vortex ring alt + vsi

[levo]

Sorry to trouble you again Nick .

Is it posible you could elaborate more on BVI .

Once again Thanks.

Levo.

[Whirlygig]

as it sounds awfull

Not when SASLess did it in the Huey at Helitech last year!! You had to be there!!

Cheers

Whirls

[NickLappos]

levo,

Any wing produces a trailing vortex ("wake turbulance") due to the spillage that creates a neat little sideways tornado at the tips. The blade is of course a wing, and it produces a tip vortex. Because the whole flow field around the rotor is downward, this tip vortex descends as it leaves the blade tip.

The vortices can be seen when you hover over grass, they make the rapid beating of the grass that marks each blade passage.

In level flight, as the rotor turns, the next blade avoids the previous blades vortex because the downward drift of the vortex causes the miss. If the rotor is tilted back a bit, as in a quick stop, the "next" blade can hit the vortex and cause a bit of vibration and a slight "slap".

The most famous bit of Blade Vortex Interference (BVI) is the Huey "blade slap" in gentle descents at about 80 knots. Often the noise during translational roughness is BVI.

The blade stresses are not terrible during BVI, usually high speed flight and maneuvering are much harder on the blades.

Here is a blade tip vortex photo.




Here is a rotor blade shedding its vortex periodically:



Here is a NASA photo of one vortex path:

[levo]

Nick Lappos

Many thanks .

Levo.

[eagle 86]

Taught many years ago at a very respected School: conditions for entry into INCIPIENT VR - KIAS < 20, ROD > 200ft/min and a SIGNIFICANT increase in power. Watch VSI - I assure you you will see a rapid increase in ROD - hold these conditions and you WILL get into VR. What's happening? At the tip decreasing angle of attack (loss of lift) due re-ingestion of tip vortices, at the root increasing angle of attack to beyond stalling angle (more loss of lift), in the middle - an area of blade that is producing a diminishing amount of lift. Modern helos are less susceptable because of better blade design.
GAGS
E86

[[email protected]]

eagle - but I bet you didn't hold it - I bet you recovered as soon as the increase in RoD was apparent. That for TopendTorque is a demo of entry to and recovery from incipient VRS. It may well be what ToT is doing at 80' but it's not full VRS at all and frankly I wouldn't be doing an incipient demo at anything less than 2000' just in case.

[eagle 86]

Crab,
I think you and I probably trained at the same School - never said to hold it - but if you do then hang on it will be a ride and a half. Only last week I endorsed a chap on an A119 - among other tricks was IVR - I had 4,500 feet AGL!
GAGS,
E86

[topendtorque]

nigelh you are right, English is not my first tongue, low level rotary flight is, mainly of the ’47 dialect but more recently the last few k hours, I’ve included some robust superlatives to my vocabulary in the R22 inflection of the repartee’.

And no, I probably wouldn’t be able to afford the squirrel job; maybe I’ll try to put together a few gravel rashed pennies from the tougher chapters of my life as a down deposit. I am sure you recognise that I am not being derogatory with that, and I have already had a go at em, they don’t do much for me.

It may well be what ToT is doing at 80' but it's not full VRS at all

For Crab, no I don’t do fully blown VRS from eighty feet, heaven help us. We start the learning process from at least 2000’. Three reasons for that;

1) The instability of the T/R couple with the descent frightens the subject, they need frightening.

2) It takes all of that to get a macine (an R22 at least) to enter fully blown VRS maybe three times if you are lucky before finishing at 500’, and

3) There is plenty of time for the subject to observe the altimeter which will indicate just how long it was in VRS. This is usually no more than eighty feet, seen as a quick flick on the altimeter, after the event. The standard recovery usually does nothing more than flatten out the vertical descent long after VRS has departed.

It is important for the subject to understand that the most frightening thing about VRS is the incipient stage where the first symptom is that little sinking feeling in the pit of the stomach. This is not just another phenomenon that is occurring during the learning of this low level mystique and it’s OK if it happens because if it gets bad the C & T guy will take over!! Oh no, if you want to fail just think and act that.

This is why we play with the incipient stage, recognition and absolutely automatic recovery before it develops. Very important stuff for people who might have their attention diverted elsewhere, being shot at, cows running amok, targets turning downwind from gun or camera, fugitives trying to hide from the nite sun under brush when the wind is all over the joint, a camera man bellowing –this way that way, etc.

So as the subjects are being trained for an operational environment at below 100’ when their attention may be diverted elsewhere, then it follows to reason that I will first demonstrate at that height and less, the pitfalls, then over time try to trap them into ‘situations’ where they WILL get hurt if THEY don’t recover.

I wouldn't be doing an incipient demo at anything less than 2000' just in case.

As far as pussy footing around doing incipient VRS at above 2000’ Jesus Crab, get real, I had you pegged well above that sort of comment.
All it is is demonstration, awareness and covered practice, without getting tooo bogged down in theory.

It follows on from the downwind theory, you know, downwind, decreasing airspeed and descending are the same as drinking, driving and eventually death.
It’s the practical that counts, not some high falutin’ drummed up philosophy, which may well be dodgy.

I thoroughly applaud any institution that trains its subjects in the recognition and recovery from VRS, before it happens.
It will get you and it will hurt, which is why I use the statement –“notice that if you really stuff it up then you seldom fall more than eighty feet, so if you enter at fifty feet it is only ever the last thirty that will hurt”- end quote.

It usually gets the message across; my business is to protect people.

Can I go back to nigelh again, VRS and EOL’s and other known emergency exercises is all about the philosophies of circumstances that are a matter of chance not choice of our trade. I agree that Low G pushovers are absolutely stupid, and a manoeuvre of choice.

We once had a top dog check pilot from down south who (unbeknown to us) was demonstrating the low G pushover and then the recovery. This is until he came up against one youngster whom he frightened straight up and who said – “do that again an I’ll f’n choke yer”.

Exercise was finished, ops manager complained to, top dog smart a##e sent on next plane south and youngster well applauded.

We later heard that during the argument that youngster had said, ‘the reason I don’t wanner do it is because it’s in that book under yer f’n seat that it’s dangerous and I ain’t gunna do it, get it.’ Nuff said.

EOL’s are often a matter of chance not choice, just a couple of days ago a B206 went into the ocean off Queensland, the turbine spooled down I hear. How many threads here are started with the subject being an emergency landing, all sorts of aircraft?

And Nigel you are an old ’47 hand, if that is true, look me in the eye and tell me that you have never had an emergency auto. Of course you cannot. So we practice them, which is best done at the ab-initio stage and then usually it’s only needed for ‘differences of type’ later.

I have another saying- ‘practised dexterity in the areas of known emergencies will usually lead to automatic recovery in the areas of unknown emergencies.’ I have found that doctrine as cheap insurance. A salutary repast for all low level disciples.

I'm with you on this one, recovery from VRS is practised at the incipient stage, because the real McCoy is too unpredictable and therefore, too risky.

Hihover, you have thoroughly confused me, just what is dangerous about the manoeuvre, as long as it’s accomplished well away from the firmer terror?

Have ye never descended from a high rise in an express elevator? It's all the same, you descend in your own cocoon of air in ignorant bliss of the surroundings and at the bottom - you stop - the doors open - and - you step out into fresh air
No need for the canaries in your head to beat themselves into submission. You’ve obviously never practised the idea.

The blade stresses are not terrible during BVI, usually high speed flight and maneuvering are much harder on the blades.

I sincerely hope that this is the case as deliberate blade slap to project noise well ahead of you when moving mooies is a very much used tool. Of late we have heard it may contribute to pulling the skin off an R22 blade???

[hihover]

Topend -

You may wish to consider cutting the drama queen bullsh*t with your "subjects" being shot at and cows distracting their attention , just for a few minutes, and consider what you are saying.

Flying a helicopter vertically downwards from 2000' and trying to force it to enter a state which is known to have caused accidents is risky, surely, through your macho, no poofters hardshell, even you must grasp that. No matter how good you are at doing it.

I don't think anyone is saying that you cannot recover, certainly what I am saying is that, if you teach your "subjects" how avoid it in the first place then you ought to be able to dispense with this unnecessary training risk. You could be falling into the trap of thinking that because you can enter and recover in an R22, that the same will work for all other models, it may not. There are many variables affecting VRS and recovery from it.

[3top]

Hi all,

I don't know what all this fuss about emergency training is.

VRS is not a scary thing to practise if you do it right - at least 2000' AGL, better 4000.
It only really gets scary when you get it where you shouldn't - short final, coming down too fast, going too slow and no ALTITUDE left to recover.

I always had a hard time to get into a decent VRS with any Robinson, but once in it, it is easy to demonstrate, that increasing power doesn't help, just makes it worse. It is this demo that is worth gold, as it drives the fact home, that power is worthless at this stage.
No one mentioned to enter Autorotation to recover - watch your MRRpm though. A proper demo and pre/after discussion of VRS will make any student aware of the dangers of a incorrect decent procedure.
At 3000-4000 it is no problem, but you will normally not get it there.
I never teach revocery by Auto OR Forward flight, it sounds like you have a choice of needing altitude OR go around. I ALWAYS point out, that you WILL need altitude, no matter what. I need a minimum of 400 feet from full VRS to "fly-out" - it also shows, that you have about 0% chance to get away if you get on short final.....

Same goes for EOL (does anyone do REAL EOL? I assume engine idle would be the thing...). How are you ever get someone to do a good auto if you never get them on the ground? Are you planning on damage anyway?

The situation seems to me like 15-20 years ago in fixed wing: "We don't need spin-recovery, because we train our students to never get into one!"
Well, WHY then is spin-recovery back in the syllabus??

Face it, initial training will always carry more risk for Helo-body-damage than regular flying. Well, part of the game.

Nothing to do whether your first ride is R22 or something else.
B-47s got lots of damage in training long before Frank knew about the R22 himself...

I am not in favor of NOT demonstrating Low-g anymore either, as it is a rather interesting experience. If you are never in it you really have no idea what to expect. So why is there no more low-g demo?
Because of the US-system of basic flight instruction. A 150-200 hr chap gets to instruct the next guy, having really 0 experience. He gets low-g demoed, next thing he shows his "new skills" to his student, chickens out (panics) half through the demo and ....crash.
No wonder FR pulled the plug on it.
I agree on this though.
What they should do, is demo it at their Safety-course, with the factory-instructor (Tim Tucker or whoever else is there now too....).
It is a rather harmless occurance, IF you know how to recognize and meet it, like retreating blade-stall and VRS.

Same should go for the VRS, don't let the green instructor kill his first student and himself with it, but let your 1000 hr+ chief-flightinstructor or better yet your 18K+ retired pro, turned instructor do the more complex/demanding demos and instruction. Cost? So what!!

Until you have full motion sims (I hear there is even a R22 sim under development...) realistically perform advanced emergency maneuvers, there is no substitute for the real training, including VRS, full-touchdowns autos, running ldgs/tkoff, stuck pedal, hover-autos, etc.

Enough ranted......


3top

[[email protected]]

TopendTorque - out of interest, the British military hasn't demonstrated vortex ring for very many years and doesn't even demo incipient VRS on any of it's many courses.

Why? Because, as Hihover says, if you teach your pilots how to avoid it then they won't put themselves in it.

I would suggest that military pilots operate in a far more demanding low-level environment than cattle musterers, especially with current ops in hot and high locations.

Yet we don't keep losing aircraft to VRS, how strange?

I have still to be convinced that you are actually demonstrating fully developed VRS in your aircraft - you imply you can enter and recover 3 times entering from 2000' and not going below 500'. That sounds like incipient VRS since fully developed VRS would give you several thousand feet per min RoD.

I know only 2 people who have encountered full blown VRS and they both took several thousand feet to recover from it.

[Thomas coupling]

There is definitely a misunderstanding of philosophies here:
IF you understand the aerodynamics of fully developed VRS, then you will naturally understand that the control of that helicopter has been removed from the pilot. It is an UNSTABLE state where the pilot has to wait for the a/c to remove ITSELF from VRS. It may choose NOT to do so!!!.
Fully developed VRS provides the following symptoms:
loss of control in all three planes.
ROD well in excess of 4000' +, even up to 8000'/min.
All qualified instructors (military anyway) are taught this in ground school. It has been removed from the UK syllabus for atleast the last 20 yrs, because instructors were crashing helos. Even then, they were demonstrating in ignorance, but atleast they started at 6,7,8 thousand feet in preparation for the horrendous ROD's.
Either you are talking about incipient VRS where you can still use the cyclic/coll/pedals and are experiencing ROD's of max 2000' ish, or you are extremely naive and plain negligent.

[oldpinger]

3Top-

Full motion sims don't have the flight modelling (yet I think) for vortex ring. I managed to crash the motion system on one once 'demoing' VRS. I also remember Incipient VRS generating pretty a impressive ROD. on the annual QHI check.....

[topendtorque]

Thoms and Crab,
You guys have been around a bit.

care to explain just how after a rotor disc has fully developed its own bubble of vortices separate from the surrounding air mass, (VRS) which of course allows the disc complete with helicopter to fall at a diabolic rate, and then retain that bubble in the face of the equal and opposite diabolic rush of relative airflow?

and still be doing it many thousands of feet later? what were those idiots doing to fly into the gound (as you say) doing VRS exercises. see next para.

Care to also explain how an out of control helicopter can be flown out of VRS (as they say) by applying a cyclic control input?

Helicopters in VRS and near vertical descent trying to find VRS are still operating at 1G, unless i've missed something?

[eagle 86]

TT,
In the old days VR was near impossible to recover from - witness all the pictures of crashed Sycamores shown to us at the School. Modern design means that forward disc is more achievable - "blows", if you like, the vortices off the disc hence lessens ingestion - restores angle of attack, lower lever reduces angle of attack inboard to below stalling angle - requires lots of alt agl. Still wouldn't push a modern helo into fully developed VR.
GAGS
E86

[mountjoy]

I guess we've all seen the footage of the blackhawk, tailwind onto a moving deck, to the experts out there, was that VRS/SWP induced crash ?

[Thomas coupling]

Topentorque: eagle has it about right. The older heads on helos only allowed you to "regain" control, if, IF, that is...the selection of fwd cyclic was long and sufficient enough to "blow" the turbulence going on around the root and emanating outward...back away from the disc and placing you into cleaner air, thus allowing some form of recovery. Newer head design makes the above statement happen more quickly.
If you are able to confidently apply fwd cyclic in the knowledge that you will come out of VRS...may I suggest you are not in fully developed VRS

[[email protected]]

Topend, the theory goes that the root end of the blade is stalled because of the high AoA caused by the low speed of the inboard section and the high RoD.

The tip and outboard section is in its own recirculating vortex and not producing any useful lift.

This leaves the bit in the middle of the blade (a ring if you will, hence the name) which suffers unpredictable airflow due to the wandering of the tip vortex and the root stalled area.

The airflow coming at the disc from underneath is therefore somewhat erratic and causes random pitch, roll and yaw making control of the helicopter much more difficult.

This is one reason why some people say enter auto to escape VRS but you need a lot of height to do it.

Forward cyclic, if held long enough, should fly you out but , as TC has said, you really don't have much control at all.

The answer is to avoid the entry conditions and know how to recognise and recover from the incipient stage.

I think modern helicopters with higher hinge offsets and therefore more control power are probably easier to recover in - having said that they also have higher disc loadings and therfore need a higher RoD to get into VRS in the first place.

[topendtorque]

I've been away a bit, playing with CAT (18 degrees C) and carby heat (two thirds applied all day), winter rain, export boat deadlines, all fun stuff.

here is a reply.

Thank you for that EAGLE and others. With the hindsight of modern light rotorcraft behaviour and design I’d be delighted to discuss those old machine characteristics sometime. The sycamore (did they have tapered blades and or retreating leading edge?) must have been a funny animal. I dreamt about one once which in my comatose state turned out to be a Dutch windmill cavorting around. Maybe not so strange?

It seems that we are at cross purposes. You guys are talking about a characteristic which was described by ‘eagle’ and all the old text books as “settling with power”, with old style blade design and perhaps is also an inherent characteristic of some new designs, where you hurtle downwards at speeds up to 80 knots and unable to steer the damm thing around.

I am not.

Thomas Coupling quite rightly points out the confusions that arise in definition and I agree. We have all been subject to it, maybe because of references to the older generation blade / disc designs and the slowness of the text books / instructors to adapt to the change of new designs???????????

Of course we must also differentiate between types of helicopters as Nick Lappos points out in a recent article of his in the latest issue of a leading magazine.

Very few people experiment with these phenomena to the fullest extent which is fair enough if it generates a known and dangerous situation on types known for it.

Even on light types most people go to the edge of the ‘predicted’ (whatever that might be in the imagination)and teach students to stay away from that big dark phobia out there. Thus guaranteeing that neither the students nor they will ever progress for their own sake to really know their machine. That is dangerous for those who live in the low level environment.

However I am going to stick to my guns on this issue as it is one that we in the mustering fraternity meet often and unless one is prepared for it then it may hurt.

Old mate with his macho talk can bug off; perhaps he could study the video of the Seaking crash on the deck of a ship with its obviously unloading disc and tell me that there have never been any problems with incipient vortex ring state in his navy.

What I do, is demonstrate how VRS will hurt and then make sure the student can demonstrate his/her recognition of the incipient stage and then, their ironclad recovery from it, or, they don’t progress; simple.

I am talking about a situation that bites you in the backside if you are not careful which is a devastating descent for a short while, which happens with VRS. It is well described in the B/CA mag January ’84 issue by Dan Manningham, where he talks about “power settling” as a vortex ring state. Excerpts as follows:-

“Engineers refer to power settling as ‘vortex ring state’ because some air cannot escape from the rotor disc and becomes trapped in a ring shaped bubble that envelopes the outer rim of the rotor disc isolating it from the flow of undisturbed air. The vortex ring state occurs when the descent is nearly vertical and at a moderate rate. ---- but at some combination of moderate vertical descent speeds and moderate power setting, the upflow from the descent is approximately the same as the rotor induced downwash, setting the stage for vortex ring interference at the rotor tips. 300 to 600fpm is the range at which most helicopters are susceptible to power settling. Once the vortex ring has been created the helicopter will settle due to a loss of lift at the rotor tips. But if power is added to arrest the rate of descent, the increased energy at the tips will just add to the vortex ring and further degrade total lift.—---If the condition is encountered at a very low power setting, a rapid power application of full power may destroy the vortex bubble.. Nevertheless, if altitude is available the best technique is to push over to increase forward airspeed. At 15 to 30 kts the rotor will no longer fly in is own vortex-----“

This is what I demonstrate, as below.

One of the things that worried the heck out of me when I first started exploring the issue was what might happen to the relative AofA at the emerging recovery stage.

About then we started to hear about the horrendous R22 stalled blade stories. They had as their Highest Common Denominator, Low RRPM, and consequently not enough power to overcome drag and or retain blade rigidity.

Quite obviously while the machine is totally engaged in VRS in its totally isolated bubble of descending air then it is as safe as a house, the blade tips are not stalled. It is controllable in a disc sense because the blades are still operating at normal AofA attitudes. The disc has no idea that it is re-ingesting air, only we do because we are sinking with it.

However inside and with that bubble the machine must be falling at the acceleration of gravity which will only last for as long as that bubble of recirculating vortices can withstand the friction from the upcoming relative airflow.

THERFORE: - complete VRS will NEVER last for long, a couple of hundred feet in the biggest helicopters I suggest.

But what happens next? The M/R blade has to transition from that safe-house of normal AofA operation to one of a very high AofA coupled with a high G loading, which is required to slow the already gained massive RofD, not a happy situation.

At that developed state there is one coupled, advocated recovery, 1) collective down and 2) cyclic forward.

Re;-

1) a) Collective down presents the disc in flat pitch, as long as it presents itself purely as an unstalled item then it should prevail with some lift to slow the RofD down. I thought about the windmilling airplane propeller in flat pitch, it simply acts as a brake and slows things down, so to should the rotor disc as long as it retains its flat centrifugal plane of integrity.

b) By experimenting with quick stops I found that it didn’t matter how severely I did them, by presenting a flat disc to the relative airflow, there was never a problem with blade stalling.

c) All the time in the VRS practice I was juggling with the collective to keep the machine in VRS as long as I could.

2) a) This was more interesting. I found that inside the bubble of air the machine immediately started to fall toward the C of G, especially laterally, unless it was loaded symmetrically and then it fell toward the right (I.E. the gyroscopic precession in the southern hemisphere).

b) As I was initially doing it in a G5 by myself (for those who are not aware the driver sits to the left of the G5) I loaded the left litter and the fall to the left was very pronounced. I found that I could apply right cyclic to hold it in.
I found it easy to manipulate the disc at any time in VRS, so the forward cyclic for recovery is immaterial, any which way will do.

c) A tilted disc certainly makes the transition into clean air quicker and reduces the RofD quicker than from a flat profile when the VRS state simply “blew away.” This to my mind was because the reduced AofA of a disc tilted towards the relative airflow gave me a far lower AofA and thus a far higher performance value to work with rather than struggling with the disc still at high AofA and not wanting to go anywhere sitting flat ar**ed in a vertical descent.

I concluded that applying forward cyclic (or in any direction) is an ‘after the event’ process, which merely serves to help flatten out the RofD quicker if one has applied it earlier.

I reckoned to get out of any “worse situation”, one simply must use the further resources that one has, I.E. 1) roll toward gyroscopic precession, 2) use T/R to present the airframe as a drag moment to help push the rotor disc over toward the relative airflow.

It seems with the “settling with power” discussion with the old style rotors that what is needed is to get the AofA down by getting the disc tilted toward the relative airflow.

I understand the exasperation with the ‘tradition’ of trying to push over “forward” with cyclic. That is usually the slowest response direction of the cyclic. Certainly some quick rolling moment is required. With 8,000fpm = 79kts up yer trousers leg, yer could stick yer arm out the window perhaps?

Or maybe it might have been easier to attempt recoveries as explained above?

I think I should leave you guys to your discussions about “Settling with Power” where you do not talk about a single ‘vortex ring state’ but instead talk about a phenomenon that I have been unable to replicate in the R22 and B47 despite many hours trying.

I assume that you refer to the old textbook and the old style machines where the blade stalls from the root outwards and every thing turns into a big rout after that, no inferences meant.

Certainly any suggestion that a ‘fully developed VRS’ is where the disc is carrying one big vortex downhill at 8,000fpm is absolutely ludicrous. Perhaps your phraseology needs rephrasing.

The beginnings of either phenomenon could well be induced by a high DA situation where not enough performance, in either the rotors or engine, is able to arrest all measure of descent. This can be a perfectly safe environment and is often encountered. The trick is to keep that descent below 300fpm and not in the discs’ own rotor induced downwash. Shooting down its own tube, so to speak.

But, shooting the tube can be done anytime, with distractions aplenty and changing wind cues invisible.
That's sort of where i live.

[Shawn Coyle]

One of the first cues to vortex ring state that appears to be missing in this discussion is low frequency airframe vibrations - unlike those you get when transitioning back to the hover.
Always been the first cue in any of the numerous demos of vortex ring state I've ever done.