> If you know everything about both frames all the time, then why is there ever > a need for any adjustment figure? Everything was calculated, all needed > reference points were known already from the VRM to the FDM, you didn't adjust > anything there was no need.
The 3D model and the FDM are each know about their *own* frame. The FDM knows nothing about the 3D frame, and the 3D modeler frame knows nothing about the FDM frame[s]. Remember, we are talking about code here. It only does what we tell it to. The FDM side exists separate from the 3D code and vice versa. We have to tell them about each other, as it were. > Some of these factors weren't known at first if people can draw planes of any > relative size in the drawing program. You are figuring out the size > relationship somehow, it is not set if they can just draw the model any size. > If one person can draw it twice as large as another, and you can match the FDM > to it, then you are matching more than just the nose point somehow. You are > making your scale bigger somehow, or shrinking their drawing, if you can match > the bigger model. If not then you have to move your nose around to match, then > change every motion constant in the FDM to make a large drawn model act large, > and a small model act small. And even then the modeler has to draw it the right > size, they have to make the model 30 feet if it needs to be 30 feet. This is probably where the misunderstanding is coming in. It is a convention that the 3D modeler "composes" models in units of inches, or that the model is composed in units of feet and scaled by 12. It is known that the JSBSim FDM (for instance) uses dimensions of inches in the structural frame (an industry standard). The size relationship is a standard, it is a convention, it is not figured out dynamically. Maybe Jim can chime in, here. It is obvious that if the models were scaled in some odd, non-standard way, we'd need to fix them. > I know you're doing it correctly. The models, once adjusted, look and act > properly. It is your explanation of how the two reference frames fully match is > what's lacking. How come everyone else gets it? > You say no no we only adjust the nose, when really you are > doing other things too to match the visual and FDM frames with your adjustment > figure, even if you don't see it. You are making every point match in scale in > both frames, not just the nose. Your adjustment figure is from the nose in one > frame to a reference in the other. Your scale is either known > set or adjusted along with this adjustment through other factors. I don't have to deal with scale. If I *was* to create my own 3D model, I know that I need to use units of inches, and I know the frame that the FDM describes the aircraft in, so I could create a 3D model that would work out of the box. Obviously we have to live by certain rules. And we are not and never have said "no, no, no we only adjust the nose". What we intend to do - and what we do - is to correctly place the 3D model so that it's "virtual" CG is co-located with the CG as known by the FDM; but, we do this in an indirect way via the common VRP, out of necessity. > And when you get a new model, and move it around with your nose adjustment > figure, and aligning your reference point, you are aligning your CG's even if > you don't say you're aligning your CG's. That's the *idea*! Let me repeat: the 3D model really doesn't care about CG - it just wants a yaw, pitch and roll, and to be translated to the proper location. > Yes, they fall into alignment when you > get your reference points matched and your nose was in the right place. But how > is it you know when your reference point looks like it matches? By the CG. Yes. That's what we've been saying: the Visual Reference Point (VRP) is located by the FDM because the CG is known in structural coordinates at any time, and the VRP is know because we specified it in structural coordinates, so we always know the relative vector from the CG to the VRP (remember, the CG floats). By placing the nose (the common VRP) of the 3D model at the world-space coordinates that the FDM determines, the CG of the 3D visual model is located correctly - it's co-located with the CG as it is placed also according to where the FDM says it should be. The 3D coordinates that comprise an aircraft visual model do not change. If the nose VRP is located at (10,0,20) then it will always be at (10,0,20). If the FDM model for an aircraft has the common VRP located at -15,0,400 then it will always be *there*. The FDM calculates the position of the CG using the 6DoF equations of motion. The relative vector from the CG to the VRP as calculated by the FDM is the glue that ties the two frames together. > You > may look at only your reference point. But when you move the plane, and you're > saying 'hey it looks like we got the nose and reference point right', how do you > tell? > You judge it's motion relative the motion of the CG. You are matching > the CG of the FDM to look in the right spot. Even if you say 'No, we are only > looking at this other spot swinging around the way it should', you are really > looking at it swinging around another point, the CG, when you check it visually. In the physical world, the aircraft rotates about the CG - that's what the equations of motion are all about. That's what the FDM does. The 3D modeling code is separate from the kinematics and dynamics stuff - all it is concerned with is proper rendering. It is up to the visuals people and the FDM people to find the best way to communicate the proper position of the 3D model. > Like it or not that is how you are almost guaranteed to be aligning the visual > model. There is very little else to accurately gague in a visual model in > motion than 'is the model rotating correctly around the CG?' When you're > aligning the visual with your number you're aligning the CG point, even if you > do it through reference to some other reference point. Now you've got it. That's the whole idea. Unfortunately, in practice, you will find that it is not straightforward to simply match where the FDM *knows* the CG is, with where a "dumb" assortment of vertices and normals "thinks" the CG should be. There has to be a reliable, common VRP. > Regardless, there is a 3 or 4 sentence explanation of exactly how every point > in your FDM frame, and the entire visual model frame, match. This includes > matching any model of any size. It can be said easily, without saying "We only > align the nose, you're not getting it." You are leaving something out of your > objective explanation of how they match. You put it all in no doubt with the > relative points match since it works in the program. But there is a simple > overall description of how it matches that is much more accurate than 'we just > align the nose'. No wonder you've discussed it to the point of being blue in > the face if you haven't got the simple full reference match description to > rattle off. Jim can talk to you about scale - that's never been a problem from my point of view. > I am not the one not getting it. There is a simple relationship between the > frames, not described fully by just saying 'we're fixing the nose'. You have a > correct reference to reference model in your head no doubt. You likely have a > correct idea of how the sizes and shapes match up. But you do not give the > simple explanation of 'this forest matches and aligns completely and is size > matched to this other forest by these these 3 or 4 steps'. You keep describing > how this tree relates to that tree relates to that other tree and so on. You're > so busy trying to explain every little tree you don't give out the simpler > explanation of how everything is adjusted by your points and adjustments not > just your nose. There is a simple transform formula for every point in the VRM > to fix to every point in the FDM to align and scale for any size drawn model. > And you're not giving all of it when you say you just align the nose. Like I said above: the FDM structural frame is in inches, with the axes aligned as stated. The 3D visual modeler either knows our scale, or we adjust the scale to match in the modeler. As far as scale goes, I believe we are one for one. > Again, no need for reply. I will come back in a day or two with a complete > description of how all points are transformed by your 'nose point' and your > adjustment figure, and how any scaling is accomplished or if it's fixed from > outside convention. It is fixed. > And how you can adjust the adjustment to position any size > visual model correctly. I'm pretty sure you do it by moving your adjustment > number till the nose and reference point align and are the correct distance, and > I'm pretty sure you tell they're aligned by looking at the model in the program > and seeing it. It is often done from engineering drawings. It's not imprecise. > And I'm pretty sure that is done by when everything's rotating > around the right reference point and it all looks right, and that's the CG as > the main reference point for looking to see if everything's rotating around > right. Even though you say you're not visually aligning the model by the CG but > really your nose adjustment to some other point. We of course want to align the visual 3D model so the "virtual" CG of the 3D model coincides with the CG as known by the FDM. Due to the variables we have explained ad nauseum, it is more expedient to communicate a known common and *fixed* point in each "world". Jon -- Project Coordinator JSBSim Flight Dynamics Model http://www.jsbsim.org _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
