Day X

I was fortunate one day to be standing on the sidewalk with my audio recorder when a copter flew over at low altitude - slapped it on some music.
I used a short segment, placed it on audacity, copied it, then reversed the track, then pasted the original forward track at the end of the reversed track. Made a nice loop. It works for copters because a reversed copter sounds much like the forward segment. So tah-dah - a circling whirlybird.