I'm very bad explaining things, but I will try it anyways
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First of all, set your Main pivot point where you think the CG (Center of Gravity) of your airplane is. Then open "c172r.xml" and edit the following code:
<location name="CG" unit="IN"> <--- You also can use [unit="M"] to use meters instead of IN(inches)
<x> 39 </x>
<y> 0 </y>
<z> 36.5 </z>
</location>
Set X,Y,Z to "0", so the CG will be where your Main pivot is. (If I remember well, I had a problem here with X axis, so I set it to 1.0 [<x> 1.0 </x>]).
I also used to set all X, Y, X of <location name="VRP" unit="IN"> and <location name="EYEPOINT" unit="IN"> to 0.0
Then you scroll down and under <ground_reactions> and you have:
<contact type="BOGEY" name="NOSE">
<location unit="IN">
<x> -6.8 </x>
<y> 0 </y>
<z> -18 </z>
</location>
<static_friction> 0.8 </static_friction>
<dynamic_friction> 0.5 </dynamic_friction>
<rolling_friction> 0.02 </rolling_friction>
<spring_coeff unit="LBS/FT"> 3800 </spring_coeff>
<damping_coeff unit="LBS/FT/SEC"> 400 </damping_coeff>
<max_steer unit="DEG"> 10 </max_steer>
<brake_group> NONE </brake_group>
<retractable>0</retractable> <------ Set this to 1 if your landing gear is Retractable.
</contact>
This is the NOSE wheel (you can add many wheels you want, by duplicating the <contact> section renaming each under name="NOSE" and obviously write the correct coordinates).
You have to write the coordinates of each wheel under X, Y, Z. Note: The coordinates are exactly the same as in 3ds Max, except In Max, Y Negative == X Positive in FDM I think in Blender in the same, if not, use trial and error, that always works
Note: All coordinates are relative to the Main Pivot point.I hope you understand my poor explanation
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