Perhaps you should try some spelling variations:
"unmanned aethereal vehicles" » "unmanned aerial vehicles", "unmanned ethical vehicles"
"unmanned several vehicles" » "contained several vehicles", "unmanned aerial vehicles", "unmanned naval vehicles", "unmanned several articles", "unmanned several vices", "unmanned general 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 aethereal vehicles\") OR (\"unmanned several vehicles\"))".
• Removing quotes may allow a broader search: (( unmanned aethereal vehicles ) OR ( unmanned several vehicles )).
• Adding a wildcard symbol may retrieve word variants: (("unmanned aethereal vehicles") OR ("unmanned several vehicles"))*.