Jon Berndt wrote:
Jon Berndt wrote:


Model Reference Point (MRP):  This is the reference point that is agreed
upon by both the aircraft modeler and the 3D model builder.

I'd vote for calling it the "Visual Model Reference Point" because the term model can still be used for the 3d model and he flight model.

Erik


True.


Note this fact. If you have the CG point as the reference, then scale
matters very little to the motions of flying. Only the point reference makes the models match reasonably well. You still need a distance measurement to know the scale and work out CG shifts, where the nose, wheel, and tail really are, etc though. Even the control forces off. But the motion point is not, for both rotation and translation.


You aren't just using the nose point. No matter how much you trick yourselves into saying that you're only using the nose, and just 'fixing it', you are really providing a distance reference back to another point. It takes one point with full orientation, and another point to completely match two reference frames. There is no way to get around that fact. What you are really doing is having the nose point and a difference reference, and then working everything out from that bringing everything into alignment. That includes most importantly for visual motion to FDM motion match the CG.

If you look across all your models of all scales, you will see that your adjustments all bring the CG point of the visual model to the CG point of the FDM. The CG to nose distance will be different, but that's in the offset combined with whatever the FDM does with the offset to find the CG point.


Take all of the models. Put all of the nose points at exactly the same spot. put the FDM nose point there. Note that you have a range of CG's based on how large the plane drawing is. And your 'nose fix adjustment' is the distance to some other point. Then your FDM works out the CG point of each size model from that other known adjustment and your CG is in the right place in both models. We already put all the nose points into one spot. If your alignment system only needs the nose point to match in the FDM and the visual model, then set the FDM nose to a known point, put all the visual nose points there, and your models will all work no matter what size. This doesn't work, even though all of your nose points are now correctly known. It doesn't work because you need a nose point known, plus another number to some other reference point. This other reference gives you scale, and then put all other points in their place. Most importantly to visual motion, it lets you put your CG where it goes on the visual model.


That was my original point, it is impossible to have just the nose point on the visual model and not have anything else and have everything match regardless of scaling between the FDM and the visual. You have to have some other reference fix, that lets your FDM bring it's CG into alignment of where it should be on the visual model. That you are hiding it from the model creator so they don't have to find the CG of the visual model is a great idea. That you are hiding it from yourselves though by saying you're 'fixing the nose point' isn't as good. You're sliding everything around with this adjustment, not just the nose point. And while it brings everything into line, the single most important thing to how the model moves is getting the CG correct. You don't have some arbitrary fixed point on the nose of both models that makes everything line up. You have a fixed point on the visual model only. You then move it around on your FDM model with your adjustment, until the CG is where is should be for the visual model so the motion looks correct. If you double the scale of the picture, you have to adjust your nose to CG distance through your adjustment, even if that uses some other common reference point to get the CG in the FDM. Your adjustment is fixing where the nose should be, relative to some other point, so that that other point is where it should be, so that the CG is now also calculated in the right position in the visual model.

You don't have a 'nose point does everything' system. That adjustment isn't some minor little tweak to your system. That 'adjustment' carries every bit as much weight in aligning the system as the nose point. Using the nose point first lets the model creator make the model without reference to scale, which is great. But it just means that someone on the FDM side is fixing the scale later through the adjustment, not that the nose point is the only thing and nothing else matters. That adjustment is just as critical to the alignment of the systems as the initial point. It takes both, the nose point isn't 'it' like some of you have tried to imply. There was something else to your system, which there had to be to bring the FDM and the visual model frames into full alignment. No single point fix at the nose or even the CG can completely align two frames.

Alan






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