Re: [Flightgear-devel] Landing Gear discussion
differential equation sense). Fixing *this* by interpolating the force function over small velocities leads to a stable but non-physical solution that exhibits the drift problem that was talked about. Ah, OK. So did I get this right, here's a tradeoff between the ``physically correct'' model and fixing the stiffness? I.e., stiffness was ``fixed'' by using a physically incorrect model? -Gerhard -- | voice: +43 (0)662 642934 *** web: http://www.cosy.sbg.ac.at/~gwesp/ | | If emailing to [EMAIL PROTECTED] doesn't work, please try [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Landing Gear discussion
be skidding at any given moment. The notion of holding forces at zero makes intuitive sense, but underneath that it has very little physical meaning. On the contrary. I haven't followed this discussion too closely and I'm no physicist either, but this sounds to me exactly like static vs. gliding friction. I still think the rooted spring model has the most promise, although it too is complicated by the fact that rolling wheels will skid in only one dimension. Isn't that a bit of oversimplification? My car will skid in at least two dimensions if I try hard enough :-) And a small amount of skidding will always be present. -Gerhard -- | voice: +43 (0)662 642934 *** web: http://www.cosy.sbg.ac.at/~gwesp/ | | If emailing to [EMAIL PROTECTED] doesn't work, please try [EMAIL PROTECTED] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Landing Gear discussion
Hi, I'm no math or phisics genius but I was wondering if anyone has tried making the friction logarithmic. As in high friction at slow speeds and quickly dropping to normal friction. This is just a suggestion. :-) Stephen ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] Landing Gear discussion
Hi, I'm no math or phisics genius but I was wondering if anyone has tried making the friction logarithmic. As in high friction at slow speeds and quickly dropping to normal friction. This is just a suggestion. :-) Stephen Actually, there is some truth in this. To show yourself, try turning your car while going forward very slowly. Then try it again going faster (in a parking lot - be safe!) You'll see that the turn radius is different between the two runs. I need to turn my Mustang very slowly because it has an abysmal turn radius. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Landing Gear discussion
Gerhard Wesp wrote: On the contrary. I haven't followed this discussion too closely and I'm no physicist either, but this sounds to me exactly like static vs. gliding friction. Yes, there are separate coefficients of friction for the static vs. dynamic case. But these are only different numbers. The problem is that the *model* (the algorithm used to compute results) must change in the static case. In the dynamic (skidding) case, it is enough to know that the coefficient of friction will be proportional (by the dynamic coefficient of friction) to the normal force into the ground, and be in a direction opposite the velocity. This works. The problem is that in the static situation, this model breaks. It's still physically correct, mind you, but it leads to numerical instability in the algorithm because of the very high velocity derivatives of the force function (i.e. the problem is stiff in the differential equation sense). Fixing *this* by interpolating the force function over small velocities leads to a stable but non-physical solution that exhibits the drift problem that was talked about. Isn't that a bit of oversimplification? My car will skid in at least two dimensions if I try hard enough :-) And a small amount of skidding will always be present. It's not a simplification, it's a complication. A static spring in two dimensions requires storing a location and computing a distance. What do you do when the location has changed becaues the tire was rolling? You need to use the velocity and compute a 1D distance from where the tire would have been. Ick. Andy ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
re: [Flightgear-devel] Landing Gear discussion
Jon Berndt writes: In the end, it could turn out that a physics-based approach is not worth the effort, and we should simply make the aircraft do what experience tells us a real aircraft would do. As either you or Andy mentioned before, the problem is the transition. Improving the steering gain might help a bit, though. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
re: [Flightgear-devel] Landing Gear discussion
Jon Berndt [EMAIL PROTECTED] said: If there were no winds at all, that might help. Otherwise, it doesn't work at all. Jon Let me expand on that. If you do come to a stop, and there are no winds at the moment, then the winds come up after you have stopped, then having reduced the forces as your velocity goes to zero, you won't have any resistance to the wind, and you'll start sliding again. That makes sense. Thanks. So then what would happen if you artificially introduced resistance at the same time (near zero velocity) in a manner similar to a partially applied parking brake? Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
re: [Flightgear-devel] Landing Gear discussion
Jim Wilson writes: So then what would happen if you artificially introduced resistance at the same time (near zero velocity) in a manner similar to a partially applied parking brake? The problem is that if the landing gear produces opposing forces or moments that are too great, the plane starts moving or turning in the opposite direction. They have to exactly balance for the plane to stand still. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
re: [Flightgear-devel] Landing Gear discussion
David Megginson [EMAIL PROTECTED] said: Jim Wilson writes: So then what would happen if you artificially introduced resistance at the same time (near zero velocity) in a manner similar to a partially applied parking brake? The problem is that if the landing gear produces opposing forces or moments that are too great, the plane starts moving or turning in the opposite direction. They have to exactly balance for the plane to stand still. That's what I got out of Jon's earlier post. I don't think I'm asking the right question (and I probably won't this time either :-)). Can't we bring in some sort of damping factor that would just render the aircraft stuck at very small velocities, but would still allow it to become unstuck if a great enough force was applied? A sort of automatic parking break that gets applied gradually starting at 0.01 fps and slower. The current brake seems to do a good job of holding most aircraft still. Rather being something that is realistic I'm thinking about some arbitrary value that would be derived through trial and error by the modeler. My apologies for the stupid questions. Needless to say I've just barely scratched the surface only having really studied a specific portion of the YASim code when playing with the p51. Best, Jim ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Landing Gear discussion
Jim Wilson wrote: Can't we bring in some sort of damping factor that would just render the aircraft stuck at very small velocities, but would still allow it to become unstuck if a great enough force was applied? A sort of automatic parking break that gets applied gradually starting at 0.01 fps and slower. The current brake seems to do a good job of holding most aircraft still. Rather being something that is realistic I'm thinking about some arbitrary value that would be derived through trial and error by the modeler. I was amazed at how tricky this got a year or so ago when I was experimenting with it. I agree that we need some kind of damping at slow speed. Essentially, the gear forces have to become a special case, reducing forces and moments towards zero but never beyond into the opposite sign. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Landing Gear discussion
David Megginson wrote: I was amazed at how tricky this got a year or so ago when I was experimenting with it. I agree that we need some kind of damping at slow speed. Essentially, the gear forces have to become a special case, reducing forces and moments towards zero but never beyond into the opposite sign. For the record, I tried it too. It didn't work for me either. :) The problem again comes down to the fact that it's a multivariable problem. There are multiple gear objects, and only some of them may be skidding at any given moment. The notion of holding forces at zero makes intuitive sense, but underneath that it has very little physical meaning. I still think the rooted spring model has the most promise, although it too is complicated by the fact that rolling wheels will skid in only one dimension. Andy ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
[Flightgear-devel] Landing Gear discussion
David M. wrote: Unfortunately, not -- when the JSBSim and YASim aircraft are rolling, they are still far too much affected by the wind. In real life, even with 30 kt gusts, you can usually taxi a 172 or Cherokee around as if it were a car. Personally, I do set the controls appropriately just in case (I'd hate for that one gust to get under the wing and flip me), but I have not seen it make a noticeable difference. Thanks for the input. When I get a chance, maybe I'll try increasing the steering gain. In the end, it could turn out that a physics-based approach is not worth the effort, and we should simply make the aircraft do what experience tells us a real aircraft would do. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
