> I'm still very confused with the axis used, and the model is appearing
> behind the actual FDM A/C

In the aircraft config file, locations are given in "structural frame". Structural 
frame
in an aircraft has the X axis pointing backwards, the Z axis up, and the Y axis out the
right side. The origin can, technically, be anywhere. Usually, it is near the nose. The
exact location does not really matter, because *internal* to the FDM, contact point
distances from the CG are computed and used - again: _internal_ to the FDM only the
difference between two points is used.

> values used are:
>    AC_CGLOC     921.1  0.0 -18.0
>    AC_AERORP    921.1  0.0  0.0
>    AC_EYEPTLOC  0.0 -30.0 75.0
>    AC_VRP       0.0 0.0 0.0
> the  3d model has nose at (0,0,0)
>
> I understand it this way, please correct what's wrong:
>
> AC_CGLOC - this is the center of gravity, from which all other points
> are related to

Sort of. The CG is the point about which the equations of motion are based - the CG is 
the
point about which the forces and moments act, and about which the aircraft rotates.

> AC_AERORP - aerodynamic reference, should be near the CG, so it has a
> similar value

Yes.

> AC_EYEPTLOC - this is where the view is placed when in cockpit view,
> related to(??) CG (??)

This is just the point where the pilot's eyes are located. I do not think this is used 
in
FlightGear, but it is used in JSBSim to also calculate the accelerations that the pilot
feels. The pilot's "sensors" are located near his/her eyes, too.

This is given in inches as other measurements are, and in the same coordinate system as
CG, reference point, etc.

> AC_VRP - point related to the CG where the nose of the 3d model should be
> A 3d model with nose made at (0,0,0) in the editor should have a VRP of
> (0,0,0) and this should be enough to position the model properly

This is the point that the 3D modeler and the aircraft flight model designer agree 
upon as
a reference point. From the FDM point of view, really only the CG is of interest - i.e.
the latitude/longitude/altitude that is reported by the FDM refers actually to the
specific lat/lon/alt of the CG. After long discussions some months ago, we decided 
that in
order to make things work out "better", the FDM would report the position of some 
common
point that the modeler could easily see. That point was agreed upon as the nose of the
aircraft. So, the VRP is simply the (x,y,z) coordinate (in inches) of the nose of the
aircraft in the same coordinate system being used for reporting the CG, aero reference
point, landing gear locations, etc.

The modeler would then make sure either that their 3D model had its origin at the 
nose, or
that the offset from the origin of the 3D model to the nose (the agreed upon visual
reference point) is given. I forget which file is used to report this 3D model 
"offset",
or "skew".

The end result of using the VRP is that the FDM always reports the position of the nose
(considering the orientation and position of the aircraft), and if FlightGear places 
the
3D model so that the nose of the aircraft model is at the lat/lon/alt reported by the 
FDM,
then everything works out. But, the camera (I believe) looks at the nose of the 
aircraft,
too, so it appears as though the aircraft rotates about the nose, so there is probably
some adjusting that needs to be done to the camera angle ... I forget, and that's not 
my
area of expertise.

Jon


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