DCS Mi-8: Vortex Ring State Tutorial

[@ErikScott128]

I'm debating just how much detail I want to go into here before everything I type just starts becoming annoying rather than helpful. The problem is, vortex ring state, like so many other things in a helicopter, does not lend itself to simple explanations. I'll start with comments at various time stamps, and then follow up with a summary of my own. 0:48 It's not that you're rate of descent is "so high..." it's more that your rate of descent is beginning to equal the rotor's induced velocity. Many people say the rotor is descending into its own downwash. 1:03 The S-64 may be able to "power out" of VRS, but that's likely because it is comparatively light compared to other similarly sized aircraft. It would not be able to do that while doing what it is designed to do: carry heavy loads. 2:35 So, the whole thing with the root vortex is simply unimportant for this discussion. The turbulence associated with the presence of the hub and fuselage largely destroys the root vortex. It is true, however, that net airflow may by upward at the inboard section of the blades, so there will be another vortex, but it is not about the blade root, but somewhat more outboard. 4:40 So, I suppose this would relate to the idea that VRS occurs when your descent velocity roughly equals your induced velocity. Normally, the tip vorticies are washed down with the induced velocity (Here's an excellent video showing tip vortices and wake contraction kzbin.info/www/bejne/oqTPkqWQnrd9nsk ). If the induced velocity equals the downward velocity, they will be washed up into a single torus ring encircling the blade tips. This is a gross simplification, but it's not bad for the layman. 7:52 Collective doesn't really make things worse (you'll notice you arrested you descent at -10m/s) but it doesn't help either. It merely keeps you in VRS rather than allowing you to transition to an autorotative or windmill brake state (more on this later). 11:30 This may be nitpicky as I probably just picked up on you mis-speaking, but VRS is not an aerodynamic stall. The blades may stall if the pilot uses too much collective pitch, but this is not inherent in VRS. I can appreciate the similarities as experienced by the pilot, however; in both cases, the correct response is to do the opposite of what your instinct might tell you. 12:38 Absolutely not. The requirement is that the rotor must be in a thrusting condition and in vertical descent close to (~70% to 125% of) the rotor's induced velocity. You could enter VRS during an autorotative flare that you entered too high and which didn't properly arrest your vertical speed. If you then pull collective, you could enter VRS just as your rotor slows down, making your day even worse. 12:59 Yes, this is true. Auto-rotation and VRS are different vertical flight states. However, auto-rotation is not the same as un-powered flight. 13:16 "High rate of descent" isn't really true. I think you've demonstrated that VRS occurs at a pretty low rate of descent, especially compared to rates of descent you can safely achieve in forward flight. At sufficiently high rates of descent, you will exit VRS and enter turbulent wake state and then windmill brake state. 13:31 Pretty much right. Pilots like to talk about ETL like a special point where everything changes. As engineers, we don't really think like that. But there are enough cues available to the pilot to make this a good rule of thumb. Essentially, if your velocity is more vertical (downwards) than translational (ie, your descent angle is greater than about 45 degrees) then you're prone to VRS. Okay, now I'm just going to ramble a bit. If you haven't seen this video, I highly recommend it. kzbin.info/www/bejne/fpvIg4Z6qN5gpKc So, VRS is a very unstable state. It's actually difficult for a pilot to remain in it for any significant length of time without pitch and roll oscillations threatening to tip you out of it. The problem is when it happens close to the ground, where you don't have to remain in it for any real length of time before you hit the ground. It can be though of as an airbody surrounding the rotor system which grows, bursts, reforms, grows, and bursts again. Within this airbody, there is no net change of momentum of the air, it just kinda gets sucked down with the helicopter. The air is very turbulent, and as the body grows and bursts, there are a lot of thrust and flapping fluctuations that make for a rough ride. Depending on who you talk to, there are 4 main states of vertical flight: climb, vortex ring state, turbulent wake state, and windmill brake state. Hover exists on the boundary of climb and VRS. In reality, a small amount of vertical descent is tolerable before entering VRS. In his book, Ray Prouty distinguishes slow descent as its own condition, and doesn't consider the turbulent wake state, instead combining it with VRS. Regardless, there's an entire region of vertical flight between hover and a descent rate of twice the hover induced velocity where momentum theory (probably the most important and foundational theory of helicopter aerodynamics) breaks down. Empirical data may be used to fill the gap, but the fact remains, it's a very difficult state to model analytically. Beyond a descent rate of twice the induced velocity, you enter windmill brake state, for which momentum theory provides good answers. The point I want to make here is that you will exit VRS and enter an autorotation if you keep your collective low and remain in vertical flight. VRS is not characterized by a descent rate greater than some velocity, but actually a *range* of descent speeds. Increasing collective acts to keep you in VRS. As for what you need to know as a virtual pilot, I think you got it pretty much right: avoid descents at low or zero forward speed. If you enter VRS (Characterized by a turbulent, unsteady vertical descent), enter translational flight to exit the vortex encircling the rotor. As before, I hope you found this helpful.

[@vsTerminus]

Very helpful. These are as you say complex problems that don't lend themselves to simple explanations and here I come as a virtual wannabe pilot trying my best to do just that :P Thank you for sharing your knowledge, I hope others reading this will find it as useful as I did.

[@snowman374th]

Wow. A lot nothing to read. Good job. pros everywhere you look... lmao

[@HarryMcW]

Very good! VRS got me yesterday... I guess it is like getting caught in a rip current, have to go sideways, not fight against it.

[@krisdellalawson5484]

great explanation mate, Im a fixed wing pilot and just couldn't figure out what was going on when Started flying the Mi8, I managed to get it through trial and error but its always good to have a solid understanding of what is actually going on. I manage a good transition to hover on most flights but would still get bit from time to time, the hard numbers you give should now keep me out of trouble. Thanks.

[@adrienbanha3004]

My 3rd flight ended up exactly like that, VRS and then the engines and generators stopped working, I had no idea what caused this but thanks to you, I have this knowledge now

[@sam0899gamer]

Excellent video! Very informative and great quality! Keep it up!

[@valuedhumanoid6574]

I just got this module and although VRS is an issue with ALL helo's, this one seems to play havoc with the phenomenon more than others. The first landing ended up with me actually starring at my tail. It had been chopped off and I landed 10 feet in front of it. That's when I went lookin' for a solution, hence, me discovering your tutorial series on this wonderful bird. She's a finicky gal and harsh mistress. But conquer her I must.

[@BlindxSide]

My first foray into DCS has been with this Hind, and I'm almost certain this is the exact issue I've been running into that causes me to lose lift and promptly slam into the earth. Even as you described, I attempted to raise collective as a fix and now know it's not the correct procedure. Thanks for the informative video! love your content:)

[@vsTerminus]

Yep, that sounds right. And you have it harder in the Hind because ED hasn't added the buffeting and shaking yet. So you don't get much indication that VRS is happening until it's too late. Keep an eye on your vertical velocity indicator and make smooth collective movements and you'll be ok! Good luck!

[@OctoBooze]

Not knowing you've slowed down enough to need to really worry about it can be a real pain if you're focused on trying to nail the landing spot. Keep at it, you should develop a feel for it soon enough!

[@mnordavind]

Your channel it's a gem, thanks!

[@tallkien2250]

I tried the Vuichard recovery technique and it works really well in both the Hip and the Huey. You should do a video on that too. Really love your vids though. Thanks and keep it up.

[@vsTerminus]

Yea, I want to revisit the VRS video at some point and include the Vuichard technique as well. You're right, it does work and I think there is value in demonstrating and explaining it.

[@energiam880]

the kamovs are also very good for powering out of VRS. Including the ka50 in dcs

[@vsTerminus]

I need to log more time in the Shark eventually. It's such a cool machine. I'll definitely experiment with VRS in it when I get the chance.

[@TheGranicd]

Recently got Mi8 and was confused why is this happening because with Ka50 I had no such problems.

[@Sky-yl8sy]

11:41 Is that flower the coat of arms of Erusea?

[@SpeeedyZach]

Great videos! Thank you! Watching them all :)

[@Roebuck325]

my understanding is not that it's little pockets at the end of each blade, it's that the air is pushed out then it's sucked back in from above, but the recycled air is hitting the center of the aircraft where it can't be used.

[@airspeed_alive]

Awesome tutorial thank you

[@peterwassmuth4014]

ThX! Yepp! 100% quality.

[@fastfiddler1625]

Main point at the bottom. I keep bouncing between thinking VRS is way over done in DCS and the ground. Convoluted jokes aside, I don't think that's the issue. Helicopters are a lot more somatosensory involved than airplanes. I am still figuring this out, being a fixed wing guy IRL. But say, in a fixed wing aircraft, you can feel when the airplane is about to stall. You learn to feel and avoid this state. VRS is basically a stall but for different reasons. In a helicopter, you learn to coordinate collective and pedal, or you constantly swing left and right every time you adjust collective. In a similar manner, prop pilots have to use rudder any time angle of attack is high to adjust for P-factor among other things. In DCS, you see this with an uncentered ball in the P-47. In a real plane, this feels uncomfortable. The long and convoluted point I'm making, is we're all just jumping into these complex machines and learning them on the fly (oh yes I did). Real helicopter pilots don't talk about VRS that much because they know how to fly the things. VRS is something the pilot did to the helicopter, not the other way around. One thing I know I keep doing in the Hind at least is this. I go for the aggressive stop, which is already difficult enough in the Hind. If you don't know what translational lift is, it's time to look it up. But there's an important thing to remember here. When you slow down, you lose translational lift. If you don't PREEMPTIVELY add collective as you enter hover state, you have already lost; both lift and the battle. It's important to monitor your vertical speed, but be careful about utilizing the VVI/VSI for this. An old fashioned VSI has a lag time of 6-9 seconds. It is a TREND instrument. Your altimeter is instantaneous. I've figured out this is my biggest problem when I'm in the heat of battle trying to make a landing and I'm focusing on where I want to set down. And then I go into a state of RUD. Rapid Unplanned Disassembly.

[@mischi9203]

5:52 imagine that happens to you in reallife, scary asf

[@jimothywhimothy8683]

How do you turn get the mirrors in the mi-8?

[@vsTerminus]

Unlike most other modules, if you have mirrors disabled in the main menu options they get completely removed from the helicopter. You have to enable that option, and then you'll be able to toggle them on and off like normal once you load in.

[@indyjons321]

I've never experienced VRS in any other helicopter more than in the Mi-8.

[@vsTerminus]

It's definitely modeled in the others (though, in the Gazelle you can just power out of it so it doesn't count), but the weight of the Hip makes you so much more prone to it. Combine that with the slow spool up time on the engines and it's definitely a recipe for disaster if you don't stay one step ahead.

[@nobeltnium]

@@vsTerminus I wonder you can power out of VRS in Gazelle because it's a light helicopter, or because DCS modelled it not accurate? In the Huey i tried to power out of VRS but fail every time (or maybe skill issue from me)

[@vsTerminus]

@nobeltnium nah, my understanding is the kind of power to weight ratio needed to just power out of VRS only exists in like the S-64 Skycrane. The Gazelle does not have that kind of power, the flight model was just simplified and did not accurately reproduce VRS.

[@RoamingAdhocrat]

petition to rename it Vortex Doughnut State

[@gregorypruitt5068]

Do you apply any curves to your controls? If so, what are the numbers and why do you apply the specific curves if any?

[@vsTerminus]

I don't currently use any curves in the Mi-8. I found them useful when I was learning because I think most people naturally tend to make larger corrections and movements than they need and it takes time to learn the "fine touch" you need. Curves are also useful if you have low end or worn out hardware with large deadzones, or even in things like the Warthog HOTAS with its stiff centering spring. There is no shame in using them if they help you. If you find you are wobbling around a *lot*, try putting a +10 curve on your Pitch and Roll axes. You can experiment from there, but I wouldn't go higher than +15. The hip *shouldn't* require you to adjust saturation values, at least I've never personally felt the need to do it.

[@ricbish]

Good explanation of VRS but I think it’s over modeled in DCS. It happens way too often especially in the Hind.

[@snowman374th]

Time to hear from somebody whose working on this helicopter's VRS problem and tail snapping... ME!!!! VRS deaths = 32 with 80plus hours of flying it. DCS SAR mission. Caucuses. Collective/lift Dead zone = 0 Xsat = 82+ Ysat = 86+ Curvature = 0 NO MORE VRS if you watch wind direction. Pitch Dead zone = 1+ or 0 depending on ones habits of flight. Xsat = 45+ Ysat = 45+ Curvature = 90+ you can go 100, and it should be fine Roll Deadzone = 1 or 0 again depending on ones habits. Xsat = 45+ Ysat = 45+ Curvature = 90+ you can go 100 but it should be fine Rudder Deadzone = 3+ to 5+ Xsat = 0 Ysat = 0 Curvature = 0 Try this and let me know what you get. If you play DCS come see me in SAR caucuses. I'll help you out. If I am not working, I am flying. LOL Until the war starts

[@snowman374th]

This WILL cure TAIL SNAPS or TAIL ROTOR LOSS

[@snowman374th]

NO MORE CRASHES

[@tomf5450]

You Don't power out of a vortex ring state... the more the power the greater the vortex. You have to side slip to destroy the vortex. The simple version