Jim wrote: > Before we get too worked up about this... It has absolutely > nothing to do with modeling the aircraft. It is only a reference > point for positioning the 3D model in the scene. All the nose is, > sans pitot tubes and other items that are not centered, is a > location easy to identify on almost all aircraft.
Yep. There's another concern here, though - one that is a bit more difficult to grasp. Someday(tm) JSBSim will have a multi-body type of capability (carrying food packets to drop, water "bombs" to put out fires, or whatever). This is of no concern in this thread other than that when a payload is released, the CG changes instantaneously. This is one reason why tracking the CG is not a good idea (as well, fuel burnoff moves the CG). Before going further, it is important to realize that the FDM (JSBSim, at least) reports the position of (*integrates* the position of) the CG. That is, ALL forces and moments take place about the CG, which affects the motion and future location of the aircraft CG. Now, the position of the CG of the aircraft is not really calculated specifically. It is integrated over time given the velocity - i.e. the previous location is altered by the movement since the previous time step in order to arrive at the current position. So, in the case of a payload being dropped, a step change in location is not seen (as far as JSBSim is concerned). This parallels reality. In the case of a visual model, we need to be careful how we relate the FDM model and the visual 3D model in order to preclude seeing a step change when the vehicle has a mass/balance state change. With this in mind, now, we go back to the stated problem. Let's say we (FDM/JSBSim) are reporting the lat/lon of the nose of the aircraft to FlightGear's visual subsystem. We know where our CG is in lat/lon, and we know where the nose is relative to that. So we report in lat/lon/alt coordinates the position of the nose. Now, let's say we drop a payload and the CG shifts. In the FDM, the position in lat/lon/alt is still the same. However, the location of the nose has now shifted with relation to the CG, because the CG has shifted. If we report the new nose position to the FlightGear visual subsystem, there will be an instantansous shift in the visual position of the aircraft. Follow? Jon _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
