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using an integrated design and simulation environment such as MATLAB /
SIMULINK. ‘Off-line’ in this context means that the analysis does not have to be performed
in real-time and does not yet include piloted flight-simulation. In a later
stage the control system will have to be evaluated in a real-time pilot-in-the-loop
simulation environment, to enable test-pilots to assess the handling qualities of the
automatically controlled aircraft. In particular, the pilot–aircraft interaction should
be examined thoroughly, especially if the pilot will be actively involved in the aircraft
control loop (e.g. for fly-by-wire systems).
Based upon these results it is possible to select the best solution if there are more
feasible methods to fulfill the mission requirements. If the results of the on-line and
off-line analysis are completely satisfactory the next step in the design process will
be the implementation of the control laws in the flight control computer(s) of the
aircraft. The actuators and sensors in the aircraft must be installed (if that hasn’t been
done already), thoroughly tested, and calibrated. For some purposes, e.g. aircraft
certification, it may even be useful to test the complete control system in an ‘Iron
Bird’ test-stand arrangement with hardware-in-the-loop simulation capabilities.
Special care has to be taken to prevent errors due to the conversion of control
laws when moving from one design phase to the next. In particular, the transfer of
control laws from the simulation environment to the flight control computers of the
aircraft needs to be carefully verified, to ensure that the control laws used in flight
match those analyzed on the ground.1 In order to reduce the risks of such conversion
errors, it would be beneficial to be able to couple at least the complete FCC software,
but preferably also its hardware, to the real-time flight-simulator and/or off-line design
environment.
After successfully concluding the simulations and ground tests of the hardware and
FCC software, the FCS can be evaluated in real flight. In an ideal world this phase
would only be a straightforward verification of the previous results, but in practice
it will often be necessary to return to a previous stage in the FCS development for
fixing errors or fine-tuning the control laws. It also may be necessary to update the
mathematical models if deficiencies in these models are found during the in-flight
1During the Beaver autopilot studies some dramatic examples of conversion errors were encountered,
luckily for a large part before the flight-tests were started. Still, some minor errors remained undiscovered
until the actual flight-tests! References [28], [37], and [38] provide ample food for thought for
future FCS projects.
1.4. FCS design environments 7
evaluations. Quantitative results from the flight tests need to evaluated on-ground
to confirm the correct control behavior. Obviously, the off-line CACSD environment
that was used for the FDC design can also provide the required computer support
for this post-flight analysis.
The iterative nature of this FCS development cycle should be acknowledged: at
any stage in the process, the discovery of a fault, design error, or previously unrecognized
uncertainty might require the return to a previous design stage. Figure 1.3
summarizes the FCS design process. It illustrates the different design stages from
ref.[26] and the more detailed divisions presented in refs.[12], [31] and [35], and it
clearly acknowledge the iterative nature of the whole process. On the left-hand side
of the figure the models and tools (software and hardware environments) are shown,
while the right-hand side shows the design stages themselves.
1.4 FCS design environments
It is obviously very important to make the transitions between the different development
phases as straightforward as possible, in order to reduce the number of transition
errors which inevitably will arise if the tools for the different phases are not
compatible (Murphy’s Law), and also to reduce the time needed for the FCS development.
For this reason, there is a need for an integrated software environment that provides
full access to all required mathematical models, as well as a large range of
linear and nonlinear control system design and simulation tools, preferably from a
single PC or workstation. The software tools for the off-line analysis should be able
to effectively communicate with eachother, the tools for real-time pilot-in-the-loop
simulations, and with the flight control computers of the actual aircraft. Access to
the mathematical models and the control systems should be made as easy as possible,
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FDC 1.4 – A SIMULINK Toolbox for Flight Dynamics and Contro(8)
