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
----- Original Message -----
Sent: Sunday, April 25, 2004 10:26 AM
Subject: Re: [Flightgear-devel] Jsbsim trim


On Samstag, 24. April 2004 21:31, Andy Ross wrote:
> It's a misfeature in the gear modelling.  YASim has pretty much the
> same behavior.  Both FDMs model gear force as a function of "skidding
> velocity", which is fine for dynamic solutions.  But a gear that is
> planted on the ground is capable of "holding" an aircraft at zero
> velocity, which doesn't work with the current FDMs -- zero velocity
> produces zero force.
>
> What's needed is code that, at low speeds, uses a spring model for
> gear force based on the distance in position from where the gear is
> "stopped".  Which sounds easy, but in practice is awfully hard.  I've
> gotten started on this several times, and never produced useful
> code.
Yep, kind of true.

Jon and I are working on a solution to this. At least for JSBSim.
It is not only the gear modelling which plays a role here, the time
integration of the equations of motion play a role too.

There is code on a development branch in JSBSim's cvs which addresses this and
also works well so far.

And Andy, If I remember right, I believe that it was a comment of you about
the problem beeing stiff, that made me look into JSBSim's timestepping ...
;)

   Greetings

         Mathias

--
Mathias Fröhlich, email: [EMAIL PROTECTED]

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