> On 11/22/2007 09:10 AM, Jon S. Berndt wrote: > > > At this time, JSBSim zeroes out the wind velocities while at rest on > > the ground, to prevent slippage. >
John Denker replied: > Aha. Thanks for the info. > > FWIW that sounds like *two* bugs to me. > 1) Maybe we need a better model of static friction and quasi-static > rolling friction so as to properly handle the behavior of tires > gripping the ground. > 2) We do need the real wind and real airspeed vector while on the > ground. There are many reasons for this, including things like > the weathervaning tendency, which is an important part of real > crosswind takeoffs and landings. For the best ground handling performance in JSBSim, ALL forces that act on the vehicle are zeroed out at rest on the ground, and wind effects are ramped in as velocity increases. Trust me, I've looked at this problem for years, read various tech papers on it, seen how other sims do it, and talked to experts. There's always a workaround of some kind. If you really think about all aspects of this, it quickly becomes a big headache. What happens when you are at rest, and you have wind forces acting on the aircraft? The wind can come from any angle, so the aero model has to be very good and all-aspect. The normal force at each gear can then be different than simply distributing the normal forces based on a static analysis. What happens if the combined moment on the vehicle and the translational force causes only one gear to go past its ability to hold traction on the ground? When you start programming this to take care of the general case, it just gets out of hand. With that said, it did occur to me that it should be possible to use one set of data to do gear modeling, and another set to report values (such as sideslip) for instrumentation. I'm thinking off the top of my head, now (as I'm in the middle of cooking and baking with my wife for Thanksgiving), but it seems we ought to be able to separate out the values used for gear modeling and those reported as output for display. I'll try to remember to look into that. Jon ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
