Dave, i'm about half way through that Leishman paper. It is interesting to note that a practical coaxial with vertical rotor seperation will only suffer about 22% increase in power over widely seperated rotors, and not the 41% often quoted. To my mind this provide further justification of the X2 design over the lateral seperation of say a V22.
For a given disk loading, hence power required for HOGE with a specific payload, Sikorsky's X2 concept will always be able to land in places too tight for any tilt rotor. Also the lower roll inertia means X2 based designs will be better suited to landing on the heaving deck of a ship. The numbers just add up.
Maxtork, this paper details how you estimate coaxial performance:
http://www.baldwintechnology.com/MTR_AHS06.pdf
Expect to see X2 spawn a new generation of transport concepts...
