One thing that may be confusing is that the VRP setting given by aeromatic is
wrong. In the JSBSim configuration file If the CG location is X, Y, Z,
then the VRP location is -X, -Y, -Z.I had thought that AC_VRP defines the
location of the VRP, however it actually defines the location of the VRP
*from* the CG (?). I never noticed it in the T-38 and other smaller
airplanes because the effect is hard to see. In a big airplane like the 1049
you can see it.
The above may seem authoritative, but I'm really only 90% sure it's correct :)
I know you have all been waiting impatiently for another VRP thread.
Dave
No. The VRP defines the location of an agreed-upon reference point in structural
coordinates. The CG, eyepoint, gear locations, etc. are all defined (in JSBSim)
in
structural frame. By convention, we've agreed that the nose is typically a good
reference
point, because it is (or should be obvious) to both the 3D model designer and
the FDM
designer. The CG generally cannot be used, because it moves - sometimes that
movement
could be profound.
Think of it this way: the structural frame is a fixed, solid, coordinate frame
that
permeates the aircraft structure. The structural frame we use MUST have X
positive out the
back, and Y out the right wing. The Z axis completes the right-handed system
positive
upwards. The _origin_ is what is usually found to be confusing. Often, the
origin is
located by having the X axis be coincident with the fuselage centerline, with
X=0 at the
tip of the nose - but THAT IS NOT A REQUIREMENT. If the origin is 200 inches in
front of
the nose, then the VRP could be defined as (200, 0, 0). If the 3D model designer
understands that, the aircraft model can be placed with the nose at the
location pointed
to by JSBSim. The VRP is the registration mark that relates what is reported
by JSBSim
and what part of the 3D model is placed at what location in the 3D world.
Within JSBSim, the equations of motion are all done relative to the CG.
However, JSBSim
can send to FlightGear the lat/lon/alt of ANY desired point on the aircraft, at
any time,
in any orientation (it's not hard). We just have to agree on WHICH point is
being sent.
That's what the VRP is all about.
I pray to God that explains it for the last time! :-)
Jon
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