Ok, I never remember which way the
reveal works
..
So you may need to experiment with this a bit...
But, you can make a trigger covering the are on which you want the air unit to engage the troops, make it be activated by the side of the troops, then in the on Act. field, put:
"
chopperName reveal _x" forEach thisList
Oh, and you might wanna make the choppers/whatever to fly quite low (flyInHeight)...
Now, I'm not guaranteening this will work, just a thought...