Vortex Ring / Settling with power (Merged) [Archive]

UNCTUOUS

20th Jan 2001, 23:26

Also known as recirculation vortex.
Onset tends to be easier/earlier on twin and three-bladed helos (than on four/five-bladed say) simply because of disk solidity producing a weaker TE vortex sheet. Blade loading and downwash on two-bladed is going to be higher, leading to easier formation of the recirculation characteristic of the vortex ring state.

Of interest is the early tilt-rotor paper wherein academics urged more blades than the tilt-rotor's present three-bladed arrangement. However, unfortunately the weight, inertia and complexity of hooking up four blades controlled that outcome - and we know the rest. Because of the 95 degree tilt capability that was included because of ground manoeuvre considerations, yet is usable airborne, the Osprey is able to arrange to get into a VR state quite a bit earlier. i.e. at 95 degree tilt it has essentially arranged for it's own downwash wake to earlier (and more closely) approximate its approach flight-path (think about it). Add an arcing-over tendency due to close-in pad-edge obstacles and a bit of tail-wind at height - and you're easily in that ball-park.

Unfortunately the asymmVR nature of the tilt-rotor's VR encounters precludes recovery once the condition is encountered. Instinctive application of differential collective (as if to pick up a dropped wing) exacerbates the problem - much as simply raising the collective does in a helo. But in the Osprey's Marana accident time-line, it is evident that the roll-rate/nose-drop was totally without warning and non-recoverable. That was borne out by the lead aircraft crew-chief who witnessed that final evolution. Those who say that an emergency "conversion" would be a recovery solution are simply not familiar with what the Patuxent River test-pilots have been writing about their experiences.

As Offshoreigor says above: "the biggest problem is that if you don't react it increases in intensity exponentially." This is true in a helo but unfortunately it's irrelevant in a tilt-rotor. But it's interesting to still read the Marine Corps General's briefings wherein he reassures everyone that VR is not a problem for the Osprey - because of the amount of excess power the MV-22 has. Right sir, as you say sir.

Skycop

20th Jan 2001, 23:48

RAF CFS taught me that the vortex ring state affects the rotor induced flow by increasing it (the air above the disc has already been through the disc before as it is now recirculating outwards, upwards, over and down!). From the standard velocities diagram, this has the effect of decreasing the angle of attack of the blades whilst increasing the rotor drag.

The clearest description I have come across is from R. W. Prouty in his book "Helicopter Aerodynamics".

He shows, in a series of diagrams, the airflow in vortex ring. This is shown as a "doughnut with a hole" shaped (my interpretation - SC) vortex encompassing the entire rotor disc tip path (similar to a huge smoke ring as blown by some smokers, but upside down here, as the airflow direction is downwards at the centre) with the blade tips in the centre of the "dough / smoke" itself. The downwards / induced flow within the vortex is inboard from the blade tips and the corresponding "up draught" is outside of the tip path plane. He goes on to describe how underneath the "doughnut" is a higher pressure air bubble, fed from the downdraught flow within the "doughnut". The bubble cannot be sustained and randomly bursts below the aircraft and then quickly re-forms, causing the random rolling and pitching moments associated with this phenomena.

Prouty states that "from flight and wind tunnel tests, unsteadiness starts at about one-quarter, peaks at three quarters,and disappears at 1 and a quarter times the hover induced velocity". (Beyond this it is theoretically possible for the disc to go into autorotation but don't try this at home as you will probably hit the ground first - SC).

Easier seen with the aid of the diagrams than described though - I hope this can be understood!

Acknowledged to R.W. Prouty - Thanks!

[This message has been edited by Skycop (edited 20 January 2001).]

lmlanphere

21st Jan 2001, 02:04

thanks to all the posts above, however I still don't have a clear understanding of why the tip vortex intensifies. anyone have that answer?

Flight Safety

21st Jan 2001, 09:50

Lmlanphere, I'll guess at this and someone with more knowledge can correct me.

I think the effect intensifies because the air currents have formed a vortex (in this case a "ring" vortex). Once a vortex is formed it's almost self-sustaining as long as the pressure differentials that created it, are still acting upon it.

When a vortex is formed it creates a very organized airflow pattern in response to the pressure differentials acting upon it. As long as the pressure differential remains, the velocity of the air flow increases within the vortex in an attempt to equalize the pressure. Anyone familiar with a tornado can readily understand the velocities that can develop in a vortex. Higher velocities mean more air mass is being moved in the attempt to equalize the pressure. It's this ever increasing velocity that plays havoc with the AOA and increased drag on the rotor blades.

I might add that even when the pressure differential is removed (such as a wing tip vortex on a FW aircraft) the vortex will sustain it's organized airflow for a time due to its rotational momentum. Without the energy input from the pressure differential, the vortex will dissipate. However with sustained energy input from the pressure differential, it intensifies.

------------------
Safe flying to you...

[This message has been edited by Flight Safety (edited 21 January 2001).]

SPS

21st Jan 2001, 15:14

Pac Rotors will hopefully agree that I have fully explained why the effect increases
as I have fully described it in graphical terms in my book.

I firmly beleive I have the answer and although I have never had the opportunity to Read Ray Prouty's book (one day, hopefully!)
I have come up with a similar description as
Skycop describes (I used a doughnut simile thinking I had thought of it... Bah !)
I also used the triangle of ingredients that offshoreigor has mentioned, as taught to me by Mike Green of CFS.

Give me a day to get other stuff done (like work!) and I'll see if I can convince my computer to allow me to post the graphics on the thread...!

By the way, I too am not a fan of the description "settling with power'

It's to cuddly and friendly, it does not impart the seriousness of the situation.

'Settling' is what happens to the contents of your coffee jar during transit. Vortex Ring State as a title is a little more threatening, a little more violent, and so it IS!

SPS

21st Jan 2001, 15:45

And by the way implanere, ALL of the factors you mentioned in the beginning of your thread are involved. The intensification of the 'doughnut' effect is directly due to an upward incoming airflow (does not matter whether that is due to ROD or updraft, the effect is identical) which 'pushes' the doughnut up to disc level, when previously it would have been held below the disc (if a/s is low or zero)therfore increasing recirculation of air that has already been accelerated downward. All tht remains is to consider the effect of that upward incoming airflow on the ROOT area of the blade(s) (as you say, it increases a of a and brings that part of the blade to stall) and the anatomy of Vortex Ring and its exponential development is explained. and of course, the more it develops, the more ROD increases....

You can easily demonstrate the effect with a desktop fan. If you rig up some way of opposing the fan's induced flow with another (stronger) airflow it will begin to develop recirculation and its RPM should increase audibly as it takes less 'bite' at unaccelerated air. Also interesting is what happens when you ADD to its induced flow with another airflow, this time from behind. As the induced flow of the fan is increased RPM will again rise audibly. This shows the effect of recirculation in a different (but less accurate) way.

I may struggle to get graphics onto this post as my software is not easy to convert (any tips here would be welcome, but I'll have a go. If not I could post them on my website for a short time) Anyway, I'll try first !

SPS, still wondering how they managed to convince us that 'Nestles' were always called 'Nestle' (accent on the E)
because in the 'Milky Bar Kid' adverts it was always NESTLE'S milky bar.......

[email protected]

21st Jan 2001, 20:12

The AoA along the length of the blade is increased by the rate of descent flow (or updraughting when hovering in the mountains). As the root has the highest AoA it is the first to stall whilst the increase in pressure on the underside of the tips (as the airmoving upwards meets the air moving down ) intensifies the ever present tip vortex.
The tip is encountering recirculation and the root is stalling, leaving the remainder of the blade to do the work of keeping the aircraft in the air. If no action is taken the overall loss of lift will cause an increase in RoD and make the situation worse. If the pilot tries to arrest the Rod with lever he increases the AoA - the recirculation at the tip gets worse and the stalled area at the root moves outboard. At the point where rotor thrust no longer balance weight, the aircraft accelerates rapidly downwards ( as much as 8,000 fpm on some types).
It is at the incipient stage of vortex ring where the aircraft first catches up with its own downwash, before the RoD begins to develop that the warning signs - an increase in vibration and or blade noise (slap) - can be felt. The American term Settling with Power seems more appropriate to this stage than the fully developed "Doughnut" of the Vortex Ring state.
Two factors affecting the susceptibility of an aircraft to vortex ring are
a. All Up Mass - for the same aircraft you will have a higher AoA at a higher AUM and therefore be closer to root stall.
b. Disc Loading - aircraft with a high disc loading have a higher downwash speed and therefore need more RoD to catch up with their downwash.

The generic "less than 500 fpm RoD below 30 kts IAS" is still the best way to avoid vortex ring - I have talked to people who have survived the fully developed state and it's not a place you really want to be.

SPS, in answer to Nestles v Nestle, it was the good old advertising whizz kids who decided the British Public would not buy something that sounded distinctly foreign (especially French/Swiss). These are the same tossers who have conned the Post Office into changing their name to Consignia. How can allegedly intelligent businessmen believe some of the bo**ocks the marketing consultants tell them.

JoePilot

21st Jan 2001, 20:34

Fully developed it is very stable - quite elegant really, quite hard to stay in, needs very stable homogeneous air - arguably the fastest (of three) way down. (allegedly actually used in Vietnam by a friend of mine)

SPS

21st Jan 2001, 23:28

They changed the name of the Post Office to
WHAT!!??

Good post by the way, think you've boxed it off. All that we need to do now is recoveries, how and why they work. You or me?

Well, better get to work....I'll Consignia this......

SPS

22nd Jan 2001, 07:52

They changed the name of the post office to WHAT?!!

I don't know, I'm away for only 7 months this time and look what happens!

Anyway, it doesn't rhyme or even fit. 'NESTLE milky bar...' indeed.

Anyway, good post, looks like you have it boxed off. All we have to do now is recovery
methods and why they work.

You or me?

Anyway, better hurry and Consignia this...

offshoreigor

22nd Jan 2001, 10:30

SPS:

RE: Recovery.

It doesn't matter what type you fly, the only difference will be the intensity of the VRS.

The most effective way to recover is to reduce power, depending on the severity of the situation, up to autorotation. What this does is it reduces the intensity of the Vortex and re-establishes cyclic effectiveness allowing you to increase your airspeed above translational speed and thus leave the vortex behind the disc rather than back up through it.

Having said all this, hopefully you had enough altitude in the first place to recover! It's not the fall that will kill you, it's the sudden stop at the bottom.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

CTD

22nd Jan 2001, 18:00

Saw an Mi 26 with a dump truck in the back crash and burn on the runway in Steung Treng, Cambodia in 1993 after settling with power. Or was that..."Settling in spite of power"? All 11 people in the cockpit got out ok.

Lu Zuckerman

22nd Jan 2001, 19:56

Are the Westland Lynx and the EHI 101 suceptible to vortex ring state? It is my understanding that their "BURP" blades were designed to minimize tip vortices.

BERP/BURP they both sound the same.

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The Cat

[This message has been edited by Lu Zuckerman (edited 22 January 2001).]

[email protected]

22nd Jan 2001, 22:05

Joe Pilot - reports and tests of Vortex Ring suggest that it is far from stable in the fully developed state. Apparently a bubble of higher pressure air forms and then bursts regularly but in unpredictable directions giving random pitching and rolling. Additionally, the disturbed airflow from the main rotor plays havoc with the flow through the tail rotor causing yaw fluctuations.
Stable? - No, A fast way to get down? - yessireeebob!

SPS - the only recoveries are 1. A lot of forward cyclic to gain speed then apply power once out of the dirty air, or 2. Lower the lever to enter autorotation - but you need a lot of height and may overspeed the Nr!

Lu - The BERP blades have a very powerful tip vortices, just like a swept fixed wing - the tip losses were so high in the hover due to Blade Vortex Interaction (BVI) that the had to add the anhedral "droop" tip to spit the vortex out of the way before the next blade came along.

VLift

22nd Jan 2001, 22:57

Have been in "setteling with power" in a Hughes 269A (not a fun aircraft to teach it in as the vibration level puts the viewable world out of focus), a two bladed Bell and, a CH-47.
Have done it at 3000' and 13,500'. I'm not an expert but, my position differs with some.
If you are going down with no power applied and no forward airspeed and you don't feel too badley about it all, you are in a descent.
If in the descent, you apply up collective or forward TCL and stall the inner/slower portion of the rotor system where some air direction has already been up, you increase the stalled area where the second (at the inner portion of the rotor system) Vortex Ring has been for as long as the air has been going up through the rotor system instead of down and, you now feel very badley about it all, you are in setteling with power. More power/pitch, larger area where the air is going up rather than down meaning more of the blade is effectively stalled and not performing. Point being, you may be able to get the aircraft to go down fast enough to get a flow of air up through the rotor system near a the center/slower downward flow area. But, you are not "setteling with power, in my humble opinion till you make the mistake of increasing the power/pitch and increase all that has been only a potential bad thing to this point. When the pitch of the blade increases at the root, it increases at the tip. Which means it creates more/faster downflow of air until it exceeds design limit.

Recovery: correct what got you here. Forward airspeed and reduce power/pitch till the rate of descent slows noticably and the vibrations smoothe out. Then add power to stop the descent and fly out. Adjust as necessary for nearness to the ground. In a tandem rotor aircraft give thought to going to the side to find the clean air for the rotor system as forward or aft cyclic will only keep one of the rotor systems in setteling with power while the other has reverted to vortex ring state when the power/pitch was reduced. Two rotors fore and aft or lateral makes no no difference. The rotors don't know they have a twin, they stall/recover as individuals.

[This message has been edited by VLift (edited 22 January 2001).]

[This message has been edited by VLift (edited 22 January 2001).]

RW-1

23rd Jan 2001, 00:48

My understanding in recovery is to lower power as well, however just the act of moving forward cyclic (and gaining forward A/S) would begin the recovery process, as soon as the rotor "comes out" of the vortex ring, you begin flying again.

The Nr Fairy

23rd Jan 2001, 02:08

So what's the quickest way out of VR then, other than avoidance wherever possible ? Forward cyclic AND down collective, or just one followed by the other ?

offshoreigor

23rd Jan 2001, 07:06

NR:

As anyone will tell you, once you are in fully developed vortex ring state, not the inscipient stage, the cyclic tends to have little or no effect until you reduce power (ie. enter autorotation). So for my money, it would be reduce power followed very shortly thereafter by forward cyclic.

I also agree with the comment on the Chinook recovery with sideward cyclic.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

Randy_g

23rd Jan 2001, 14:00

I personally don't like the term "settling with power". In some ways it is correct, but to be pedantic isn't that what we do when we land ?? :) We don't land in autorotation (at least I don't), so therefore when landing, we let the a/c settle using power.

During some re-current training (Hughes 500D), my instructor and I did some VR work. We went up to about 4,000 agl and slowed down to below trans lift, and began a descent. It was quite interesting to watch the vsi begin to rise, even though we were increasing power. We lowered the power, then eased the cyclic forward. Once the shaking eased, and airspeed started to recover, we flew out of it.

Cheers

Randy_G

If you can't stand the heat...

then turn up the air conditioner !! :)

[This message has been edited by Randy_g (edited 23 January 2001).]

SPS

23rd Jan 2001, 14:03

Won't be differing with any of that!

Will only add that recovery method has to be 'tailored' to height available. At 3000'
that's one thing, at 75' into a confined is something else. Poor speed/power management is the major cause but a downdraft would be very unfortunate at the latter height.

I stick firmly to the 30/30 rule, no more than 30(0) ft per minute if speed is less than 30 kt. You won't fall into any trap then. I have also heard that it cannot develop at approach angles of LESS than 30 degrees but have had no time to go into that.
Maybe someone else has?

VLift

23rd Jan 2001, 18:59

Reduce power, forward cyclic; forward cyclic, reduce power. I agree.
It's much like the fellow who showed up to work in a new duty site at 6000' in the summer, having arrived from a long period at sea level.
I said " The book tells you that the 180 auto has several steps. Collective down, pedal adjust, cyclic into the turn, check rate of turn, etc, etc. Not correct. At this elevation it's one step or you won't finish the turn till your on the runway" He didn't believe that. Half way through the turn I caught sight of the man with no forhead that I've seen a few times before. From cheek bones to hairline he was nothing but eyeballs.
To take a position, I do agree that you won't get what you need from forward cyclic till you reduce the collective.
The worst aircraft for VR and S w/P for me has been the CH-47. He was flying, I was digging for a map (not doing my job of paying attention). There were no vibrations to alert me out of the map case. When I looked up we were falling like and anvil out of trim, so smooth. Not what anyone who has flown a few Chinooks would ever expect in terms of vibratory level. He was still operating well within the bliss that ignorance provides.

ShyTorque

24th Jan 2001, 00:28

Lu Z,

As I understand it the BERP blade was designed to overcome problems with compressibility at the high speed end of the flight regime. They are in effect a swept tip but to reduce twisting moments the rear of the blade shape is modified too.

offshoreigor

24th Jan 2001, 00:45

SPS/Randy_G:

I agree with you completely, settling with power is not the correct term for VRS (Power Settling).

A classic example of Settling with power is the young hotdog trying to impress his onlookers and starting a return to target into wind! The resultant course reversal into a downwind return sets you up for a classic settling with power situation. Too heavy for the ambient conditions without enough power to recover.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[This message has been edited by offshoreigor (edited 23 January 2001).]

[This message has been edited by offshoreigor (edited 23 January 2001).]

Flight Safety

25th Jan 2001, 23:57

When I posted the earlier comments on why a VRS intensifies, I wanted to add the following comments but chose to wait to see if the comments would be anywhere near correct.

A vortex also has the property of being "semi-static". By this I mean the rotational energy and velocity of a vortex causes it to stablize its position in the airmass that it was created in. However it will move with the prevailing winds and air currents that move the airmass it exists in. That's why it's "semi-static", it remains positionally static within the surrounding airmass, but moves when the surrounding airmass moves.

I think a very good example of visualizing a vortex ring occurs when a prop driven aircraft is taking off in high humidity air. In these conditions you can "see" the prop tip vortexes being generated by the prop tip, by seeing the moisture inside those vortexes. I think this happens because the rotational energy of the vortex causes a low pressure area inside the vortex (like in a tornado or hurricane) to develop, and the low pressure area causes the inside air's "dew point" to be reached and the inside air "fogs". You also notice that the visible moisture soon dissipates as the vortexes dissipates.

Being able to "see" these circular "ring" shaped vortexes on the prop tips makes it easier to understand how vortex ring state develops in a helicopter. If you've ever seen this, you know that the visible circular vortexes move towards the rear of the aircraft in a spiral corkscrew shape as the aircraft accelerates down the runway. On occasion you can even see this corkscrew pattern when a prop driven aircraft has its engines under power but is stationary with its brakes applied.

The corkscrew pattern is being developed because the vortexes are stationary within an airmass that is being moved towards the rear of the aircraft by the force of the propellers. Now use your imagination and suppose that a rather hefty tug were put on the nosewheel of the aircraft. Imagine that a pilot in the aircraft pushed the throttles forward on a very humid day (so you could see the vortexes) and the tug operator began to push the aircraft backwards until the speed of the aircraft moving backwards began to equal the speed of the airmass being driven backwards by the propellers. The ring vortexes on the prop tips would no longer be stretched out in a corkscrew pattern, but all of the vortexes being generated would now accumulate in one spot around the arc of the propeller, within the propeller driven airmass that is now stationary with respect to the rearward moving airframe. I hope you can visualize this.

In a helicopter "settling with power", this is exactly what you have. The rotor is driving down an airmass around the helicopter's airframe, while the airframe itself is falling down at a rate that is at or near the airspeed of the decending air column generated by the rotors. The rotor tip vortexes are no longer being spread out in a corkscrew pattern, but are being allowed to accumulate at the rotor. This causes the effect of "vortex ring state" to intensify as the rotor tip vortexes have nowhere to go, and they just keep adding energy to the vortex ring that already exists.

I agree completely with the methods of escape described in the other posts on this topic. Since the vortex ring is "semi-static" within the descending air column, then the best method of escape is to cut collective and/or power and to push the cyclic forward, leaving the vortex ring behind. Side stepping the vortex ring with side cyclic could also work, which the Chinook pilots seem to find helpful. For a tiltrotor, forward cyclic would seem to work best as this would seem to remove both rotors from the vortex rings at the same time.

Please correct me if any of this sounds wrong.

------------------
Safe flying to you...

[This message has been edited by Flight Safety (edited 25 January 2001).]

[email protected]

26th Jan 2001, 00:27

Flight Safety - A.W.T. ('Appy With That) (quaint British Army expression denoting satisfaction with a statement, theory or state of affairs)

JoePilot

26th Jan 2001, 17:02

Agreed. Although 'lever down' is part of the generally accepted recovery , IMO , it is more of an attempt to make clear in the mind-set of the 'victim' that 'normal' rules no longer apply and raising lever in response to increased sink rate is futile. Once fully-developed I have found a very stable elegant aerodynamic condition (v. high R.O.D.) and have still had good cylcic authority (Offshore; not also true in your types? inertesting.) to maintain or 'fly out' of V.R. - min hight loss recovery being Attitude (only) till VR eliminated then Power up and nose up to climb - recoveries which reach more than 30-40 kts are just energy (height)inefficient. IMO.
Auto recovery is nice (but does not minimise height loss).

Thomas coupling

26th Jan 2001, 20:35

Offshoreigor: Goldie / s61 / New york, early 90's. I believe!!!!
And they all walked away too....

http://www.gograph.com/Images-7298/AnimatedGif/redstar.gif

------------------
Thermal runaway.

offshoreigor

27th Jan 2001, 15:56

TC:

Spot on! And Goldie is still as funny as ever!

Did you know they use his DND video documentary in CRM these days? The way he describes it, it was the perfect setup for the fall, no pun intended.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

the coyote

8th Jun 2002, 20:38

After reading the discussions on the Blackhawk accident threads, could it be possible to get into an unrecoverable VRS? Say a helo is in a full blown VRS with a huge ROD of 4000+ fpm. Could it be possible that:

1. The rotor is completely stalled and so cyclic pitch changes will have no effect on disc attitude, or

2. With such a huge ROD, even with the collective on the floor, the ROD airflow is large enough to give a resultant relative airflow and angle of attack that exceeds the stalling angle on the rotor blade?

I have been in many doozies in the training environment and have always effected the recovery before the ROD exceeded 3000 fpm, but I always presumed some parts of the rotor disc were still doing something right. Any ideas or test data to prove or disprove my theory?

Nick Lappos

8th Jun 2002, 21:13

(Edited to reflect light disk loading helicopters)

The state where the rotor is in VRS is strictly defined by the ratio of the descent rate to the downwash velocity of the specific helicopter. Only within that boundary is VRS possible. One caviat is that we read RoD with a static instrument, which can go flakey when the wash around the fuselage is skewed beyond the normal certification envelope.

For modern helicopters, VRS is unlikely below vertical speed ratios of .5 and beyond ratios of 1.5. For an R-22. descent rates less than about 700 fpm are below the VRS threshold, and descent rates beyond 2100 fpm are above VRS. For Black Hawk, descent rates of about 1200 FPM are below VRS and greater than 4000 FPM are above VRS. The Vortex in VRS is produced when the downward velocity matches the rotor's downwash, so the exact match causes the air to simply wrap around the rotor tips. Lift is lost quickly, and the bottom falls out. If the power is left up and the condition is stable, the descent rate is constant and very high (about equal to the downwash velocity - maybe 1400 fpm (robbie) to 3000 fpm (Hawk)). The thrust variations might be 20 or 30% thrust (producing low frequency turbulence that is about .3 G's).

If you lower the collective in VRS (and you have lots and lots of altitude) you will simply get into windmill brake state and autorotate at about 5 to 6000 feet per min.

For data and photos, see my web site at:

http://mywebpages.comcast.net/llappos/

During VRS, typical modern helicopters retain some cyclic control. The rate of descent will make the horizontal tail try to pitch the nose down, which will help you recover. Reduced collective and nose down will produce a fast exit from VRS. Increased collective will only help if the aircraft has a great deal of excess power, not at all likely in anything but an empty machine with powerful turbine engines.

Note from the data on the web site that VRS is unlikely in a purely vertical descent, some forward speed is needed, maybe 8 knots. In practice, it is difficult to attain and hold VRS, as the condition is unsteady and tends to break of its own if any disturbence is induced. That does not mean it can't do harm, since the first 1000 feet of drop might be several hundred feet too many!

The Nr Fairy

9th Jun 2002, 06:44

So that's a "no", then ? :D

tgrendl

9th Jun 2002, 16:40

Great thread,

Nick, thanks for the charts and especially the picture of the little midget helo on your site.

Imagining myself as very small and inside the little helo in the picture I was wondering about the recovery at that point.

The state is fully developed,

Cyclic not making desired effect,

Collective has become excercise machine for left arm,

Disc not happy, pilot not happy.

But the tailrotor can still provide translating force to the fuselage. Tractoring it (and the disc) away from the bad ju-ju happening at the time.

I'd think that a fair amount of pedal input (not enough for a spin) should slowly edge your disc out of it's nasty state to the point where the other controls start to take effect again.

Holes in the idea?

Obviously pedal input topping the engines would be a concern.

Nick Lappos

9th Jun 2002, 20:33

tgrendl, for most modern aircraft, cyclic is still quite effective, because you do not need to be developing thrust to develop control. The exception is a teetering rotor, which needs thust to develop lateral or longitudinal control power.

In any case,reduce collective, push the nose forward, and fly out. It would be the rare case that everything fails to respond. If you get stuck, be sure the collective is fully down, and you should slide down into autorotation.

Here is a very good web site that has flow depiction of the rotor flow as a descent progresses from gentle descent to VRS and then to autorotation, it was put up by Dr. Gordon Leishman of University of Maryland (and a Brit, I believe, formerly of Westland). Scroll down to the single rotor cases which are well labeled, and run the movies. Note that he shows how little forward component is needed before the VRS is broken. For those on dialups, it might take a while to download each. I would be glad to post some stills from them if anyone needs that done.

http://www.enae.umd.edu/AGRC/Aero/vring.html

Vfrpilotpb

10th Jun 2002, 16:53

Last year 2001, I was with a very experienced Ex mil Cfi who showed me what can happen in a very fast descentVRS(Heli was R22) we went very high(5000ft) and he showed me a rate of decent to 3500 fpm, recovery was achieved by simply completely closing the collective, after recovery to height again it was my turn, and the same sort of recovery was made, It was very fast, very intimidating but after the event I feel able to cope with this problem( if it arises) in the future, brilliant for the old ticker!!:eek:
Regards Peter RB

3 D

10th Jun 2002, 20:18

A simple question perhaps:

Why do we need a small amount of forward speed to induce VRS, why doesn't a purely vertical descent induce it. I can see from the data that it doesn't just wondered why ?

cheers all
:D :D :D

Gaseous

11th Jun 2002, 00:24

I recently did an Enstrom Type conversion and the instructor could not get any sort of VRS, even when lifting the lever from the bottom in low speed auto to 38 inches MP. The aircraft just stopped decending and started to climb. No fuss. He had several goes all with the same result. This was NOT the case last time I did it in a R44. Anyone got any ideas why the difference? Articulated vs teetering? Rotor RPM? Conditions on the day?
I have no idea.

Nick Lappos

11th Jun 2002, 02:11

(Edited to accurately reflect Robinson and Black Hawk downwash velocities, based on the coyote's calculations)

VRS is only encountered when the downward speed of the helicopter matches the downwash velocity from the rotor. In the cases mentioned by vfrpilotpb it is likely that the VRS was passed through, and then a zero knot autorotation was flown, thus the 3500 ft/min descent rate.

The flow around the rotor is established by the balance between the rotor's push on the air, and the upwind matching that push. Tales of VRS sort of locking the rotor up aerodynamically, and causing tremendous fall rates are simply not true. Those descents are experienced, we can be shown them, but they are not VRS. The reason for VRS is that the rotor downwash is pushed upwards by the free stream and then recirculated back down through the rotor. If the ROD is very much higher than the downwash velocity, the free air just passes through the rotor, and you have to raise the collective to keep from overspeeding the rotor.

For Gaseous, the autorotation first could be a difficult way to experience VRS. If you are into auto, the rotor must be transitioned to powered state, and you must try to trim the descent rate at somewhere between the range of 50% to 150% of the downwash speed. The rate of descent varies a lot depending on how heavy the disk loading is. For a Robbie, the VRS range is 700 fpm to 2200 fpm, for a Black Hawk, it is 1400 fpm to 4000 fpm.

One of the difficulties comparing what your instructor shows you with the actual VRS is that there is no telling what your instructor knows about actual VRS, and there is not standardization of techniques for the demo. The texts are poor, and much pilot lore surrounds the maneuver. I am sure many well intentioned instructors show a descent, some vibration and then an awesome vertical autorotation, and call the whole thing VRS. Why not, mine did in flight school back in 1968.

For VRS to be established in a rotor, the rotor must be lowered to about half the downwash speed under powered conditions. By 75% of the downwash speed (R-22 = 1100 fpm, H-60 = 2000 fpm) the VRS will show its head, the thrust will oscillate (you will feel low frequency vibration like turbulence, with maybe 3/10 of a G of magnitude, really big) and the aircraft will pitch and roll somewhat, the cyclic will be sloppy, and raising the collective will not necessarily produce a reduction in ROD. If the descent is increased to about 150% of the downwash speed, VRS is gone, clean air passes through the rotor, and you are in a vertical descent.

Any nose down (or even lateral tilt to slide out sideways) will help break up the VRS, and a climb will probably start (or at least the rate of descent will reduce somewhat. If you are falling at 3000 or 6000 feet per minute, you are not in VRS, you have slid through it and are now into a vertical descent or autorotation.

the coyote

11th Jun 2002, 07:52

Nick, based upon your formula to calculate the downwash velocity and the following data:
R22, at MAUW=1370lbs, rotor diam=7.7m/25.26ft, I calculate the downwash velocity to be 1440 fpm. Am I right? Is the 0.002378 number in your formula, is that specific to a particular rotor system or a constant for all helos?

When I used to demonstrate VRS to students, I would get it into the hover (obviously with at least 3000 feet to spare) and in the R22 this would usually require pretty much max power. I would then very slowly lower the collective. As the ROD got to around 500-900 fpm or so, (and sometimes you would have to gently search for the downwash with cyclic), bingo you would be into it and most times so quickly you would lose your stomach. All the normal control sloppyness, pitch and roll and vibrations just prior to it. On a windy day often you couldn't get into VRS no matter how hard you tried for obvious reasons. From your formula it seems like a higher ROD is necessary but experience tells me otherwise, maybe I am under-estimating the lag in the VSI? Or does the downwash velocity vary considerably depending upon the power being applied at the time?

Thanks for your input and a Disclaimer for students: DONT do the above solo.

Nick Lappos

11th Jun 2002, 09:38

the coyote,
You are exactly on. The disk loading of the R-22 is very light, so it does not need to push the air very hard to get its lift. The downwash velocity of 1440 is correct. The .002378 is the density of air at sea level. The density is important because the formula simply matches the downward air momentum change needed to make the upward lift for the rotor. With denser air, less velocity is needed, and vice versa.

OK, so we estimate that you get into the beginning of VRS at 50% of the downwash speed. That means by about 700 fpm ROD you should start to get VRS effects. You have observed that at 62% of that speed, 900 fpm, you are fully into the effect (that is the orange area on the plot on my website.) It all checks very nicely.

I think it all fits. The misleading thing about my post is that I have assumed downwash velocities more in the range where turbine helicopters are built, note the use of a Jetranger for the website chart. I will edit the post right now to add Robbie numbers! I will also put a Robbie scale on the chart right now.

Turbines have higher disk loading bcause they have more power, so they can afford to spend more making higher downwash velocity. I will eventually add that to the web site, too.

3 D

11th Jun 2002, 16:56

Not tryingto be cheeky by asking again but why is it that we need as Nick said around 8 knots of forward speed for VRS why is it less likely in a vertical descent ?

Nick Lappos

12th Jun 2002, 01:23

3D,
I didn't ignore your original question, I just can't answer it. I do note that the data on Dr. Leishman's site shows no such forward speed bias, but his data is rotor only.

I have a gut feel as to the answer. The steep descent makes the fuselage and tail want to tuck nose down, with the forward moment provided by the horizontal tail. To stay level, some aft cyclic is necessary to balance the moment. This aft flapping could result in the rotor wash having a forward bias, requiring some slight forward speed to keep it reingesting.

That is pure speculation, but it has a certain ring to it, I think.

Nick

3 D

12th Jun 2002, 14:47

Thanks Nick Thats a very good explanation I think.

jungly

12th Jun 2002, 15:41

in attempting to minimise the airspeed ie:transverse flow across the disc, we tend only to think of IAS.

it is possible that in your example a floww across the disc existed (eg: left,right or even tail wind) hence no VRS.

i have no time in the enstrom but the bell47 'sometimes' displayed similar inconsistancies.

Shawn Coyle

14th Jun 2002, 00:39

The only way I have been able to repeatedly demonstrate the onset of vortex ring state (and I don't want to get to the fully developed version, thank you very much), is to decelerate downwind (at a sufficient height above the ground to permit a timely recovery)
Find out where the wind is at altitude (watch the clouds drifting by, or know it from some other source), but downwind is the only way I can get this to work.
Start at 60 KIAS in level flight - reduce the power to something slightly below the power for level flight at that airspeed and maintain altitude.
As the airspeed passes through about 20 KIAS, there will be increased airframe buffeting, unlike any you get anywhere else. The helicopter may have uncommanded pitch, roll and yaw (i.e. you're not moving the stick and it's dancing around on you). The rate of descent will be 500-800 fpm, maybe less.
Increasing the collective might increase the rate of descent, but this is not very repeatable.
WHen you've had enough, smoothly, but positively lower the nose to 20-30 degrees below the horizon and fly out of it.
Doing it into wind won't work.

Irlandés

14th Jun 2002, 02:38

Nick,
when you say "That is pure speculation, but it has a certain ring to it, I think.", I assume that was a totally unintentional pun! :D

Shawn,
I'm a little confused by you talking about being upwind or downwind when trying to enter VRS. I mean if you're starting off at 60 knots IAS what difference does it make what the air is doing relative to the ground? I mean, you're still flying relative to the airmass. I'm not trying to pick holes, I just don't get it! Maybe I'm a bit slow today. :(

Irlandés

P.S. I really enjoyed your book! :)

Nick Lappos

14th Jun 2002, 03:27

Irlandes,
The pun was intentional, I settled upon it easily.

Shawn,

The maneuver that you describe sounds like a way to force the rotor into VRS with a combination of desent and deceleration, all quite par, for it forces the downwash/upwash fight that creates VRS.

Coyle mentions this as one of the scenarios that the V-22 can possibly find, where little ROD is needed.

I, too, find the need to be downwind interesting, but I assume that you have really decelerated to effectively zero velocity, and perhaps even a bit rearward. This could also explain the wobbling, as you pass through ETL and then into rearward flight. Where is that omni-directional airspeed system to help settle this powerful discussion?

Nick

Irlandés

14th Jun 2002, 03:30

Nick,
I had a look at your website (very good!) and have a couple of questions. In your formula for downwash speed, you have the three variables, weight, density and disc area. I would have imagined that angle of attack and possibly rotor rpm would have to be in this formula somewhere. If I'm on the ground at 104% (in the Robby say), with no pitch, then my downwash velocity is zero (correct me if I'm wrong). As I increase pitch and subsequently AOA, my downwash speed increases until such point that I can take off. So I see a downwash speed varying with AOA. By inference I assume my downwash speed in a descent will be lower than in straight and level flight everthing else being equal, thus permitting me to descend. So is your formula valid for a concrete flight situation or is it valid for all flight situations? My worry is that I could calculate my 700fpm (50%) and 2100fpm (150%) as being the avoid descent rates for a given all up weight and air density, only to find that these can dramatically lower, with decreasing downwash speeds due to lowered collective.

I may of course be talking utter non-sense. :confused: If you could point me on the straight and narrow, I would be very grateful!

Irlandés :rolleyes:

Nick Lappos

14th Jun 2002, 04:21

Irlandes,

The formula is a simple translation of the speed change that the rotor must make on a column of air in order to make the hover thrust. It is only an approximation (a good one) but it is restricted to the case of hover. The factors that you describe are of course necessary, but they tend to describe the details that operate the wind machine. In the end, small angle of attack on a high speed rotor, or large angle of attack on a slow one all must move the air, and it is the net momentum change on the air that determines the thrust of the rotor.

In a climb or descent, the downwash speed is the same, as long as the ROD or ROC is steady state. Recall that lift equals weight as long as the flight is unaccelerated. It takes a bit more thrust (downwash) to initiate the climb, and a bit less to start the descent, of course.

I will make this change to the web site to clarify the limits of that formula.

Thanks!

Nick

Irlandés

18th Jun 2002, 04:44

Thanks Nick for that reply. I've been mulling over what you said and sorry to whip a dead horse but I have one more question if you feel up to it.

Is rotor downwash speed measured relative to the plane (cone?) of the rotating blades or relative to the undisturbed air mass surrounding the helicopter?

I mean, if a helicopter with a theoretical downwash speed (in an OGE hover) of say 1400 fpm is descending vertically at say 400 fpm. Is the downwash velocity such that the downwash is approaching the ground at 1800 fpm (the vector sum of the two) or is is that the downwash velocity relative to the blades is actually 1000 fpm which added to the 400 fpm descent rate gives a total downwash velocity (again relative to the approaching ground) of 1400 fpm??

I feel this second solution would tie in nicely with both your explanation and my possibly misguided idea of downwash velocity (relative to the blades) reducing with reduced collective.

Thanks again!

Irlandés

Shawn Coyle

18th Jun 2002, 14:42

More on being downwind to demonstrate Vortex Ring State.

I'm not sure where or when I picked up on having to be downwind to get this to work. All I know is that having done it many times starting downwind and having it work reasonably well, and tried it several times into wind and having no results, is that downwind works well for the demonstration.
The aim of the demo is to educate the student on what the symptoms of VRS are, not explore the depths of the monster.
Avoidance should be the key!!
I've investigated at least one accident where VRS was the culprit and the board of inquiry didn't even consider it - shows how little the matter is understood.
This thread is actually pretty awesome in what it is explaining, and I'd like to thank Nick Lappos especially.

hihover

18th Jun 2002, 17:52

Nick and Shawn - I have read your posts and have found them very interesting and enlightening. As a 7000 hour pilot and examiner, I thought I had this Vortex Ring thing sussed. It turns out that I didn't quite know as much as I do now and, as with just about everything else in aviation, there is much more going on than Lofty Marshall explained to me at CFS.

The main point that I note from your discussion is that the larger helicopters with more power and higher disc loadings seem to have significantly different entry parameters to VRS. As a simple operator, instructor, then instructors' instructor now examiner, I have been lulled into the general train of thought that rates of descent more than 300 fpm when below 30 knots are not acceptable. Whilst this is a good policy to adopt and has always kept me well clear of the offending phenomenon, I now realise that there have been situations where I possibly could have achieved something from which I had chickened out.

I will continue to emphasize the 300fpm and 30kt alarm bell as I still feel this is a point beyond which, very careful consideration is required.

Shawn, having taught (to aspiring instructors) and demonstrated
to others how to get into and out of the danger zones, I generally used the downwind approach or downwind hover (at recoverable altitudes), to show how easily things can go wrong. I think the visual cues from downwind help to mask the onset and make the demonstration more effective. In addition, helicopters do not generally "like" being downwind at slow speed and this increases the pilot workload, control inputs and the likelyhood of a rate of descent inadvertantly building.

You readers and writers of Greek flute music have given me something else to think about and I am grateful, however, I think we all agree that exploring the envelope in that direction should be left to those paid and taught to do so under controlled circumstances and for the rest of us, no rates of descent in excess of 300fpm when below 30 kts will keep us in a nice cosy corner of the envelope.

Thank you all for a very interesting thread.

hh

Vfrpilotpb

18th Jun 2002, 20:21

Shawn,
Good evening,

The Ex Raf, and now Cfi/examiner who spent quite a lot of time explaining and actually demonstarting this VRS state to me on one of my annual's headed the Heli into wind, brought the heli to a high hover and when there slowly started to shed power by pushing the leaver down very slowly, we felt the entry into VRS by the vibrations and uncontrolled yaws, at this stage the power was slowly reintroduced and we could see the huge/fast loss of altitude, it was recovered as you have stated by pushing the nose down at the same time as pushing down on the collective and after the airspeed built up to around 15 Knts the power was reapplied and we flew out of this VRS state, but to reiterate we started by flying and hovering into wind, frighteningly it seemed to me the only ref to loss of many hundereds of feet could really only be judged by the instruments, until we were flying again.

UNCTUOUS

18th Jun 2002, 20:48

The downwind part is easily apparent to me, because that was not only my first frightening encounter with VR but also the probable cause thereof.

I was doing a low speed recce of a potential confined area at the base of a deep gorge, in the bend of a river and it was obvious that it would have only limited approach directions, and they would be relatively steep. There were also some power-lines around (and a flying fox wire-borne cross-gorge funicular close-by). Because I wasn't planning to terminate in an OGE hover but just do a slow flyby for my crewman to take some photos, I disregarded the wind - which wasn't really significant anyway, although it was certainly about a 5 to 9 kt downwind component....and the air was quite smooth. It was only as we came down to the level of the rim of the gorge that I started paying attention to the rate of descent, because it seemed excessive. I pulled in some collective (UH-1B) and the rate of descent and blade-slap increased quite noticeably (being below the level of the surrounding high terrain). Half my mind was concentrating on the constraints of the flight path for a good photographic pass, but the other half was becoming increasingly concerned with the plummeting aspect. Because we were quite light I wasn't really concerned at this stage, more perplexed really. At about the stage where I was beginning to get ground rush I had pulled it in to max torque but hadn't done anything really about the flight-path, still fixated on the photography aspect and still inexplicably intent on maintaining the angle so as to bypass the pad at a good ground-surface recce height. We may have actually been arcing over somewhat because of that downwind approach effect.

This all happened over the space of no more than 35 to 50 seconds and you have to remember that I'd slept through the VR and retreating blade stall lectures and still had only about 50 hours on choppers. Dumber than dog-droppings really. At the point where I threw it all away and looked desperately for survival solutions I wasn't thinking engine failure or anything machine related, because everything sounded right and felt right, it was just that we were in a hell-bound and cable-cut freight elevator. To this day I think that my instinct of "overshoot" was a valid solution (i.e. I didn't consciously lower the nose with the cyclic as an answer to VR - because I hadn't realised that that was what it was). At about the same time, around 400ft AGL the river, we entered the narrow bend of the gorge and the air changed direction and speed with a bit of turbulence - and it was as if we'd jettisoned a very heavy load. I felt a real surge of lift upon the rotors. Might have been the venturi effect of the gorge narrowing or just the change in local wind velocity but it was something akin to the Hand of God as far as I was concerned. I was quite shaken by it, later discussed it with a few people smarter than I and it all then became clear.

The real clue to the significance of the approach being downwind is that I was flying in relation to the ground and so enhancing the probability that I'd be staying in my own downwash. Highly probable that it was an arcing over approach because of the overshoot trend effect of the downwind component. The very same consideration applies to what happened to the MV-22 at Marana, except that he arranged for the downwash to be on his flight-path by fully utilising that 95 degree nacelle-tilt capability (to kill his wingman overshoot). Think about just what that extra 5 degrees does. Great for slowing down, but it also puts your downwash right in place, beneath and ahead, ready for recirculation.

So I think that "downwind" is very relevant to your chances of encountering vortex ring. Been there, done that, still got the stained undergarment.

Those unfamiliar with the night-crash of the Puma VH-WOF on oil-rig approach to a tanker's helideck off the Coast of Western Australia might care to download the 170 kb file from

this url (http://www.iasa-intl.com/pdf/VH-WOF.pdf). It has some good lessons to be learned, first person commentary in there (as I recall). It may have been that a moving vessel's deck has the same effect as a downwind component? Too late to think that one through. Anyway, it sucked in two experienced rotary drivers, so it's worth reviewing.

Nick Lappos

19th Jun 2002, 02:06

Irlandes,

The downwash velocity is measured relative to the free stream ie, the air mass that the helo is operating in. The air motion is that which the rotor induces on the air in order to generate the lift. If the aircraft is rising or descending, the downwash velocity is a constant relative to the air around. That is why the VRS state can occur, the downwash velocity matches the descent rate to start the VRS process.

Imagine a swimmer in a body of water, stroking with his arms. As he strokes, he is thrusting packages of water backwards to pull himself forward. We can estimate the drag of the swimmer if we could measure the backward flowing stream of water he leaves behind. The momentum change on the water is required to create the forward propulsive force.

Nick

Steve76

8th Mar 2003, 18:45

Hi Nick,

It seems the link to your vortex ring page is not working. Any chance of it being reinstated?

http://mywebpages.comcast.net/llappos/

Hilico

8th Mar 2003, 19:13

I may be taking things a little off-course here, but can anyone enlighten me on the subject of vortex ring? I have flown helicopters at two difference schools (I now have the grand total of 11-ish hours rotary time).

At one place, the first thing they said to me, while we're preparing the coffee, not even seen the a/c, is AVOID THE VORTEX RING STATE. Below thirty knots, DO NOT let the ROD get beyond 500fpm. I later calculated this equates to an angle of six degrees.

At the second place, it did not have the same emphasis. Bailey volume 1 describes it as though you do all the wrong things and then recover, like incipient spin recovery in a fixed-wing. I've also seen a description of VR as airflow attaching and disappearing in a flickering manner the entire length of the blade (wind-tunnel tests thankfully), which sounds like something one would want to avoid.

Can anyone wise me up? Moderator, don't hesitate to squash this if you think it's inappropriate.

The Nr Fairy

8th Mar 2003, 20:07

Hilico :

Read all about it in any decent helicopter aerodynamics book, then get it demonstrated - if you're in the UK it needs to be done as part of the PPL syllabus, and is quite fun. If your instructor is game, see it from lowish level - 800' - 1000' with a smartish recovery to see what it looks like when you're most likely to encounter it.

And as for six degrees - angle, shmangle ! You can't see what 6 degrees is like from inside, but you can see IAS and ROD - use them.

Heliport

8th Mar 2003, 20:21

Not inappropriate at all Hilico - and welcome to the forum.

Vortex Ring occurs when you encounter your own downwash, and can happen even if your rate of descent is not high.
It’s most likely to occur if you have (1) a low or zero forward speed when (2) descending at a medium rate and (3) a high power setting eg a steep approach where the column of air remains underneath your helicopter. With a rate of descent matching the speed of the downwash, there is no angle of attack, the blade root stalls and thus you have no lift.

To get out of it, you reduce power and go into forward flight or autorotation, but you’ll will lose a lot of height anyway.
You avoid vortex ring by keeping forward speed while you descend, or by descending more gently.

The symptoms sound terrible (vibration, buffeting, pitching, yawing, rolling, accelerated rate of decent and temporary loss of cyclic control) but you’ll learn in training how to avoid it, how to recognise it at the incipient stage - and how to recover safely if it does happen.
(It's more like a stall in a fixed-wing, than a spin.)

[Edit]
As for the instructor who started your introduction to flying helicopters with a briefing on the dangers of vortex ring state - find a better instructor or school. :rolleyes:

Whirlybird

8th Mar 2003, 20:34

Hilico,

You need three things for vortex ring:

1) Low or zero airspeed
2) Power applied
3) a high rate of descent; books vary, but probably more than 400 ft/min

Note you need ALL THREE of these. So if you get rid of one, you solve the problem. So all you normally have to do is move the cyclic forward and get some airspeed.

They say it's most likely to happen in a steep approach, probably to a confined area. However, most people are ready for it then, and IMHO it's most likely when you're concentrating on something else, eg at low speed or a high hover while a passenger tries to take photos or similar. But as The Nr Fairy says, you'll do it in training, probably quite a lot, so you'll get to know what incipient vortex ring feels like.

As for what happens...well, the root of the blade stalls, and the tip has no lift due to increased vortices...read the books if you really want to know.

Nick Lappos

8th Mar 2003, 21:13

Steve76,
I switched providers, but will erect that set of pages in the next few days.
NIck

Hilico

8th Mar 2003, 22:01

Thank you all. I'll also have a look at Nick's site when it comes up in a couple of days.

sprocket

8th Mar 2003, 23:08

For those interested while waiting for Nicks page to be restored, this link will take you to info on the vortex associated with the osprey accident a couple of years ago.
It has been posted here before.

That Vexing Vortex Ring (http://www.angelfire.com/me/swissair111memorial/modded/ThatVexingVortexRing.html)

PS: Steve, got the calendars two days ago. Thanks mate!:ok:

OldDuffer

8th Mar 2003, 23:33

As for a high rate of descent required for Vortex Ring State, I would point to an accident involving a Canadian Sea King in1992 at an airshow in Schenectady NY in which the Helo started in the hover at about 100 ft on a hot day. The video is around. Someone on these threads will have a link.

pohm1

8th Mar 2003, 23:42

Whirlybird,
I'm not sure that a 'high rate of descent' is required to enter Vortex Ring. The figure of 400fpm is used widely, and that seems to be a relatively slow rate of descent, ie 2.5 mins per 1000'.

Heliport

9th Mar 2003, 10:02

Hilico
Although you'll experience vortex ring in training, and read about it in the books, you're also very welcome to post questions here.
We've had some very good threads which started with a simple student or PPL question.
Someone gives the 'book' answer, somebody else says the 'book' is wrong and .............. etc

pohm1
I agree. It may be more likely to occur at a high rate of descent, but I stand by my 'medium' rate of descent posted above. Although the books often say 500' per minute, I believe it's been known to happen as low as 300 fpm. It will be interesting to see what Nick's page says.

Happy Landing !

9th Mar 2003, 11:00

less than 30 Kts air speed.
Greater than 300 FPM decent.
power applied.

Take one of the above away = No Vortex ring (Or settling with power if your in the USof A)

Another way NOT to get into it by the sounds of your post, is to engage another instructor !

Safe flying.....

Question: Is it possible to for the tail rotor to enter vortex ?
Humm:confused:

Nick Lappos

9th Mar 2003, 16:35

Here is the link to a discussion with data on VRS

http://nicklappos.0catch.com

For Happy landing! the Tail Rotor certainly experiences VRS, usually at about 75% of its downwash speed. Try to go 25 knots rightward sideways in a Boelkow 105 and you will see the pedals dancing while you try to hold heading. In the S-76 it is about 25 knots as well.

Hilico

9th Mar 2003, 19:23

This is brilliant. Can I now throw something else in.

Do you mean 'power applied' or 'pitch applied'? What would happen if you're in auto (say an EOL), and having levelled the skids, keep 10 knots across the disk and lift to cushion the landing?

I don't suppose VRS would have much time to develop, and it isn't the kind of thing I'd want to try at altitude (I've done the exercise in a -300 with bringing the rpm to the bottom of the green, and the gauge became more and more interesting the further down it went).

Steve76

10th Mar 2003, 04:14

Thanks again Nick.
;)

The Nr Fairy

10th Mar 2003, 06:37

Hilico :

"Power" applied. In autorotation, the airflow is mostly UP through the disk. In powered flight, the airflow is mostly downwards - yes, I know it's a simplification, but hey, it's early.

No power = no chance of physically forcing air into an already disturbed airflow.

No pitch at bottom of auto = strong chance of a sore back !

ShyTorque

10th Mar 2003, 09:24

Excellent stuff on Nick's pages, never seen that detail before and it was a timely reminder why I never became a test pilot :O

Hilico,

The best "pilot's perspective" I was given about how to visualise vortex ring was this:

Imagine a doughnut of air (annular / ring vortex) encompassing the outer part of the rotor disk, with the rotor mast in the hole in the middle of the doughnut. The air travels downwards through the disk, then out and upwards over the outer edge of the disk then back in over the tips and recirculating down again.

From this it might be easier to see why forward airspeed prevents the vortex ring building around the disk.

I had it demonstrated very briefly to me once in military training and some years later, very spectacularly at night by the relatively inexperienced pilot in the RHS. I was a QHI on type but acting as co-pilot on the flight. At the end of a long night he was attempting to achieve a hover at around 8,000 ft (military observation job). At very low IAS he allowed the ROD to build up, which I pointed out, but he took no recovery action and he eventually lost the plot. The aircraft suddenly began random pitching and rolling and the VSI went hard to the bottom stop so I yelled "airspeed" a couple of times more before I intervened and pushed the cyclic well forward for him. We flew out a few seconds later, having lost well over 3,500ft.

Winnie

10th Mar 2003, 10:57

To anyone interested:
Now it was explained to me once, that you may experience VRS at the bottom of an Autorotation, while flaring, particularily in a slight downwind situation. Now I've been told that this is not possible, yet I have read about it in the aerodynamics column in R&W. perhaps Nick would share his views on this?

Thanx
Winnie:O

Nick Lappos

10th Mar 2003, 13:00

The concept of VRS is intriguing, all the more so because of the miseries of the V-22 these days. The web site I put up attempts to explain the basics, I will try to add to it as we see new questions (such as this!)

Autorotation is called the windmill brake state, since the flow (going upward through the disk) is driving the rotor, and the rotor serves as a windmill. There is no such thing as VRS while in this state. When the pitch is pulled at the bottom of the auto, the rotor goes from Windmill Brake State to normal powered state, but the descent rate of the helo in this brief transient region is nowhere near that needed for VRS, nor do I know of VRS in auto touchdowns as a problem when we do the procedures during development test work.

coalface

10th Mar 2003, 15:19

Hello Nick,

I have held an anenometer close to a hovering Puma and have recorded speeds of circa 60kts of wind and it certainly felt like it.

The calculated downwash speed using your formula comes out at only 47.76ft/sec (circa 28kts). The S92 is very similar. These figures seem very low.

I know the downwash will be deflected by ground effect etc which will distort an anenometer reading but this seems a huge difference. Have I made a calculation error or is there another factor i am not taking into account ?

By the way, your website must have taken lots of hits as your host has closed down access to it due to it exceeding its download limit. Lots of interest in vortex ring which is very healthy !!

Happy Landing !

10th Mar 2003, 17:13

Thanks Nick....

interesting concept. Would it therefore lead to a stall ? and if it did, what could be the net effect given the worst case senario?

Happy !

Nick Lappos

10th Mar 2003, 18:19

Coalface,
The formula is one that assumes the rotor produces a uniform stream of moving air, and the velocity is a constant, based on the weight of the machine (thrust produced).
In reality, there is a distribution of little streams within the wake that have various velocities, and these can be considerable greater. I would not be surprised to find strands of 60 knots but would be surprised if they were very large. On the other hand, when you are on the ground and the stream has bent 90 degrees along the ground, especially if a wind has helped concentrate the flow, then there might be plenty of concentration.

In measurements of the S-92, we found knots mostly in the 40's, which is close to the theoretical.

Remember that if the downwash is faster, this would make VRS even less likely, which is what sparked this whole thread.

Happy landing! The VRS is totally unrelated to rotor stall, which occurs at maximum lift for the blade, usually near maximum speed. I can popst some of that stuff if you'd like.

Nick

Happy Landing !

11th Mar 2003, 07:38

Thanks Nick that would be great if you could,

Realy useful stuff. I hope the thread initiator is still reading it and taken on your comments.

You can't teach experience !

Happy !

Nick Lappos

11th Mar 2003, 10:27

If you don't get in on the first try, keep checking back on my website each day. As there is no free lunch, that free site shuts down at 20 megs of data each day, probably about 30 hits.
does anyone out there have any web space they are not using? If your ISP provides web space, I could post on it, if you'd let me. My cable service does not provide web space.

Nick

Chiplight

11th Mar 2003, 17:57

Hi Nick,I've got a Question (http://was.kewlhair.com/hammer/html/induced-flow4.htm) regarding inflow during autorotation.

Thanks

Chiplight

BlenderPilot

11th Mar 2003, 18:29

The last time WE discussed this I left the thread with a whole different view of VRS, and from what others have written in previous threads it seems to me they have all gone to the previous paradigm of VRS, or some of us are still confused.

CONTRARY to what some have said . . .

Its EASIER to get into VRS . . . . .

WHEN APPLYING LITTLE POWER, the less power you use, your downwash velocity is less and easier to catch up to, the MORE suceptible to VRS you will be, .

WHEN YOU ARE LIGHT IN WEIGHT, the lighter you are, the lower disc loading, the lower downwash velocity, lower ROD required to get into VRS, EASIER to get into VRS.

WHEN AT LOWER DENSITY ALTITUDES, the lower the DA, the lower the downwash velocity, the rotor doesn't have to move as much air to produce the same amount of lift thus less downwash, harder to get into VRS.

VRS depends largely on Disc Loading and Downwash velocity.

I understand you have to catch up with your own downwash (match downwash velocity with ROD) in order to get into VRS,

Let not again get "VRS/Settling With Power" confused with "Settling With Not Enough Power"

I think I'm right aren't I?

UNCTUOUS

12th Mar 2003, 06:43

Didn't really see any mention of it above so thought I'd mention that in my view a steep downwind approach is a great way to end up settling into your own downwash.

My first exposure to it was in a UH-1B and I encountered it as I approached with a mild downwind for a low flyby inspection of a prospective LZ at the junction of an L-shaped gorge. It was only the latter perspective on the high terrain that made me aware that something was seriously wrong. Introducing lots of collective just accelerated that downward vector and induced a very distinctive sonorous beat in the Huey's blade-slap. As I'd probably slept through the lectures on retreating blade stall and VRS, I can only conclude that it was a 19 y.o.'s superior reflexes that made me shove the nose down and go for an accelerating disc. Or maybe the recovery was helped by an enforced turn away from the gorge walls.

It was very attention getting and adrenalizing and I've no doubt that if it had been at night or with NVG's, I'd not have picked up the cues early enough to have escaped. As I recall, the a/c was very light-weight (just me and 800lbs of gas, no winch).

Vortex what...ouch!

12th Mar 2003, 19:59

I also had the misfortune of experiencing VRS.

I was practicing transitions, upwind, then turning downwind then upwind and slowing for landing. On this particular day the wind changed without my noticing, not by much but enough to make a difference. I found myself in VRS after making the turn upwind. Slow with a high rate of descent equivalent because of the change of wind direction. Very Scary I can tell you, although it was only afterwards I realised what had happened.

That feeling you get when you drive over a bridge and leave your stomach behind. Well that was it. I hit quite hard but fortunately I was only about 6 feet high straight and level and so no damage was done, except for my nerves. I was very scared for a few minutes afterwards until I realised what had happened.

My experience is limited and so I did not realise entirely at first how I had got into it. I now understand fully after talking to other pilots about what I did wrong.

I was too low to do anything about it, then that is probably always the case, but more importantly too unaware of how easily it can strike.

I am so grateful for people like Nick et al who come on here and take the time to explain to us novices the multitude of ways we can kill ourselves in helicopters if we do not take the time to understand them fully.

I got away with it, but only just. There are so many other ways I could get it wrong, but hopefully will learn on here enough not to do so.

Nick Lappos

13th Mar 2003, 17:02

Chosing words carefully, let me say that the experiences described below were settling, and they did have power, but they were not VRS (or settling with power) aerodynamically, at least not to start with.

The classic downwind turn is a killer because the aircraft is slowed through zero knots and then accelerated backwards. This drives the rotor through the peak power point, where there could very well be a big power shortfall. The aircraft starts dropping, the pilot pulls what power he has, the bottom falls out and he calls it settling with power.

VRS or settling with power requires that you descent to eat your downwash, so the descent speed must already be there to start the VRS. The situations described start from a mild descent, and with the big changes in power required due to the downwind condition, the descent begins because of a power deficit (This is NOT settling with power, as confusing as the words are).

helmet fire

15th Mar 2003, 02:14

Apart from the excellent stuff on Nick's site, there are some intresting discussions of VRS and the issues asked about here on two previous threads:

Black Hawk accident (http://www.pprune.org/forums/showthread.php?s=&threadid=55067)

and

BH Accident not VRS (http://www.pprune.org/forums/showthread.php?s=&threadid=55245)

offshoreigor

15th Mar 2003, 07:49

Hello All, Its been a while.

Excellent topic.

Just one thing I'd like to point out, maybe it's symatics, who knows.

I keep hearing people associate "Settling With Power" with "Vortex Ring State". IMHO, and many others in the Check & Training department, settling with power is not VRS, but a case of the A/C simply not having the power available to arrest the ROD on landing, i.e. "Hot and heavy above RTOW to an offshore platform with little or no wind".

I think where the confusion comes in is related to an old military expression "Power Settling" which is in fact the same as VRS.

What do you think Nick?

Cheers, :eek: OffshoreIgor :eek:

Nick Lappos

15th Mar 2003, 15:57

Igor,
I wish that were how it is, because that is the best way to describe the two different things. In a perfect helicopter world, Vortex ring is aerodynamic downwash eating, and settling with power is too much weight or too little engine. Unfortunately, the "official" definitions sort of trap us.

SWP and VRS are the same, according to the FAA:

http://www.geocities.com/flyingmouse1/Chapter_9.html

By the way, I put the VRS web site up on a real server, where it won't shut off:

http://www.s-92heliport.com/vrs.htm

note that even the FAA is confusing the two things, as they warn you to avoid, "hover out of ground effect at altitudes above the hovering ceiling of the helicopter" as if that is VRS.

OH well.

The important thing for ppruners is to make sure they have enough power to perform the hover they intend. If they do, then they can make a vertical descent with relatively low ROD with no worries about VRS, until they get to a ROD of about 50% of their downwash speed.

I have been in a real case of VRS during some Cat A vertical rejected takeoffs, where I was at about 8 knots forward speed, and at 25 fps ROD, when the bottom fell out, and I was suddenly doing 50 fps when passing through 50 feet (that means 1 second to crunch time, sports fans!) I pulled an armfull of collective at about 5 feet, hit the top collective stops at 1 foot, and whacked the ground at 13.6 feet per second (twice the design sink speed) bounced up and (to make sure I had learned my lesson) hit again at 8 feet per second. To its credit, the S-76 was a lot stronger than advertized, otherwise someone else would be writing this. We wrinkled one stringer in the tail cone.
Theoretical VRS should have been about 25 feet per second.

jellycopter

16th Mar 2003, 11:21

I used to instruct new pilots to the Puma in a high altitude night hovering role. As part of the instructional package, the incipient stage of Vortex Ring was induced to:
a. Demonstrate how a relatively benign manoeuvre could rapidly go wrong.
b. Enable the new pilots to recognise when things were starting to go wrong.
c. To demonstrate how to recover from the situation if ever they got into it.

The scenario was a stabilised into-wind hover at night at about 8000ft using a hover-meter. Allow a slight forward drift to develop (1-2 kts G/S). In order to correct the forward drift, the cyclic beep trim would be beeped back once to give a slight nose-up attitude to arrest the forward movement. Then WAIT. Relatively quickly, the forward drift would be arrested but if no further corrective action was taken, the helicopter would begin a slight rearwards drift; and this is where the problem begins. As the rearward movement increases, airspeed reduces and translational lift reduces accordingly inducing a RoD. It doesn't take long, maybe 10 seconds or so, for the RoD to catch up with the downwash and you're looking at the incipient stages of VRS. (ShyTorque; do you recognise this mate?)

The interesting point in my experience, having done this demo dozens of times was that the Puma was very predictable. Allow the rearward drift and RoD to develop without any corrective inputs, watch the VSI (RCDI) increase to 750fpm and then feel the rapid increase in vibration level, note the yaw fluctuations as the T/R bit into the very disturbed VRS airflow, see the aircraft attitude randomly fluctuate without cyclic input and most startlingly, watch the VSI peg off-scale low. To recover, slightly lower the collective and apply forward cyclic. Within a couple of seconds and a few hundred feet (maybe a thousand+ if you are slow to react) the aircraft would happily be flying again, ready to climb up for the student's attempt. Except on one particular occasion........

Same scenario as above. Going through my usual instructional patter and explaining just before we hit 750fpm RoD what is about to happen. Except this night, for a reason I am still unable to explain, something completely unexpected, and scary happens.

Instead of the VSI showing 750fpm and then the usual vibration and undemanded pitching/yawing as the VSI pegs off-scale, this time there is initially no-vibration at all. Infact, the whole thing went eerily smooth; there was no pitching, rolling or yawing for a few confusing seconds. My 'slick' instructional patter dried up! However, the VSI still pegged off-scale low. After what was probably about 3 - 4 second (I wasn't counting!) the vibration I was expecting appeared as did some very harsh pitching/rolling and yawing movements. The engine RPMs (NGs) were fluctuating wildly. Perhaps scariest of all however, was when I tried to recover by taking my usual conditioned actions; the cyclic didn't not want to respond. I had to apply full forward cyclic but the helicopter just didn't want to pitch nose-down to fly away. After a few more tense seconds it slowly began to respond and the ASI reluctantly showed the magic 30 knots whereupon I applied full power and recovered to the climb. My altitude at recovery was 1800ft. We'd lost 6200 ft.

What's the moral of the story. Well, for what it's worth in my opinion. VRS or Settling with Power or Power Settling, whatever you want to call it, is not predictable at all. There are many variables as Nick L so elequently covers on his web-site. It is beyond the vast majority of pilots to compute all these variables and decide where, on any particular day, the boundaries are. So, stick to what you've been taught and ALWAYS keep your wits about you on approach, or indeed, when hovering OGE.

I normally agree with Nr Fairy's posts, however, as for his suggestion about getting an instructor to demonstrate the incipient stages of VRS at low level to make it more realistic, I entirely disagree. If you decide to go down this route - the best of British luck to you, you'll need it one day! The same logic could be applied to a fixed wing stalling / spinning demo - "lets do them on finals because that's when we're most likely to encounter them for real" - I don't think so!

Finally, Nick L, or anybody else care to try to expalin why I experienced a few seconds of super-smooth descending flight before I entered fully developed VRS? J

Nick Lappos

16th Mar 2003, 13:39

the new VRS web site:

http://www.s-92heliport.com/vrs.htm

jellycopter,

Thanks for the great post, it is this stuff that we need more of, where actual events are described in enough detail to help distill the real story. Let me try to describe what was probably happening aerodynamically to your helicopter, which I believe is entirely consistent with the data on the web site:

As you describe, the demo you normally give was really not full VRS, mostly, but nibbling (even biting) at its edges. The downwash velocity for a 13,000 lb Puma (Is this an OK value?) is about 32 ft/sec (1950 ft/min). If we follow the rule of thumb for VRS, we can see that we should start getting worried about VRS at about 50% of the downwash speed, and by 75%, katie bar the door.

With a torque fall=off of 20% in the demo you gave, you might get from 750 to 1000 ft/min descent, for most helos (sort of exactly the opposite as a forward climb, where the ETL gives you that climb rate.) So in most helos, if you shed ETL by decelerating, and make no collective/power adjustment, you will get 800 ft/min descent. For a Puma that is probably 40 to 50% of the downwash velocity, right at the edge of our calculated VRS speed. Note that one of the first effects of VRS nibbles is to increase the power required (see the cartoon I drew of the power required curve, where there is a hump at 7 knots as you descent at the .5 Vdown speed). This means that at this descent rate, the bottom begins to fall out, and you get increasingly more power shortfall, and therefore more descent rate (if you don't raise the collective). You ended the demos in the past with an acceleration forward, and all was fine.

The yaw wiggling is the rearward flight, mostly, but as you enter vrs, the yaw is in concert with the torque jumping, as the rotor flow changes rapidly enough to make the engines work up and down to keep up with the varying rotor torque. Those torque jumps are a fine indicator of VRS. Note the load factor trace on the V-22 as it gets into VRS, where the smooth slow oscillation (probably the product of a poor autopilot which is not optimized at that regime) becomes a choppy, random one. The load factor gets screwy as the rotor develops randomly oscillating lift and therefore oscillating torque.

In your one massive descent case, the initial descent was a bit more, and the conditions were perfect for the death spiral of descent/power required rise/descent to push you right into full VRS where the simple recovery you used was not enough. Your cyclic effectiveness loss was probably due to the rearward flight effects, where the horizontal tail might have stalled backwards, and the rear part of the disk also lost more lift that the front, leading to the inability to get the nose down. The Puma is a relatively low control power machine, anyway, and you were certainly outside of its normal envelope. Were it daytime, you could have pedaled it around so the nose pointed into the wind, which would have helped things (but at night, the disorientation would be a real problem!)

I believe the old pilot lore of lowering the collective is actually wrong, and more likely to harm you during recovery with greater altitude loss. If you have adequate power to climb vertically at your weight, raise the collective during the recovery, which increases your downwash speed and helps break the VRS. The old experienced Sikorsky test pilots who taught me said that, and it always worked for them. In the old days with piston powered helos, this was never the case, and increasing the collective could cause rotor droop, which is probably the last thing you need!

In a high powered helo, I would increase the collective to max power, but not droop, lower the nose and get out of dodge that way.

This set of events seems to show that the old rule of avoiding the 50% downwash region is a swell idea.

peter manktelow

17th Mar 2003, 04:07

...seem to remember when doing my basic with the RAAF on UH-1...many moons ago

"Below 200 feet then max RoD 200 fpm when below 20 knots"

That seemed to be an easy set of numbers to remember and was aimed at avoiding vortex ring....must have been easy to remember as that was in 1970.......bloody hell...time do fly when you are having fun

Hingeless Rotor

18th Mar 2003, 21:15

Nick,

Just confirm, the lowering of the collective is still a positive way to exit VRS, even in the example above. Why would it not be the most efficient means?

I must admit, it never occurred to me that in some machines it is possible to pull lever to exit. Great food for thought.

Cheers.

Garry M

19th Mar 2003, 20:16

Nick

I carry out mainly longline vertical reference work where we seem to flirt with VRS on a fairly regular basis. I have always found that increasing power and applying right cylic and right pedal the most "minimum altitude loss method" of VRS recovery although it has always seemed slightly at odds with conventional wisdom. Your explanation above has helped explain why. In your opinion is this as good a method of recovery as is available or should we looking for other flight profiles. I have played with many, but the above procedure seems to work the best for Anti-clockwise turning rotor-systems.

The Nr Fairy

20th Mar 2003, 06:54

jelly :

I'll have a word with the instructor I flew with on the day - he's someone I count as a true professional, and I don't think he'd put me (or him) in a dodgy position. Once I have his reasoning I'll re-post.

As for the event itself, it was the end of an hour of VRS. Why the CAA insists on an hour of VRS demos as part of the modular course, I don't know - do it three or four times, it takes all of 30 minutes. The "low-level" was at least 800', if not higher, started into wind, over the airfield, and the surprise was the nose-down attitude I used to recover as promptly as possible. If I ever get into VRS low-level, I'd like to think my recovery actions would be instinctive if I let it get that far !

One last thing - I'm chuffed we seem to agree on most things. I reckon that's pure luck - at my experience level I'm only too willing to let other people tell me how not to kill myself.

jellycopter

20th Mar 2003, 09:19

Nick L,

you wrote "The yaw wiggling is the rearward flight, mostly, but as you enter VRS, the yaw is in concert with the torque jumping, as the rotor flow changes rapidly enough to make the engines work up and down to keep up with the varying rotor torque"

Just to clarify; during my demos, we rarely encountered a negative airspeed. These demos were flown at 8000 ft AMSL in the UK where there is usually always a wind blowing. You'd be hovering over a fixed point with usually 15 - 40 kts airspeed. By allowing a rearward (ground) drift to develop with no collective lever input, the loss of ETL causes the helicopter to descend - all the time however, maintaining a positive airspeed. In this regime, I believe the yaw 'wiggling' at the incipient stages of VRS is due to disturbed flow over the T/R. Once fully developed in VRS however, I entirely agree with your torque jumping argument.

As regards your suggested recovery method of applying collective, I have quite serious reservations. I can see that this recovery would work during the incipient stages of VRS when only a proportion of the disc is stalling - provided you have sufficient power available to pull the extra pitch. However, in the fully developed stage (as I obviously encountered as described above) I beleive it would be entirely wrong to pull in the collective to recover. If you follow through the conventional wisdom of the doughnut of airflow on the outer portion of the disc causing increased induced flow and reduced lift. Couple that to the stalling of the inboard portion of the disc due to high alpha caused by the RoD - by raising the lever, you can only exacerbate the situation.

This leaves us with a bit of a problem. Do you raise the collective to recover during the incipient stages and achieve minimum height loss. Or do you use the same recovery action as you would during the fully developed phase - lower the lever slightly and pitch nose down to recover the airflow but lose more height. With my experienced pilot hat on, I reckon I might be able to decide, fairly accurately which method to use and when. However, with my instructors hat on, we are all aware, things need to be suitable for the lowest common denominator ie. the low hour ppl (no offence intended to low hour ppls - i was one once!). If you give the low hour ppl a choice at such a critical time, Sod's Law says he will make the wrong choice and kill himself. If on the other hand you lay down in tablets of stone a set of actions to take in a particular set of circumstances, chances are, if he recognises the problem, he'll fix without conscious thought.

What I'm trying to say is, to recover from VRS, either incipient or fully developed there needs to be a standard recovery action that will work in all events. The conventional recovery of lowering the lever slightly and diving for airspeed works - period. If on the other hand, you raise the collective to recover, it might work, it might not. This depends upon how quickly you recognise the VRS condition and how much power the aircraft has in surplus.

The only problem the conventional recovery gives us is when we encounter the condition at low altitude. In this regime, you may have insufficient height to recover using the conventional method, then again you might! Likewise, by raising the collective to recover, it might work, it might not. This is why, once again set in tablets of stone (well, more like soft jelly (that's jello I think in the US)) we have a region of the RoD and IAS envelope which we avoid at all costs. J

Nr Fairy

I take your point about the 1 hour of VRS in the syllabus but I can't do anything about that.

However, what I can do is try to give people (usually my student's) the benefit of hard earned experience. As far as VRS is concerned, on the Puma at least, I got pretty familiar with it's behaviour. Nonetheless, one day it bit me. Luckily, my 3 other crew members and I are here to tell the tale (we even made the video!). As I said in my original post, we used to recover from the incipient stages in hundreds of feet, and occasionally just over a thousand. However, I always practiced from 8000ft; and it's just as well I did. Much lower and we would all be dead!

I guess, in a Robinson or other light helicopter, you can practice VRS recovery all day long from 800ft and I dare say you'll get away with it 99 times out of 100. It's the time when it takes you by suprise, as it did with me, that you'll have insufficient height to recover and you will die.

If it's any consolation, the VRS demo I had from a Robinson instructor (at a premier (?) UK heli FTO) in an R22 was not VRS at all, not even the incipient stage. The only symptoms we had were a high'ish RoD and low IAS but none of the accompanying vibration, yawing, pitching and rolling. The recovery was the 'conventional method' and the aircraft flew away immediately. I would wager that you could have done that demo quite safely from 800ft as it wasn't really Vortex Ring.

Incidentally, the UK military do not demo VRS to ab initio students. They simply explain the risk areas and encourage students to avoid them. This seems to have worked successfully for years now, so why change? J

Jcooper

20th Jan 2004, 13:35

Do you mush Settling with power and vortex ring state together or do you keep them seperately? Just wondering as I was always taught that settling with power is the same thing as VRS and then they had another term settling through power which is when you have the power there but your descent rate was to great to be stopped? Just wondering...

Shawn Coyle

20th Jan 2004, 22:09

I don't know of a good, clear definition for 'settling with power' that would make it easy for the pilot in the cockpit to say that he was experiencing that or whether he was experiencing vortex ring state.
Most people say settling with power is when power available exceeds power required, but that can happen without the symptoms of vortex ring state.
Unfortunately the terms get mixed up all the time, adding to the confusion.
I avoid the use of the term settling with power as it doesn't add any clarity to the discussion. But that's my personal opinion.
If someone can come up with a clear distinction between the two, I'd be happy to include it in the next edition of the book.

IHL

20th Jan 2004, 23:33

Practical examples of settling with power:
1) Aircraft is able to Hover in ground effect (just) at SL. It departs and then attempts to land at 8000 feet, maximum power is applied but it is insufficient to arrest the rate of decent.

2) The aircraft at gross weight attempts to land with a 15 knot tail wind. Maximum power is applied but insufficient to arrest rate of decent.

3) If you have ever come into a landing spot and had to over torque to get stopped at the bottom, that is settling with power.

MightyGem

21st Jan 2004, 03:42

JC, this subject has been discussed at some length on previous occassions. A search on "vortex ring" will bring up many results. A good starting point would be here (http://www.pprune.org/forums/showthread.php?threadid=83820&highlight=vortex+ring)

Heliport

21st Jan 2004, 04:11

Various VRS/SWP threads merged.

Skycop

21st Jan 2004, 05:07

Glad to see this old thread of mine is still going, chaps! :ok:

Jcooper

21st Jan 2004, 12:21

sorry for not searching. my bad. thanks for bringing it to life heliport

Shawn...I personally liked settling through power. That made more sense in my head.

Shawn Coyle

21st Jan 2004, 22:47

One of the principles that has to be used when writing emergency procedures or insisting on warning or caution lights is that the symptoms must be clear and easily understood so that the correct action can be taken.
Can't have two separate and distinct procedures for the same indications, for example. Also can't have the same indications for two different problems.
Hence the question - what are the symptoms, in the cockpit, that the pilot will see to know whether he has 'settling with power' or 'vortex ring state'?
Given that many helicopter pilots can't describe what they'll see in the cockpit with VRS to begin with, how can we help to educate them about the difference.
Having re-read this wonderfully done discussion, my suggestion would be:
Symptoms:
Power / Engine Indications - at maximum power or engine limits
Airspeed - zero to 20 KIAS
Rate of Descent - 200-500 fpm/ Not slowing down / Not being arrested
Rotor RPM - May be decreasing
This might be called 'settling with power' but most probably should be called 'power required exceeds power available'.
Recovery action - maintain collective position, increase throttle in piston engine helicopter / lower collective to regain rotor RPM apply forward cyclic to gain airspeed. Reconsider attempt to land.

Symptoms
Vibrations - low frequency airframe buffeting
Airspeed - less than 20 KIAS
Airframe - may have uncommanded pitch, roll and yaw
Rate of descent- greater than 500' fpm
Power - initially may be less than any limits of airframe or engine
Raising collective results in increase in rate of descent
May be downwind if wind is known

Recovery action - apply forward cyclic and hold till airspeed increases, simultaneously increase collective (this based on Nick's advice)

I realize this appears to violate the first part of my post- the recovery action appears to be much the same for the two situations, but they are slightly different.
Further suggestions?

IHL

22nd Jan 2004, 02:13

Shawn: wouldn't VRS be noticed/happen earlier in athe approach
( you would need the altitude to get into fully developed VRS) while settling with power would be evident during the latter stages just prior to touch down.

heedm

22nd Jan 2004, 09:56

Good point, IHL. We could probably go a step further and say that SWP is avoided by confirming power, checking charts, and proper recce's while VRS is avoided by using proper technique.

My point is that if you're properly educated, both of these shouldn't occur, or at the very least surprise you.

Matthew.

helmet fire

22nd Jan 2004, 14:59

I think you are hitting the nail on the head Shawn. As I posted in the Black Hawk threads, I feel there really needs to be a change of terminology to avoid what is clearly confusing the hell out of everyone. That terminology was changed in Oz more than 20 years ago, and nowadays it is rare to hear this endless discussion and all its misunderstandings.

As I suggested in the Black Hawk thread, we can clean up the terminology simply by calling the situations what they are, and thus promote increased understanding (which is the ultimate goal anyway):

Vortex Ring State should be called Vortex Ring State, and no longer called settling with anything - because it is far from a "settling experience".

Running out of power should be called Insufficient Power because that is what it is. The fact that you then settle to the ground also happens with engine failures - but we dont refer to them as settling without any power! I go for "insufficient power" in preference to Shawns suggestion of "power required exceeds power available" because it says the same thing with more brevity.

Thus the bloke who tries to come to the hover and then smites the ground would NOT be "suffering" power settling or settling with power, he would have entered VRS or he would have attempted a manoeuvre with insufficient power, depending upon the circumstances which, (as should be blindingly obvious by now) are vastly different for each case.

Whaddyareckon?

:8

Hughes500

22nd Jan 2004, 15:43

Something for you instructors !!

Just for interest while talking about vortex ring. Last week did lesson 15 vortex ring with a student in a 300. Normal briefings and demos from 3000 ft. Asked the student to hold 18 inches MP and height so he obviously is pulling back the speed. As we came below about 20 kts ias with 18" the ac began its normal vibration followed by pitching and rolling with rod at 700fpm. Asked student to recover, he did by snapping the lever up to 25"map. I was't expecting his yank so couldn't initally couldn't overcome the increased pitch!!!!

The result was impressive language, a 300 falling back to mother earth on its side ( very unerving looking out of the side window directly down). To recover was full lever down and full forward cyclic, it took a long time to ( seemed) get the ac out of vrs with about 500 ft to spare before stuffing in.

Result now take a strong pair of cycle clips and alot of pressure on collective to stop it coming up !!! I would have to agree with other posts in incipient vrs cylic first, in full vrs collective down followed by cylic and hope you have the height !!

Shawn Coyle

23rd Jan 2004, 03:51

This will be duly noted in the next edition of the book. I like 'insufficient power' and the other bits of advice.

Ross Mathieson

24th Jan 2004, 14:15

I once deliberately entered a Vortex ring, as a demonstration to a student, in an Iroquois (Bell 204) at about 3000 ft. The entry was achieved by hovering downwind, descending and introducing power. When established the aircraft descended at approx 2500 ft/min, shook and rattled, and the cyclic was slack and ineffective. A moment of panic ensued when I discovered that the supposed recovery technique of selecting forward cyclic had no effect whatsoever, and we had already lost more than 1000 ft. I selected full pedal (left I think), and after a second or so the aircraft suddenly spun around into wind, and I was able to dive out and recover. From then on the Vortex ring topic was discussed in the classroom only, as far as I was concerned.

Helinut

24th Jan 2004, 17:50

It is a while since I have done ab-initio instruction, but the point was made during my instructor training that the aim was to show the symptoms of the incipient stage of VR, and then the recovery. It seems to me that there is a real purpose and value in going this far for the student - if for no ther reason than to make himm/her aware.

I remember several occasions when instructors I knew were flying together vying with each other to get a higher and higher rate of descent in VR before recovery - it was never a game that I joined in on. My experience of even the incipient stage was that it was so unpredictable that I really did not want to go further. I am not at all sure that we know enough about VR to play with it in this way - certainly an area for the test pilot in my book, and a high starting altitude to the exercise!

PPRUNE FAN#1

24th Jan 2004, 23:30

It's funny, really, to hear all the theoretical discussions of SWP/VRS. Those of us who've actually gotten into true VRS know (like Ross and "Hughes 500" found out) that VRS can be pretty...let's call it "anxiety-producing" since we pilots don't like to admit that something was scary.

Let's be honest, if you get into actual VRS below 500 feet you are going to crash.

On the other hand, SWIP ("settling with insufficient power") is probably more recoverable due to the fact that the rotor is still behaving in a conventional, predictable manner. All you need to do is get through ETL and you're flying again. But does it matter?

The trouble is, the line between SWIP and SWVRS is a hazy, indistinct one. The former can turn into the latter in a heartbeat. It matters not whether you think your a/c is more susceptible to one or the other. Operationally, the technique used to avoid both is the same.

And that is? Simple really. Pilots who find themselves on approach to an LZ with very low airspeed and power applied (i.e. non-autorotative) better be on their toes. You better know where the wind is coming from, and you better be super-attentive to the cues coming from the ship.

I absolutely hate downwind, back-below-ETL approaches. In the course of my career (60,000 landings, conservatively), I've had to make a few when no other alternative was available. They make me very uncomfortable. And I hope I never do get comfortable doing them. Because I never want to be sitting there thinking, "Is this just SWIP or true VRS?" If that thought ever crosses my mind, it means that I've screwed-up, big time. For whatever reason (luck?) I've yet to think it.

NickLappos

25th Jan 2004, 02:10

Either case (Settling with insufficient power or vortex ring state) are unlikely to sneak up and bite you, as both take a reasonable amount of cue-ignoring by the pilot.

For "insufficient power" the training technique that helps might be something I have taught for a while - the controlled power approach. Simply said, it is a progressive slow down and power check as you make the approach so that you never find yourself committed to a hover while overloaded. Most instructors teach something like this, perhaps not formally, but certainly trying to spark the judgement. These techniques are obvious to experienced pilots who have flown overloaded helos for a living. I operated an AH-1G in Vietnam that could never hover above 1 foot at max power, any technique but the one below spelled doom:

1) know the power you can pull, the max power, before you start the approach. Easy way, at 500 feet above the landing, just increase collective, climb, and note the max power (where you reach the first limit, of course). Since you are somewhat higher than the landing point, the check will be slightly conservative. Note that power, and don't forget it.

2) As you enter the approach, spot the max power on the gage. As you slow the aircraft down and keep on the approach line, watch the torque begin to increase. Slow down gradually and note the power increase as you pull up on collective to keep the approach angle constant.

3) As you get quite slow, perhaps in the 25 knot range, the power will start approaching the max power that you observed. Slow down carefully, feeling yourself increasing power as you slow. One knot is an appreciable change. If the power gets within 5% torque of the max, be very careful not to slow any further unless you are SURE things are healthy. Most insufficient power accidents are made right there, when the pilot slows abruptly and inadvertantly commits to the hover, without having enough hover power.

4) If you reach the max without assurance of being able to arrest things in the hover, go around. Simply keep the power where it is, gently lower the nose a hair, and you will start to accelerate. Increase power to the max and you will gently climb. It is impossible to "fall through" on approach if you keep the power and speed under your direct control.

5) recall that a bit of ground effect will help you at the bottom, so if you have almost max power and a very gentle descent rate, all will calm itself as you wash into the hover. Note that the peak power you pull should be at about 10 feet or so, and it should be almost exactly the hover power (maybe 1% more at the most) if you have done things right. I have made this a game with students, to see if they could sneak the aircraft into a hover with only hover power as the max pulled. It is great sport, and teaches the soft technique that will keep you able to fly at the edge of your aircraft's performance.

6) Note something interesting as you practice the sneaking into a hover - the control that causes you to pull too much power at the bottom is almost never the collective, it is the cyclic, because you are decelerating too rapidly, and the "whoa, Nelly!" flare at the bottom will always cost you more power than a steady hover.

7) As solid confirmation that you have the controlled power approach down pat, try making a go around during the approach with your hand off the collective. Here's how - at some point, just remove your hand from the collective, and then gently lower the nose maybe 1 degree. As you accelerate, the extra speed will get you closer to best rate of climb speed, so you will start to gently climb. If you are rushing the approach, you will sink quite a bit before you climb, a sign that the approach is a bit too fast for a maximum performance technique.

8) You are ready for graduate school in this technique if, on a calm morning you can actually plan a touchdown at a spot with your hand off the collective while decelerating through the 30 knot point. Gentle speed changes will change your approach angle, and the smooth cyclic control will help you a bunch.

Here is some discussion on VRS:

http:www.s-92heliport.com/vrs.htm

vorticey

25th Jan 2004, 06:30

still dont understand why the VRS graphs say that a 0 airspeed decent is less lickley to enter VRS than at 5- 10 knots?? maybe it enters auto before VRS can develope?
anyone expain?:confused:

CJ Eliassen

26th Mar 2004, 17:25

Charlie,

Some things to remember about VRS is that the airspeed and ROD required to enter VRS change depending on the aircrafts weight, density altitude, and rotor loading.

A lightly loaded helicopter is more likely to get into VRS at a slower ROD, but conversely will experience ETL and leave VRS at a lower airspeed. The same helicopter with a heavy load will need to be descending at a higher rate in order to experience VRS. But then this helicopter has to travel faster to achieve ETL and escape VRS.

A heavily loaded rotor versus a lightly loaded rotor works in the same fashion. The higher the rotor loading, the faster the ROD must be for this helicopter to enter VRS but the airspeed must also be higher to experience ETL and escape VRS.

The higher the density altitude, the greater the ROD must be to enter VRS. But again, the airspeed to achieve ETL and leave VRS is higher.

Vorticey,

Yes, the rate of descent that you will enter VRS decreases slightly as you increase forward speed, then it increases slightly before hitting ETL. This has to do with the fact that your rotor is now tilited forward and your vertical downwash is lessened. Since your downwash is now slower, you will experience VRS at a slower ROD than at 0 airspeed.

NickLappos

27th Mar 2004, 06:03

It is somewhat frustrating to know that those who write the guide books for various governments, as well as those who write the tests are schooled in the same pop-aerodynamics course.

There is no helicopter on the planet that will experience VRS at 35 knots. (The end of that sentence is a period)

Take a gander at the plots shown at the web site I posted a while back, when this thread was young:

http://www.s-92heliport.com/vrs.htm

The data is excellent stuff, created by some good NAVAIR engineers in briefings on VRS in the V-22.

CJ Eliassen

27th Mar 2004, 15:10

Charlie,

What do they say the correct answer is?

Nick,

Several helicopters can experience VRS at 35 knots. Helicopters with highly loaded rotors at gross weight and high altitude may experience VRS will above 35 knots. However, it won't be at 300 or even 500 feet ROD. For example, an CH-53E at gross weight and 10,000 feet can experience VRS up to 38 knots. The ROD however, would have to be between 3800 and 4800 fpm.

CJ Eliassen

27th Mar 2004, 17:43

Charlie,

Ask them to prove it too you. Do you have the formula for VRS?

C.J.

heedm

27th Mar 2004, 18:46

Nick, the page you referred shows data for VRS on the V22 occurring between 30 and 42 KIAS.

I completely agree with the written test creators being students only of "pop-aerodynamics" courses. I believe this is the greatest hurdle in explaining many things helicopter. The introduction and conclusion of those courses should include a caveat explaining that the discussion is meant to be simple not truthful.

NickLappos

27th Mar 2004, 20:08

I appreciate the idea that a helo or tilt rotor can "experience" vrs, or that it "occurs" at 30 or 40 knots, but let's define "experience" or "occurrence".

The data shows that the V-22 had nibbles (5 to 10% torque variability - the blue data points) at about 45 knots (at a speed equal to the downwash velocity). This is not VRS, it is the beginnings, where they wanted to stop the tests to avoid losing roll control. Note that the reds were at about .75 to .85 of the downwash speed, where the torque variability is 10% and the tests were aborted. This is not VRS, it is the first sign of it.

For a helicopter, with disk loadings about 1/4 that of the V22 (2 to 7 lbs per sq foot instead of 22 to 25 psf) the downwash speed is much less, and so the vrs forward (and downward) speed is much lower. In a Bell 206 you would have to slow to about 10 or 12 knots forward speed to experience real VRS, and could get some nibbles at 15 to 18 knots.

The written test question is simply wrong, fellows. The idea that 35 knot flight is close to VRS is another way of perpetuating the myth, and allowing misunderstanding and superstition to cloud the learning experience.

When I took my FAA Flight Instructor written, I refused to answer one question. It asserted that in a climb lift was greater than weight (the least-wrong multiple choice). I wrote a brief critique of the question, with an explanation of why, and I got credit for a correct answer.

I know the regulators in several countries. Good people, professional aviators and great instructors, but they get their aero from the old piloting texts written by those who they follow. Few aero engineers dabble in the training publication world. A guy like Gordon Lieshman or Gareth Padfield could help square these publications away. Maybe I'll ask them to do so next time I communicate with them!

BTW let me congratulate the ppruners here, this is a great discourse, with erudite observations and sharp minds. I love it when someone posts something and someone else reads it, goes beyond it and makes us all actually think!!

CJ Eliassen

28th Mar 2004, 13:04

Nick,

Who is defining experience and occurance here? And what's the definition of nibble?

If a helicopter begins to experience VRS at 35 knots, no matter how small the loss of lift, the helicopter will slow down. By slowing down, it enters VRS more deeply. So corrective action must be initiated no matter how small the experience, occurance, or nibble.

I definitatly wouldn't call VRS a myth.

NickLappos

28th Mar 2004, 14:17

CJ,
The myth is that at 35 knots a helo can experience VRS. As long as the guidebooks our students use have that bunk in them, our students will not understand what they are doing. This is second cousin to the now-standard belief that a 300 fpm descent can trigger VRS.

The data I posted shows that the max you can get at 18 knots in a helicopter is a 5% torque fluctuation. That is what I said, and what the data proves.

If you want to believe that a helo at 35 knots can experience VRS, please go ahead, nobody will stop you. Maybe you could even write some exams while you are at it!

You will be wrong, but nobody will stop you, after all, they didn't stop the last guys who did it, the ones that charlie used when he started this topic!

CJ Eliassen

28th Mar 2004, 16:03

Nick,

I gave an example of a helicopter that can experience VRS at above 35 knots.

One reason books explain it in this manner is that we don't use sophisticated equipment to determine airspeed or ROD. Vertical speed indicators and airspeed indicators are subject to many many errors. Therefore, in consideration of safety, most books overstate the regime in which you can experience VRS.

I have experienced VRS in an R-22 at a ROD indicated of 400-500 FPM, but in theory, I should experience VRS until I achieve a ROD of over 900 FPM.

So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.

PPRUNE FAN#1

28th Mar 2004, 17:30

CJ Eliassen:So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.I agree with Nick that helicopters will never see VRS at 35 knots. By definition, to get into VRS the rotor must be back below ETL. There is no vortex ring during translated flight. I've never understood why the books use such hard (and obviously wrong) numbers for defining VRS, which is why so many pilots seem to not understand it. I agree with you CJ, that the instruments we use in the cockpit are very imprecise. In my opinion, what the texts really ought to be saying is:

a) Speed below ETL
b) Some RoD
c) Some power applied (not autorotative descent).

The trouble with the above is that there is no Translational Lift Indicator- nothing to look at in the cockpit that would definitely tell us whether we are in translated flight or not. Ergo, the conservative "Look out below 40 knots!"

There have been many times in my career whilst making landings to offshore oil platforms in grossed-out BO105s on hot, calm days (or when the light wind was variable) when I've wondered if the shaking, shuddering and rivet-loosening was the normal BO translational event or the onset of VRS? Either it never was or I just got lucky.

Finally CJ, I got quite a chuckle out of trying to imagine an CH-53 in the flight conditions you described. A RoD of 4000+fpm with a forward speed of only 38 knots? While that might work in theory, I would imagine that such a manuevre would be impossible to perform with any sort of power applied (have to defer to real '53 pilots there though). But it sure would be exciting to watch, eh!

CJ Eliassen

28th Mar 2004, 23:42

PPrune Fan,

Why is it so hard to believe that a 45 degree angled descent can't achieve VRS when at 35 knots? It can happen and has happened.

NickLappos

29th Mar 2004, 01:51

Charlie asked:
".......how do you calculate what sort of airspeeds the aircraft is going to be susceptible to VRS (the airspeeds corresponding to the red area on your B206 graph)? I understand how to estimate the descent rates using the formula, just missing the link to figuring out the airspeeds"

Charlie, the forward speed scales on those plots are in ratios to the downwash speed, so 1.0 is simply 1x the downwash speed (which is calculated using that formula shown in the beginning) That formula is simply an attempt to approximate the speed of the downwash needed to produce the lift equal to weight. It assumes that the momentum transferred from the air to tha aircraft has produced a uniform stream of air.

Sorry that it takes a while to download. I can break the page up into chunks if that would help, or I can email the plots to you separately.

CJ Eliassen

29th Mar 2004, 13:05

Charlie,

In the spreadsheet I sent you, Vi 1 is the airspeed that the helicopter experiences ETL. On the chart it is represented on the bottom as Vi/101.4.

Hope this helps.

NickLappos

29th Mar 2004, 14:12

charlie,
You ask ", why isn't the vertical band making up the red section of the chart more to the left, or more to the right? What is it that has fixed it at those points? I suspect "testing" is going to be the answer, but I was hoping there would be a mathematical model for it, or a rule of thumb like 50-75% for the descent rate"

Those plot points that are colored red and greenon my website are made from flight test points on a V22 test. They are not where VRS is truly developed, they mark the outer boundary where VRS just starts for a helicopter that is slowing down from forward flight. It could be possible to test from a steady zero knot hover outward, if you wish, and that would scatter some points from the left side toward the center. Those points are not VRS, they are associated with small torque variences (note the percent in the legend). The actual boundary for VRS is inside the oval considerably far from the test points. See the first plot on that page where the orange circle marks the real VRS boundary.

To calculate the downwash speed (which is the reference for VRS for a particular helo) just take the disk loading in pounds per square foot and multiply by 210, then take the square root. That will give you the feet per second for the 1.0 speed value.

To be conservative, if you take a descent at 50 to 75% of that descent rate, you can avoid VRS.

Here is an example For an R-22, disk area 498 sq ft, GW 1370 lbs, disk loading is 2.75 lbs/sq ft. 2.75 x 210= 577. The square root of 577 is 24 ft/sec, which is the average downwash of an R-22 (24 ft/sec is about 14 knots, or about 1440 ft/min).

This means that the R-22 can't get VRS above about 11 knots forward speed (75% of the downwash velocity), but also that it can't get VRS in a descent less than 700 ft/min (50% of the downwash velocity).

davehearn

29th Mar 2004, 15:26

went out with my instructor to get vortex ring experience , on return to airfield was on final and got us into it at 300ft ended up forward cyclic and whizzing along at 5 ft agl at 50knts for go round on next approach did exactly same again. i like a 50ft hover and lower it down gently. r22s can easily get into vortex, buy the time the vibrations have started its usually to late!

CJ Eliassen

29th Mar 2004, 18:17

I have gotten into VRS at 5 feet before. :) Tail rotor VRS:)

[email protected]

30th Mar 2004, 04:48

Davehearn - you are only getting incipient VRS experience not the fully developed variety or you would never recover from 300'. Frankly your instructor is a clown to even do the incipient stuff so low - what is he going to do if it goes a bit far and starts to get into VRS proper?

ShyTorque

10th Apr 2004, 22:53

A useful link:

http://naca.larc.nasa.gov/reports/1958/naca-tn-4330/naca-tn-4330.pdf

And to think I could be at the pub..... :8

DualDriver

9th Jun 2004, 07:29

I hope there are some heli instructors out there that can help me on this one.

When demo-ing vortex ring, I find that it is quite hard to get the robbie (22) into this state.

Here's what I do:
I reduce power to about 13-14 inch MAP,
Reduce speed to about 20 kts and a ROD of about 500-800 feet/min.

But I just can't get the symptoms to show CLEARLY. Is it just that the robbie is designed this way or am I doing something wrong.

Advice and Input, Please.

DD::confused: :ok:

breakscrew

9th Jun 2004, 07:57

DD,
I agree with Mr S; alternatively you could try entering the manoeuvre downwind. (Need lots of height and only let the ruddy thing enter the incipient stages!) However, there have been times when I have tried to demo it and it really won't go. It is usually when I have a newish Beta 2 and a light student.
Good luck.

BS :)

Ascend Charlie

9th Jun 2004, 09:45

Come to a complete hover at 2000'agl. You should be pulling 20+ inches, maybe 23. Ensure zero airspeed. Wait. Don't descend, don't move forward.

Then lower the lever a little to start a gentle descent.

Then the fun starts.:p

HOSS 1

9th Jun 2004, 10:05

Are you doing these with an instructor? (I hope).

If yes, find another instructor !!!

If no, find an instructor !!!

From your description of the maneuver, it doesn't seem you or your school has a "procedure" for demonstrating this.

For my students I use:

Establish Hover at an altitide that will ensure recovery by 1500ft AGL (read: Plenty high)

Slightly lower collective to achieve ~500fpm descent

When buffeting starts (or other such excitment) increase collective to show it will not arrest rate of descent

Pitch over to regain airspeed (remove one of the three conditions of VRS)

Establish Vy climb

And, this is what the Examiner in our area wants to see on check rides.

Fly Safe.

HOSS-1

DualDriver

9th Jun 2004, 10:35

Thanks for all the replies, hope there are many to follow. I'd like tyo see all the various ways and opinions.:ok:

HOSS-1...

I AM the instructor. The way I described above was the way I was taught and the way it is done is SA. Thanks for the input though.

I am going to be trying these different ways, I think that Ascend Charlie has a good tecnique going.

Keep it coming:ok:

Robbo Jock

9th Jun 2004, 11:46

The two instructors that have demo'd it to me have used slightly different scenarios, but basically the same technique:
(Both start very high!)

John - set up a downwind approach to a field a long, long way off (normal approach profile, but we are very high) and patters the approach including "wash the speed off", "wash the speed off" until suddenly it all goes horribly wrong. His theory being that a downwind approach is where you're likely to get into the state anyway.

Stuart - Pretends to be a photographer: "left a bit, no, right, slow down slightly, come left, nono too fast, right slightly..." then suddenly you're doing the dance.

Both have an advantage in that they are 'real life' examples that show how easily you can be distracted and get into the situation.

Never had any trouble getting into it, maybe because of my non-too-sylph-like figure!

TIMTS

9th Jun 2004, 14:31

Start at atleast 1,500ft....2,000 better...The FAA test standards says you have to recover before 1,000...nothing about how high to enter, so high is good.

Pick a prominent landmark and fly towards it...lower the power down to around 15-18" mark....depends on a/c an weight, and keep altitude. This means you have to come back a bit on the cyclic...keep that vsi at 0...it shouldn't take to much input...
This will bleed off airspeed, while keeping altitude.
When you get lower than ETL, you will start to loose altitude...due to low airspeed, power and rate of descent you will know most likely enter vortex ring state...
Works for me anyway.

Timts

TeeS

9th Jun 2004, 14:52

The idea of trying this from 1500/2000 ft scares the hell out of me. 4000' maybe!

Am I just getting old!

TeeS

dammyneckhurts

9th Jun 2004, 20:33

Hello all,

I am from Canada, spend most of my time looking down a long line, and flying in the mountains. I find myself in the kind of work environment that frequently lends itself to vortex ring situations. I am not an instructor, but I can perhaps throw a few ideas on the table that I wish I knew when I was starting out. Some of this will be redundant for people, and I apologize for any inaccurate terminology that may be used in current instructor speak.

It's important that students have a thorough understanding of exactly what Vortex Ring is. They need to know:

-What 3 factors you need to get into it,
-How to recognize it,
-How to get out of it.

I just wanted to elaborate a bit on some of these points and add to the concepts that the basic exercise teaches.

To get into it we need:

-Little or no airspeed,
-Rate of decent,
-And using power.

If you don’t have all 3 present no worries right? Well not always!.....Up flowing air gives you the same situation as being in a descent. Hovering at the top of a ridge can be brutal if the air is screaming up the slope at you from below. First off you are surprised at how little power you need to hover, you feel the mushiness of the tail, pull a bit of power and all of a sudden it gets real ugly. Same mechanics as the practice scenario, but different causes and your not 2000' off the ground!

Next part would be on how to recognize it. Most will talk about shakes and shudders, tail is all over the place etc..All very important of course, but the reality is that if you are on very short final and it gets that bad, you are in big trouble unless you make immediate and drastic action to recover.

In production work your going as fast as practical (not possible), floating in on minimal power and pulling power on very short final. Unless your SURE that you have no V.R. concerns its always good to do a little check with the collective just before the final pull. This is the crucial part... when you do this your looking to see how the helicopter is reacting and how it feels under your ass. If all is good you should feel HEAVIER in your seat when you pull a bit of collective. If your anywhere close to entering V.R. you will not get that "heavier" feeling as your weight will stay neutral. If your closer to V.R. you will feel lighter in your seat...all this happens WELL before you begin to notice the usual signs of V.R. that everyone talks about.

Now put this into the hovering on the top of the slope scenario previously mentioned. Your hovering 40' off the trees (or your longline load is 40' off the trees) trying to decide if your going in or not. Your at a low power setting due to upflow air, make sure you have some air under you so you can abort if necessary and give the collective a bit of a pull, are you heavier or lighter in your seat?

Finally a few comments on how to get out of it. We are all well versed on that one, I just wanted to expend a bit on the practice scenario that we all learn with. The reality is that if it happens unexpectedly at 100' or less, there is not a lot of room to work with. Another reality is that unless your in full blown V.R., it doesn’t take much to get out of it. Generally just getting out of your downwash by moving a few rotor widths will do it. (try to go crosswind if you know your downwind as it started). What if your are already below treeline going into a pad and dont have the room to maneuver? One thing that can help is to stir the cyclic in a circle. This disperses your downwash a bit and can help a lot in the recovery.

Awareness that you are working in conditions conducive to V.R. , early detection followed by swift and accurate recovery techniques is what its all about concerning V.R.

The emphasis though should be on awareness and detection.....all the recovery techniqes in the world wont help a guy that repeatedly stumbles into in unaware.....he wont live long or he will be so scared after the first time he will have a hard time in a production work environment and will be quick to shut it down as he gets out of his comfort zone.

Conditions that are most conducive to V.R. are working in the bush with light and variable winds. You don’t always have the luxury of any form of wind indication (no water, leafy trees etc). If you want to be quick in these conditions you need tools to detect early V.R., and be comfortable with rapid recovery methods. In conditions like this the "collective check how does it feel in the seat thing" allows you to stay well ahead of the game. Always do the check early enough to get out of it if it doesn’t feel good. This kind of thinking allows you to work downwind, and in a variable wind condition with confidence and swiftness in a production environment.

Can you tell that I am sitting on a fire contract and its pissing rain? Too much time on my hands..
Thanks for reading my ratings....just wanted to offer suggestions on how to teach a new pilot to think about the whole concept and bring it a little closer to the ground so to speak.....

Signed;

Dammyneckhurts

RDRickster

9th Jun 2004, 22:13

dammyneckhurts, great stuff... it's good to hear about techniques from different operations! Nicely put and you relate your message well (easily to understand).

:ok:

moosp

9th Jun 2004, 23:19

Now that's what I call a first post. Good info and a good read.

Anyone else heard of the "Stir the cyclic" technique" in a confined area?

Steve76

10th Jun 2004, 00:05

moosp,

Yep, heard about that one before. Seems to make sense and previous discussions on the subject have elaborated on the benefits of "spilling" air out of the disc. My problem in life was trying to get copilots not to stir the stick :)

There is a massive amount of info on PPRUNE about VRS, be sure to do a search and read Nick L's posts. It is actually harder to get into than one thinks....

Now 'settling with power' on the other hand..... :hmm:

Good stuff dammyneckhurts. Where are you sitting waiting?

Nigel Osborn

10th Jun 2004, 01:40

A point that generally doesn't get a mention is that the tail rotor is not stalled or in VRS.
We used to teach pedalling the tail rotor or put a boot in if you have the space to get some sideways movement as it doesn't matter where you get airspeed from, forwards or sideways as long as you get some.

DualDriver

10th Jun 2004, 07:12

dammyneckhurts...

YOU SHOULD BE AN INSTRUCTOR!! Experience like that is great. Thanks for the input. I agree, GREAT READ!!!

DD:ok: :ok: :ok:

2Sticks

10th Jun 2004, 13:06

As a relative newcomer, Dammyneckhurts post was fascinating to read - certainly something to take to post PPL(H) update discussions with my instructors. Thanks for the input.

2Sticks

Whirlybird

10th Jun 2004, 16:19

dammyneckhurts,

Great post.:ok: Really useful stuff. Hope you'll post a bit more often when you have bad weather.

Thomas coupling

10th Jun 2004, 17:01

Stick with the correct descriptions, otherwise people will walk away with the wrong idea on a very dangerous situation.
You can't demonstrate VR, because the pilot cannot get out of it!!!
What is demonstrated is Incipient VR where the symptoms begin to show (vibration, high ROD developing, stirring stick etc). It is only during this phase that the pilot still has control of the a/c.
If the ROD gets excessive (4000+) and the stirring of the stick does absolutely nothing, the a/c starts to nose down, up, yaws, then you have forever lost control of the a/c:ooh:

Starting height is your saviour. I wouldn't demonstrate IVR below 4000' personally because it gives a safety buffer for most things to go wrong.....
Dodgy doos...............................

dammyneckhurts

10th Jun 2004, 18:04

Gentlemen,

Nice to know my ramblings are appreciated!

As Thomas Coupling says some of these concepts are well above the student level when it comes to actually demonstrating it, but none the less it's a crucial skill for some segments of the industry. What the answer is I have no idea....it's unfortunate that we have such a huge void in our skill level when we come out of flight school, and what we need for the average customer.

I just would like to offer that there is a huge difference between an intentional Incipient (thanks for the correction Thomas) Vortex Ring Demonstration at altitude, and the reality of what happens in some work environments where a driver is working in conditions conducive to V.R. on a daily basis.

I have yet to experience unintentional IVR at any altitude above 400', much less 2000' or 4000 '.

It seems that most students come away from flight school with the basic understanding of VR and how to get out of it so I guess its safe to say that instructors have done their job....?

If pilots have the tools to avoid and recognize low level VR, they wont need the recovery skills they learned at 4000' as often.

If I were to demonstrate it to someone I would most certainly do it at altitude, but once the recovery technique is mastered, it is equally important to have a clear understanding of avoidance and recognition in the working world.

Steve76....I am in Saskatchewan doing a lot of couch surfing and spending way too much time on this darn puter!

Signed;

Prayin for sun

Thomas coupling

11th Jun 2004, 12:28

You are demonstrating IVR without a shadow of doubt.

VR: massive ROD (>4000'/min)
No response from the very sloppy controls (cyclic and coll)
Seriuos pitching and rolling (no input from pilot)

No-one demonstrates VR!!! They can't. They'll die:E

MOSTAFA

11th Jun 2004, 15:59

Not wanting to sound boring but TC is absolutely spot on I cant believe the casulal tone of this thread. I last commented on "Helicopter crash at Newcastle" thread I believe my comments are equally valid here. Repeat,

Never flown a R22, I think you are a pretty plucky bunch of aviators to get into one. Please, I am not trying to be rude, just an honest appraisal from someone who had Her Majesty's budget to learn with many years ago. I know a lot of people didn't have that luxury.

Of course everyone must wait until they publish the accident report, but there is nothing wrong with using the replies on this thread to learn from, thats usually how we imbibe, from others. Sadly, a lot of the times that we learn are from mistakes, but I do not think that they should ever get personal. There but for the grace and all that! Should we not let the licencing authorites worry about age and the likes.

The only thing that really concerns me from reading this thread is the almost casual use of one of the most alarming exercises in helicopter aerodynamics, Vortex Ring. Thank God, nowadays its only talked about during instruction, isnt it? I should also make the point reading this thread that the R22 is generally talked about in the training or private hire role. I am certainly not trying to upset any commercial R22 operators out there.

I am certain that before any QHI sends a student off on their first solo, the student fully understands the symptoms, the requirements and most importantly the the recovery from a vortex ring state. Do people actually still go out and intentionally put helicopters into vortex ring? We stopped it in the military about 20 years ago and I have never done ot commercially. Why? because its dangerous, the stresses placed on the airframe are scary and telemetry shows that it is usually very expensive. If any of our readers believe they have experienced it below a 500' then they truly did not experience an established vortex ring state. Unless of course they crashed or were very fortunate indeed.

We all know there are several high risk areas, sadly most of us operate in those areas daily. There are also factors which aggravate those high risk areas and quick/emergency stops certainly fall into that catagory. But surely, they are purely advanced coordination exercises only!

Other than on continuation training (isn't that planned and all exercises at least thought about prior to entry). Other than an emergency situation thats going badly wrong or landing in mountainous areas with extremely bad up and downdraughts or operating in a high hover (R22) what possible reason is there for anybody to get themselves into a vortex ring state. Accidents do happen and sadly will continue to, food for thought and hopefully, discussion

Gonna get off my soapbox now

the coyote

12th Jun 2004, 06:58

TC

Wouldn't you be talking about total rotor stall rather than VRS then?

I am no aerodynamics guru but doesn't VRS by definition still require the rotor to be creating downwash which is then 'recirculated'?

If it was VRS couldn't you recover by entering autorotation and removing all downwash? Or is the ROD so huge that the angle of attack on the blades with no pitch angle will still be beyond the stalling angle?

I have had ROD's well beyond 2000fpm on the R22, no amount of power will arrest it, and recovered by entering autorotation and then getting airspeed. Entering autorotation will stabilise the aircraft, give you back more cyclic control making it easier to recover airspeed. And this was in the training environment by the way.

MOSTAFA

In my opinion, no amount of discussion can replace the experience of it and recovery (with regard to training) whether its incipient VRS or not. Surely rotary flight training must involve aerodynamic recovery of the wing, just like stalls in fixed wings?

MOSTAFA

12th Jun 2004, 09:03

Hi coyote, crass statement I know but do it in a simulator, you might not be able to simulate it exactly but it will sure scare the sh*t out of you. Havn't seen them for ages but there used to be two really scary films of aircraft in a developed vortex ring state, some where so badly stressed in the recovery they went straight to the scrapheap, another just broke apart on the recovery and some very brave testers died trying to give us the information needed to operate safely.

Ascend Charlie

12th Jun 2004, 09:07

Of course you can get out of it.

Roll off the throttle, bottom the lever, enter auto and you drop out of it. The vortex is only there while the blades are trying to throw air downwards.

Heliport

12th Jun 2004, 11:03

Welcome to Rotorheads dammyneckhurts.
Great post. :ok:

Heliport

MOSTAFA

12th Jun 2004, 12:02

Quality answer AC nobody says you cant get out of it, only just how unpleasant and downright dangerous it can be! Interesting thought "roll off the throttle" please explain. Have you ever tried it in a commercial a/c with two or even three engines running. Gimee a clue just how much height you are gonna lose before you can recover the said engines to be able to do flyaway or are you advocating an engine/s off landing, possible but really,really scary in a ? ton machine Completely agree with the recovery action of lowering the lever and cyclic fwd, or anything else come to that matter that makes the ASI leap into life . Not trying to be picky just interested thats all. Its a shame I cant remember where I saw those films, Boscombe Down probably.

NickLappos

12th Jun 2004, 16:15

Maybe time to weigh in here:

True VRS is recoverable, and is not a mysterious death trap. The rotor needs more power at a certain combination of descent and forward speed, usually much more than the engines can deliver.

Lets attack each wive's tale:

1) In VRS, increasing the collective does not make the aircraft fall faster.

2) In a hover, you cannot get VRS until the aircraft has an appreciable rate of descent, usually beyond 700 feet per minute, likely about 1200 fpm or more.

3) In a hover, you can descend vertically without getting VRS.

4) In incipient VRS, you can raise the collective and fly up out of it, if you have enough excess power.

5) At high altitude, it is harder, and takes more descent rate, to get into VRS.

6) Most events people call VRS are actually where they haven't sufficient power to hover, so they settle down and hit the ground and call it "settling with power" and "VRS" and it gets captured that way in accident reports.

None of the above will be believed by most ppruners, because all we have been told about VRS comes from the same flawed source, which says the opposite. The popcorn "aerodynamic" guides we are given are written by old line, experienced IP's (none of them schooled in aero) and have less than 1 gram of true aerodynamic knowledge in them. They are useful, they capture good old school of hard knocks info, of course.

All this being said, there is little wrong with believing the worng stuff, since little of what we now do needs to have straight down descents, yet. However, when we actually get to use helicopters IFR as they can be used, one method will be the 90 degree glideslope to land in an obstructed, congested area. When we do that, the entire helicopter pilot population will scream "VRS" and we will have a hell of a battle explaining all the above, again, until somebody understands!

yet again, see www.s-92heliport.com/vrs.htm

Let's keep this thread alive until 2100!

Steve76

12th Jun 2004, 16:46

YES!
VRS rears its ugly head.....
Thanks Nick for the gospel again. I think reprogramming the helo community will take a little longer than 2100.

Dammyneckhurts: Just on my way out to Alberta to do exactly the same.... have you started your fire dance yet? whaddabout chain smoking with a casual attitude to litter?? Cigars burn longer eh?

HYLTONSPENCER

12th Jun 2004, 16:46

Try a higher rate of descent if you are at sea level. Keep plenty of height as insurance. Start at 1500 Ft. Speaking with a expierenced instructor would be a good start.

dammyneckhurts

12th Jun 2004, 18:37

As a newbie here I had no idea how many threads there were on VR wow...or should I say I(incipient)VR....

Lots of talk/arguments about defining exactly what it is, and how dangerous it is etc. Not a lot of tales/advise from people that actually experience lowlevel IVR on a somewhat regular basis unfortunately.

Nick: You mention that you have to have an appreciable rate of descent for VR, I would like to add that you get the same effect hovering on the side of a mountain with big up flowing air. This is a borrowed quote from an esteemed college on another site...."5 mph wind moving vertically up the slope is equivalent to a 440 ft per min rate of decent" I haven't done the math myself but as many a mountain driver will attest, there are many days you just cant go into pads perched in a big upflow area for this reason...and 5 mph is nothing! Nick you have a massive amount of info on the subject, thanks for sharing it !

Steve:.....My box of Cubans is still unopened....you have to actually get up in the air to put them to use! Just when I thought it couldn't get any wetter the sky opened up again yet again.
:{

DMNH

MOSTAFA

12th Jun 2004, 19:37

Nick, sorry fella but let me WEIGH back in.

1) In VRS, increasing the collective does not make the aircraft fall faster.

In an established VRS raising the lever further will make the a/c accelerate downwards, fall faster, as you are stalling more of the rotor from the root thus exacerbating the ROD.

2) In a hover, you cannot get VRS until the aircraft has an appreciable rate of descent, usually beyond 700 feet per minute, likely about 1200 fpm or more.

If you have a rate of decent beyond 700' per minute you aint in the hover but descending vertically.

3) In a hover, you can descend vertically without getting VRS.

Of course you can.

4) In incipient VRS, you can raise the collective and fly up out of it, if you have enough excess power.

Of course you can very, very carefully but I dont recommend you try it to often.

5) At high altitude, it is harder, and takes more descent rate, to get into VRS.

Explain please, at higher density altitudes you will be using higher power settings to hover. Less power margin to try to recover.

6) Most events people call VRS are actually where they haven't sufficient power to hover, so they settle down and hit the ground and call it "settling with power" and "VRS" and it gets captured that way in accident reports.

I think you are confusing overpitching with? Surely everybody looks at the graphs in the manuals for HOGE. 5%, 10% etc

By the way the bloke who taught me POF was an TP from ETPS with more than 1 masters in aerodynamics. It scared him!

Thomas coupling

12th Jun 2004, 20:00

Nick,
I know and respect your credentials, but at the end of the day, you are, just like the rest of us, human and not 'perfect'

I am afraid this thread could end up like the one we all had with you regarding "ground effect" and the endless arguments and counter arguments for and against. I didn't (like so many others) believe your explanation then and I most certainly don't believe your explanation of VRS now:uhoh:

I support Mostafa 100% here, and will defend my understanding of VRS to the hilt.

VRS is NOT RECOVERABLE, not now, not never. There is only ONE reason you can survive fully developed VRS and that is IF the helo falls out of the aerodynamic condition it finds itself in of its own accord (and provided it does that before you run out of bernoulli's:\

VRS is where most if not all of the blade is 'STALLED'. The pilot may have initiated an auto but the stalled areas along the blade (innermost section near the root and the tip) now grow to envelope most of the surface of the blade. When that happens, the helo will accelerate to excessive ROD's as the rotor tumbles inside its own dirty air.

I believe VRS requires some fwd airflow over the blades to initiate it (induced airflow etc), therefore your statement about VRS in hovers being impossible might carry some truth...I'd need to check my notes.

Let the show begin.:E

NickLappos

12th Jun 2004, 21:56

Thank heaven it is Saturday and we can all sit around the cyber-stove and just jaw a bit.....

In real flight, VRS is not a steady state condition. It is actually very hard to find, and not at all steady. It has absolutely nothing to do with any stall event of any kind. The angle of attack of the airfol is very low, thank you, and the flow through the rotor is quite unstalled.

What VRS is is a recirculated flow where the rotor works quite nicely to push a package of air downward, but the outside air simply walks that air package around the outside of the tips so it can get sucked back into the rotor. In effect, the rotor is simply recirculating the same air again and again. If we stated it simply, the rotor is now behaving very selfishly, and not taking much new air, and not letting all the air it pushes mix with the free stream. This means it is not transferring much momentum with the outside and thus the thrust drops somewhat. Since the state is so very unsteady, the indications are torque or MP jumps, eratic behavior, pitching and rolling, and the like. On incipient vrs, just lower the nose, increase power to max (do not droop the rotor) and it will walk out.

I will dig up a great graphic simulation of vrs on a rotor and post it. You can see the downwash get slowed as it enters the free stream below the rotor, and as the rod is increased, you can see the vortex reenter the rotor and form a beautiful ring.

TeeS

12th Jun 2004, 22:49

Admittedly I have only basic P of F knowledge and a bit of ‘A’ level physics, but surely, if the rotor aerofoil is stalled then it is producing next to no lift. Therefore it is not throwing any airflow downwards to be re-circulated, if there is no recirculation you are not in vortex ring!

My image of the vortex ring state consists of the rotor disc producing lots of downwards moving air, now introduce a rate of descent (or upward moving air) and the downwards moving air can’t dissipate, instead it moves outwards and upwards where it finds a nice low pressure area immediately above the rotor disc, gets sucked in and round it goes again, and again, etc. Now we have the rotor disc ‘flying’ quite happily in its own ring doughnut of air – unfortunately, the doughnut of air will go wherever the rotor disc goes i.e. downwards!!

Downward airflow through the disc will surely reduce the angle of attack on the blades. Sorry, I can't see how you can disagree with Nick's explanation.

TeeS

NickLappos

13th Jun 2004, 01:56

There is some great info out there, but the data is for engineers, so it might be way too pointy-headed for us drivers. I can try to translate it into english, however. The points the below is intended to show is
1) That true VRS does not occur at 300 fpm, it takes a lot more descent rate.2) that VRS is not a destruction of rotor lift as we know it (like "crossing the streams" in Ghostbusters!) but rather a place where the thrust is unsteady and the power required goes up a bunch.

The paper that says it all is here, done by Dr. Gordon Leishman, who is a real pro at the U of Maryland:

http://www.enae.umd.edu/AGRC/Aero/AHS2002_Leishman.pdf

Vortex ring is a state where the recirculation makes the rotor less efficient, so it chews up more power making the lift.

Rate of descent for vrs:
Here is a rotor in a steady hover. See the ring of tighter vortexes forming at the bottom, which seems to form "just because", and is probably the reason why we feel the puffs of downwash from a rotor, instead of a steady stream as it flys by:
http://s-92heliport.com/hover.jpg
Here it is as the rotor is descending at 0.15 times the downwash speed (225 fpm for a R-22). Note the downwash has been stopped and a ring forms below the rotor, at the place where the downwash (which is slowing down as it disburses) matches the upward wind:
http://s-92heliport.com/descent .15.jpg
Here is a sequence as we descend ever faster, all the way to incipient vrs:
at 675 fpm, the rings are less steady, and closer to the rotor:
http://s-92heliport.com/descent .45.jpg
at 900 fpm, the rings are more compact but still push away from the rotor:
http://s-92heliport.com/descent .6.jpg
at 1050 fpm, 0.7 times the downwash speed, the rings start to get closer to the rotor, and sometimes puff through it. This 3 shot sequence takes place in about a second (six rotor revolutions!) so these rings are fast. The pilot would feel a 1 per second torque variability, cyclic and collective control problems, and lots of yaw, due to the torque spikes. This is incipient vrs, for the first time in this sequence:
http://s-92heliport.com/descent .7.jpg

For such descents, the power rise is shown below, with the curve illustrating the amount of increase in induced power needed to trim at flight thrust (full lift). Induced power might be 30% of the total power, so you can estimate the total power increase as maybe 1/3 of the amount shown. Note that for a descent at half the downwash speed (that is the -0.5 on the horizontal scale), the induced power would increase by about 40%, so the total power might fluctuate at about 15% above normal. The curve is experimental data, cited in Dr. Leishman's paper.

http://www.s-92heliport.com/leishman.jpg

Here is an animated file of a rotor seen from the side at Dr. Leishman's web site. The animation takes a rotor from steady hover, then has it go faster and faster downward until it gets into VRS and then past VRS into autorotation (windmill brake state). Each blade's tip vortex is plotted as a different color green, red, orange, yellow. As the descent rate goes from steady hover to VRS and then into autorotation, see how the ring forms below the rotor then is pushed into the rotor as the descent rate increases, then finally the ring drifts away upward in smooth autorotational flow:

http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif

If the animation does not work, here is the link so you can just go directly to the site:
http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif

Also, the thrust of the rotor does not change and make the rotor stop producing lift. here is a plot of the thrust for the experimental rotors as they are forced to go through descent, VRs and finally autorotation:

http://www.s-92heliport.com/thrust.jpg

One place where I must admit I was wrong, even leishman says that some inboard sections of the rotor experience stall in vrs. As I have always said, we all learn on pprune!

The detailed discussion from Dr. Leishman, and dozens of animated gif files are shown at his fantastic web site:
http://www.enae.umd.edu/AGRC/Aero/vring.html#Case%201st

I should point out for pprune's benifit, Dr. Leishman is a transplanted Brit!

MOSTAFA

13th Jun 2004, 10:30

TeeS, you mention you A level physics so just spoke to my daughter, doesn't usually help!!! albeit, she gave me a wonderful sum once about slope landings and how helicopters could land on excessive slopes by compounding the slope. Anyway, you dont have an equation without various weights, gravity, various accelerations, chords, washout/shape/taper types of head and a few other things I can't even spell she's a Dr MPhys (Hons) MSC AMinstP DIP IPEM, so far but not an aerodynamicist. Not sure I am disagreeing with Nick or he with me. Other than one of terminology.

Nick, nice animation and very graphical, a good learning lesson but cant see how it proves your point I believe it generalises, pity the max rates of descent are only half what you quoted hope you sort of agree with my my last sentance to TeeS. Enjoy

vorticey

13th Jun 2004, 15:01

good working diagram nick!
thomas, what would the rate of decent be to stall the blade (not just the root) at 100% rpm? pretty scary i would imagin, can your downwash be that fast? otherwise you would be in auto, wouldnt you?
when learning to fly, VRS had me scared so my instructor took me out to show me it. we had the needle peged at 2000 fpm and came out quite easily. we used a fair bit of collective too, dont think we got to 7000 fpm though.

NickLappos

13th Jun 2004, 15:26

MOSTAFA,
Thanks for the questions, I redid the post above to more clearly show the progression to more descent. I hope the new organization is helpful. If the group can follow all this, I will transplant it to my web site to add to the more general stuff there.

Your daughter sounds wonderful, when will she start making enough money so you can retire, buy that helicopter and just fly around? Unfortunately, the unsteady nature of the helo rotor wake make it a candidate for simulation, and not equations, as Dr. leishman has done. His work does include a family of helicopters all with different chords, radius, etc. They have different properties, but follow the general behavior I have extracted here. Pass the web link for Leishman's site to her, she will blast right through it, I am sure.

As a note, in flight test I have descended vertically through vrs to vertical autorotation several hundred times, so I can confirm that this sequence IS what rotors do.

Winnie

14th Jun 2004, 13:38

For those who'd like good way of getting into it, check out Shawn Coyle's Cyclic and collective. I tried that one, and IT WORKS!

Gregg

14th Jun 2004, 14:07

I hate to sound like another "Nick Lappos fan", since I work for another company and spend most of my time trying to argue against him behind the scenes....

But- why must everyone argue with his discussion on VRS? What he is putting out is as close to gospel as it gets.

I can verify that what he is trying to teach us is the same information that is taught at the US Naval test pilot school (where there are just a few people knowledgeable in helicopter aerodynamics). His information is backed up by flight test data by BHTI and Boeing as well as Sikorsky. Recent flight testing on VRS has focused on the V-22 and its somewhat unique issues because of the potential effects of VRS on only one of the two prop rotors, but the V-22 data agrees with what Nick is teaching.

The arguments are entertaining, but don't confuse "my instructor says" with actual flight test data backed by aerodynamics lab work (wind tunnels, CFD, and other simulation.)

On the other hand, the arguing does help, because it prompts Nick to give his lessons again, which is a valuable refresher training...

MOSTAFA

14th Jun 2004, 14:21

Gregg, I believe its just one of those topics where terminology Americanism/Britism dont compute. If you read the posts I have put on this thread I dont think we disagree on anything except the gap between you and the ground. I cannot spk for piston drivers, never been one and I also think the FTGs or the FCUs, depending on types or both make a difference. Continue enjoying but be sure nobody is trying to pull down Nick, quite the opposite his threads are very informative, always food for thought. His last animation even got me talking to ETPS again.

Shawn Coyle

14th Jun 2004, 14:45

Somehow I think I've written the following before...
Demonstrating the incipient stages of Vortex Ring State is the RW equivalent to stalling a FW airplane. You learn what the symptoms are in training, and then learn to avoid it for the rest of your flying career.
I have only ever been able to get the symptoms when entering at sufficient height AGL (more than 2,000' AGL, just like stalling), and from a downwind condition. Start at 60 knots, reduce the power slightly below the power to maintain level flight at 60 knots and decelerate maintaining altitude. At about 20 knots, the first symptom will appear (low frequency airframe vibrations). Next the aircraft may (and note the may) exhibit uncommanded pitch, roll and yaw (i.e. don't move the stick or pedals and the aircraft wanders around). Not always repeatable. Rate of descent is 500- 1,000 fpm. Adding collective may or may not increase the rate of descent (not always repeatable).
Recovery is by smoothly, but positively lowering the nose about 20 degrees below the horizon and flying out of it. Safe to do as described.
Learn the symptoms and recover at the first sign of the symptoms.
Any time the induced velocity equals the airflow coming into the the rotor disk, something has to give. The induced velocity (the downwash velocity if you will) is not uniform across the disk when hovering, so the effect will probably start at the hub and move outboard as power is increased.
Something that should be done on transition to a new type, and at a later stage of PPL training as well, just so you remember it. Doing it too early risks losing it in all the other stuff you have to remember.

MOSTAFA

14th Jun 2004, 15:32

B*gger me Shaun!!! thats another British'ism, nobody is disputing what you are saying about the onset of any type if incipient stall or your words about recovery. I for one have had enough, enjoy.

[email protected]

14th Jun 2004, 21:21

Anyone ex-brit mil should have been taught the same thing about VRS and it goes as follows:

1. Heli in the free air hover - root of blades at higher AoA than tips due to washout and rotational speed - total thrust of disc equals weight so hover maintained.

2. Rate of descent eg 500' - 1000' fpm allowed to develop (for whatever reason) - tips start to experience recirculation (loss of thrust) root of blade reaches Cl max and starts to lose thrust (stalled aerofoils still produce thrust) - net thrust of disc reduces and RoD increases. At this stage power available maybe sufficient to overcome RoD - if not then settling with power is occurring and step 3 is next if the ground doesn't get in the way.

3. As RoD is allowed to increase, more of the tip experiences recirculation and more of the root stalls -compounded by raising the lever which spreads the stall outboard from the root - further loss of thrust and acceleration downwards.

4. Eventually, only the central portion of the blade that is neither stalled or in recirculation is producing any useful thrust - nothing like enough to match the weight so the ac continues to accelerate downwards. The air trying to escape round or through the rotor gives random pitch, roll and or yaw and makes recovery from just forward cyclic very difficult.

Recovery - lower the lever, to auto if possible and try to achieve airflow across the disc with forward cyclic.

MOSTAFA - Nick is right, in fully developed VRS raising the lever will not make the ac fall faster as it is already going down as quickly as it can - however, in the incipient stages, raising the lever can exacerbate the problem and take you into fully developed VRS.

Nick's stuff is excellent, but for mere mortals a more than adequate explanation is in AP3456

MOSTAFA

14th Jun 2004, 21:31

Read it from the start, read my words J

Rich Lee

14th Jun 2004, 22:43

I attended an HAI Instructor Pilot Refresher clinic in the year 2000 where I sat through a very interesting presentation by a Bell test pilot. He told those in attendance that it is impossible to get Vortex Ring State (Settling With Power) in a Bell 206. He claimed to have the test data to prove it. I can't remember the fellows name but it was a very interesting discussion. I don't buy his theory, but many walked away from the presentation believing it was true.

NickLappos

15th Jun 2004, 00:30

I think the way it could be said, Rich, is that the VRS you experience in a properly powered helo is not something that will cause it to descend unstoppably or lose control. The crane driving test pilots at Sikorsky used to describe vertical descents until deep into vrs, then lifting collective and climbing straight up out of it. Done by professionals on a closed course, do not try this at home!

I got a great case of vrs doing short field landing OEI in an S-76 once. Engine cut at 140 feet, passed thru 75 feet at 50 feet per second rate of descent. Hit at 13 fps, bounced. Makes you shake the hand of the airframe designer, vigorously!

[email protected]

15th Jun 2004, 06:40

MOSTAFA - I have re-read the thread and your posts - settling with power is not the same as overpitching - overpitching is when the Nr decays due to insufficient engine power to keep driving the rotors - settling with power can occur without Nr decay, you simply don't have enough power to arrest your RoD. Overpitching may rapidly follow settling with power or be combined with it as the pilot tries to stop the ground from smiting him but the 2 events can happen quite separately.
Get Alan Wiles on the subject and he will tell you about VRS in a Scout and losing 6000 plus in the recovery in NI.
All of Nicks other points are valid, as ever, and we have been through the terminology issue several times on these threads.

The civvy world teaches incipient VRS recognition and recovery, the military doesn't, although the Sea King force used to do it at 10,000 feet, it is now covered in the simulator (much safer).
I have been shown and carried out the demo in an R22 and a 206 and frankly it's a bit of a non-event. Taking it beyond the incipient stage is in my view practise bleeding and the risks outweigh the benefit - at the point at which pilots are blase about VRS, someone is going to get hurt.

The argument about updraughting air producing VRS is cobblers - in an updraught your pitch settings are much lower (sometimes in autorotation) so the recirculation at the tips is much less and the root AoA much lower. I have sat in more updraughting air than I can remember in several different aircraft types and the only time a descent occurs is when the wind gusts or drops and you are a bit slow to react. The only problems with approaches to mountain sites in these conditions are that even with the lever fully down, a big updraught will send you upwards and once you move out of the updraught your power required increases very quickly, especially if there is any turbulence caused by ledges/ridges which can produce a local downdraught (like rotor streaming). We teach SAR pilots to fly in the updraughting air in the mountains because it is so much safer/comfortable.

Thomas coupling

15th Jun 2004, 08:18

I think the only person missing in this discussion is: Prouty!!!

Great to get such a professional response.

However, in an attempt to clear the misunderstandings:

I think we all agree there is such a thing as incipient VRS and then there is VRS. [Let's forget about overpitching and settling with power for now!].

We all agree that symptoms for both, are subtly different.

[I have read Leischmanns report on VRS and several others, too.]

What I have yet to see in print, is that VRS is neither demonstrable , nor survivable (at low level).

If we all accept that VRS is a development of IVRS, where nearly all of the blade surface is stalled and that the controls are behaving erratically and randomly, how can one take control of the aircraft to be able to recover? The helo, in its 'uncontrollabe state' might decide to flip backwards or roll inverted??

IVRS is, I believe, misconstrued as the "full monty" that is to say, people think they are either being shown VRS or are experiencing VRS. Not true, they are experiencing IVRS.
No-one to my knowledge would demonstrate fully blown VRS with all the trimmings, surely :uhoh:

Crab: I am ex mil, and often demonstrated IVRS to ab initio's? Don't the mil do it any more?

MOSTAFA

15th Jun 2004, 10:02

Last one, for what has been a most enlightening discussion. Crab@SAAvn thanks for your input both TC and I have entered into this debate and again I think added nothing but good old fashioned honest experience. Its a shame really that we all write under usernames because it doesn't really allow the reader to guage any experience. Perhaps its time to come out, perhaps its time to go back to school and take english lessons because like you, I have read all the posts again and cannot see anywhere, where anybody disagree's other than pilots having IVRS demo'ed to them. I advocated the use of a simulator and that people should be extremely wary of allowing the aircraft into a VRS. Like, as you quote "Get AW on the subject" I have on numerous occasions having served with and for him for the best part of 25 years. It would be impossible not too having a good 3000hrs on the Scout myself. The best aircraft I have ever instructed on to demonstrate overpitching in manual throttle. Perhaps that is why I am so reticent about playing around with IVRS, anyway enough of my ranting. I shall continue to enjoy reading in between lifts

212man

15th Jun 2004, 11:08

At the risk of being shot down (or even entering irecoverable VRS!) could someone explain what difference being downwind will have when conducting this demonstration (IVRS) at a suitable altitude?

I fully understand the significance of being downwind close to the ground and allowing IAS to decay due to external cues, but surely if at N thousand ft plus,, and using the IAS as a reference for the entry conditions there should be no relevance to the wind direction?

Not wishing to get into a 'downwind turn' debate!

[email protected]

15th Jun 2004, 11:35

MOSTAFA - if you worked at MW between 94 and 01 you probably know who I am - I have retained the username after going back to crabland so as not to confuse myself and others.
We did have a thread a while back where posters detailed their experience and aircraft types - there weren't many who didn't add to the list.

212 man - I think it's easier to make people fly at low speed downwind as they will have ground references telling them they are still going forwards - if you tried to get them to do it into wind on a strong wind day they would have to fly backwards relative to the ground which some might find more uncomfortable. Other than that, low IAS is low IAS whichever way you are pointing.

TC - it was done by the RN and the RAF on the Sea King but has since been canned in favour of the simulator version which is not so scary. As far as basic flying on Gazelle, I dont believe it was ever taught as part of a syllabus - certainly not by the RAF after 83 when I went through training.
You're certainly right about not surviving VRS at low level but this thread started with IVRS demos at height. Even with lots of fresh air between me and Terra Firma I wouldn't demo full VRS - eeeessss ffargin' crazeeee!!!

MOSTAFA

15th Jun 2004, 12:24

Crab@SAAvn, always read your posts, always informative but this post didnt start with IVRS unless your computer says something different to mine. It has, as always drifted in and out of the subject, perhaps I'm just sh*te at english and cant read. Hope you are enjoying yourself away from MW, miss it?

Shawn Coyle

15th Jun 2004, 13:50

Why does the demo of IVRS work downwind and not into wind? Wish I knew.
But it does work downwind. Never had it work into wind.
By the way, I had the experience of dissecting a UK AAC Lynx accident several years ago where VRS was pretty clearly the culprit. The fact that it hadn't been considered by the Board of Inquiry at all went a huge way to convincing the Treasury Solicitor that the rather junior pilot was not completely to blame for the accident (his widow was being denied a pension as some military folks were saying he was to blame for allowing the helicopter to crash).
All the symptoms were there - turn to downwind with a descent from about 200' AGL, lots of pitching and rolling and heavy vibrations. Controls appeared to not be responding. No attempt to get out of it by flying away. Helicopter hit the ground with almost no forward motion - total length of wreckage was 90'.
VRS by the way is recoverable if you have enough altitude, as is evidenced by several people who have had it and walked back into the office without assistance of a nylon letdown.

My experience is that the symptoms of VRS are not recognized by most pilots - they can tell you the theory, but not what they are going to see in the cockpit.

[email protected]

16th Jun 2004, 05:04

MOSTAFA - the first post, from dual driver is about demoing vortex ring and is clearly talking about incipient not fully developed - he just doesn't call it incipient.

I do miss MW, I had a lot of fun there and worked with some top blokes - I don't miss the hangovers from the Officers' to Sgts' mess visits!!! I am regularly at Wattisham checking the SAR flt there so I still get to see Lynx and Gazelle regularly - I got an hour in a Gaz recently and it was fabulous - very different from the Sea King!

MOSTAFA

16th Jun 2004, 14:02

Nah..............but its good to know you are ok.

[email protected]

16th Jun 2004, 20:00

MOSTAFA - pm me to let me know who you are

TeeS

16th Jun 2004, 20:16

Crab, you said "The argument about updraughting air producing VRS is cobblers"

Surely, the pitch setting is usually low when you enter a rate of descent, from the hover, prior to entering Vortex Ring! So, when you are sitting in a hover in a column of ascending air and then pull an armfull of collective, why can't you end up in Vortex Ring if the rate of ascent is high enough?

TeeS

dammyneckhurts

16th Jun 2004, 23:46

Tees- Yes its possible....well Incipient VR anyway. I have been there a few times. The air has to be directed up at you from pretty much straight from below. Hovering beside a cliff, machine is twitching and buffeting, trying to lower a load onto a ledge with a longline and anytime I pulled a bit of power to see how it was feeling, the bottom fell out. Nasty. Seeing that the bottom only falls out when you pull power has me leaning in the direction of IVR as the culprit. Other ideas anyone?

Of course generally speaking 99.9% of the time, up flowing air in the mountains is easier than work in than the down flow side. But there are rare exceptions...

ground effect

17th Jun 2004, 00:09

check your Private Messages Mr Neckhurts:ok:

[email protected]

17th Jun 2004, 06:07

Tees - the classic VRS setup starts with you in a OGE hover which implies rather high pitch settings - you don't have to lower the lever very much at all to induce a RoD so your pitch setting are still high. Then when you raise the lever to arrest the RoD your pitch settings are higher than they were in the hover - now you have a powerful tip vortex and very high AoA at the root, a RoD, low IAS and power applied - all the ingredients required!

In an updraught you already have lower pitch settings to maintain the hover and the only danger is the variable nature of the updraught - if you don't react quickly enough to it reducing then you may well find your self with a RoD you didn't want and have to apply a lot of power to arrest it - in this case you are in a similar situation to the normal free air hover and VRS could ensue.

BlenderPilot

7th May 2006, 05:47

Just recently I read an article in a well known helicopter magazine by a Mr. Mott Stanchfield in which he regarded that one of the conditions that would most likely promote VRS would be "high altitudes near or above the helicopters HOGE ceiling"

And then I thought if learned anything on this forum (from Nick Lappos) is that VRS is actually more difficult to enter at higher altitudes because to produce the same amount of lift you need to move air downwards at a higher velocity the higher you are, thus your downwash it going faster and it's more difficult to equal in a descent to enter VRS at higher altitudes.

I wonder if the article is keeping alive some of those old myths that say that the higher you are the easier it is to enter VRS, confusing Settling With Power, with Settling For Lack of Power.

I have lately seen a couple of articles on this subject, at Rotor and Wing in which also the author mislead the readers into saying that it's actually easier to enter VRS when hot and heavy that when your flight condition is the opposite, and this is just not true, its the other way around acording to this . . . .

Same thread earlier . . .

Thoughts on the matter, I think we should clear this up once and for all.

Nick Lappos

VRS is only encountered when the downward speed of the helicopter matches the downwash velocity from the rotor. In the cases mentioned by vfrpilotpb it is likely that the VRS was passed through, and then a zero knot autorotation was flown, thus the 3500 ft/min descent rate.

The flow around the rotor is established by the balance between the rotor's push on the air, and the upwind matching that push. Tales of VRS sort of locking the rotor up aerodynamically, and causing tremendous fall rates are simply not true. Those descents are experienced, we can be shown them, but they are not VRS. The reason for VRS is that the rotor downwash is pushed upwards by the free stream and then recirculated back down through the rotor. If the ROD is very much higher than the downwash velocity, the free air just passes through the rotor, and you have to raise the collective to keep from overspeeding the rotor.

For Gaseous, the autorotation first could be a difficult way to experience VRS. If you are into auto, the rotor must be transitioned to powered state, and you must try to trim the descent rate at somewhere between the range of 50% to 150% of the downwash speed. The rate of descent varies a lot depending on how heavy the disk loading is. For a Robbie, the VRS range is 700 fpm to 2200 fpm, for a Black Hawk, it is 1400 fpm to 4000 fpm.

One of the difficulties comparing what your instructor shows you with the actual VRS is that there is no telling what your instructor knows about actual VRS, and there is not standardization of techniques for the demo. The texts are poor, and much pilot lore surrounds the maneuver. I am sure many well intentioned instructors show a descent, some vibration and then an awesome vertical autorotation, and call the whole thing VRS. Why not, mine did in flight school back in 1968.

For VRS to be established in a rotor, the rotor must be lowered to about half the downwash speed under powered conditions. By 75% of the downwash speed (R-22 = 1100 fpm, H-60 = 2000 fpm) the VRS will show its head, the thrust will oscillate (you will feel low frequency vibration like turbulence, with maybe 3/10 of a G of magnitude, really big) and the aircraft will pitch and roll somewhat, the cyclic will be sloppy, and raising the collective will not necessarily produce a reduction in ROD. If the descent is increased to about 150% of the downwash speed, VRS is gone, clean air passes through the rotor, and you are in a vertical descent.

Any nose down (or even lateral tilt to slide out sideways) will help break up the VRS, and a climb will probably start (or at least the rate of descent will reduce somewhat. If you are falling at 3000 or 6000 feet per minute, you are not in VRS, you have slid through it and are now into a vertical descent or autorotation.

KikoLobo

7th May 2006, 07:03

the conditions to encoutner SWP... or Vortex Ring state?

1.- NEar zero airspeed
2.- Descending more than 300FPM
3.- With Power

If any of this three conditions is not met, you can't get into VRSTATE?

Am i wrong?

Revolutionary

7th May 2006, 07:17

Hasn't this issue been settled a long time ago?

Pun intended.

MightyGem

7th May 2006, 07:29

KikoLobo, correct apart from the ROD. Read the quoted text in the post above yours.

[email protected]

7th May 2006, 07:45

Kikolobo - I think one problem is that the RoD figures used to define the conditions required for VRS haven't changed in 40 years (maybe more) from when helicopters had low disc loadings and therefore low downwash speeds. Add to that a safety margin and, as Nick says for the R22, the bracket starts at 700fpm so if you always react at 4-500 fpm you will pretty much be safe.

Broadcast Control

7th May 2006, 15:33

When entering VRS I found that the torque rose without any control input. I have not seen this mentioned as an indication of VRS and wonder if anybody else have experienced the same. My guess is that the increased drag on the blades shows up as a increase in torque for a given collective setting.

BlenderPilot

7th May 2006, 19:03

KikoL,

That set of conditions is true, but for most of today's helicopters it's way too conservative I would think.

My point is that for example in a Bell 407 that has a very high disk loading to enter VRS you would have to descend with at least 1,000 plus feet per minute to even get near VRS, plus you would need to have at least some forward airspeed, very little but some forward airspeed.

Then another thing that is a myth about VRS and it keeps on being told is that the the higher altitude you are or the heavier you are the easier it is to enter VRS, and this it simply not true, it's the opposite way around. (athough it would be a lot easier to Settle for Lack of Power in these conditions)

To enter true VRS you must get close to your downwash velocity, and the higher and heavier you are, the faster your downwash speed will be, the faster you will have to descend to catch up with it and actually get VRS.

A lot of people at all levels seem to be confused by this, and always blame VRS for not being able to stop a descending aircraft because of high DA, weight, etc.

Remember that you can have power to hover OGE at 100% power, but if you are in a descent you will need more than a 100% to kill the downwards inertia of the machine, if you don't take that into account it could easily mean bent skids.

Nick Lappos created a document in which he clearly showed all of this, and where he gave the VRS numbers for a Bell 206, I remember that he demostrated that the ONSET of VRS in a heavy 206 started around 900 FT/MIN descent, and the center of the VRS was at somewhere 1,500 FT/MIN descent.

I sincerely thank Nick Lappos for having previously explained this to all of us.

Skycop

7th May 2006, 20:39

Glad to see this old thread is still generating interest - my original post was nearly 7 years ago! :ok:

paco

8th May 2006, 01:20

Belnderpilot

"plus you would need to have at least some forward airspeed, very little but some forward airspeed."

Surely (sorry for calling you shirley) the point of the low speed mentioned in the conditions required for VRS is that you are essentially in the same column of air. If you had some forward speed you would be away from the column and the vortices would not be hanging around the blade tips.

The problem is that the old thinking is still being used for exam questions and there's no real scope for teaching the correct stuff without prejudicing a student's results.

Phil

Lama Bear

16th Dec 2007, 14:25

There seems to be a lot of speculation in this thread by pilots that have never done any production long line work.
In my experience it is possible to enter "settling" with the three stated FAA parameters being met. Altitude and gross weight have had no effect in my experience. In fact most of my occurrences happened with an empty hook and I was trying to salvage a poor approach, a little fast, a little down wind and a little step. The termination has put skid marks in my shorts in a Lama, UH-1H, Jet Ranger, Hughes 500 and Allouette.
Nick refers to it as "over pitching" but I am unfamiliar with that term. I will tell you that the only solution is to increase airspeed with a quick cyclic move, usually to the 2 o'clock position for me. I am sure that to enter true, steady state Vortex Ring, Nick is correct. However when you are close to the ground with your line or the ship, just the beginnings are enough to ruin your day.
Go ask any old seismic pilot.

offshoreigor

20th Dec 2007, 12:36

:cool:
Hi ALL,
Been a while since I posted but here goes!
Nick, I read all your wisdom posted, but I feel a little reluctant to tell a newbie that "if your flying this type or that type" you wont have a problem with VRS.
My simple philosophy has always been and always will be, watch out, VRS WILL BITE YOU IN THE A$$ IF YOU LET IT.
But thats just me.
Cheers :cool:OffshoreIgor:cool:

NickLappos

20th Dec 2007, 13:21

The problem with believing and teaching the myth is that you teach the student to cure a performance problem (too little HOGE power margin) by not descending. Sort of like telling a guy who is playing with dynamite to use earplugs.

The vast majority of mythical VRS stories are experienced by folks who hovered OGE with too little power margin, settled down and scared the crap out of themselves. It is impossible to get VRS in a helo with high power margins, unless you go down so fast vertically that you deserve to die.

Teach students to have 10% power margin before they hover OGE. Teach students that VRS will happen if they descend at more than 700 to 1000 feet per minute. Then you are teaching the truth, and in a way that will prevent accidents, not explain them.

Lama Bear

20th Dec 2007, 13:28

Nick said

"It is impossible to get VRS in a helo with high power margins, unless you go down so fast vertically that you deserve to die."

Before we go any farther here, just how much long line time do you have? Do you consider a Lama with 60 gallons of fuel and no load to have adequate power margins?

NickLappos

20th Dec 2007, 14:26

Lama bear,

What is the power to hover OGE and what is the maximum power available? What is the maximum vertical climb rate at that condition?

Lama Bear

20th Dec 2007, 14:52

At 8,000 feet MSL, 25 degrees Canadian and 60 gallons of fuel the Lama is capable of climing and decending vertically with 1,500 lb loads. With no load the verticle rate of climb is better than 2,500 feet per minute. Nobody that has ever flown a Lama has considered it under powered.

I have never gotten into "settling" with a load on. I have had loads that I could not hold and have had to let them land so I could regroup. Apparently I am more careful with a load on than with an empty hook.

NickLappos

20th Dec 2007, 15:09

What is the power to hover OGE and what is the max power and what is the gross weight (without the 1500 lb load that it can handle?)

What is the power margin with the 1500 lb load HOGE?

victor papa

20th Dec 2007, 15:20

Can I ask a related question pls? We have a huge amount of questions at work with conflicting answers between the web and the manufacturer. We just started operating an EC130. Can the fenestrom stall or get vortex ring/LTE? What would be the warning and rectification or prevention? Sorry if I diverted!!!!

Lama Bear

20th Dec 2007, 15:22

Nick

I'm at work and my Lama manual is at home. I will jpg the IGE and OGE charts to you tonight. But if the aircraft can handle a 1.500 pound load in those conditions it stands to reason that it has more than adequate power without the load. The seismic and core drills that I moved were broken down into 1,200-1,500 loads. The Lama moved them day in and day out.

But before we get too far into Lama peformance I have had the same phenomonen in the UH-1H with the 1,800 hp engine, the Hughes 500D, and the Jet Ranger series. It is not machine specific. Ask any logging pilot and they will tell you that they make their money at the edge of settling.

Lama Bear

20th Dec 2007, 15:36

Nick

Opps. I found a jpg of the Lama OGE chart. I am unable to attach it here. Apparently I don't have that authorization? I can send it to your web site.

Shawn Coyle

20th Dec 2007, 18:30

The symptoms of vortex ring state can be encountered any time the downwash velocity equals or is very close to the rate of descent.
As the downwash velocity will vary along the span of the rotor blade, the symptoms can be encountered in more conditions than just the absolute classical case.
When entering using the method I described (much) earlier, the downwash velocity is initially pretty low, because you're at a reduced power setting. As you start to descend (due to power used being less than power required to maintain a hover), you may be in a (dare I introduce a term here) partial vortex ring state - that's what makes the demonstration not totally repeatable. If, as you start to descend, you were able to match the average downwash velocity from the rotor, you'd have the classical 'total' vortex ring state.
I've never been lucky (?) enough to get that to happen in all the demonstrations I've done, but the symptoms that show up are sufficiently convincing that all the pilots I've done this with have said they've come away with a better understanding of the scenario.
To recap - anytime the downwash velocity equals the rate of descent you should encounter (some of) the symptoms of vortex ring state. Learn those and either avoid the situation if possible, or know when and how to get out it.

manfromuncle

20th Dec 2007, 20:40

Nick Lappos: As a CFI I am interested in how you personally would teach VRS/SWP in a single engine piston helicopter? How would you explain it to a newbie student, what points would you emphasise? How would you fly the demo?

I teach this 'maneuver' a lot and I would welcome your opinion

NickLappos

20th Dec 2007, 21:04

manfromuncle,

Good question, it is easy to comment on what we do, harder to say what we should do!

Let me start out by saying the lesson is OGE hover safety, and the lesson has two different, and equally important pieces, Performance and VRS. I would bet 95% of all hover descent accidents are the result of performance /overpitching and NOT VRS:

1) Hover PERFORMANCE
A) where the power needs of OGE vice IGE are discussed and demonstrated (use a light helo, and trim IGE, note power, then trim OGE and note power. Pick 3 IGE hover heights and then plot the power post-flight. Look up OGE performance in the RFM to orient the pilot to the weight effects. Then discuss altitude temperature and wind effects.

B) Then discuss marginal OGE hover, and the insidious danger of over-pitching, especially how the rpm decay leads to even lower power so that the fall is accelerated.

C) Then discuss how low rpm leads to Loss of Tail Rotor Control.

D) Then show how an OGE descent at moderate rate can be flown up if one has some good power reserves to just raise the collective. The point to be made is that one can descend at 300 or 500 fpm in any helicopter with sufficient power reserves, and simply raise collective and fly up out of the descent. This is known to all military pilots when they operate NOE, and is not a mystery. The student should be shown, however, that a descent needs somewhat MORE power than a steady HOGE, so that a 300 FPM descent will need about 5% more power than a steady hover. (This is the fact that fuels the "VRS" demos we all are shown, and the reason why descents lead to overpitching).

2) Vortex Ring State
A) At 3000 feet or higher, slow to a steady OGE hover. Begin a descent at moderate vertical speed and accelerate the descent until you are at 75% of the downwash speed. (I can show how to calculate this for any helo). At this ROD note the lurches as the thrust bucks by +10%, and the aircraft has torque surges of 10 to 20%. This is incipient VRS, and all that is necessary to show, fly out of it by lowering the nose to accelerate and break the reingestion that is feeding the VRS. The lesson is vastly different that the above OGE power lesson.