This brings back the discussion a couple weeks ago on the 747 origin. The following is what I got from that thread:
1). The reported "origin" is arbitrary in relation to the FDM's internal workings...ie how pitch/roll/yaw is calculated. The exact position of the 3D model origin can be calculated to be anywhere on the aircraft. 2). On a fixed wing aircraft, if origin is anywhere but between the wings the external model views that follow along with the aircraft (e.g. chase view) will not look correct. The camera is tied to the origin. If the origin is at the nose or tail then the plane "looks" like it's wagging when it pitches. And if the origin is too high or too low again the model can "appear" to be swinging like a pendulum or sliding when it should simply be rolling. In "reality" the model rotates correctly no matter where the origin is defined. This is only a visual effect of translating the camera position from the origin and there's pretty much no way to avoid it other than placing the origin as suggested above. It seems that after the now numerous past discussions, that we should be considering the visual effect first when it comes to 3D representations. This is only about the 3D model. Also, this location will differ substantially with different aircraft (consider the C172 with its wing forward and high, the swept centrally located ERJ-135 wings, or the Concorde's delta wing. It would be hard to make a general statement other than "usually center of the fuselage, and somewhere on the wings" or "just so the model's behavior looks reasonable". I hate to bother the engineers with this appearances stuff. Having grown up in a family of engineers I know how hard it can be to pick out a matching tie in the morning (which explains why most don't wear ties these days :-)). Best, Jim Jon Berndt <[EMAIL PROTECTED]> said: > > That's the hard part. The POH and the TCDS give the standard origin > > on the X axis (the weight and balance reference datum), and the origin > > on the Y axis can be assumed to be the centreline of the plane, but > > where do you put the Z origin? For a single, the thrustline of the > > propeller might be a good choice; > > Yes. > > > alternatively, the cabin floor at the firewall (right in front of the > > rudder pedals) should work for any aircraft, > > ehhhhh ... > > > and the ground sort-of works, as long as you're consistent > > about strut compression. > > yecch. > > > I'm not big on rules and standards, but we > > could publish a list of friendly suggestions for all aero and 3D > > modellers: > > > > 1. Put the X axis origin at the published weight-and-balance reference > > datum. > > 1) Respect the manufacturer's structural coordinates, if known. If not, > place the X axis origin at the published weight and balance reference > datum. If not known, place the X axis origin at the tip of the aircraft > nose or propeller hub - whichever is farther forward (not including probes > or antennas). > > > 2. Put the Y axis origin at the centreline of the plane. > > Yes. > > > 3. Put the Z axis origin [where? the ground?]. > > Place the Z axis origin as specified by the manufacturer's structural > coordinate system, if known. If not, place the Z axis origin at the tip of > the nose or propeller hub that the X axis origin is located at. This would > place the origin for the entire axial system at the nose or propeller hub. > > Maybe there should be a few sensible systems and we can specify which one > is to be used. For the FDM, as is known, its the relative distances that > need to be known, with the stipulation that the X axis is positive > backwards, the Y axis is out the right wing, and the Z axis completes that > using the RHR. > > Perhaps the above have problems, too, but it I agree that some kind of > agreement should be reached. > > Jon > -- Jim Wilson - IT Manager Kelco Industries PO Box 160 58 Main Street Milbridge, ME 04658 207-546-7989 - FAX 207-546-2791 http://www.kelcomaine.com _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
