ive found a way to get infantry to attack helos not tried it the other way around but could be worth a go
get the chopper give it a waypoint openfire, engage at will and combat mode
switch the WP to hold i think it is, make the chopper fly rather slow and make it fly at about 30m using this flyinheight 30 should do it
add a couple of reveal commands and it should attack the infantry aslong as they opose a threat i think works for me to get infantry attack choppers