The category "<rotary_aircraft_type>" can be expressed as follows:

<rotary_aircraft_type>

ah-1

ah-1j

ah-1js

ah-1s

ah-64

ah-64s

black hawk

black hawks

ch-46

ch-46s

ch-47d

ch-47ds

ch-53

ch-53s

chinook

chinooks

civilian chopper

civilian choppers

civilian copter

civilian copters

civilian helicopter

civilian helicopters

civilian helo

civilian heloes

cnn chopper

cnn choppers

cnn copter

cnn copters

cnn helicopter

cnn helicopters

cnn helo

cnn heloes

cobra

cobras

frog

frogs

kiowa

kiowas

neutral chopper

neutral choppers

neutral copter

neutral copters

neutral helicopter

neutral helicopters

neutral helo

neutral heloes

non-combatant copter

non-combatant copters

non-combatant helicopter

non-combatant helicopters

non-combatant helo

non-combatant heloes

oh-58d

oh-58ds

stallion

stallions

uh-60

uh-60s