Re: [Jsbsim-devel] C-172 Performance --- More comments

[Gregory P. <gpi...@so...>]

David



No offense, but this attitude will doom JSBSim to failure.



When you start using excuses to lower the quality of your input data, doesn'
t it invalidate all the high quality equations in JSBSim all of you have
worked so hard on?



Realistically, 90% of JSBSim's input parameters effect less than +/- 1% of
the aircraft's performance. So why use them then?



Both Ron and myself have created a lot of FS flight models. We found it
really does pay off if you try to carry as high a precision as possible. The
end product will really show this when your user flies it. We've also seen
products with a "we don't need that much precision" that have been total
disasters.



Ron and myself go to extreme lengths to make the input data as accurate as
possible. The people that fly our FS aircraft can tell the difference. If I
release a P-51 that does 436mph instead of 437 at 25,000ft the users will
scream. My competition uses Cessna stability derivatives for all their
fighters, we try to calculate the aircraft's true stability derivatives. The
users can and do feel the difference. But the real pay dirt is when they fly
against each other online and start complaining about the same issues the
real pilots experienced when flying these aircraft.



The point I'm trying to make is the JSBSim developers need to aim high. If
they want JSBSim to gain wider acceptance, possibly create a cottage
industry for JSBSim consultants or used in a retail product, their attitude
has to always be to go that extra mile with their data.



Just my two cents from the cheap seats



Gregoryp





----- Original Message ----- 
From: "David Megginson" <dav...@gm...>
To: <jsb...@li...>
Sent: Thursday, October 14, 2004 4:28 AM
Subject: Re: [Jsbsim-devel] C-172 Performance --- More comments


> On Wed, 13 Oct 2004 21:12:00 -0500, Ron Freimuth <ron...@at...>
wrote:
>
> >     As far as 'accuracy goes', I'd disagree that ones flight model
doesn't
> > have to be that accurate, 'since  real AC vary all over the place'.
Seems
> > to me that a good flight model should represent the ideal AC.  One fresh
out
> > of the factory.  Not one misrigged, with inaccurate instruments, etc.
> > Ideally, a pilot could compare an accurate PC AC with what he flies.
And
> > see how much has real AC has deteriorated from a factory fresh model.
>
> My point was slightly different -- I'm suggesting that we try to
> choose an appropriate precision for our sample.  For example, my
> height probably changes by as much as 5 mm from morning to evening, so
> there's no point measuring it to within 0.0001 mm, or even 0.1 mm.
> Two brand-new planes straight off the assembly line are still going to
> handle differently, so even in the best case, there's no point
> measuring more accurately than the likely variation between them.
>
> As a pilot, you must notice this problem with flight training --
> student pilots have to plan a cross-country predicting our exact
> cross-wind correction, to the degree, our exact groundspeed, to the
> knot, and our exact ETA at each checkpoint, to the second.  That's
> good practice in using the E6B, of course, and it's a good reminder to
> be aware of winds aloft for fuel management, but it has very little to
> do with actually flying a plane cross-country.  I've never seen the
> winds aloft *exactly* the same as forecast, and as you mentioned, the
> plane becomes lighter as it goes anyway, so your TAS at the end of a
> 3-4 hour leg is going to be a few knots higher than your TAS at the
> start.
>
>
> All the best,
>
>
> David
>
> -- 
> http://www.megginson.com/
>
>
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