Oh it's an airplane frame
Just a few questions and observations then:
Where do you plan to attach your 4 engines?
How do you attach the wings and what is their composition?
In a conventional design the plane derives strength and stiffness from ribs, stringers and skin (like a thin-wall composite beam) or an monocoque stress-skin. Is it your intent that in your design the stiffness comes from a bending frame of 0.5" tubes?
Anyway, pending a better understanding of how your design works, I would in the first instance model your frame as simple beam elements (available in FreeCAD)
Where to apply static loads and supports depends on where your engines sit and the flight mode (lift off and landing vs horizontal flight). Also in a very skinny design (which you are aiming for (?) dynamic loads need to be considered.
Finally, you may well find that appurtenances (like the wings) provide much greater strength and stiffness than the skinny tubular structure itself.
Hope this helps.