Mart,
The Coaxials were/are popular with the Russian navy because of their smaller "landing box". But what has this got to do with the subject of rotor aerodynamics; or with a hub spring for that matter? You keep moving the goalpost.
Nick,
'Disk loading' is an averaging of the loading over the surface of the disk. I agree, that the simplistic Momentum Theory, and its components of 'disk area' and 'disk loading' etc, are fine when comparing apples to apples, such as single-rotor configuration to single-rotor configuration.
However, Momentum Theory's simple 'gap equation' for the coaxial configuration and 'gap and stagger equation' for the tandem configuration are inadequate in defining effective disk area, disk loading and power loading etc for their respective comfigurations.
Interestingly, Stepniewski was required to use 'an extended' coaxial and tandem Momentum Theory to compare; the cold-jet-driven rotor configuration, the single rotor configuration, and the ABC Intermeshing configuration, in his final publication.
I strongly feel that today's rotorcraft would be much more advanced if the past 60 years of rotorcraft research had been spent on twin-rotor configurations and not on some quaint concept called a tail rotor.