Hey @yul1,
I had a quick look at your simulation, but couldn’t dig deep into it so far. What you have to consider when only applying the symmetry conditions, so leaving the drone in a statically indeterminate state is that you have rigid body motions to consider as the first eigenfrequencies of the system. You can check on the rigid body motion in y-direction in your first simulation, that has a rather small eigenvalue (order e-02, e-03).
In reality of course the correct value is exactly zero as the stiffness matrix has zero eigenvalues for rigid body motions (infinite wave length) and becomes singular at that point.
The simulation behaves not very stable at that point. Also the load in the static step does not really make sense (gravity) as the structure is not fixed and is simple moved, although this should not change the stiffness matrix as it is a rigid motion. I hope this already helps.
All the best
Alex