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

[David M. <dav...@gm...>]

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/