Info and discussion on the above.
At the opposite a two-bladed rotor like that of MTOsport autogyro (figure 1 and table 2) has a low solidity of 3 %.
The thrust is used, as for an AWES using it:
For an AWES both options can lead to some advantages. A low solidity autogyro rotor can be suitable when the thrust is used as for yo-yo systems, allowing both lightness and lifetime.
I was thinking people here might have started wondering about this after seeing my simple two-bladed rotors. I remember Ken Visser, one of this study’s authors, as well as a couple of the people listed in the references, from our old wind group of people who actually knew what they were talking about. It seems these days everyone who knows anything has either been told they are not allowed to post on the web (NREL etc.), or they got tired of telling people the facts only to be called names for their trouble (Paul Gipe, etc.). Really, those adept at wind energy would say they already knew the results of the study without reading it.
That said in autogyro mode, according to gma (in French language), the solidity (called “fullness” as translated from French language, a full rotor disc having a fullness of 1) cannot be lower than 0.025, so 2.5%, a little less than 3 %: this is due to the blade ability to withstand the load factor of the disc compared to its increased speed, because of the tension with the centrifugal force (perhaps also because of the forces of the full rotor disc such like lift and drag).
As a result more the solidity (or fullness) decreases, more the blades become heavy to undergo the load.
I like this paper.
Off-topic here, but it seems to also say the SG6043 airfoil is perhaps best for small HAWTs. Searching for that in your favorite search engine gives many results. This one for example:
For the same rotor solidity, a higher blade count means higher aspect ratio blades
Those long slender spans are efficient
I like that their new rotors are literally off the chart.