Charlie asked:
".......how do you calculate what sort of airspeeds the aircraft is going to be susceptible to VRS (the airspeeds corresponding to the red area on your B206 graph)? I understand how to estimate the descent rates using the formula, just missing the link to figuring out the airspeeds"
Charlie, the forward speed scales on those plots are in ratios to the downwash speed, so 1.0 is simply 1x the downwash speed (which is calculated using that formula shown in the beginning) That formula is simply an attempt to approximate the speed of the downwash needed to produce the lift equal to weight. It assumes that the momentum transferred from the air to tha aircraft has produced a uniform stream of air.
Sorry that it takes a while to download. I can break the page up into chunks if that would help, or I can email the plots to you separately.