Thanks Nick for that reply. I've been mulling over what you said and sorry to whip a dead horse but I have one more question if you feel up to it.
Is rotor downwash speed measured relative to the plane (cone?) of the rotating blades or relative to the undisturbed air mass surrounding the helicopter?
I mean, if a helicopter with a theoretical downwash speed (in an OGE hover) of say 1400 fpm is descending vertically at say 400 fpm. Is the downwash velocity such that the downwash is approaching the ground at 1800 fpm (the vector sum of the two) or is is that the downwash velocity relative to the blades is actually 1000 fpm which added to the 400 fpm descent rate gives a total downwash velocity (again relative to the approaching ground) of 1400 fpm??
I feel this second solution would tie in nicely with both your explanation and my possibly misguided idea of downwash velocity (relative to the blades) reducing with reduced collective.
Thanks again!
Irlandés