Been ruminating helicopter rules for the US forces.
1) Each unit needs a helicopter each to move (ie a "platoon" would need 2 and a "company" 4 choppers) . This would make them the same as motorised (unarmoured) troops.
2) Only Air Cavalry units would have dedicated helicopters assigned to them (making them equivalent to cavalry.) These choppers would only carry their assigned units and no other.
3) Everybody else (artillery, ARVN, bog-standard grunts) will have to take helicopters from a general pool if available. There will never be enough to go around, I am thinking of a ratio of one Huey to every three (non Air Cav) unit. So for example a US army of 16 units would have a total of 6 helicopter models of which 2 would be in dedicated Air Cav units.
4) Units carried by helicopter may move, dismount and battle OR mount and move (as unit card) This ends their turn.
5) To counter the greater mobility of the US forces, The NVA would have 2 heavy MG units for every 3 helicopters the US has.