Priciples of flight
Here's a different idea to explain this then..... At normal cruise speeds, the effect that we are discussing is minimal, due to the relative speed of 100 kts cruise vs 20-30kt wind. Now lets decel the helo down to 40kts for the reccie as in the original question, and a 30kt HEAD wind. (Example a Frog Helicopter with head turning Clockwise when viewed from above)
Advancing side of the disc is at a speed of Rotational velocity + 30kts (max at the 9 o'clock position), causing FlapBack (Disc High) in the 12 o'clock, and the Inflow axis will cause a 2 o'clock disc high position. If the wind is turbulent or the turn is tight (low A/s therefore tight Radius of Turn) As the helo is turned at a low speed, the Inflow Axis will be close to the 11-5 o'clock position across the disc = half past 2o'clock disc high, therefore initially the helo will be rotating the fuselage into the disc high region - meaning to stop the helo decelerating, the cyclic needs to be tilted forward - just as in overcoming flapback. If this does not happen quickly enough, the helo will decelerate - in the example towards 30 kt and the danger of Vortex ring...........
Now, did anyone follow that???
As a continuation, Why does a helicopter in the hover next to a hill experiencing updrafting air, not get into Vortex Ring?? Hover = Low airspeed, Updrafting air = Rate of Descent Flow, = Hover = Power applied.