At some risk of off-topic posting:
"Making a low noise helicopter is an art, and costs weight and performance. A slow main rotor is great, but it stalls at low speed, weighs more and doesn't autorotate as well."
I have often wondered whether a hybrid powertrain would help with this. The downside is definately cost, bearing in mind the objective here is to reduce noise complaints. A high RPM electric motor (ie light weight - ideally a disk design) is used in parallel with the turbine. A minimum mass set of LiMH batteries, optimised for high power/weight is also required. We are talking a few seconds worth of power only - energy/weight or range is normally the limiting factor for batts.
The motor provides a very fast transient RRPM response, when collective requires, to overcome turbine lag. This could allow improved cruise efficiency in addition to low noise. The fast RRPM response still allows good manouvreing margin, without any pilot flight mode selection. Without doing the sums in detail, my guess is that the performance advantage would more than make up for weight penalty. Long term cost (other than fuel savings) is harder to estimate, but turbine would not need to operate so near to its TET limit which must be good.
In combination with weak collective servo additional benefits might be fast automatic entry into autorotation. There may even be some energy reserve, pending calculations, that gives the pilot more options. It may even help overcome the H-V curve limitations, by providing enough power for a controlled landing.
Mart