On Thu, 2004-02-12 at 12:53, Jon S Berndt wrote: > On Thu, 12 Feb 2004 15:35:59 -0500 > David Megginson <[EMAIL PROTECTED]> wrote: > >Jon S Berndt wrote: > > >I'm not sure I see how this helps -- the model code still doesn't > >know where the CG is, so it still doesn't know where the centre of > >rotation for the model should be. > > This is precisely *why* the nose is used as a reference point. If the > scene graph (is that the right term/subsystem?) places the aircraft at > the spot reported by JSBSim -- that is, where JSBSim says the nose of > the aircraft is -- the perceived CG of the 3D aircraft as viewed in > the scene will fall exactly in the spot that the FDM says it should be > at. > > Look at it this way: the FDM tracks the motion of the CG, and the > rotation of the aircraft about the CG. The FDM knows teh location of > the CG at any point in time, as well as the euler angles of the > aircraft at that point in time. If we were to report the location of > this CG to FlightGear, and IF the origin of the 3D model was allowed > to shift (by some magic) and always be coincident with the "virtual > CG" in the 3D model, then we'd all always be happy and everything > would match up fine. The problem is, the CG shifts and the 3D model > coordinate system can't. > > Since the FDM knows (or can calculate) where the nose is at all times, > we simply report the nose location to FlightGear, and by convention > FlightGear places the 3D model's nose at the point reported by JSBSim > - the CG falls into place as needed IFF the 3D model is defined (or > scaled/rotated/translated in the scene graph) correctly as described > previously.
And said nose location *includes* any translation the nose experiences due to the aircraft rotating about the cg. In other words, if you could move the aircraft such that only the pitch angle changes (not terribly real, but humor me) and that pitch angle change results in the nose rising 10 feet, then the FDM reports the nose location as the CG altitude + 10 feet. So now, if the 3D model code positions the nose of the aircraft at that location and pitches it to the corresponding angle, the CG will not have moved at all but the nose will. And that means that the model will appear to rotate around the CG, just like it should (in free air, at least. On the ground is a different thing) > > Takeoffs and landings would look fine, etc. > > Jon > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- Tony Peden <[EMAIL PROTECTED]> _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
