Hello Nick,
I have held an anenometer close to a hovering Puma and have recorded speeds of circa 60kts of wind and it certainly felt like it.
The calculated downwash speed using your formula comes out at only 47.76ft/sec (circa 28kts). The S92 is very similar. These figures seem very low.
I know the downwash will be deflected by ground effect etc which will distort an anenometer reading but this seems a huge difference. Have I made a calculation error or is there another factor i am not taking into account ?
By the way, your website must have taken lots of hits as your host has closed down access to it due to it exceeding its download limit. Lots of interest in vortex ring which is very healthy !!