Dave,
Prouty sums it up nicely for us sad engineering types in his "big red book of helicopters" (Or was it "Helicopter Performance Stability and Control" - i'll put down my crayons and check). He has an equation on page 26 that for ideal twist goes:
Figure_of_Merit =
1 / (1 + ( 1.5*Tip_Speed / (Cl/Cd)*SQRT(2*Disk_Loading/Air_Density) ))
This equation is a rewrite of the equation he actually derives:
Figure_of_Merit = 1 / (1 + ( 1.5*SQRT(3) / (SQRT(Solidarity_Ratio)*(Cl^1.5/Cd)) ))
The results are plotted for ideal twist on page 28 (fig 1.13), and show without doubt that the higher the solidity ratio the higher the figure of merit. I was hoping for Nick's comment on this, but he gave up on me as a lost cause and ran for the hills - i get that a lot.
My interpretation of the above info is that a helicopter runs at peak efficiency when the blades are operating just below stall, for maximum Cl/Cd. Using wide chord blades then allows a nice low tip speed, by chosing a low Nr for hover. Forward flight Nr is a compromise on fixed Nr conventional helis, due to the need to avoid retreating blade stall (particularly increasing loading during sharp manouvres
). Counterrotators do not have this constraint so can reduce Nr to keep advancing blade airspeed ~constant.
Mart