"Believe it or not, downwind is a good way to AVOID VRS because the rotor must have a slight FORWARD velocity to develop true VRS (between 7 and 12 knots forward, and ROD of about 800 to 1500 fpm,"
Before everyone gets too complicated with something simple, it may be best to repair to the whiteboard before the flight, which is what i do when FIRST discussing DOWNWIND THEORY with so called qualified pilots.
As Nick says 7 to 12 knots.
Same as the books say at or below ETL
Same as it happens, see below,
Helicopter is proceeding in the same direction as the wind.
Say the wind is 20 knots.
Say the helicopter has entered the procedure at 15 knots IAS.
it now slows to zero knots Groundspeed.
After it has gone past the zero knots IAS it then proceeds to a positve disk airspeed of twenty knots (from behind
).
Big mobs of room in that equation to find VRS at +7 to +12 knots. All is needed is to add descent at the appropriate rate.
In essence that is downwind theory.