A Few Words About VRS

[Nearly Retired]

Those of you who know me know that I can never say "a few words" on any topic. But I'll try.

 

It seems that many young pilots have an irrational fear of vortex ring state. They bring it up in odd places in discussions of flight.

 

In another thread, one pilot recounts his first solo flight. He tells about how he was on finals, and he glanced over at the airspeed indicator and noticed that his speed was just a smidge under 30 knots...you know, on the verge of vortex ring state.

 

I was, like, what?

 

I've heard other pilots, too, ask the same questions of me when I talk about some things I've done. "Weren't you worried about vortex ring state??" they ask, eyes wide.

 

Well, um, no.

 

Listen, you CANNOT get into vortex ring state if you are above ETL. Cannot. Got that? Cannot. If you simply stay above ETL you are immune from recirculating your own downwash.

 

If you are in an OGE hover, you are not "on the verge" of VRS. Even if you are in a descent from an OGE hover, you are still not "on the verge" of VRS.

 

Let's review: To get into VRS you need:

 

To be below ETL

To have power applied

To be in a descent (generally) greater than 300 feet per minute.

 

In other words, you have to be coming down fairly vertically with respect to the air mass/wind you're flying in. You have to match the speed of your downwash to get that perfect "donut" of air recirculating.

 

In calm winds, this means coming pretty much straight down. If you're making a steep approach with a tailwind, your descent need not be vertical - you could get into VRS with quite a bit of forward motion (if not airspeed). So, dummy, don't make downwind approaches!

 

It just seems to me that pilots today are being taught that VRS is this horrible thing that must be feared! ...That it is not only common but very easy to get into. It is none of that. Be aware of it, yes, but do not be afraid of it. And don't think it'll happen just because you lose all your airspeed and fall back below ETL on the approach.

 

So. What is a pilot to do? How does the hapless helicopter pilot avoid getting into VRS?? Easy.

 

Keep the nose into the wind - or at least keep some air moving laterally through the rotor system until you are ready to come to a hover (which hopefully will be an IGE hover not OGE).

 

Do not make vertical approaches at "healthy" rates of descent with a lot of power applied. Why would you do that, anyway?? More than 300 fpm down is really moving if you're vertical. Ease on down the road, baby!

 

Do not think that the helicopter is always "on the verge" of going into VRS. Learn how the damn thing works out in real air, not classroom hot air provided by instructors who've probably never experienced VRS for real. Then go out in a ship and try to get into VRS...if you can. It's not easy to demonstrate with predictability. (Even the V-22 test pilots who were exploring the Osprey's tendency to get into asymmetric-VRS couldn't come up with hard numbers as to when it would happen - just like regular helicopters.)

 

Oh, and the mushiness and vibration you get as you fall back below ETL in an OGE hover don't count. That's not VRS. VRS is: Pull up on the collective and the ship descends faster.

 

Oh, and don't confuse settling-with-power with VRS. Don't put the ship into a steep descent from which you do not have enough engine power to stop. That's not the rotor's fault. That's not VRS.

 

Thank you, rant over. Carry on.

[pilot#476398]

Ditto!

[Joe_P148]

...

[aeroscout]

Saying VRS is not settling with power seems like a difference without a distinction.

The way I understand it is the onset of VRS might not put you into a condition of settling with power, but settling with power is characterized by a fully developed VRS.

The 3 requirements for settling with power will fully develop VRS.

Not being disagreeable, just offering my understanding of the phenomenon.

It is a good topic to talk about.

I did see a BK awhile back on approach and it seemed like they were hovering in taking forever with little to no closure to their intended point of landing. i was able to talk to the pilot afterwards and he said his company's policy was no more than 200fpm descent on approach.

[Jaybee]

I don't have available to me but was taught by a very reliable source that the Navy did a bunch of testing and they proven beyond a reasonable doubt that if you don't exceed a 30 degree approach angle than it is impossible to get into settling with power (headwind of course). 30 degrees is pretty steep !

 

Even funnier, my first year on the job as a tour pilot I was the lowest hour guy so of course everyone knows more than me. Long story short this Guard pilot I worked with kept trying to convince me that he was right and I was wrong. Funny because I told him no matter what he says with his whopping 500 hours of experience I wasn't going to believe him over the 30k hour pilot with 40 years of experience who taught me how to fly number one. Number two he brings in this pamphlet he got from HAATS (High Altitude something something) and he's babbling on and on pointing at this graph, shoving the book in my face - I took the book, looked at the graph, turned the page and bam ! right there in his own little Army book it say something about not exceeding the 30 degree approach angle. That sucker wouldn't even look at his own book when I tried to show him ! Ha !

[Eric Hunt]

Sadly, that's what you get when the instructor only has 100 hours more than the student, passing on myths and misconceptions without having any experience to back things up.

 

The school I used to work at had a tribe of junior instructors who didn't really understand what VRS was about, and on final one of the checks ran "Airspeed greater than groundspeed (good, they are into wind),rate of descent less than 300fpm (not really necessary, but avoids problems) Power in Hand" and they pulled up on the collective (aircraft pops upwards briefly) to confirm in their mind that there was sufficient power to terminate the approach. However, they were doing this at 30+ kt with power down around 25% (B206).

 

I told as many as I could that this check was meaningless because they were still very low on the power curve, and of course the machine would pop up if you pulled from 25% up to 45% or whatever. What they needed to do was get MUCH slower than 30kt, get further to the left on the curve with that drag rising, power coming in, and then they should be assessing how fast that lever was coming up. If they were at 15kt and the lever up in their armpit, it meant that they DIDN'T have any power in hand, and should stay above translational, nose over and fly away with the remaining power. A few light-bulb moments happened, but others said "but that's the way WE were taught" and kept doing it.

 

 

Same with the dreaded LTE!!!

[pilot#476398]

"That's the way I was taught" Terrible way for a commercial pilolt to explain why they do something a certain way!

[gary-mike]

I had a brand new designated stage check pilot give me a hard time, and ultimately send me back to my instructor. One of the things he checked on his three strike rule was my decent rate on base, at 50 kts, I still had plenty of ETL, and power, I just made up for the altitude I needed to lose to set up for final. I don't fault the guy, it wasn't one of my best performances behind the stick but, that one got to me.

[d10]

Everyone should read the previous SWP vs VRS debate before we go down this road again. Lots of other great learning points in that thread as well.

[iChris]

I don't have available to me but was taught by a very reliable source that the Navy did a bunch of testing and they proven beyond a reasonable doubt that if you don't exceed a 30 degree approach angle than it is impossible to get into settling with power (headwind of course). 30 degrees is pretty steep !

 

 

Yes, less than 30º will work.

 

Everyone should read the previous SWP vs VRS debate before we go down this road again. Lots of other great learning points in that thread as well.

Edited February 20, 2014 by iChris

[Nearly Retired]

Look, let's face it, the similarities and differences between SWP and VRS will be argued until the end of time. But it's silly. For they are the same thing. And then again not.

 

I did not claim that they are exclusively different. All I'm saying is that if you get into a high rate of descent at the bottom of a very steep and/or downwind approach, you *may* find yourself without sufficient power to arrest the descent. You will "settle with power" right down to the bent skids. You may or may not have downwash recirculation but it doesn't really matter, does it?

 

Over the years I have made many steep approaches in overloaded helicopters where I was not 100% sure I'd have enough power to stop at the bottom. What did I do? For one thing, I planned the approach so that I could go right to the ground if need be - if it wouldn't hover. Secondly, if you get to a power limit and can't pull any more pitch, the other option that's always available is to roll the throttle off and keep pulling the collective up to cushion - but that's really a last, last resort. And no, I've never had to do the latter but once in a while I've done the former.

 

We have to remember that SWP and VRS don't happen up at altitude where there is time to sort things out and recover. That'd be nice, but it's not reality. Both of these phenomenon occur at the bottom of messed-up approaches. And at that point the difference between the two is irrelevant - you're probably still gonna crash.

 

Yes, you can demonstrate VRS. Climb up and come to an OGE hover with the wind on your tail. Lower the power and start a vertical descent. Give it time and you may find yourself in a situation where the collective doesn't seem in phase with the lift.

 

But remember! In a downwind OGE hover, there *will* be a lot of vibration and mushiness of the controls. This doesn't mean you're in VRS. But you're probably close. "On the verge," if you will. The vibration may be coming from the tail rotor or stabilizers back there that aren't used to seeing wind from the rear.

 

In a downwind hover, the Sikorsky S-55 tail rotor beats and bangs so much it almost seems like the engine - which turns at a similar rpm - is coming apart. It's quite startling at first. And a Bo-105 Bolkow vibrates so much on a steep, slow approach that you really won't know if you're about to enter VRS or not.

 

Also, please realize that you'll probably have to be up near full power to get VRS. I know that THE BOOK says that you only have to be using more than 20% power or whatever, but getting into real VRS will usually take a LOT of power. Hey, maybe you'll get lucky and find VRS at 50% or 60%. That hasn't been my experience. Usually, the lower the power, the harder it is to get recirculation.

 

In the end, my position is that it doesn't matter what you call it. VRS...SWP...potato...potato. If either of them happen to you below 300' or so, you're probably going to crash. (And if you get into VRS or SWP above 300' agl, turn in your pilot's certificate because you really don't deserve to be flying one of these things. Sorry, but you're just not qualified. Please go be an accountant or something.)

 

So avoidance is the key, right? Let's talk about an approach where you might find yourself in SWP/VRS. And no, we're not talking about "normal" approaches here. We're talking about operations into sites (confined areas or offshore platforms and such) where you have to come in steep and slow.

 

Make sure you know where the wind is coming from...MAKE SURE! Then, keep some of it flowing laterally through the rotor - preferably front-to-rear but sideways is okay too (just not back-to-front). Don't let a big rate of descent build up as you fall below ETL. Try to not make any power changes from 300' down - let it settle into a hover without yanking in a bunch of pitch. Keep the rate of descent under control all the way down - and prepare to go right to the ground if an increase in power doesn't cause the helicopter to stop descending.

[WolftalonID]

So in laymens terms..... Smoooooothhhh on the controls?

 

I think I faintly remember that phrase some where in my training.....faintly....it must have been over the excessive screaming and white knuckles while learning to hover! Hahaha

 

Maybe it works on an approach too....I will have to try that!

 

Seriously though, yes smooth controls work wonders in everything we do up here.

 

Thanks for the useful posts, and thanks iChris for that diagram. Its a good way to show visually what we mean on approaches.

 

My now ex chief pilot has been a utility pilot for a living during summer and does fire fighting. He always liked to point out the difficulty in putting a training robbie with skinny little wussy blades in SWP, vs his big ol bombshell blades of his bell.

 

Then added in a proper scenario where it is likely to happen and has often to many pilots.

Being in a hurry, rushing in with a bambie bucket, high pitch, quick stop maneuver, and high rate of decent to dip and go.

 

Its a mind set of "i gotta get there faster..." And then all skill goes out the door as the maneuver gets ugly real fast. Not to mention snagging the bucket and end up in the river like a cowworker of his did last spring.

 

Slow, smooth, safe.

[hooked4life]

Great discussion. I went back and read the thread from last year, lots of great points made there. One thing I'm a little confused on in this area which may or may not be related is this; What is going on when you quickstop or flare downwind?

 

I remember hearing through all my training that we never air taxi to a quickstop downwind because you can get into settling with power. I may be imagining things, but it seems that was the deal.

 

I have to admit that I'm guilty of the "because that's what I was taught" mentality and it seems to me that if you flare with a tailwind your vortices would be blown out in front/under you, but as far as aerodynamics go I don't really understand what is going on there. I've never tried it to find out what exactly what would happen.

 

I was under the impression that VRS is caused by a second set of vortices forming on the inner portion of the rotor disc which form due to a high rate of descent >300fpm. It doesn't seem like that would be exactly what was going on there while lowing collective in the flare. Can anyone comment on this?

[aussiecop]

Woo Hoo! I finally stirred the pot enough to be square in NR's cross hairs with my rookie story! lol (just kidding NR, I see your points and as a relative amateur comparatively, you have me on most points, but that's what being a rookie pilot and learning from your mistakes is all about). That's what I love about forums like this. Sure, some of you are crotchety old wind bags, but I learned long ago, that the old salty pilot with the cantankerous disposition will have experience to share if you can crack that tough exterior.

 

I sure did learn from it, and it never happened again. You may be right based on the comments here, it may have been SWP versus VRS, but when you're on your first solo, both might kill you due to inexperience. In hindsight, I know my approach angle was wrong and I wasn't paying enough attention to my instruments, as they would have told me I was about to get deeply in the sh*t if I didn't react. Thankfully the seat of the pants feeling was enough to jerk me into a quick reality check in which I made adjustments and came out the other side a little bit wiser.

[Nearly Retired]

Aussie, I didn't say you were in SWP versus VRS. You may not have been in either of them! This is what I'm trying to warn people about: Don't be predisposed to thinking that you're always "on the verge" of or actually in VRS.

 

Helicopters do strange things. Maybe the ship was just settling on you at low airspeed and an application of power would have stopped it (or did). Just because you feel a little "sag" on the approach doesn't mean you're suddenly in SWP/VRS. Like I've said, if you really get into either SWP or VRS below 300' or so, you're probably going to have a hard landing. Because while retreating blade stall is self-correcting, SWP/VRS is *not*. YOU, the pilot has to take the corrective action and fly out of it before things get too far out of hand. Which they will in a heartbeat.

 

"hooked," you most certainly can do a downwind quickstop. And yes, your downwash will be blown out in front of you as you terminate. And yes, that certainly would be a good setup for VRS. But it won't necessarily happen...if you're lucky.

 

Remember, a rotor doesn't *just* work by downwash/thrust. There's the lift component coming out the top as well. As long as the lift is positive we stand a good chance of continuing flight. Can a rotor produce lift without downwash? Err, what happens at the bottom of an auto in the flare with the collective on the bottom? You get plenty of lift but not a lot of downwash, eh? At least, not downwash in the traditional (hover) sense. A helicopter rotor in forward flight obviously does not produce the same type of downwash as in a hover.

 

But the downwash out at the outer edges of the disk (tip vortices) sometimes can and do recirculate, putting us into the dreaded VRS regime. If you're lightly loaded, and engine power combined with the thrust supplied at the inner portion of the disk is sufficient, you *might* be able to power-out of VRS. But that's the trouble with helicopter flying: There are no absolutes. Fixed-wing pilots know precisely at what attitude their wing will stall. We helicopter pilots do not have that luxury.

 

Can you do a downwind quickstop? Sure. You're starting from above ETL, and you actually lower the collective as you begin the flare, right? Okay, no problem so far - whatever downwash you're producing is still behind you. But recognize that the nose attitude necessary to get it stopped is going to be high. And it might not stop if the wind is strong. As you drop below ETL and begin to level and pull, two things are going to happen. One, you actually might get some recirculation of your tip vortices. Two, even if you don't get any recirculation, momentum may carry you right through to the ground, and you'd need the power of a CH-53K to get it stopped. Momentum's a bitch! Me, I wouldn't be messing around with downwind quickstops. That's like asking for trouble.

Think of the rotor as a parachute. It works great when the wind is from below and in front - doesn't work so well with the wind on the top and behind you.

[aussiecop]

No sweat NR, all good information. You are that crusty old salt type I was referring to, but in a good way. You old dogs always have plenty of knowledge to pass on. Hopefully some of the younger low timers take notice of some of the great information on the many threads here that focus on actual operational concerns. Way I figure, the internet has made it so much easier for high time pilots to pass on their knowledge.

 

I don't really understand guys who say they pay no attention to flying forums (yep, I've heard that verbatim a few times). Sure there is a healthy dose of people who get a little giddy in the no consequence grey area of the internet, but overall, it's a great bunch of guys that share some amazingly valuable advice and information.

[Joe_P148]

One thing is like to add to this, is that DA and GW seriously adds to the onset of getting into these conditions when you get up to 6000 ft plus it feels like your rotor system has about a 2 second lag and is barely holding altitude. When you are at sea level in the pattern you can get out of tricky situations, either with the proper techniques or by shear luck. I've seen recoveries using both techniques., when you get into dusty, high, mountain conditions I.e Afghanistan every tool you learn about avoid g settling orVRS seriously comes into play.

 

As I see it, I don't care what you call the condition as long as you how to avoid it or recover from it. At the end of the day, Aerodynamics is theory and you never know how your individual airframe will handle it.

[helonorth]

If any of this is new to any pilot past the private stage, I would say that either you went to the wrong school or the quality of training has deteriorated significantly in this industry. This is as basic as it gets. And don't do any downwind quick stops or put yourself into a situation where you have to (a downwind air taxi comes immediately to mind).

[Wally]

I feel disloyal, but I have to suggest there are very enlightening discussions of VRS, SWP on other websites. A Nick Lappos quote from one thread at http://www.pprune.org/rotorheads/107985-normal-approach-descent-rate-vrs.html:

 

"The 300 fpm is actually quite wrong, but has rested unchallenged for decades. The actual downward speed of the helo has to be about 50% of its downwash speed, which is usually quite a bit faster than 300 fpm.

Most VRS/settling with power accidents are actually the problem of trying to hover out of ground effect with too little engine power, but the understanding of this is not pervasive in the community, even among accident investigators (especially when they read the books that quote 300 fpm as the VRS boundary!)"

 

Me, I'll continue to believe I'm about to develop VRS and actively fly away from the cliche boundary conditions of steep approach and high rate of descent with power. Lots fewer surprises that way.

 

And some quibbles with posts-

I don't believe rearward (TR first) flight is any more or less susceptible to VRS.

You can get into VRS at reduced weights, it just takes finesse and more patience building the descent rate and applying power.

High DAs don't increase your chance of VRS, but it does increase the chance of "SWP".

Edited February 20, 2014 by Wally

[helonorth]

I also agree that 300 FPM thing is wrong and that the downwash velocity/decent rate is what matters. Also, VRS is very easy to demonstrate, at least in the 300. I did it dozens of times. Even at 2000' AGL, with plenty of time to recover, it's a real attention getter. The controls are mush and you are literally falling from the sky.

[iChris]

I feel disloyal, but I have to suggest there are very enlightening discussions of VRS, SWP on other websites. A Nick Lappos quote from one thread at http://www.pprune.org/rotorheads/107985-normal-approach-descent-rate-vrs.html:

 

"The 300 fpm is actually quite wrong, but has rested unchallenged for decades. The actual downward speed of the helo has to be about 50% of its downwash speed, which is usually quite a bit faster than 300 fpm.

 

Most VRS/settling with power accidents are actually the problem of trying to hover out of ground effect with too little engine power, but the understanding of this is not pervasive in the community, even among accident investigators (especially when they read the books that quote 300 fpm as the VRS boundary!)"

 

I also agree that 300 FPM thing is wrong and that the downwash velocity/decent rate is what matters. Also, VRS is very easy to demonstrate, at least in the 300. I did it dozens of times. Even at 2000' AGL, with plenty of time to recover, it's a real attention getter. The controls are mush and you are literally falling from the sky.

 

Based on flight and wind tunnel test that have been documented, that is correct. The thrust instability as a result of the unsteadiness of this flow state start as the descent rate reaches about 25%, peaks at 75%, and disappears at 125% of the induced velocity (ν_i) of a given helicopter. We’re looking at the 50% - 75% range as more realistic rates for VRS; consequently, most of these so-called VRS accidents are a result of insufficient power availably.

 

The higher the disk load (DL) the higher the descent rate must be to enter VRS.

 

Lowering the gross weight reduces the (DL) lowering the descent rate for entering VRS.

 

The induced velocity (ν_i) is a function of the helicopters disk load given as:

 

ν_{i (ft/sec) =} 14.5√gross wt/π r² or 14.5√DL (sea level standard day)

 

Edited February 21, 2014 by iChris

[Wally]

iChris

If I understand your post, the 14.5 is rule of thumb lb. sq. in., ISA/sea level.

Less dense air, induced velocity reduced for identical weight? My head hurts...

[iChris]

iChris

If I understand your post, the 14.5 is rule of thumb lb. sq. in., ISA/sea level.

Less dense air, induced velocity reduced for identical weight? My head hurts...

 

In more technical terms we’re referring to the ideal hover induced velocity (v_h) which is: v_h(ft/sec) = √(T/2ρA). Were T= thrust, ρ= is air density, and A= rotor disk area.

 

However we can break that down to make the math easier:

v_{h(ft/sec) =} (√1/2ρ) x (√T/A)

 

To reduce it even more, we’ll reference our work to sea level standard day.

 

Air density at sea level is .002377 slugs per cubic foot. Now we can solve the first part (√1/2ρ) = √(1/.004754) = √(210.34) = 14.5

 

We’re left with 14.5√T/A. Since thrust must equal weight in any steady state flight condition, we can substitute the gross weight for the term “T”

 

Now all that remains is 14.5√(gross wt./disk area) or simply 14.5√(DL)

 

Just makes the math easier.

Edited February 21, 2014 by iChris

[Spike]

I got a C- in math…. Therefore, when the machine starts to shake like a mo-fo and I see my IAS zero’d out and the VSI is pointed at my left knee, it’s time to make a correction…………

[WolftalonID]

Funny how a small circular muscle in our lower regions can out think and tell you somethings not right before our advanced brain muscles can!! Lol