That thing is at best only marginally stable
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I’m no expert but I remember from my RC airplane days that a flat bottom airfoil was the most stable profile. That is what this drone has.
OTOH, the wing has little or no dihedral angle — which is to say it extends straight out rather than rising towards the tips. A flat wing with low dihedral is a less stable configuration IIRC.
And it looks like the distance from the CG to the vertical stabilizer is unusually short, which I could see reducing stability.
But to my eyeball it doesn’t look fundamentally unstable.
What is it that makes you say it’s unstable?