"Jon S. Berndt" <jonsber...@comcast.net> wrote: > The wheel slip angle, for instance, varies wildly in JSBSim when no > filtering or other compensation is used: [...] > If the forces are ramped down to zero below some threshold, the dynamics > improve greatly:
This one shows the nose wheel _velocity_ over a timespan, the other three diagrams are showing the slip _angle_. To draw a reasonable conclusion it would be interesting to see the same measurand, say the slip velocity for all of the involved cases and not only for the nose wheel but for for the main gear as well. BUT, basically, I'd say this is not the point at all - these are interesting theoretical approaches to the problem, but for the 'real' user who's getting veered off the apron while he is simply doing a proper run-up at a windy location, this is entirely irrelevant. > So, I have to ask you: what is "proper tire/ground reaction"? With a few > lines of code I've reduced the ground reactions artifacts for the f-16 to > very small levels - and I really haven't optimized them, yet. They could > maybe be improved even more. > > So what is the cost/benefit to adding more code to do gear modeling more > precisely? How much would the user really notice the difference? If we model > the ground reactions to such high fidelity, should we then also model the > aero characteristics using the Navier Stokes equations? There are always > tradeoffs in simulation. In the case of JSBSim, I am hoping to model ground > reactions so that the end user is satisfied with the response. I'll tell you that quite a bunch of users is unsatisfied but many of them take the current situation as given because they didn't know that a proper solution had been available. So, please correct me if my conclusion is at fault: 1.) The current, unfortunate state of tire/ground reactions in JSBSim, the fact that FlightGear's default aircraft is slipping over the tarmac is _NOT_ caused by the fact that there's nobody around wo knew better .... as alledged by this posting of 2007-11-22: "Jon S. Berndt" <j...@hal-pc.org> wrote: >> Maybe it would help to talk to the CDM guys instead of the >> FDM guys. >> http://www.google.com/search?q=car-dynamics-model >> http://www.google.com/search?q=car-dynamics-model+nascar >> >> I reckon the car-dynamics guys have a pretty good model of >> static friction and quasi-static rolling friction. After >> all, that is their raison d'tre. Whatever they've got >> is surely more than good enough for our purposes. > > Yes. I have looked at car dynamics casually. It would be nice to get > one of those guys onboard. As far as I can tell, this posting had been written _after_ a patch to implement proper simulation of tire/ground reactions had been submitted to JSBSim. 2.) The current state is also _NOT_ a question of having a "budget" to pay someone doing rocket science - 2009-02-03: "Jon S. Berndt" <jonsber...@comcast.net> wrote: >> * Jon S. Berndt -- Tuesday 03 February 2009: >> > It's a tough problem, [...[ >> >> Maybe we should hire a rocket scientist? :-] > [...] > With our *huge* budget? :-) NO, it's simply due to the fact that the FlightGear project decided to tie themselves to the development goals of JSBSim of which the 'project steering committee' decided against adding proper ground reaction code - which had been availale for free - for the sake of a smaller line count. FlightGear is suffering from a 'political' decision which has been made at JSBsim. .... which brings me back to - oh - my own words, written last month: > Overall, I think some day "the crowd" should start making up their mind > about wether relying on an externally maintained FDM is still the way > to go. Developing a copy of the FDM _in_ FlightGear might return a much > higher benefit at reduced effort. Just for the sake of completeness: Think of a scenario on the carrier, you're approaching the catapult from behind .... oh, too far, you're idling the engine, letting the wind blow you backwards while you're slowly manoeuvering your aircraft right into the catapult position solely by nose wheel steering. That's what I call proper simulation of tire ground reactions. FlightGear _could_ do that - it they didn't depend decisions made at JSBSim. Cheers, Martin. -- Unix _IS_ user friendly - it's just selective about who its friends are ! -------------------------------------------------------------------------- ------------------------------------------------------------------------------ Create and Deploy Rich Internet Apps outside the browser with Adobe(R)AIR(TM) software. With Adobe AIR, Ajax developers can use existing skills and code to build responsive, highly engaging applications that combine the power of local resources and data with the reach of the web. Download the Adobe AIR SDK and Ajax docs to start building applications today-http://p.sf.net/sfu/adobe-com _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
