This stuff can get confusing, and I hate to reopen a big can of worms, and I didn't really pay attention to the JSBsim VRP discussion a while back.

With YAsim you can pick any point as the (0,0,0) point and reference all your dimensions and positions from there. YAsim handles this just fine and has no bugs that I am aware of in this area.

Most likely where you are getting confused is that YASim reports the (0,0,0) position as the model position to FG. Again, this is no big deal if as an aircraft designer, you make the (0,0,0) position of the model correspond to the same (0,0,0) position you use in the FDM configuration.

Where you might be getting confused is with the view system. If (0,0,0) is the nose, then all the external views will center around the nose. The visual effect is quite compelling and makes the model *appear* to be rotating around the nose. But it's not! YAsim is handling everything correctly, the model is getting placed correctly, the model is getting rotated correctly. It only looks wrong because the view is always forced to center on the nose. When the nose pitches up, you don't see it because the view stays perfectly centered on the nose. All you see is the entire aircraft pitching down. That *looks* very wrong, but it's an optical illusion. The quickest way to fix this is to offset the view center location in the aircraft-set.xml file (see the p51 or pa28-161 config files for examples.)

If you want to use the same 3d model for multiple FDM configurations which choose different reference points, then that makes life a lot harder. I think that is what the JSBsim VRF is for.



Melchior FRANZ wrote:

* Jim Wilson -- Wednesday 04 August 2004 02:55:

Melchior FRANZ said:


When I noticed that the skids didn't work correctly, I found this VRP bug.
I shifted the model so that the VRP would agree with the YASim origin
and changed the view look-at point accordingly. This fixed the skid problem.
But now it became obvious that yasim turns the bo around the origin (nose
tip) rather than the CG! This was masked by the VRP bug.

Actually it wasn't, and has nothing to do with VRP. VRP is just JSBSim's way
of shifting the location where longitude, latitude, and altitude are reported
at. It has nothing to do with YASim. This actually enables JSBSim to report
location at something other than 0,0,0.

I know. But still, YASim's origin (the nose) is the (unchangeable) VRP, or what JSBSim calls "VRP", right? And the 3D model would also have to be moved such that the nose is at YASim's 0/0/0, too. But the bo105 does only turn around the CG if the main *rotor axis* wrongly is at YASim's *nose*.

YASim on the otherhand always reports at 0,0,0. We've often talked about
standardizing with the "Nose" at 0,0,0 and reporting the location
(lon,lat,alt) there.

I know. I read most of these discussions (and understood some of them. :-)

And now I'm out of ideas. Is it a YASim bug? A bug in YASim's rotor parts?
A bug in my brain?

Yes, it is a bug in your brain. Actually everyone's brain. I've added the
"solution" to the Flight Gear wiki.

Not so quick! I knew about the target offset and had considered it, but still the model rotated around the nose: I had the bo105 parked on the ground with the nose over a taxiway corner, then I slightly increased the collective, turned the bo with pedal input, and looked at it from above. The bo clearly rotated around the nose, if I had shifted the 3d model correctly in place. That was hardly and optical illusion.


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