Perhaps you should try some spelling variations:
arterial vehicles » aerial vehicles, aethereal vehicles, aerial vehicle
several vehicles » several articles, several vices, several cicles
general vehicle » central vehicle, energy vehicle, lateral 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 (((several vehicles) OR (general vehicle)))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "unmanned ((arterial vehicles) or (((several vehicles) OR (general vehicle))))".
• Adding a wildcard symbol may retrieve word variants: unmanned ((arterial vehicles) or (((several vehicles) OR (general vehicle))))*.