How were helicopters done?

Anybody know what technique was used to animate the helicopters with their smoothly counter-rotating rotors and their formation flying that seems to pay homage to Apocalypse Now?

The rotor motion is fully detailed; it's too complex and too smooth to be simply hand-painted. At the same time it's obvious most of the shot is non-CGI; I really wonder if a CGI of just the rotors can be married into the otherwise-non-CGI scenes that seamlessly.