loop simulations (for stable aircraft), and understanding the influences
of the modelling assumptions is very important, and linear simulation of the
aircraft model may also be required at this stage [24].
The next step is to define the controller architecture. In the initial phase of the FCS
design, control system design tools based upon linear system theory can be applied
to linearized models of the aircraft and its subsystems; modern CACSD software
provides the required computer support. Although the linear control system design
and analysis techniques will provide insight in the essential behavior of the FCS,
only relatively small deviations from the equilibrium state are permitted before the
results start to deviate from those of the real aircraft.
Luckily the main purpose of many FCS control laws is to keep the deviations
from the equilibrium state as small as possible, e.g. in order to maintain a certain altitude
or heading, but there are other control laws which require large deviations from
nominal values, e.g. selecting a new reference heading or altitude which differs con6
Chapter 1. Flight control system development
siderably from the original value. For this reason, detailed nonlinear simulations will
be required, in order to validate (and possibly enhance) the results from the linear
analysis and design phase. Gain scheduling functions need to be implemented, to
ensure that the FCS will work well over the compete range of the flight-envelope
for which it is designed, taking into account a suitable safety margin. Also, signal
limiters will be required to deal with certain physical limitations (e.g. the maximum
deflection of control surfaces) and for safety reasons. If a robust design is to
be achieved, much attention should be given to understanding nonlinearities and
model uncertainties.
This off-line analysis, which should cover a wide range of velocities and altitudes
and all possible aircraft configurations, can be performed on a single PC or workstation,
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