Regarding the virtual Delta 3 hinge: this will have a significant and beneficial documented effect on disk response to pilot input and will effect the Lock number consequently modifying the aerodynamic phase lag. The collective/pitch couple is a red herring and unrelated to the Delta 3 question. The aerodynamic collective/pitch couple is a function of airspeed (advancing Vs retreating blade), it does not occur in a still air hover. Any pitch of the aircraft in the hover attributable to collective inputs will be due to downwash or similar effects.
There cannot be a comparison between the Bell and Robinson heads because the Bell does not have “coning” hinges. In the case of the Bell the two blades are, for all intents and purposes, linked and will provide mutual damping and can be expected to have a natural frequency of 1R. The Robinson is a different design as Lu points out, but the text books still have validity. You must take the virtual Delta 3 component and the relationship between the 3 hinges (teeter and coning x2) into account when calculating your phase lag.
Any modern non pure teetering head is likely to have a phase angle other than 90 degrees. You can rush out with your protractors and pencils and check. I have flown teetering, articulated, semi-rigid and rigid headed helicopters and believe me you can’t tell what the phase angle is until you measure it, and then it will change with density altitude. Even then it is academic; you look out and fly the aircraft, let test pilots decide if it has adequate handling qualities and believe me if they don’t like it you won’t get to fly it.
The only point I would make, and this is purely my opinion, is that during low G manoeuvres the dynamic relationship between the teetering and coning hinges may become confused which is a good enough reason not to enter a low G regime. However without flight testing (which may have already been done) the answer is pure conjecture.
There is a lot of supposition dressed as fact in some of the posts. We have heard a lot from Lu and I eagerly await Mr Robinson’s’ reply, although I do expect “Lock”, “Virtual Delta 3” and “Teeter and coning hinges” to be mentioned!