I haven't followed the whole thread, I do not know what the ROD was in the final stages of the accident, but this might shed some light as to whether it was IVRS or VRS or not (as I suspect)!
VORTEX RING STATE
To calculate downwash speed (reference for VRS for a particular Helicopter) take disc loading in LBS/Sq.ft × 210, then take the square root of answer, this will give you speed in ft/sec. (100% downwash speed)
To be conservative, if you now take 50% of this you will avoid VRS.
At 50% you may feel some conditions of IVRS, at 75% you will be in IVRS.
Example for S76.
Formula: Disc Loading (Lbs/sq/ft) × 210 = Χ. √ Χ = ft/sec.
S76 Disc area: (π×r×r) r =22ft
3.14×22×22 = 1520²ft.
Disc Loading = AUW = 11700 = 7.70Lbs/sq/ft.
DISC AREA 1520
7.70 × 210 = 1617. √1617= 40.2ft/sec.
40.2ft/sec = 24kts = 2412ft/min Downwash speed.
50% of 2412 =1206 ft/min.
S76 @
11700 Lbs Downwash speed = 2412ft min. 50% = 1206ft/min.
11000 Lbs Downwash speed = 2339ft/min. 50% = 1170ft/min
10500 Lbs Downwash speed = 2286ft/min. 50% = 1143ft/min
10000 Lbs Downwash speed = 2232ft/min. 50% = 1116ft/min
9500 Lbs Downwash speed = 2173ft/min. 50% = 1086ft/min
9000 Lbs Downwash speed = 2116ft/min. 50% = 1058ft/min
Goodnight.
