CRAN,
The textbook referred to is Principles of Helicopter Aerodynamics by Leishman. In section 2.4.6 he covers the coaxial and tandem configurations. He references experimental measurements for closely spaced coaxial rotors by Harrington (1951) and its review by Coleman (1993).
In addition, a comment by Leishman and the graph in Figure 2.20 suggest that even the revised calculations of the coaxials performance may still not fully portray its advantage over the single rotor. Perhaps Kamov's 15%, which you mention, is closer to the truth?
This anomaly has bugged me for a number of years. I have felt that perhaps the initial 1.41 factor came from comparing the twin 2-blade coaxial rotors against a 2-blade single rotor, and not against a 4-blade single rotor.
The 10% gap and wake contraction comments may be relevant to the intermeshing configuration, as well. This is because approximately half of the intermeshing's upper blade thrust (the outer half) is subject to similiar conditions, The intermeshing has, obviously, the additional advantage of a larger total disk footprint.
Thanks for the information.
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My comment about yaw control in the coaxial was not very clear. The coaxial has excellent yaw control, which is done by varying the lift, and more importantly the drag, between the two counter-rotating rotors, while maintaining a constant total lift. During autorotation, the airflow through much of the disks is reversed. A particular pedal input would now cause a yaw in the opposite direction, if it were not for 'pedal switching linkages' that automatically take place in Kamovs at the onset of autorotation.
Dave J.
Symmetry is beautiful