Mart,
You're correct. My reply was very short and doesn't properly address the subject of stability. The web page
UniCopter ~ Trim, Stability & Control is an attempt to address this subject in respect to the UniCopter.
Your original statement was
"Dave you really must think about hand-off hover stability. I don't see how synchropter will be improved over conventional rigid. " I suspect that you are correct. An intermeshing (synchropter) helicopter will probably not be any more stable than a single rotor helicopter; if they have identical rotor types. (Teetering v.s. teetering.) (Offset flapping v.s. offset flapping.)
The UniCopter should be a different story.
The two rotors are extremely rigid and they are rigidly mounted to the rest of the helicopter. Their rotational inertia will want to resist (hopefully not too much or not too little) the pitching and rolling of the helicopter. In addition, because their axii are close to being aligned and they are rotating in opposite directions, the gyroscopic precession of one will offset the gyroscopic precession of the other.
A little elaboration. You are correct in that the above will only reduce the rate of pitch or roll. However, I believe that this will be an improvement over conventional rigid rotors. In addition, the UniCopter's absolutely rigid rotor will provide the pilot with a very fast cyclic response to correct the pitch or roll.
Concerns by you and others are desired, because they cause an assessment or reassessment of potential problems. If there is any disagreement with the above, please fight back.
Dave
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A not so little sales pitch.
slowrotor is very interested in safety.
Theoretically, the UniCopter should have trim, stability and control that is quite similar to that of an airplane. In fact, I think that the rotors could be fully stalled out, just like an airplane, and then the craft be put through a stall recovery.