Understand now, Nick. C.N.Keys covers this well in "Chap III: Forward flight; Sect 6.1: Rotor Stall Limits Methodology".
Interestingly, you could almost put forwards a case for increasing single rotor RRPM with speed to keep rotor advance ratio down (ignoring compressibility). Maybe control system reducing RRPM in hover, for longer duration. I certainly understand how increasing sigma helps overcome the reducing Ct/Sigma with mu.
Still struggling to work out why hover power goes up with solidity ratio. I take it the blade section and twist is chosen to minimise high speed cruise power. This means section min drag AOA is above hover AOA @ 0.75 radius (although this implies camber - reflex perhaps to avoid divergence). Clearly good twist for high mu will be bad for hover.
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Definately, unloading the retreating blade is the way to go. I find myself wondering how an intermesher developed with 15% effective offset hinge would perform. Certainly there would be negligable risk of blade clash, unlike coaxial on startup with "wrong" pedal applied
. The other possible advantage is that, for upper advancing, you get inbuild lateral dihedral without the need of Comanche style air speed sensors.
Hmmm, maybe Dave has a point...
Mart