Hi,
the correct cg for a SF25C is 0.143 .. 0.334m behind the wings tip
(measured 0.52m from the center line).
Regards,
Maik
am 15.08.2011 22:07 schrieb TDO_Brandano -:
BTW, you can get a pretty good idea of where the CG is on a plane from
the landing gear position. On a tail dragger the CG will be slightly
behind the main wheel. Too far back and the tail won't have enough
authority to lift for takeoff, too far forward and the plane will nose
over when braking, or ground loop when rolling on the ground. In this
specific model it might be a little further back than most, since it
has to keep the tail on the ground with the weight of the passengers.
I believe the CG will be approximatively coincident with the pilot
position, since in a motorglider that will be the largest variable mass.
> Date: Mon, 15 Aug 2011 13:18:53 -0400
> From: grne...@gmail.com
> To: flightgear-devel@lists.sourceforge.net
> Subject: Re: [Flightgear-devel] New aircraft: SF-25
>
> 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
>
>
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