Perhaps you should try some spelling variations:
arterial vehicles » aerial vehicles, aethereal vehicles, aerial vehicle
several vehicles » several articles, several vices, general vehicle
central vehicle » control vehicle, central pedicle, 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 (((several vehicles) OR (central vehicle)))).
• Adding a wildcard symbol may retrieve word variants: unmanned ((arterial vehicles) or (((several vehicles) or (central vehicle))))*.