Vortex Ring / Settling with power (Merged)
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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
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
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reply to gaseous
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.
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.
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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.
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.
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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!
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!
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!
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!
Last edited by Irlandés; 14th Jun 2002 at 02:51.
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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
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
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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.
If you could point me on the straight and narrow, I would be very grateful!
Irlandés
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.
If you could point me on the straight and narrow, I would be very grateful! Irlandés
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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
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
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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
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
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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.
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.
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VRS
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
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
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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.
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.
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downcast by the downwind
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. 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.
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. 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.
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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
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
Nicks website
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/
It seems the link to your vortex ring page is not working. Any chance of it being reinstated?
http://mywebpages.comcast.net/llappos/
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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.
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.
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.
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.
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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.
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.
Last edited by Heliport; 8th Mar 2003 at 21:06.
The Original Whirly
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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.
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.
