Dave, i'm only just printing off the paper.
I'm also planning to read Leishman's book this year. I'd be interested to know more about your rotor spreadsheet - like could i have a copy?
Although the rotor will be operating near to stall, the FM of 0.5 is probably about right. The ideal blade taper planform for hover has chord inversely proportional to radius, which these aren't. X2 likely chose it's planform to avoid reverse flow aerofoil divergence, and will have been optimised for cruise not hover.
Besides the work on aerofoil section optimisation for laminar flow had only just been done by
Ludwig Prandtl, so was not widely known. These blades are flat, so will transition at a low reynolds number - ie they are draggy.
The final difficulty in all of these designs seems to be controlablity. The small radius rotors is a hint at the future difficulty of providing sufficient structural stiffness in a rotor blade, only later overcome by Cierva...
Slowrotor, to estimate engine power it should be thought of as a pump. For this era, assume the valve timing was about right for the RPM chosen, but had no acoustic boost from exhaust or inlet. So you can get power from ideal volume flow, hence mass flow, hence power for ideal stoichiometric (14:1 by mass). Assume overall efficiency between 10% to 20% to get useable shaft horsepower.
http://en.wikipedia.org/wiki/Petrol#Energy_content
I'll read the paper over the next day or so to provide more useful feedback.