Some notes that might be of value.
2) If a co-axial rotor system fully stalls, can it be recovered by flaring and inducing airflow through the disk, or do the blades simply fold up (as I was taught is the case with the R-22 and Bell 206 teetering head systems)
I suspect that on a craft with two main rotors, which are vertically or laterally separated from each other, and which are extremely rigid, it may be possibly to utilize the same method of stall recovery as is used on a fixed-wing aircraft, i.e. drop the nose.
3) Autos: I understand that yaw control becomes an issue during the flare, ..................
You are obviously aware of how yaw control operates on a Coaxial during autorotation, But for others who are interested,
here is a 'description of operation' presented on PPRuNe by HDW a couple of years ago. As you mention, the flare is the interesting part. HDW describes how the flare is handled on an Intermeshing. Unfortunately, this ability to used opposed longitudinal cyclic is not available on the Coaxial.
4) Flight envelope-wise, what conditions must exist for the two separate rotor disks to be at risk of collision?
The rigidity of the rotors will obviously play a major roll in determining the gap (vertical spacing) between the two rotors. The following rough gap-to-radius ratio may be of some interest. Then again it may not.
Ka-50 rotor radius 23.8 ft Gap between rotors looks like being somewhat over 4 ft. Gap/radius ratio = 0.168
Sikorsky S-69 (XH-59A) ABC Rotor radius 18 ft. Gap 2.5 ft Gap/radius ratio = 0.139
On this page and its linked pages is information related to the minimum tip-tip gap experienced, etc. etc. I think that it was 11 inches.
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Ewan.
I would, perhaps presumptuously, recommend the Intermeshing configuration for consideration, in conjunction with the Coaxial during your preliminary design process. In the minds of most people the Intermeshing configuration is perceived a craft with a 'soft and mushy' control, but with a good lift. This is due to the direction that Kaman took the configuration. It is not due to
Flettner, nor to
Kellett's aspirations.
As you probably know, Prouty mentions the "Synchropter" in his book 'Military Helicopter Design Technology'. His short review of the configuration is interesting, however it does not assess the configuration in the light of today's requirements and technological advancements. I would not fault Prouty, since knowledge of the intermeshing configuration has been very limited, however his last paragraph needs some expansion.
In addition to the yaw control situation in autorotation, which is shared with the coaxial, he says;
"Another problem for the synchropter is the difficulty in using more than two blades on each rotor because of the blade strike problems. This is one of the reasons that the three-bladed Kellett synchropters were not successful.".
~~ The Kellett 3-bladed XR-10 crashed when a pitch link failed, killing the pilot, Dave Driskill. The mechanic who parachuted out of the craft and survived reported the situation.
~~ In addition, from The Luftwaffe Profiles Series No.6 on the Flettner Fl-282;
"two three-blade rotors - was tried out on the test bed. It proved to run extraordinarily smoothly, however this was not a consideration for military use."
~~ My opinion; Today's ability to have extremely rigid rotors makes it possible to produce 3-bladed Intermeshing rotors; and within certain spatial constraints it should be possible to produce 4-bladed Intermeshing rotors.
Just some thoughts for consideration when looking for the very best.
Dave