On Wednesday 19 Jan 2005 02:24, Curtis L. Olson wrote:

>  Yasim has a "magic" solver that is sometimes sensitive to specific
> inputs.  In the back of my head I imagine a little robot trying to climb
> to the highest point on the map by always going up ... but then coming
> to the top of a smaller hill and getting stuck.
>
> The solver tunes the lift and drag coefficients to make the aircraft hit
> the numbers you specify ... so if you provide engines that are too weak,
> you will end up with a super "slick" model which an incredibly efficient
> wing ... thus it can still hit the numbers but has really slow
> acceleration and climb.  On the other end of the spectrum, if you
> provide too much power, you end up with a high drag, low lift model (so
> you don't blow past the provide performance numbers.)  This will give
> you great ground acceleration and probably great climb, but will still
> top out at whatever numbers you specify.
>
> So once you have your basic YAsim model flying, you can tune things like
> rate of climb by adjusting actual engine output.  You can tune
> roll/pitch rates by adjusting the size or effectiveness of the control
> surfaces.
>
> I'm not convinced you could get a YASim model close enough in every area
> to get FAA level 3 certification or higher, but you can get a really
> fine flying model in most regimes with a bit of tweaking and
> understanding (at least at a simple level) how various configuration
> options relate to each other.
>
> The other thing that confused me early on was how YAsim handles weight.
> I don't remember the rules well enough off the top of my head to
> summarize them here, but the solver solves at 80% fuel load I believe.
> This means that unless you are very careful with your fuel load and the
> weight the solver uses, you won't hit your performance numbers exactly
> ... those number only are for one particular aircraft weight.  Once you
> figure out how to control the weight the solver uses and figure out how
> to configure the aircraft at that exact same weight, you do hit the
> performance numbers dead on.
>
> For someone like me with zero aeroengineering background, YAsim is a
> *really* fun tool to play around with.  After a few hours with it, I
> almost feel like I understand it enough to build pretty plausible
> numbers.  When it comes to stability derivatives and aero coefficients,
> I'm still pretty much as clueless as the day I was born.
>
> Curt.

Thanks for the advice; the B1900D FDM is really coming on now.

I've got her flying the 'envelope' and I'm managing to balance out the flight 
characteristics nicely.

Something I noticed early on is that the mass needed distributing for things 
like Engine+Gearbox sets and Maingear etc as Yasim just evenly places the dry 
mass otherwise.

I do agree that Yasim is great fun to work with - feels like I'm learning a 
lot.

A bit more flight testing and then I will show what I have got and you can all 
'shoot me down' ;-)

Dave Martin

Footnote: It appears that the B1900D props *do not* counter-rotate after all.

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