Perhaps you should try some spelling variations:
"unmanned arterial vehicles" » "unmanned aerial vehicles", "damaged arterial vehicles", "skinned arterial vehicles", "unmanned aethereal vehicles", "unmanned aerial vehicle"
"unmanned arial vehicles" » "unmanned aerial vehicles", "unmanned area vehicles", "unmanned marine vehicles", "unmanned aerial vehicle"

Try widening your search to All Fields.

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

• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"unmanned arterial vehicles\") OR (\"unmanned arial vehicles\"))".
• Removing quotes may allow a broader search: (( unmanned arterial vehicles ) OR ( unmanned arial vehicles )).
• Adding a wildcard symbol may retrieve word variants: (("unmanned arterial vehicles") OR ("unmanned arial vehicles"))*.