Jim Wilson wrote:
Dave Martin said:
On Thursday 20 Jan 2005 16:13, Jim Wilson wrote:
Dave Martin said:
On Thursday 20 Jan 2005 14:42, Jim Wilson wrote: getting an aircraft working
is about 2 parts theory and 1 part voodoo (the part that the basic"Any sufficiently advanced technology is indistinguishable from magic" -
formulas don't cover).
Sir Arthur C Clarke.
Ok wrong word. Let me just say that it seems to lack some magic. Setting up the p51d in Yasim was not my original intention as Jon S. Berdnt was claiming at the time I started the 3D that he had a nearly working JSBSim model. Doing the yasim model was a juggling act that started with geometric specifications, a couple software patch submissions, and then an endless number of tweaks. The "tweaks" were really comprimises that ended up producing something that was sort of close to performance specifications, but not really accurate in any respect.
Best,
Jim
Thats litterally what I've just been doing with the B1900D FDM.
The numbers in the FDM file don't all match up to the numbers in the POH but the FDM does now match with the *performance* figures in the POH - Which I hope is the right thing to aim for.
The biggest tradeoffs seemed to be in trying to balance high end performance,
(e.g. altitude, speed) against having enough drag to get reasonable behavior
at lower power settings. It seems pretty common for yasim models to glide too
much (excessive lift/insufficient drag) when cutting the power.
Jim,
I think this is more a factor of getting the right actual power output from the engine. If the engine output is too low, you are going to get up with a "slick" model that glides forever. If the engine output is too high you are going to end up with a "draggy" model that slows down too quickly. I remember you struggling with trying to model the complex p51 engine gearing correctly in yasim and maybe that never came out right and you ended up with a severely under powered model.
If that is the case, yasim will reduce drag and make your wing lift better so that you can still hit the performance numbers.
For the Citation-II I felt like Yasim did a pretty good job. I haven't really hit the validation end of it hard, but I was getting plausible performance throughout the envelope.
I think the big trick is to get the engine power output in the right ball park, then everything else starts falling into place much better.
Regards,
Curt.
-- Curtis Olson http://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text: 2f585eeea02e2c79d7b1d8c4963bae2d
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