Martin Spott wrote: > Simulating friction on the ground should be quite easy as long as you > know some parameters: You have to know about position as well as > horizontal and vertical forces of _each_ wheel. Probably this is > already there for a C172 (as mentioned above, I don't know), the rest > is sort of practical mechanical numeric almost every student in > engineering should learn in the first or second year.
It doesn't work like that. Put a book on the table in front of you. What is the friction force? Zero. (If it wasn't, of course, the book would be accelerating.) Push on it a little, but not enough to move it. What is the friction force? Equal to your push force, and in the opposite direction. Somehow, the book *knows* how much force to apply based on external conditions. You don't just calculate the friction force and apply it. You need to know what is happening to the system. For a single book and a single external force, the solution is trivial. For N gear objects applying force in 2 dimensions, this becomes a giant simultaneous equation problem. Even worse, there might not even be a solution! Big problem, right? Nope. It just means that the vehicle can't be held in place and will start to slide. Which gear are slipping? How do you tell? And those are just the complexities I understand right now. The stuff that really worries me are the bits I can't figure out: if there is a solution, is it unique? What do you do if the terrain is 3D, and the gear aren't on a single ground plane? (The friction force is 2D only, the compression force doesn't work the same way at all.) Basically, this just won't work. Sorry. The idea of switching from a sliding friction model to a static spring at low speeds is probably as good as we're going to get. But quite honestly, it's been my experience that almost all of the YASim aircraft I have worked on can be made to sit still with a little gear tuning. I'm not convinced that this is a critical feature. Andy _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
