For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.
For the record, no helicopter can experience true VRS unless it is descending nearly vertically, with respect to the relative airflow, at about 800 to 1000 feet per minute.
Starting to drift off topic a little, but what is wrong with this..................
If you fly level at a hill which has an upslope of 30 degrees (not that steep) with a 25 knot tailwind, the following should/could happen.
At 25 knots groundspeed as you approach the hill you would have zero airspeed. In fact at 50 knots groundspeed your ASI will probably read zero.
The vertical component of the wind would be ~ 11Knots or ~ 1100 FPM. Plenty right for VRS.
The VSI would read ZERO.
If the hill is 45 degrees for the same vertical component you would only need 15 knots of wind. 100 FPM is only ~ 1 Knot after all.
In this situation you are flying where the ASI won't tell you much and neither will the VSI. Where VRS can get you is where the wind blows parallel to the rising ground. It will get worse as you get closer to the hill (Up to a point where surface friction will start to affect things).It is true that VRS will not get you if you have relative flow across the disk. At these low speeds the ASI will not indicate much.
The indication which will also catch you is that you are not descending according the static instruments (ALT/VSI/IVSI) and you are using LESS power than you would be in a hover (Where hover = zero airspeed). You are descending at ~1100 FPM after all. If it feels too good to be true, that is probably right.
Also your escape route is blocked by rising ground. That is why this style of approach is very dangerous. Remember all these scenarios are for a LEVEL approach. Add in a little decent and it all happens a lot easier.
If you think these conditions don't happen, I have experienced a climb in a Bell 205 at 11,200lbs with the needles split!
Back to your subject - as pointed out, a lighter helicopter is more susceptible than a heavier helicopter, but I think the point should be a lower disk loading versus a higher disk loading. i.e. disk area/weight.
Go and talk to an experienced Cheetah/Chetak mountain pilot. Be quick as those aircraft will evidently be gone next year.
