Material
Carbon fiber sheets are most common - light and durable
Conducts electricity so everything needs to be well insulated
Blocks signals so need to place antenna in a good location
HDPE also an option but not as strong
Arms
Easier to fix aircraft (and cheaper) if the arms are replaceable
Having unibody (arms not removable) could be less machining
Shape
Dead Cat
allows space for a camera at the front (and no rotors in the way)
True X
Same stability on every access since the motors are all at the same distance
Wide X
Wider so it could make the drone lighter/smaller while still fitting a grabber and camera on the wide axis
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