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Good morning,
I took a couple of classes in Matlab/Simulink last
month and this was addressed specifically in the class. Matlab permits you
to vary timestep size as you approach the ground. It you extrapolate ahead
in time to see if any of the gear have come in contact with the ground you can
then retreat to the previous time, cut the timestep size down and then go
forward again until you capture the ground contact at a fine enough stepsize
to prevent instability. It isn't necessary to run the entire
simulation at this reduced stepsize if you can run the gear model as a faster
subtask of the main simulation. Matlab then does running checks to vary
the timestep size on the basis of a predictor-corrector algorithm (if there is a
large discontinuity it will go back and systematically chop down the timestep
size until the output is "sensible". It's possible in this modern age
to find implementation of these algorithms (Adams-Bashforth is one that I'm
familiar with. Naturally you are taking a chance on frame
overruns if you let the program decide its update rate, but then that's fixable
too in this age, using a faster processor.
When I worked with commercial airline training
simulations the common "smoke test" to see if everything was working OK was
to taxi on the ground with the autopilot running. This was the peak load
situation where any problems with overruns were most likely to show
up.
Hope this helps.
Nickolas Hein
Morgantown WV
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