I started this post looking for help with what the menu livery UI was doing to place the 3D object, because my plane sits fine on the runway but displays sunken into the ground in the livery UI so the UI is not using the flight_model.cfg contact points.
After a lot of trial end error narrowing it down to flight_model.cfg and the specific parameter I’ll answer my own question so folks can find the niche answer (perhaps they’ll have a related issue) in here:
The livery UI is using flight_model.cfg static_cg_height (in feet) to decide where to place the model relative to the UI backdrop. Works fine for most planes with the static CG 5 to 30 feet off the ground but beyond that range the rendering in the UI becomes problematic. For a glider we might just about fit but it means tweaking the CG parameter for the UI rather than the flight model.
If your model does NOT display in the livery UI for some reason (i.e. possibly not at all) during development, maybe that static_cg_height parameter is not set yet.