On Sat, 2004-01-10 at 12:14, Alan King wrote: > Andy Ross wrote: > > > Alan King wrote: > > > >>The 'nose' is a bad choice for either the viewing center or the FDM > >>center. > > > > > > Except for the obvious fact that it's 100% unambiguous. It's not > > uncommon for the FDM definition and 3D model to be done by different > > authors. Take 21 people and ask them to identify the "POS" (or > > quarter chord point, or aerodynamic center, or even c.g.) and you'll > > get 21 different answers. Any child over six can find the tip of the > > nose on a photograph. > > If the visual model doesn't have EXACTLY the same distance from the nose to > the POS as the FDM, then they are out of line, period. No common point of > reference is going to make there be any LESS error if it's distance from the POS > is not JUST as exact in visual and FDM models. And if you do have error from > one model to the other, using the nose will actually exaggerate the error, the > longer lever arm from center for the error will actually exaggerate things. It > may be a bit easier see the nose visually but the internal models both best work > from the POS. There isn't a CHOICE on whether it takes more calculations or > not, it takes more to get out to that point of reference and back. Any amount > of ambiguity between models is EXACTLY the same but worse from extra translating > when using the nose. If we both didn't use "EXACTLY 6" feet back from the nose > as our centers, then there is just as much error as if we both didn't use the > same "EXACTLY 0" as the center. You can't have your visual distance 5 feet, > your FDM distance 6 feet, and have the models match correctly. The nose is just > an arbitrary reference point, that child over six won't be any more likely to > match the things that really need matching from referencing the nose than by > finding the center directly. Couldn't matter less if you found the nose > correctly if you didn't find it's offset from the POS correctly. Physics > doesn't worry much about what point is easy for you to match up when it's > keeping a plane in the air. A plane flies from this central point both visually > AND aerodynamically. And so should the models. If any other point is used then > there is extra translating required or the models have errors. And it's easy to > introduce other errors in all the extra translating.
> > The centers need to match for no error. Using the nose REQUIRES that you > have the visual distance from center exactly matched to what the flight model is > calculating for distance to center. If it's not then you have a geometry error > in the point about which your visual rotates on the axes. No way around it. > Using the nose as a point of reference does not magically relieve you of the > obligation of knowing exactly where the POS is to match them. You still have > error if you don't. And it has worse effects since the error is geared by the > distance from the origin. I think its understood that the FDM needs to know the vector to the model's reference point in order to properly calculate its location in space. Once that's info is at hand, all that's needed are the Euler angles and the model will look just fine. > > I was simply stating the fact that the best point of calculations for BOTH > visual and FDM models is the POS. This has nothing to do at all with what is > easier to line up. That point is the simplest for all calculations. Working > from any other point for calculations adds translational offsets to the > calculations, there isn't any way around it. If you line up by the nose point, > then you have to make quite sure that the distance from the nose point to the > POS for both models is exactly the same. You can fly an RC plane by looking at > the nose for reference. But you can also bet that those who do can't fly 3D > nearly as well as those who fly by looking into the center of the wing. That > isn't my opinon, it's me relating my observation of the fact that it's much > easier to fly a plane from the center point. It is the better reference point > because it doesn't have all the translational offsets introduced by looking at > the nose. When flying any plane at high angle of attack, the nose is a terrible > place of reference compared to the POS. The wing moves straight forward in > level flight at any angle of attack without changing height. The nose moves all > over the map, changing its distance from and angle to the line of motion like > mad. It's offset from the real center makes it a poor choice for calculations. > It is FINE to use the nose to match for a point of reference, IF both then > reference it to the POS correctly. Then just do the translation once and use > POS for both models calculations. BUT, there is no 'extra' better matching from > using the nose if it isn't matched to the POS. > > > > Take one of your models referenced by the nose. Now multiply it's visual > size by 10, leave the FDM alone. It'll look silly flying. Now have the common > reference be the POS. Any visual size scaling looks great without any > adjustment to the FDM. It is the much better reference point to work from for > matching the visual and FDM models. It is where they both agree with the least > calculations and problems from any errors. Matching your nose just SHIFTS your > error over if you don't have your visual and FDM centers matched too. And the > shift makes the total error motions worse not better. > > It is perfectly FINE to match the nose, and have BOTH say that it's exactly > this far from the POS, and do the calculation once then have the models work > from the POS. But the calculation from the nose to the POS has to be the same > for both anyway. You're only hiding the error from yourself if you believe > matching the nose will be easier and have less error than finding the POS. > > And point was that the calculations for both visual model and FDM are > simplified by using this point. > > > > > Having the FDM coordinates and model coordinates match up is > > critically important for collision issues like gear compression. > > > > There is a far more important point that has to be matched than the nose. > And there was also mention of a CG reference for the FDM. POS which is COL for > the plane is the REAL reference point and gets the simplest calculations. The > CG arms for calculating the CG are from the COL, not the current CG. CG is > purposely put a bit forward of the COL for stability. COL is the real reference > point, which is the POS for a flying body. COL is the average point where the > plane is being held up by the air. Everything else necessarily happens around > this point. > > I fly RC helis and planes in 3D. I have both my mental model of the physics > and the visuals of what is seen to go by. The place they match and rotate about > with the easiest calculations is the center, not the nose. I was simply > reporting this fact. I know it's a fact I use it every time I fly, it is much > better to fly from the center point in 3D than be flying from the nose and have > to deal with it's extra motions that often don't agree with the direction the > aircraft is actually moving. I can fly by the nose quite well too. But it's > easy to see it moves around the true center like mad. Something tells me you > guys rarely fly in odd attitudes from outside the aircraft. This is a quite > solid fact for me since I see and use it often. You may never properly > understand it just from theory if you don't fly from outside, you just won't see > why you're wrong about thinking the nose is the right place to match. > > Alan > > > > > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- Tony Peden <[EMAIL PROTECTED]> _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
