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.
_______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel