--- Jim Wilson <[EMAIL PROTECTED]> wrote: > Andy Ross <[EMAIL PROTECTED]> said: > > > Jim Wilson wrote: > > > It isn't tedious at all, we can offset the origin to where we > want > > > without messing with the ac file *and* it won't affect the > > > animation. > > > > Cool. Uh, how? :) > See: > http://www.flightgear.org/Docs/fgfs-model-howto.html#repositioning > > > > Having the FDM origin at the center of gravity should improve > the > > > appearance of the 3D modeling since pitching of the model would > appear > > > more realistic. > > > > It wouldn't matter. The FDMs do the dynamics for you. You could > put > > the origin of the model on Mars* and the model would rotate > correctly > > on the screen so long as the origins agreed. The aircraft rotates > > about its "real" c.g., which has nothing to do with either the 3D > > model coordinate conventions or the YASim coordinate origin. > > > Ok, yes as long as the origin is in sync, and the fdm rotates > correctly. Just > the same if the FDM origin was at the c.g. (geometry or gravity?) > instead of > the cockpit there would be a better chance of actually having the > thing on the > runway. > > Now that I think about it the 747 was originally setup to originate > in the > cockpit, until I saw the model sitting way back off the runway > threshold at > startup. So then I guesstimated where the greatest lift or pitch > axis would > be based on the shape of the 3D model for both the 747 and the A-4. > No doubt > these are off a bit and would change anyway with load. > > Obviously it would be better to get the two origins in sycn. I can > think of > two solutions. Either move the FDM origin back to c.g. (or some > place not at > one end of long body aircraft), or create an offset for the startup > position > on the runway configurable per aircraft. BTW, where are the JSBSim > origins?
I'm really struggling with the relevance of this discussion. No matter where the origins of the coord systems used to specify coordinate input to the FDM's are, the center of rotation will *always, always, always* be either the c.g. (in-air) or a ground contact point (on-ground). This is not a design choice, it is the nature of the physics involved. The reported position and euler angles are always referenced to the c.g. (be it in-air or on-ground) unless specifically calculated otherwise. The only case of that I know is the pilot accels. This is a design choice, but is consistent with industry practice. I would recommend either a) changing the 3D model origin to be just forward of the main gear and about centered vertically, or b) adding code to FG to properly calculate the position of the origin you'd like to use. BTW, you will rarely see the c.g. used as a reference point for dimensions on aircraft. First of all, it moves in flight. Second of all, it's very difficult to actually point to its location. By that I mean that it's not hard to calculate its location, but it is hard to then walk up to the aircraft and put your finger right at that location. > > Best, > > Jim > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > > __________________________________________________ Do you Yahoo!? HotJobs - Search new jobs daily now http://hotjobs.yahoo.com/ _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
