RE: New F18 70mm Adding rudder capability
Because of the dual fins, I'd imagine the easiest thing to do is to put a servo on TOP of the plane to drive both rudders.
That servo faces UP with the control horn on top. You then use a straight two arm servo arm...
Pushrods can then be connected to your newly cut out and hinged rudders on both inner sides of the rudders.
The biggest difficulty is making a cutout for the servo, and routing the wires.
You can hinge the rudder with CA hinges, using foam safe CA, white glue or epoxy.
If you are using CA you should use the thicker stuff and do one side of the hinge at a time.
IMHO epoxy or white glue is easier though allow at least a day or more for the latter to dry.
Bare in mind doing this will really screw up the C.G. so mark it before you begin then rebalance the plane to the same spot when you are done.