reported On Vortex Ring State from active Mi-8 instructor

[cw4ogden]

And here’s the kicker, what if the mission asked me to fly that profile? 
 

What do you tell the boss, No can-do sir, that’s too steep an approach angle?  No, you would dig out some charts and figure out exactly how close to that demon you can fly.  
 

But you can’t, do that in DCS or real life, because the charts don’t exist, because the whole thing is a phony, at least with regards to the steep but well within a tactical approach style landings.  

 

So what does a real life pilot do?

 

You keep your approach angle below 30°, and you avoid going straight down at all costs.  But when you do have to go straight down you just do it slowly, using a ruler of them with no more than 300 ft./min., yes I know the gauge is measured in meters per second.  I’m sure the Russians have their own with thumb, In the US Army 300 ft./min. is designed to keep you absolutely 100% above the vortex ring envelope.

 

it works in any DA any gross weight combination.  It lead to confusion early on in this thread, so, for everyone wondering where the hell 300 ft./min. came from, either with respect to this thread, respect to hearing it used in relation to vortex ring state, that  300 ft./min. comes from the horizontal line, if you drew one up across and intersecting tangent to the top of VRS.  You physically cannot be in vortex ring state, in vertical descent at that rate of descent or slower.  
 

It works because it’s failsafe.  It’s origin is to avoid the need for charts.  Or possibly to combat the lack of specific charts. 
 

Not because it’s the rate of descent at which VRS is going to start occurring.  It’s the rate it can’t occur, basically.  

 

And for straight down it’s plenty.  It’s slow, but that’s just how you have to do it.  You can define straight down as anything beyond that thirty, because it’s going to feel like it.  But generally talking vertical descents here.  

 

Can you exceed 300 feet per minute?  Sure.

 

But at that point, you are now your own test pilot, and you better be prepared to be your own lawyer, because you are in no man’s land my friend. 
 

Joe Schuckatelli, pilot extraordinaire, can tell you it is safe at 600 ft./min., But There’s not a chart he can show you to prove it.  And it won’t be true for all conditions.
 

That’s where the 300 minute rule of thumb comes from, and explains why there is an entire lack of charts in general regards VRS, They are unnecessary for 99.99% of all flight profiles.

...

 

#1.  don’t exceed a 30° approach angle.  A standard glide slope is 3 degrees for reference.  30 is your shoes / pedals area.  Don’t come in 30 times too steep.  

 

#2 If you can’t follow rule one, you must go down slowly.  You can Probably go faster than 300 feet Per minute, but why be your own test pilot?  Stay in the known safe region, because you don’t know where the drop off is, because no one does. 

#3 If you Must land with a tailwind, apply both rules #1 and #2.

 

That is everything you need to know about vortex ring state to be a helicopter pilot.

 

if you can say you have diligently followed all three, rules and have never gotten into VRS I salute you!

 

You can get into the vortex rings like state during rapid decelerations as well, but that’s a whole other discussion.

 

It’s essentially coming backwards from the bottom of the chart in a windmill brake state, up through the bottom of the graph, it’s usually transitory and usually results in the ass end falling out.  Right before your rotor wash blows past you.  
 

That phenomenon has caused countless hard landings, (crashes were no one dies) but that hazard is nowhere near as well flushed out as VRS is. 
 

*** for the technically inclined, yes I know a hard landing has a specific meaning, I’m referring to the many Class A mishaps that were labeled “hard landings”, so as not to be alarmist about the accident rate in media coverage. 

Edited May 29, 2021 by cw4ogden

[Черный Дракул]

  

38 minutes ago, cw4ogden said:

The charts are notoriously vague and notoriously non specific to airframe, so I’d take those numbers with a grain of salt.

Me too. Sadly, there's no such chart in the manual (sorry, the best one I've found was for Mi-8), instead there's VRS warning of 

Quote

6.22.1. Попадание вертолета в режим "вихревого кольца" возможно при пилотировании вертолета на скоростях полета менее 40 км/ч с верти- кальными скоростями снижения более 4 м/с.

So, below 40 km/h and sink rate faster than 4 m/s for the base model can lead to VRS. Considering the following video of Mi-8T:

when VRS visibly starts at 5.5-6 m/s, 3 m/s could be safe. But MTV2 is heavier and can have its own quirks. 

Also, the same manual in part 4.7.1.4 (landing) note 2 declares the range of approach and descent speeds resulting in 6-13° approach angle (definitely not 30°). Again, it's for the base model though. 

In 4.7.2.1 (helicopter-style landing) descent rate below 40 km/h is limited at 1.5-2 m/s. 

 

1 hour ago, cw4ogden said:

300 ft./min

Please note it's just 1.5 m/s. 

[cw4ogden]

27 minutes ago, Черный Дракул said:

So, below 40 km/h and sink rate faster than 4 m/s for the base model can lead to VRS. Considering the following video of Mi-8T:

Yes, those numbers seem reasonable the same way 300 feet per minute seems reasonable, as an avoid criteria range, though.   Not that it is saying that’s where the phenomenon will start.  that has fudge factor built in for any number of things as well.

 

regarding approach angles, we keep talking about it like we can just choose your airspeed, rate of descent and your angle, independent of each other, when you can’t.   there’s an airspeed (groundspeed technically) for every approach angle and associated rate of descent.  Just clarifying not leveling criticism.
 

I can’t see the video on my phone, but I will watch it and let you know.

 

6 m/s down is fast.  That’s what?  1200 ft / min?  
 

I’d have guess 800 -1000 tops feet per minute for an unloaded mi-8.  
 

 

 

 

 

27 minutes ago, Черный Дракул said:

  

Me too. Sadly, there's no such chart in the manual (sorry, the best one I've found was for Mi-8), instead there's VRS warning of 

So, below 40 km/h and sink rate faster than 4 m/s for the base model can lead to VRS. Considering the following video of Mi-8T:

when VRS visibly starts at 5.5-6 m/s, 3 m/s could be safe. But MTV2 is heavier and can have its own quirks. 

Also, the same manual in part 4.7.1.4 (landing) note 2 declares the range of approach and descent speeds resulting in 6-13° approach angle (definitely not 30°). Again, it's for the base model though. 

In 4.7.2.1 (helicopter-style landing) descent rate below 40 km/h is limited at 1.5-2 m/s. 

 

Please note it's just 1.5 m/s. 

What is forward airspeed, is my question I don’t care about the rate of dissent so much is how fast is he going forward?

Edited May 29, 2021 by cw4ogden

[cw4ogden]

59 minutes ago, Черный Дракул said:

  

Me too. Sadly, there's no such chart in the manual (sorry, the best one I've found was for Mi-8), instead there's VRS warning of 

So, below 40 km/h and sink rate faster than 4 m/s for the base model can lead to VRS. Considering the following video of Mi-8T:

when VRS visibly starts at 5.5-6 m/s, 3 m/s could be safe. But MTV2 is heavier and can have its own quirks. 

Also, the same manual in part 4.7.1.4 (landing) note 2 declares the range of approach and descent speeds resulting in 6-13° approach angle (definitely not 30°). Again, it's for the base model though. 

In 4.7.2.1 (helicopter-style landing) descent rate below 40 km/h is limited at 1.5-2 m/s. 

 

Please note it's just 1.5 m/s. 

Soviet doctrine is going to be written for the Afghanistan theatre.  It’s likely never been updated, or at least changed significantly in those updates to address a problem that isn’t killing that many.

 

Wouldn’t be sad if we had 6000 foot hip performance at sea level regarding VRS because somebody misunderstood what the manual meant?

 

What if the entire problem is the fact that it was modeled on the one clause, maybe the only in the MI8 manual that’s written as a rule of thumb for operating at 6000 feet or so?

 

that could explain why it’s always felt like we’re operating at high altitude even near sea level, speaking solely to VRS not performamce.  
 

40 kph is going to be associated with ETL and maybe a small cushion or even rounded up possibly for ease of memory.  
 

I don’t know what to make of 6 to 13 degrees approach angles, to associate with anything.   I don’t know.  It seems an odd range?  Am I fine at twenty?  
 

it could mean normally, light weight etc angles up to 13 degrees are safe, but decreasing to six as you take on aggravating factors like weight or altitude.  
 

 

Edited May 29, 2021 by cw4ogden

[Черный Дракул]

49 minutes ago, cw4ogden said:

there’s an airspeed for an approach angle and associated rate of descent.

Yes, it's a number derived from these (60-70 km/h and 2-4 m/s with no wind), it's also declared in the same paragraph. I just noted that because it's a significantly more shallow angle than the one was talked of. 

 

30 minutes ago, cw4ogden said:

Soviet doctrine is going to be written for the Afghanistan theatre.  It’s likely never been updated, or at least changed significantly in those updates.

Military doctrine by army aviation and aircraft flight manuals by aircraft producers are very different areas, so the former should not affect the latter. In case it does change, all the changes to the documentation are listed at the start of the linked document with signatures of those responsible. 

Of course, here we come in contact with soviet aviator training doctrine, where learning and flying are separated to a degree. The aircraft manual alone can describe these things in general, while its theoretical or practical application can be taught in other parts of soviet aviator's path. The diagram we're looking for can actually be existing in one of the helicopter trainee theoretical textbooks or guides or whatnot -- or in specific training center or regiment's practical documentation. 

 

30 minutes ago, cw4ogden said:

that would explain why it’s always felt like we’re operating at 6000 feet or so, even near sea level.

Personally, I'd love to see @PilotMi8's point of view on the matter. He could explain it better himself. 

Edited May 29, 2021 by Черный Дракул

[Ramsay]

13 hours ago, cw4ogden said:

The charts are notoriously vague and notoriously non specific to airframe, so I’d take those numbers with a grain of salt.

I'm sure you know this but for those that don't, the generic VRS chart is given in terms of the theoretical downwash where

 

1 = Vih = Theoretical downwash = Square root of (T/(2xρxA))

 

 

A little more detail on how to calculate the Theoretical down wash

 

 

VRS = 700 ft/min is a specific value when the generic chart has been scaled/labelled for a R22's theoretical down wash.

Edited May 29, 2021 by Ramsay

[cw4ogden]

1 hour ago, Ramsay said:

I'm sure you know this but for those that don't, the generic VRS chart is given in terms of the theoretical downwash where

 

1 = Vih = Theoretical downwash = Square root of (T/(2xρxA))

I was not aware.  Thanks for the information.  What is the source document?

[sLYFa]

VRS zone diagramms exist for the MT variants, I cant post them though since my source is a training document from the russian AF. Above 40kph, there is no risk of entering VRS. Below 40kph, maximum vertical speed is 10m/s which reduces to approx. 4m/s at about 20kph where it remains constant up to zero forward speed. Now in DCS, you can go as low as 4.5m/s in a hover before entering VRS while above 40kph there is no danger of entering VRS at all. It gets intersting in between though. At 30kph, you should be able to descend at just above 5m/s without entering VRS. In DCS however, VRS occurs as early as 3.5m/s at 30kph which is even less than what you can fly in a hover. I guess there is indeed a bug in the transition zone between 40 and 20kph where maximum allowable descend speeds should be higher than in a hover but are in fact lower. I will put together some tracks and report in the russian forums. 

[cw4ogden]

2 hours ago, sLYFa said:

I guess there is indeed a bug in the transition zone between 40 and 20kph where maximum allowable descend speeds should be higher than in a hover but are in fact lower. I will put together some tracks and report in the russian forums. 

Could you post your track files and any findings here as well?

 

Understand you can't post the source, graphically, it would be something like this?   

Are there different charts for different configutations, or operating conditions, or just one chart?

 

Edited May 29, 2021 by cw4ogden

[Ramsay]

7 hours ago, cw4ogden said:

What is the source document?

 

Unfortunately I don't have an easily quotable source - I don't recall the exact web pages, some where a mix of free form diagrams, text and maths and used different variables but it became pretty obvious that Vz/Vh, Vd/vi, etc. were ratios with respect to the theoretical downwash i.e. this chart for the R22 using Vd/vi

 

 

or this using Vz/Vh (from "Model for Vortex Ring State Influence on Rotorcraft Flight Dynamics", ADA526709.pdf)  where Vh is defined on it's front page as Square root of (T/(2 x ρ x A))

 

 

 

These use equivalent scales where Vi and Vh = the velocity of the theoretical downwash

Edited May 30, 2021 by Ramsay

[sLYFa]

On 5/29/2021 at 2:10 PM, cw4ogden said:

 

 

Thats pretty much it, although the original curve has obviously much smoother curvature. I have only one chart and it doesnt state for which weight or temperature/altitude the chart applies. I assume its. standard pressure, medium weight at SL.

Here are my results from DCS (15°C standard pressure)

TOW 10000kg

forwad (kph)     vertical (m/s)

0                         5

20                       4.5

30                       3.5

 

TOW 13000kg

forwad (kph)     vertical (m/s)

0                         6

20                       5

30                       4.5

 

You can see that higher weights delay VRS onset, although I would think it should be the other way around (more blade AoA-> stronger vortices).

What is definitely out of place is the reduction in allowable descend rate between 40kph and hover.

Tracks attached

VRS_0_MTOW.trk VRS_0.trk VRS_20.trk VRS_20_MTOW.trk VRS_30.trk VRS_30_MTOW.trk

Edited May 30, 2021 by sLYFa

[cw4ogden]

28 minutes ago, sLYFa said:

You can see that higher weights delay VRS onset, although I would think it should be the other way around (more blade AoA-> stronger vortices).

What is definitely out of place is the reduction in allowable descend rate between 40kph and hover.

Tracks attached

This makes theoretical sense.  VRS is about UP-flow through the rotor.  Heavier aircraft, in general, and heavily loaded aircraft will push more air downward to stay aloft, than their lighter, or lightly loaded counterparts.

Heavily loaded aircraft, in theory would be somewhat less likely to encounter it.  But also have a much smaller available power margin to power your way out of the early phases.



Great work by the way.  I don't think my track files found the "smoking gun".  But sounds like a good place to start, for sure.  

Tagging @NineLine  (because this may be a new finding and for your track files).

 

[sLYFa]

9 hours ago, cw4ogden said:

VRS is about UP-flow through the rotor.  Heavier aircraft, in general, and heavily loaded aircraft will push more air downward to stay aloft, than their lighter, or lightly loaded counterparts.
 

That makes sense, so at least that part should be correct.

[cw4ogden]

58 minutes ago, sLYFa said:

forwad (kph)     vertical (m/s)

0                         5

20                       4.5

30                       3.5

 

TOW 13000kg

forwad (kph)     vertical (m/s)

0                         6

20                       5

30                       4.5

Assuming we are not missing something, and if your numbers are correct, something is way off.  

These numbers would indicate the curve is essentially, backwards, with respect to susceptibility and forward airspeed. 
I.e. it looks like the numbers are flip-flopped.  I would expect just the opposite, with very low susceptibility at 40 kmph, and that susceptibility increasing with decreasing forward airspeed to a high susceptibility before leveling off at 20kph or so.  At which point the value remains constant.
 

Edited May 30, 2021 by cw4ogden

[cw4ogden]

 

This would be the VRS curve for the DCS mi-8 given your data.

[sLYFa]

Yeah thats pretty much the issue. Perhaps they've got the sign wrong on the blending curve between 20 and 40kph

[cw4ogden]

I've heard a lot of people say essentially, "You're case is purely circumstantial!"  

And I will concede this point, with the caveat: it's a special kind of circumstantial.  
It's a Prima Facie case - Prima Facie -  "sufficient to establish a fact or raise a presumption unless disproved or rebutted."

In my track file, the aircraft enters VRS at 30 KIAS or about 55 kmph.  This is above ETL and above ETL by the very definition of what ETL means, you physically cannot induce VRS, period.  It's an aerodynamic impossibility.

That is not circumstantial, that is prima facie.  

I don't need to prove the MI-8 VRS modelling is wrong, you need to prove to me that the MI-8 can in fact, encounter VRS at 55 kmph.  
On it's face, prima facie - everything we know about VRS says you must be below the effective transitional lift airspeed to encounter VRS.  VRS is a phenomenon of re-ingesting the air you have induced a downward momentum. 

 

"Effective translational lift (commonly referred to as ETL) is a term used to describe the airspeed at which the entire rotor system realizes the benefit of the horizontal air flow. This happens when the helicopter's rotor disc moves completely out of its own downwash and into undisturbed air."

"A vortex ring state is when the helicopter’s downwash recirculates into the induced flow and the helicopter descends while under power."

 

The two are mutually exclusive. I don't have to prove that.  It is correct on the face of it.  My case is prima facie, a very special kind of circumstantial.  I don't have to prove this point, it must be disproven, or the findings to be shown in error.

 

Edited June 1, 2021 by cw4ogden

[Blackeye]

On 5/30/2021 at 2:45 PM, sLYFa said:

forwad (kph)     vertical (m/s)

0                         5

20                       4.5

30                       3.5

 

That seems really weird to me - perhaps it's worth its own bug report as this is not about "VRS seems too sensitive" but rather its relation to forward speed is off.

Edited June 1, 2021 by Blackeye

[cw4ogden]

1 hour ago, Blackeye said:

 

That seems really weird to me - perhaps it's worth its own bug report as this is not about "VRS seems too sensitive" but rather its relation to forward speed is off.

 

Yes, that is my beef.  

It is modeled well when entered from a hover OGE or very steep approach.  

It's off, if the most dangerous region is near ETL where, by definition, once beyond, you are not subject to vortex buildup.  Around ETL I should be very slightly susceptible, and if I do get into it, it's going to produce a rolling moment, initially at least, as the aft portion of the head encounters it first, from induced from of the forward half, which is still in clean air, all offset by 90 degrees. 

It's conceivable the Mi-8 goes through ETL much higher speed than normal helos, but I find it unlikely, and unsupported in any documentation.  But if that is the case, the modeling may be fine.  Regardless, that is the crux of my argument, something is wrong regarding VRS kicking in while you have enough forward airspeed and / or shallow enough approach angle that you should be well clear of your downwash.  

At this point, the flight model is either off, or the hip exhibits behaviors unlike any other rotary wing aircraft flying.  And it's totally undocumented and has led to surprisingly few accidents.

Edited June 1, 2021 by cw4ogden

[cw4ogden]

From the FAA handbook on helicopter flight.  Note the passages in BOLD text.  

 

Vortex Ring State Vortex ring state (formerly referenced as settling-withpower) describes an aerodynamic condition in which a helicopter may be in a vertical descent with 20 percent up to maximum power applied, and little or no climb performance. The previously used term settling-with-power came from the fact that the helicopter keeps settling even though full engine power is applied. In a normal out-of-ground-effect (OGE) hover, the helicopter is able to remain stationary by propelling a large mass of air down through the main rotor. Some of the air is recirculated near the tips of the blades, curling up from the bottom of the rotor disk and rejoining the air entering the rotor from the top. This phenomenon is common to all airfoils and is known as tip vortices. Tip vortices generate drag and degrade airfoil efficiency. As long as the tip vortices are small, their only effect is a small loss in rotor efficiency. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in circulating the air in a doughnut pattern around the rotor. In addition, the helicopter may descend at a rate that exceeds the normal downward induced-flow rate of the inner blade sections. As a result, the airflow of the inner blade sections is upward relative to the disk. This produces a secondary vortex ring in addition to the normal tip vortices. The secondary vortex ring is generated about the point on the blade where the airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disk. Rotor efficiency is lost even though power is still being supplied from the engine. [Figure 11-3] A fully developed vortex ring state is characterized by an unstable condition in which the helicopter experiences uncommanded pitch and roll oscillations, has little or no collective authority, and achieves a descent rate that may approach 6,000 feet per minute (fpm) if allowed to develop.

 

A vortex ring state may be entered during any maneuver that places the main rotor in a condition of descending in a column of disturbed air and low forward airspeed. Airspeeds that are below translational lift airspeeds are within this region of susceptibility to vortex ring state aerodynamics. This condition is sometimes seen during quick-stop type maneuvers or during recovery from autorotation. The following combination of conditions is likely to cause settling in a vortex ring state in any helicopter: 1. A vertical or nearly vertical descent of at least 300 fpm. (Actual critical rate depends on the gross weight, rpm, density altitude, and other pertinent factors.) 2. The rotor disk must be using some of the available engine power (20–100 percent). 3. The horizontal velocity must be slower than effective translational lift. Situations that are conducive to a vortex ring state condition are attempting to hover OGE without maintaining precise altitude control, and approaches, especially steep approaches, with a tailwind component.

 

When recovering from a vortex ring state condition, the pilot tends first to try to stop the descent by increasing collective pitch. However, this only results in increasing the stalled area of the rotor, thereby increasing the rate of descent. Since inboard portions of the blades are stalled, cyclic control may be limited. The traditional recovery is accomplished by increasing airspeed, and/or partially lowering collective to exit the vortex. In most helicopters, lateral cyclic thrust combined with an increase in power and lateral antitorque thrust will produce the quickest exit from the hazard.  This technique, known as the Vuichard Recovery (named after the Swiss examiner from the Federal Office of Civil Aviation who developed it) recovers by eliminating the descent rate as opposed to exiting the vortex. If the vortex ring state and the corresponding descent rate is allowed to progress to what is called the windmill brake state, the point where the airflow is completely up through the rotor, the only recovery may be an autorotation. Tandem rotor helicopters should maneuver laterally to achieve clean air in both rotors at the same time. For vortex ring state demonstrations and training in recognition and recovery should be performed from a safe altitude to allow recovery no less than 1000 feet AGL or the manufacturer’s recommended altitude, whichever is higher.

 

To enter the maneuver, come to an OGE hover, maintaining little or no airspeed (any direction), decrease collective to begin a vertical descent, and as the turbulence begins, increase collective. Then allow the sink rate to increase to 300 fpm or more as the attitude is adjusted to obtain airspeed of less than 10 knots. When the aircraft begins to shudder, the application of additional up collective increases the vibration and sink rate. As the power is increased, the rate of sink of the aircraft in the column of air will increase. If altitude is sufficient, some time can be spent in the vortices, to enable the pilot to develop a healthy knowledge of the maneuver. However, helicopter pilots would normally initiate recovery at the first indication of vortex ring state. Recovery should be initiated at the first sign of vortex ring state by applying forward cyclic to increase airspeed and/ or simultaneously reducing collective. The recovery is complete when the aircraft passes through effective translational lift and a normal climb is established.

 

 

 

Hence, my point.  VRS and flight above ETL are mutually exclusive.

 

Source: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf

Edited June 1, 2021 by cw4ogden

[cw4ogden]

For anyone following this thread, the discussion migrated to the Russian forums.  I have elected to no longer beg intrusion on a foreign language forum, as the case has been made.  I follow up only to let any die-hards know the discussion continues on the Russian Mi-8 forum through use of google translate, but my case is complete, and I hope to leave this with the community and developers as it has eaten up far too much of my life. 

I thank everyone who participated, and I will field any alibi questions and hope the discussion and testing can continue now that the ball is rolling.

 

 

[Lurker]

Every now and again you come across a great thread like this. Thanks for your efforts cw4ogden, and special thanks to sLYFa who seems to have gleaned where the problem (bug) might be originating from. 

Edited June 7, 2021 by Lurker

[cw4ogden]

6 hours ago, Lurker said:

Every now and again you come across a great thread like this. Thanks for your efforts cw4ogden, and special thanks to sLYFa who seems to have gleaned where the problem (bug) might be originating from. 

 

Thank you for the kind words.  I’d only add that we’re at the “needs investigation” point, versus a confirmed bug.  
 

 

[Lurker]

11 hours ago, cw4ogden said:

Thank you for the kind words.  I’d only add that we’re at the “needs investigation” point, versus a confirmed bug.  
 

 

 

I've read the entire thread, and it certainly looks like there is something wrong in a very narrow part of the flight envelope. Most of us need to get through that narrow part to actually land this helicopter. If we stick to the flight manual, we won't encounter any problems but it appears that there is indeed a pretty big problem if we take it to the edge of the flight envelope. Which is not beyond the realms of impossibility in the Mi-8 or any military chopper. KUDOS to you for sticking with it, I've always felt that the Mi-8 is a bit too easy to get into trouble with, but I've always thought this was down to my shitty controls\inexperience\horrible skills and lack of a haptic feedback system. Since I've gotten VR it's been a much easier bird to fly, but I've still wondered why I had to anticipate VRS so much even in some cases where it "looked" like (in VR this is especially obvious) that I'm obviously moving out of my own rotor downwash at a decent clip. Hopefully the devs take a look at this again. I'm pretty sure it's not an intended thing at all but some kind of bug that's occuring between 30-40kph. 

[cw4ogden]

 

Pulled from another discussion:

 

The first portion is a series of LTE accidents, but I call your attention to 3 minutes and 30 seconds. This appears to be a high fidelity mi-8 simulator, capable of modeling LTE, so I am making an assumption, that VRS is modeled as well.

If you watch the flight parameters just before entering LTE, I believe had this been DCS's mi-8 modeling, the pilot would have been eaten alive by VRS. Yet his limited forward motion appears to keep VRS away.