I think, with driving cars in the heli, there is one usual problem. If you place a unit ON a chopper, not in the cargo, he just falls off when it starts to move. It's the same with any other vehicle.
It tried getting my guy to start inside a flying heli by using setpos height script to the height of the inside compartment of the chopper, so i could be IN it and still run around. Didn't work. I just fell off (although if it stays hovering you can start on top of it which is pretty cool and totally useless).
I think the chopper would have to have the properties of a building in order to fly with a unit in it byt not technically in cargo...
For example, i donwloaded an LST transport ship. You can store men and tanks and stuff in it and walk around in it, but if it starts to drive they all fall off. :-\