Hi
most of the helicopters may have rotor stall during vertical descent with more than 500-700 ft/min and fwd speed less than 30 kts but this only matter when your blades are with positive angle of attack , so if you are in the same conditions but has your collective fully or almost down (0 or very little pitch ) this shouldn't happen ?
My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed .
https://www.youtube.com/watch?v=USmOqlj9Hag
There are few misunderstandings with your statement, but that is not surprising as this is a chewy subject for many people and you are not alone! I lecture internationally on helicopter operations and principles of flight so I know a little about this - nobody knows everything, believe me...and a lot of PoF is what we 'think' is happening. There is also a lot of poetic licence used to make concepts easier to understand, for example, if you are familiar with the 'vector diagram' often used to show forces acting on a blade, it is actually drawn wildly out of proportion to the true vectors (a vector having both magnitude and direction), but it allows us to explain the concept in a more simple and easy to understand way.
I shall add a few comments to your statement, which I believe is referencing Vortex Rings state fronm the parameters you mention. If you are implying Retreating Blade Stall (I don't think so?) then I can explain that also.
most All of the helicopters may have
partial or complete rotor stall during vertical descent with more than 500-700 ft/min and fwd
airspeed less than 30 kts,
but many aircraft have far higher requirements and need RoD in excess of 1500ft/min with airspeed below 10 kts, depending on muliple factors but predominantly aerofoil cross section, sweep charcteristics and disc loading. but this only matter when your blades are with positive angle of attack - wrong, this only occurs when the 2 conditions you mention are met, and there is also power applied. Positive AoA is not a required factor for this condition. A helicopter in autorotation with the collective fully down and zero pitch applied has a positive AoA due the relationship between the chordline and the relative airflow. Remember the RAF is modified due to the RoD flow from under the aircraft and this creates a positive AoA, thereby creating lift, and also drag that acts forward of the blade (at 90 degrees to lift or parallel to the RAF, whichever you prefer) to accelerate it - in short, the position of the collective dictates pitch angle,but it does not dictate Angle of Attack. - , so if you are in the same conditions but have your collective fully or almost down (0 or very little pitch ) this shouldn't happen ?
Kind of - if the collective is fully down you cannot get into VRS, but is has nothing to do with pitch or AoA, it is because there is no power applied. My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed .
Some mention that in VRS it is the root that is stalled - this is true, but a little misleading. The root of a blade is always stalled (too little airpseed and no workable aerofoil cross-section, but in VRS, whilst the root stall is larger, the tip's may also stall due to the recirculation adding to the induced flow driving the AoA to beyond the critical AoA, but it is not a certaintly.
Cheers