Dave,
"Where did this statement come from?"
I made it up!
Seriously though it is common sense. Stepniewski describes potential theory in his "Rotary-Wing Aerodynamics", which is the origin of CFD. With CFD, as with so many C.A.E. activities, common sense goes a long way to getting the right answer. The computer just tells you if you were right.
If you think about a rotor operating in forward velocity, the ideal is to have a positive pressure across the entire underside of the disk. Around the perimeter of the circular reverse velocity region there will be no flow across the aerofoil. There is therefore no way that the aerofoil can generate downwash, or the pressure gradient associated with it. The practical upshot is that the air will leak from below to above. If you consider his chapter on vortex theory you see that this means that two opposed vortices will spill off either side of the zero velocity region. Ergo you will get an effect similar to the fountain effect that Prouty nicely describes in one of his books (Heli aerodynamics, i think) - where the air leakage at the hub actually forms a miniature tornado from the airflow towards top (beanies help).
Perhaps, though, i should have been more specific with that statement. The conventional will suffer the worst, but any twin rotor will lessen the effect. Either coaxial or intermeshing have a gap between opposing rotors, so will allow the effect to occur with reverse velocity utilisation (leading to further blade slap). An interleaver will suffer less, and indeed i did wonder if this was the reason behind your recent migration to interleavers. My reason against use of RVU with interleavers is that you still need a large blade AOA in forward flight to keep a good downwash, at both root and tip, which will be away from optimum. There will be forward speed where the zero aerofoil velocity regions will coincide, but this is over the fuselage so no big deal.
Basically if twin rotor hubs are rigid, feather the reatreating blade and design the hub for the rotor lift inbalance. As i say intermeshing looks good here. Your project, your choice...
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"...tri-teetering hub is an attempt to give greater control authority to very light rotorcraft."
Right, i wasn't sure though. Must admit to still wondering what advantages this head would give over say the Sikorsky/Schweizer 300 articulating lead (rigid with lead-lag). This is generally percieved as having superior flight characteristics to the R22. The tri-teetering is a sort of half-way design.
Gyro stabilised Schweizer 300 - mmmmmmmmmmmmmmm...
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From the (rather interesting) site:
"Prouty further states, the "hinge-less" design suffers from the characterization that "they all shook". Lockheed added a fourth blade and controlled the shaking to a more pilot acceptable level."
I don't recall any provision being made for lead-lag on the 186/XH51 rotor heads. This would go some way to explaining the vibration. Indeed the original CL475 had a 2-blade rigid rotor!
Mart
[Edit:typos]