I know very little about helos so could be quite wrong. I always thought this was a matter of convenience. The first helicopters like the S-51 were flown from a single seat in the centre. I believe the original Bell 47 was flown from the left-hand seat like a conventional fixed-wing type. When side-by-side dual control became popular the collective lever was positioned in the centre between the seats. As the majority of pilots are right-handed it made more sense to fly from the right-hand seat.

That's my theory anyway.

Not all light ones are flown from the right seat. A modern example is Hughes/MD helicopters.

As far as I know, all large helicopters are designed to be normally flown from the right seat with  the exception of the Russian machines.

The first few Bell 47s were built for right seat flying but the fixed-wing pilots learning to fly them whined so much about it being unnatural that Bell bowed to the pressure and changed their production line. But, you'll notice all subsequent designs have right-hand drive.

Helicopters are designed for stick in the right hand, collective in the left and you can NEVER let go of the stick (yeah, I know, we have some with auto-pilot/autohover). The right seat concept was to eliminate having to reach across yourself to make adjustments on the console and/or panel. Take a look at the cockpit arrangement in a Bell 47J. Pilot sits centered with console conveniently on his left.

I think most pilots with 400-500 hrs experience in either fixed or rotary wing would be comfortable flying from either seat as long as critical or important adjustments can easily be made.

Cal

Many thanks Gents for your courtesy and enlightenment