I'd like some (good or bad) feedback on whether i have understood this properly. Apologies in advance of the slightly technical nature of the post. I figure some of the TPs out there will have their own thoughts on this.
I'm not convinced helicopters demonstrate dutch roll:
Fixed wing dutch roll is an undamped oscillation resulting from the lateral dihedral trying to keep the aircraft "wings level" after a disturbance. The roll-yaw coupling comes from the vertical stabiliser trying to keep the heading correct while the aircraft is sideslipping port-starboard-port. This means yaw direction should lag roll attitude by about 90 degrees, with a larger rudder area reducing tendancy to dutch roll (until you get to spiral divergence).
The problem in a helicopter is that there is a delay before the sideslip induced lateral flapback causes the fuselage to change roll attitude. This delay is caused by the relatively low effective hinge offset (compared to a fixed wing) requiring the rotor to change attitude before generating the necessary moment above the centre of mass. So the lateral dihedral has a response lag which swamps any dutch roll time constant.
The practical upshot of all this is that the instability is more likely to be PIO from the cyclic roll response lag than instability in the aircraft. So in a quatering wind, any delay in pedal position results in a cyclic input. Since the cyclic input was not really required then when the pedals are moved the cyclic needs to be re-corrected. All this means that cyclic effectively sees a disturbance, so the helicopter starts to oscillate.
Any thoughts?