Melchior FRANZ said:
> I know. But still, YASim's origin (the nose) is the (unchangeable) VRP, or what
> JSBSim calls "VRP", right? And the 3D model would also have to be moved such
> that the nose is at YASim's 0/0/0, too. But the bo105 does only turn around the
> CG if the main *rotor axis* wrongly is at YASim's *nose*.

The only difference is JSBSim has the extra feature of specifying the VRP
which is an offset from where 0,0,0 ends up being specified.  It saved
changing all the entries for existing flight model configs that had 0,0,0 at
the firewall, etc.

You could say the YASim VRP is unchangable,  but that's partly a misstatement
as there is no VRP defined in YASim.  

It sounds like you pretty much understand what is going on,  but just keep in
mind this FDM side is only about where the lon/lat/alt gets reported in
FlightGear.  We talk about "Nose" but that's just a convenient way for the 3D
modeler to know how to position the Model so it represents where the FDM
thinks the aircraft is.  You may as well put the bo105 with the origin at the
nose and then address any other issues after, because that is where it ought
to be.

BTW I noticed that the bo105 fuselage in CVS is not anchored exactly to 0,0,0
but it is offset (I think it is 0.17 forward and 0.51 down).  This should
probably be fixed.
> Not so quick! I knew about the target offset and had considered it, but still
> the model rotated around the nose: I had the bo105 parked on the ground with
> the nose over a taxiway corner, then I slightly increased the collective, turned
> the bo with pedal input, and looked at it from above. The bo clearly rotated
> around the nose, if I had shifted the 3d model correctly in place. That was
> hardly and optical illusion.

Keep in mind that exactly straight down isn't possible, and remember the
camera is moving too.  So yeah that "illusion" can work from any direction,
even with what appears to be a good fixed reference.

The solver places the bo105 CG as follows on my copy:
  -2.491, -0.000, 1.091

I'm not really sure how the aircraft should turn and what relation rotation
would have to CG.  Note that your FDM thinks the nose is 1.5 meters (add the
offset I mentioned above) below the CG.

If there is a bug in YASim this would be the first I've seen of it.  Is it
possible that you never had the model positioned with the target offset
correct all at the same time?  Did you put the target offset in the right
place, in the right file?



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