Paul:
The root of the problem - though it is not really a "problem" - is that the
FDM cares about modeling where the aircraft "is" in the world based on the
aircraft CG, and the 3D model wants to be in the correct spot in the world,
too, but how does one decide where to place it? Can one simply take the
FDM-reported CG and place the aircraft (0,0,0) point there? No, because the
various aircraft modelers can use any point they see fit to create their
aircraft model. There is no universal convention for that.
We discussed here in the group some time ago a convention. The result was
that the forward most center portion of the aircraft would be the MODEL
REFERENCE POINT ("MRP" hereafter, not to be confused with the AERODYNAMIC
REFERENCE POINT). If the FDM can report the position of the MRP (and it
can), the scene code on the FlightGear side can place the aircraft very
nicely where it is supposed to be.
This is a slight problem for the FDM, though. The FDM has to make sure that
it knows where the MRP is IN RELATION TO THE CG. This is very important to
remember that the FDM reports the lat/lon/alt of the aircraft CG. As fuel
burns off the aircraft CG moves. So, the vector to the MRP will change with
time. It's not a big problem, just a little work for the FDM - a little work
that JSBSim does not currently do.
I hope this clears things up more. Most of this thread will find it's way
into some documentation I am writing!
Jon
--
Project Coordinator
JSBSim Flight Dynamics Model
http://www.jsbsim.org
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