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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!!! |
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 !!!!!e at english and cant read. Hope you are enjoying yourself away from MW, miss it?
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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. |
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! |
Nah..............but its good to know you are ok.
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MOSTAFA - pm me to let me know who you are
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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 |
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... |
check your Private Messages Mr Neckhurts:ok:
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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. |
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. |
What about.
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? |
Hasn't this issue been settled a long time ago?
Pun intended. |
KikoLobo, correct apart from the ROD. Read the quoted text in the post above yours.
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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.
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Indications
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.
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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. |
Glad to see this old thread is still generating interest - my original post was nearly 7 years ago! :ok:
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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 |
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. |
: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: |
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. |
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? |
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? |
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. |
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? |
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!!!!
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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. |
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. |
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. |
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 |
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. |
Thanks for the post Shawn. That makes sense to me.
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Someone smarter than me feel free to correct, but it seems to be a question of momentum. Taking off from a confined area, where your obstacle is at OGE height, you can get out with less than OGE power if you apply power early to gain an upward momentum and "coast" over the obstacle. The reverse is true, your downward momentum requires extra power to stop. If you start in an OGE hover of say 200ft, and descend at 500fpm with the intention of stopping at 100ft while still OGE, you will need greater than OGE power. How much extra power you need will depend on your decent rate and how much you weigh. I don't remember off the top of my head but I think the HAATS number for a Blackhawk is every 100fpm of decent rate requires 2% of excess power to arrest. If I'm right, it is easy to see how going from one OGE height to another with only OGE available could get hairy.
EDIT: What I'm talking about is a crappy situation that is not VRS but could make you think it was. |
The point that we are all working around is that power problems in OGE are usually NOT Vortex Ring State, and that 300 fpm descents in the hover are not by themselves dangerous.
Here is a web site that does a credible job of discussing VRS: http://www.safetycenter.navy.mil/MED...n03/vortex.htm Here is a diagram from it, the values are quite valid for light helicopters. Note that it is NOT POSSIBLE to even feel slight VRS at descent rates less than about 750fpm, and full blown VRS requires 1000 fpm: http://webpages.charter.net/nlappos/vrs.jpg |
Nick
I ask again what your production long line experience is? The phenomenon is there at low power settings, low speed and low rates of decent. I and every other long line pilot I know has experienced it. Is this just anecdotal evidence in a scientific world? |
lama bear,
I am not sure what you experience but at low rod it is not VRS. Can you tell us what you experience, in as much detail as you can? I looked at the charts you sent, and they indicate that at moderate altitudes at 2500 lbs the Lama has no power issues at all, it is a screamer, so it is not "over pitching" for sure. For the record, I have not worked long line, ever. That being said, I have done one heck of a lot of tethered hover, sling loads and OGE Height Velocity engine cut testing. I do not at all doubt your experience, sincerity and capability, lama, just that what you are experiencing is VRS (unless you are descending at 1000 fpm.) |
In a previous VRS discussion someone thought he had encountered VRS while hovering beside a ridge top in an updraft.
My understanding was hovering in a 1000ft/min updraft = 1000ft/min descent and possible VRS. |
I would seriously doubt that the precise angle of descent and steady flow into the rotor to cause VRS could be found in a mountain updraft. At least not enough to cause any loss of sleep over the possibility.
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Lama Bear, is there any chance that ground effect vortex roll up might be causing some downwash to be recirculated? It sounds to me like what you are describing only happens at the start of ground effect. It might be something new...
Nick, my ABS once again stepped in this morning to catch a nasty moment on a spot of unexpected road ice. For settling with power there must surely be an arguement for parallel and series actuators in the collective. The wee computer would stop a nasty moment becoming anything more... |
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