VRS While in Autorotation

[helipilot12]

Hello,

I'm not sure if this is a bug or if it's theoretically possible, but it happens often enough for me to question it. I tried searching for previous threads regarding this, but couldn't find something with the exact issues either. Please feel free to move/close/point this thread to an existing one if one does exist.

From a theoretical standpoint, helicopter rotor discs have 3 modes of flight:

1: Normal flight (air travels down through the rotor disc)

2: Autorotation (air travels up through the rotor disc)

3: Vortex ring state (air travels down through the disc, and is then recycled around the rotor tips and back through the top towards the bottom)

 

I'm a huge believe in practicing full-down autorotations, thus I use DCS as a good platform to play around with. I am a real pilot (only ~100 hours however), so I'm not an industry veteran, and I'm well open for suggestions, corrections, and others' experiences.

 

The issue at hand essentially seems to be the ability to enter VRS while the engine is idle or off, and the aircraft is already in a state of autorotation.

To replicate this, set up at around 2000ft AGL, drop the engine to idle, and enter a direct vertical descent autorotation. Once in a stable auto at 0 knots airspeed, and 1500-2500FPM descent, firmly pull the collective to max pitch. What I would expect to happen is my descent rate being reduced, until the rotor RPM decays enough. What happens instead is a rapid increase in descent rate (Unexpected), along with the rotor bleeding off RPM(Expected).

 

As I understand it and as the industry has been teaching, the following factors must be present to enter vortex ring state:

1: Descent rate of greater than 300FPM (This factor varies widely based on aircraft, rotor loading, etc. The FAA just uses 300FPM as a safe number)

2: Airspeed less than Effective Translational Lift

3: Between 20-100% engine power (essentially some power that's keeping the rotor spinning rather than RPM decaying)

 

I have two ideas as to what could possibly be happening:

1: The firm pull on the collective is enough for the aircraft to enter VRS briefly as if it were still under power

2: The flight model doesn't have this simulated properly

 

I'm at about a 50/50 split between my two theories, and I'm open to what others may have to say. I've done autos in an R44 at ~20 knots with no flare, and the rotor RPM decays immediately and arrests the descent. I'm able to replicate this in DCS as well, and I'm 99% sure it can be attributed with there being enough airspeed to be in ETL. So the only effect I'm questioning at this point is the ability to enter VRS at 0 airspeed. I know the DCS Huey flight model has been particularly prone to VRS in the past as well, based on some other forum posts I was reading.

Again, feel free to question, critique, correct, or share your experiences, I've been meaning to ask this for a while and finally had some time to do it.

[Quadg]

 

7 hours ago, helipilot12 said:

1: The firm pull on the collective is enough for the aircraft to enter VRS briefly as if it were still under power

This.

You autorotate to maintain RPM and store potential energy in the disk.

When you pull the collective you stop autorotating and convert that stored potential energy into down thrust. slowing the disk.

and entering VRS if you are within the VRS margins.

landing normally if you are outside the VRS margins.

[cw4ogden]

@helipilot12

What you are describing is being below the VRS region, down in the windmill brake state.  

Often overlooked, one way to break out of VRS is by dumping collective (this is only an option with a lot of excess altitude to spare).  
Lowering collective to the point all the wind is coming upwards through the rotor will free the rotor from a vortex state.  And for reasons stated below, you still need some lateral motion, or you will just re-enter VRS, when you pull power back in.

If you try to come "up" through the VRS region, meaning if you try to arrest the descent, and wind up entering the VRS region from the bottom or the vertical speed axis, you will encounter VRS.

On the diagram, coming in at a 60 degree approach angle (not unheard of for autos), you can see you will absolutely enter VRS as you try to arrest the descent.

The same phenomenon is at play trying to do quick-stop landings.  You dump the collective, enter the windmill brake state, all airflow is upwards through the rotor.  But when you transition to landing, woe be unto the pilot who tries to pull up through the VRS region as a means of arresting descent.

One caveat being for hovering autorotations, If you dump the collective (amount depends on hover height), the collective pull at the bottom is not likely to induce VRS.  Full blown VRS is an equilibrium state, meaning it takes a moment to onset, even under the best conditions.  So for hovering autorotations, you will likely arrest your descent without VRS becoming a factor.  It's all happens too fast for VRS to develop.  

Edited October 22, 2021 by cw4ogden

[helipilot12]

@cw4ogden Thank you for the diagram and explanation, I assume this is probably something taught in much more depth in military flight programs, compared to a part 61 civilian flight school. I've been researching this topic on-and-off over the past several years, and have never seen that diagram, so it's certainly something new to consider and a good point to do some more research with.

I'll bring this up with a few flight instructors when I get a chance, it'll be good to re-visit this topic. It would be neat to test this further in real life, although it's not really feasible given that you'd likely end up as a pancake on the runway.

Off to do some more testing in DCS with this new information, thanks again!

 

Mods - Feel free to move this to the correct section.

[cw4ogden]

2 hours ago, helipilot12 said:

@cw4ogden Thank you for the diagram and explanation

It's unfortunately not well taught, nor even well understood in the community.  Gonna send you a P.M.

[kam]

On 10/22/2021 at 12:53 AM, helipilot12 said:

To replicate this, set up at around 2000ft AGL, drop the engine to idle, and enter a direct vertical descent autorotation. Once in a stable auto at 0 knots airspeed, and 1500-2500FPM descent, firmly pull the collective to max pitch. What I would expect to happen is my descent rate being reduced, until the rotor RPM decays enough. What happens instead is a rapid increase in descent rate (Unexpected), along with the rotor bleeding off RPM(Expected).

 

Just wondering what you're trying to achieve doing that, or is it for experimentation purposes?

I've only had one direct vertical 0 speed auto demonstrated to me in the Cabri G2, and at 500ft we gained a little bit (40kt ish) of fwd speed to do a normal auto landing.

[helipilot12]

@kam Just experimentation, wanted to test what happens when pitch is pulled during a 0 knot or below-ETL autorotation. I've done a couple autos in an R44 that started as a vertical descent, then gaining around 30kts of forward speed, and then yanking the collective at the bottom (full-down to the ground and no flare). The Huey in DCS reacts similarly to that, and satisfies what I've seen in the real world. Doing one with no forward airspeed at all in real life wasn't going to be something anyone attempts, so I figured I'd do some testing in DCS.

[skypickle]

I have been able to escape vrs in dcs by rolling the cyclic to the left, with a lot less altitude loss than dumping the collective. Even with very little forward velocity.

 

I think this has been described by Vuichard.

[fapador]

2: The flight model doesn't have this simulated properly.

I had 2 operators/  high rank militants, one former and one with 3600hours on the Hellenic Army UH-1H  have a go in Dcs huey. Long story short, they said the flight model is ~80% like the real thing. They especially didn't like autorotations and said the rotor felt like a hog compared to the real thing. they also made some remarks about level flight speed and said dcs huey has a strange over tendency to return level when banking. They described it  like the rotor is acting like a "gyroscope" to me. They also said vrs is a little exaggerated and that the huey in reality is not so prone. When they tried some aggressive maneuver flying, BOTH of them ended up with detached rotors (mast bumping) and laughed at how easy this is to happen... Also they said tail rotor is a tad on the weak side on max pedal deflection especially on the left . All in all, they liked some things FM wise,  like tendency to roll with antitorgue input and loved the graphics. Some sound effects also seemed a little off to them.  

[admiki]

OP, I have also done quite a few of 0 speed autos. Only difference is that I don't go full collective from bottom, but use as much as I need to slow down descent and then use rest of it to cushion the landing.

Helicopter rotor blades are basically wing profiles. IMO, you are not VRSing, you are completely stalling the blades. Air is coming from below and you have inertia to overcome. So when you pull full collective, that AoA on blades is huge. At low collective, those blades are acting as a brake. At full collective I would expect them to behave just like a stalled wing. Huge drag, but no lift, hence you drop down like a stone.