rotor vs main rotor

rotor and main rotor both are nouns.

Word	Noun	Adjective	Verb	Adverb
rotor	Yes	No	No	No
main rotor	Yes	No	No	No

As nouns, main rotor is a hyponym of rotor; that is, main rotor is a word with a more specific, narrower meaning than rotor:

rotor: rotating mechanism consisting of an assembly of rotating airfoils
main rotor: rotor consisting of large rotating airfoils on a single-rotor helicopter that produce the lift to support the helicopter in the air

Other hyponyms of rotor include anti-torque rotor, tail rotor.

rotor (noun)	main rotor (noun)
rotating mechanism consisting of an assembly of rotating airfoils	rotor consisting of large rotating airfoils on a single-rotor helicopter that produce the lift to support the helicopter in the air
the rotating armature of a motor or generator
the revolving bar of a distributor