On Mon, Aug 15, 2011 at 12:18 PM, Viktor Radnai <viktor.rad...@gmail.com> wrote:
> Hi all,
>
> Athanasios Goritsas (cc-d) created a nice Falke 3D model with a basic
> Yasim profile, based on the aircraft he flew with. I would like to
> develop his model further (I'm doing my PPL on the real thing atm). We
> would both like to see it in Git (under a GPL license), maybe more
> people would join in developing it further. (If you need a statement on
> this directly from Athanasios, as he's the primary author, let us know.)
>
> In the meantime, could you please check out the plane and shed some
> light to some Yasim parameters. You can grab it from
> http://www.avatarzenekar.hu/files/sf25b.tar.bz2
>
> The biggest issue with the FDM is that the plane does not seem to have
> enough drag -- it easily goes over Vne, and it's impossible to land
> without spoilers unless you stop the engine (and even then it takes
> forever). Compared to the Grob 109, I had to use an absolutely huge drag
> multiplier to force the bird down (150 instead of 2.5). 150 is probably
> a bit much -- maybe 100-120 would be enough, but I think the airframe or
> the wings are just not generating enough drag.
>
> Not sure if I did a good enough job with the fixed prop. The diameter
> should be correct, not sure about the rest. My school's falke has the 2L
> engine and it spins up to about 2600 when stopped. No idea about the
> prop's most efficient speed and horsepower values are pretty much guesses.
>
> Anyway, please let us know if this is good enough to go into the repo,
> and any suggestions for improvement. Some (working) 3D instrument panel
> would be great, but I have no idea how to make that. Any pointers on
> that would be very much appreciated. Since Falkes have fairly varying
> instrumentation, even a copy of the Grob 109's panel could be OK.
>
> Ah, and the splash screens I've made do not load. What did I do wrong?
>
> Thanks in advance.
>
> Cheers,
> Vik
Vik,
Looks like a great start. The first thing I would do before anything
else is make sure your CG is positioned reasonably. In your SF-25, the
CG is much too far back; given the forward-swept wings, it looks to be
about a meter behind MAC:
E:\FlightGear projects\sf25b>yasim sf25b-yasim.xml
Solution results: Iterations: 1292
Drag Coefficient: 10.955851
Lift Ratio: 291.677826
Cruise AoA: 1.469686
Tail Incidence: 2.793443
Approach Elevator: -0.014301
CG: x:-0.900, y:-0.000, z:0.284
Inertia tensor : 1831.357, -0.000, 78.171
[kg*m^2] -0.000, 2075.542, 0.000
Origo at CG 78.171, 0.000, 3856.738
The command-line YASim solver is showing CG at x=-0.9, well behind the
wing's root chord position at x="-0.371".
It isn't worth messing with other YASim values until CG is about
right. I'd first try to locate the real CG range from a certification
sheet or pilot's handbook and then use a YASim <ballast> element to
shift some of the plane's mass forward toward the nose. Once you get
CG better positioned, you'll have much better luck with other factors.
After CG, a couple of other things to watch in the solver: Lift ratio
is very high-- this indicates the glides-forever issue. For this plane
I'm guessing you'll want a value somewhere between 100-130, but the
actual value will depend on flight experimentation. Lots of YASim
parameters affect that value. (Note that these YASim drag and lift
numbers should not be treated as real lift/drag ratios; don't try to
make them match a real L/D ratio.) Approach elevator is much too
small-- this value means you'll need almost no elevator to hold an
approach. Look for something in the -0.7 to -0.9 range for starters.
In any case, don't try to tweak these until CG is resolved.
Wing and hstab stall AoA values look really high at 30 degrees. Most
conventional high-lift general aviation airfoils seem to be in the
15-18 range. If you know the airfoils used, you can get camber and
stall values from the airfoil data.
I have a little and rather incomplete YASim guide that might be helpful:
http://ltts.crlt.indiana.edu/grn/flightgear/
-Gary, aka Buckaroo
------------------------------------------------------------------------------
uberSVN's rich system and user administration capabilities and model
configuration take the hassle out of deploying and managing Subversion and
the tools developers use with it. Learn more about uberSVN and get a free
download at: http://p.sf.net/sfu/wandisco-dev2dev
_______________________________________________
Flightgear-devel mailing list
Flightgear-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/flightgear-devel
