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Condition for Vortex Ring

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Condition for Vortex Ring

Old 18th Nov 2017, 10:57
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Condition for Vortex Ring

I have recently read an article mentioning that vortex is likely to occur around lower than 30kt and more that 500ft/min decent.

As far as remember from school I have been told lower than 20kt and more that 300ft/min decent.

1) Who is right ?
2) I guess those figures will vary depending on aircraft, right?
3) Is there any aircraft out there that has a vortex warning light based on any combination of parameters stated above?

For the 3rd question I specially have in mind the Puma accident in the north sea a few years ago where such a warning light would have been usefull.
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Old 18th Nov 2017, 11:09
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This subject comes up very often.
I politely suggest you use the search facility on the yellow bar at the top of the page because almost everyone here has discussed it (and argued about it) many times.
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Old 18th Nov 2017, 11:49
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I ran a search vortex previously and didn't find specific post I will run it again and look more carefully, thanks

Edit: indeed I have used the advanced search this time on rotorhead only and found a few posts. Hopelly some will answer my questions
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Old 18th Nov 2017, 14:42
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The original 30 kts/300 fpm was for when helicopters were very light in weight and had very little downwash, but nowadays I believe the CAA would expect to see 400 ft per minute. It is in fact a safety margin - in reality you would need to be inside ETL (around 12 knots) where the downwash stays more or less within the rotor disc.

The figures for rate of descent will vary with the aircraft, but the heavier it is, the less likely you will get into VR because you will need to go down faster to catch up with the downwash.

Never heard of a warning light for VR..... Is that like the red light that comes on when the engine falls off a DC 10?

Phil
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Old 18th Nov 2017, 14:43
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OK, it's been a long, long, long time since I've had to work with this. The answer to your questions is 'it depends'. There are multiple independent variables(height of disk above surface, dia of disk to height, cabin size/shape, mass, and mass moments, etc) and also a number of dependent variables. One cannot use a fixed number for all shapes.

Lucky, there is a calculator for all that stuff, and damned if I can remember the name of it but something like Kutte-Jukowitz(?) formula. It's a derivative of the above variables which will predict the potential for things like ring-vortex state. You can find it in some obscure aerodynamic texts from the 50s and 60s. Maybe also now on one of those software packages for finite element analysis.

Also, while it's not that important to know the speed or descent rate, what's important is to recognize it and implement recovery very quickly. For US built rotorcraft I prefer hard right pedal, right cyclic. For the backward crowd from Yerrup rotorcraft, I would presume hard left pedal, left cycle, but that is only theory as I have no time in the backward spinning machines. Of course, if there are path obstructions preventing that 'out' you better have something figured out before things get out of hand. Rarely fatal, but it will turn a collection of spare parts, all operating in close formation to a collection of expensive metals being flung out at remarkable speed and distance.
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Old 18th Nov 2017, 22:58
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Oh yippee! Another vortex ring thread just when things seemed to be getting quiet! OP, no this has never ever been discussed before. Well, not in the last week or so anyway.
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Old 19th Nov 2017, 03:33
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Almost invariably when helicopters are put in vortex ring state the pilot has been unaware of the wind, or airspeed, or rate of descent, or power settings at the time. So the question really only holds academic merit, and as others have pointed out, that discussion has been widely discussed before.
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Old 19th Nov 2017, 05:47
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I concur with the idea of GulliBell, the question has academic merit mostly, because:

My impression, its hard to enter Vortex ring you have to be looking for it, and even still before it happens, you get so much vibrations and uneasy feeling that any normal pilot would wake up look at the instruments and make the proper correction.

if you know where the wind come from, keep your airspeed in the green, don't go crazy on your rate of descent and add bunch of power to fix it. it does not matter 300 or 500ft per minute, you are nowhere close.
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Old 19th Nov 2017, 07:06
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@Agile is correct. When teaching/demonstrating Vortex ring recovery - settling with power - or whatever you wish to call it, you need to concentrate and physically fly the helicopter and establish the necessary parameters to demonstrate the manoeuvre. The helicopter gives you clues it is not happy doing what you are asking it to do. So pilots who inadvertently enter vortex ring state are more than likely not concentrating on things they should be concentrating on immediately before entering this predicament. And when I hear excuses from pilots who pancake their helicopter in the steaming swamp late on a sunny afternoon when doing a low level pipeline inspection, and then blame it on an autopilot malfunction, yeah right, what a load of bollocks.
The big thing is, when you are low and slow in a helicopter, it is critical to know where the wind is coming from. Whether vortex ring state onset happens at 30/20/10 knots or 100/200/300 ROD or in any combination of parameters, knowing those parameters is secondary to knowing where the wind is coming from, concentrating on the job at hand, and listening to the helicopter if it starts to complain about what you are asking it to do.

And @paco, I highly respect your experience, but I'd need to think about that explanation about being heavier is less likely to result in VR. I'm inclined to disagree with you on that one. At the training weights we work in I'd need to feel a little braver - or higher - about being heavier before experimenting with that one.
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Old 19th Nov 2017, 07:21
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In theory - if you are heavier, you need more downwash, and it's more difficult to get the vertical speed to catch up with it, but I agree with you, it's not always true. I remember when we used to do base checks at 95% all up weight. I still think it's a good idea.

Phil
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Old 19th Nov 2017, 08:37
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The worst place to go in "VRS, is when you make a "vertical approach" in a small/confined area : You don't have any escape if the Vortex Happen
You have to be really aware at your rate of descent all time, but with a low flying speed : Above 300 ft/min, you enter in the Vortex zone, under 300 ft/min, no problem....So if in "ANY " case you maintain your rate of descent below 300 ft/min, you never risq to go in VRS..
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Old 19th Nov 2017, 09:48
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Originally Posted by paco View Post
In theory - if you are heavier, you need more downwash, and it's more difficult to get the vertical speed to catch up with it, but I agree with you..
I've got a hunch the accident stats will probably show that prangs where VR was the cause is more common where the helicopter is well below MAUW. If so I'd put that down to pilots probably being more vigilant of their performance parameters when slow and heavy, and thus less likely to being caught out unaware of a flight condition exposing them to VR onset. Rather than the opposite case, when light they are probably less inclined to monitor their performance numbers.

I don't agree yet that being at a heavier AUW puts you less at risk of VR onset for aerodynamic reasons. With a higher angle of attack on the rotor blades the pressure difference between the top side and the bottom side of the rotor disc is greater; the high pressure side air is more prone to want to escape around the rotor tips to the low pressure side, and thus establishing a vortex ring which quickly deepens towards the blade root. As this happens the equation of lift vs gravity rapidly goes in the direction of gravity.
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Old 19th Nov 2017, 09:54
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Originally Posted by BOBAKAT View Post
The worst place to go in "VRS, is when you make a "vertical approach" in a small/confined area : You don't have any escape if the Vortex Happen....
Speaking from experience, I can think of worse places. Like on a pitch black night over a pitch black sea when all of a sudden there is salt spray all over the windscreen. It sure gets your attention, even with the enormity of the open space around you.
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Old 19th Nov 2017, 16:16
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"I've got a hunch the accident stats will probably show that prangs where VR was the cause is more common where the helicopter is well below MAUW."

Exactly!
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Old 20th Nov 2017, 02:31
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Originally Posted by gulliBell View Post
Speaking from experience, I can think of worse places. Like on a pitch black night over a pitch black sea when all of a sudden there is salt spray all over the windscreen. It sure gets your attention, even with the enormity of the open space around you.
I say, the worst place for "VRS".... I enjoy the landing in a dark night at sea too. When you are down wind leg and when the boat due to operationnal reason go on " full black-out conditions" = no radio, no beacon, no radar, no light, and no boat in sight until some people light a cigarette...Not a big deal...but not the funniest
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Old 20th Nov 2017, 06:22
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I don't agree yet that being at a heavier AUW puts you less at risk of VR onset for aerodynamic reasons. With a higher angle of attack on the rotor blades the pressure difference between the top side and the bottom side of the rotor disc is greater; the high pressure side air is more prone to want to escape around the rotor tips to the low pressure side, and thus establishing a vortex ring which quickly deepens towards the blade root. As this happens the equation of lift vs gravity rapidly goes in the direction of gravity.
Gullibell - that vortex is present regardless of stage of flight, as I am very sure you know - but for VRS to occur, there must be re-ingestion of that vortex at the tips and that requires you to catch up with your downwash.

The downwash speed is important regarding how quickly you have to descend to catch it and, although it can vary widely between aircraft types, I suspect the difference on the same aircraft between being at 70% AUM and 100% AUM is probably not very great and probably not measurable with the standard VSI in order to avoid VRS.
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Old 20th Nov 2017, 12:39
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Like already said in this tread before, this subject "VRS" has been discussed before from just about every angles by a lot of knowledgeable people (and there are in PPRuNe) that went from the simplest explanations to the more elaborate ones, from the practical side of VRS to the theory side of it.
The consensus here seems to be that VRS is a dangerous condition to be avoided, if not recognized or understood or reacted upon it could end up in an accident and i am in agreement with this, being an experience (39 years in this) VFR, utility and VertRef pilot all my career I have experienced VRS many times during my carreer, almost unavoidable with more than 8,000hrs VertRef.
But as I said earlier VRS has been discussed from "just about every angles" but not all of them, I never heard anybody say anything good about it, so my question here is.

Does anybody here sees a good practical/useful side to VRS? just asking.

JD
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Old 20th Nov 2017, 14:45
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Fijdor:

It's like asking:

Does anyone see the useful side of a stall or spin?

PS: You probably haven't experienced 1 x VRS in your 8000hrs vertref (?).
Maybe the odd IVRS but that's about it.

PPS: VRS is ALWAYS avoidable. You obviously haven't been listening to the knowledgeable people have you?
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Old 20th Nov 2017, 14:50
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Originally Posted by fijdor View Post

Does anybody here sees a good practical/useful side to VRS? just asking.

JD
Absolutely - anyone who has done some production longline work in an AS350 would understand that VRS is VERY practical & uselful for teaching us to slow the [email protected]#$ down & pay attention to what we are doing!!
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