Perhaps you should try some spelling variations:
"unmanned arterial vehicles" » "unmanned aerial vehicles", "unmanned arial vehicles", "damaged arterial vehicles", "unmanned aethereal vehicles", "unmanned aerial vehicle"
"unmanned several vehicles" » "contained several vehicles", "unmanned aerial vehicles", "unmanned naval vehicles", "unmanned several articles", "unmanned several vices", "unmanned general vehicle"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("unmanned arterial vehicles") OR ("unmanned several vehicles")).
• Removing quotes may allow a broader search: (( unmanned arterial vehicles ) or ( unmanned several vehicles )).
• Adding a wildcard symbol may retrieve word variants: (("unmanned arterial vehicles") or ("unmanned several vehicles"))*.