> 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 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d