Assertion: rigid rotor | /r/HasProperty | developed on a series of helicopters
Facets:
value: in the 1960s
type: TEMPORAL
value: in 1970s
type: TEMPORAL
Source sentences:
text: Irv Culver of Lockheed developed one of the first rigid rotors, which was tested and developed on a series of helicopters in the 1960s and 1970s.