On Saturday 10 January 2004 03:09, Jon Berndt wrote:
> 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
Perhaps some published 'Standards' might be a good idea - I remember the
earlier discussions about the model origin but didn't realise that a standard
had been established, and so in ignorance, I haven't been following it:(
Personally, I think some published standards would be a good idea and cut down
on the number of problems that occur, but I realise that it's more work to be
done...
LeeE
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