Regarding X2, it does not matter where the prop is located relative to the rotor.
From the NACA report;
WIND-TUNNEL TESTS 0F SINGLE- AND DUAL-ROTATING PUSHER PROPELLERS HAVING FROM THREE TO EIGHT BLADES"The single-rotating pusher propellers were from 0 to 7 percent less efficient than the corresponding tractor propellers and from 0 to 10 percent less efficient than the tractor propeller tested with the wing. The dual propellers provide about the same efficiency, within a few percent, irrespective of whether they were tested as tractors or as pushers and whether or not the wing was present."
The first sentence suggests that a pusher propeller is less efficient than a tractor propeller.
The second sentence suggests that Kamov knows what they are doing when they put contra-rotating propellers on
their proposed Coaxial-ABC ~ [post #362]
Dave