a rotor blade rotated in synchronism with {TAIL ~ said second sprocket drum} to actuate said hinged door panel momentarily open on {HEAD ~ each half revolution} of said rotor blade 
a rotor blade rotated in synchronism with { TAIL ~ said second sprocket drum } to actuate said hinged door panel momentarily open on { HEAD ~ each half revolution } of said rotor blade
OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH