Ground limit and 3D model

I have now nearly finished implementing our project UAV "Manta" in 
Flightgear and am very satisfied with the result.

There is one thing though which I am not quite sure about and that is 
the position of the 3D model. It seems to be positioned correctly and 
flies pretty accurately. The problem, however, is when I start the 
flight on the runway the aircraft is placed right on the edge, rather 
than on the zebra-crossing...
It also has a tendency of going "underground" easily.

Should a correctly placed 3D model stay above ground at all times?
Does this mean that my 3D model is offset from the FDM coordinates? (the 
model is only 2.2m span, so is much closer to the runway).
Where is the centre of the FDM coordinates placed when you start the 
flight on the  runway?

Thanks! Sam


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