Re: CG placing

Jon Berndt wrote:

>>Hi!
>>
>>Just have a quick question:
>>How do you position the CG of the aircraft with reference to the outside
>>world?
>>i.e. the height from the runway...
>>
>>for exemple the fokker 100 has its CG at AC_CGLOC     627.4  0.0 -16.5,
>>but this is in the a/c reference.
>>Is it the undercarriage that  places the CG in the real world?
>>
>>Thanks, Sam
>>    
>>
>
>It might help if you see the first "Back of the Envelope" newsletter at the JSBSim web
>site.
>
>Additionally, the landing gear coordinates specify the location of the gear in the
>aircraft frame. If you know the location of the CG and the landing gear both in aircraft
>coordinates, you can take the difference and know how far below the CG the landing gear
>extend. If you are at sea level, and you set the altitude of the aircraft at 5 feet, and
>the gear hang down 3 feet below the CG, then the aircraft will fall two feet and hit
>ground. The altitude value corresponds to the altitude of the CG.
>
>I hope this helps.
>
>Jon
>
>
>  
>

Does this mean that when you start the aircraft on the runway, if there 
is no undercarriage, the CG will be on the runway?
I have read the first "Back of the Envelope" newsletter but did not 
learn much from it...
I am quite at ease with references and coordinates, (being an 
engineering student!) but I am finding it hard to get my hands around 
all the ways of positioning the aircraft:
--> values in the configuration file
--> values in the aircraft-set file
--> values in the model.xml file
--> values in AC3D....
HELP!!!

I am modeling an R/C UAV, 2.2m span and 6.5 kg
the METRICS file looks like this:
    AC_CGLOC     28.3 0.0 0.0
    AC_AERORP    28.3 0.0 0.0
    AC_EYEPTLOC  17.0 0.0 15.0
    AC_VRP       28.3 0.0 0.0
and the undercarriage is:
AC_GEAR NOSE_LG     10.0   0.0  -10.0    
AC_GEAR LEFT_MLG   37.5 -17.8  -10.0    
AC_GEAR RIGHT_MLG 37.5  17.8  -10.0    

the model.xml file links directly to the .ac model
and the .ac model in AC3D has the origin (0,0,0) at the center of gravity.

When I launch FG with the aircraft on the runway, it spins around like a 
bouncy ball and ends-up about 3m below ground....

I am quite lost!  :-S

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