Vortex Ring / Settling with power (Merged)
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Nick,
I gave an example of a helicopter that can experience VRS at above 35 knots.
One reason books explain it in this manner is that we don't use sophisticated equipment to determine airspeed or ROD. Vertical speed indicators and airspeed indicators are subject to many many errors. Therefore, in consideration of safety, most books overstate the regime in which you can experience VRS.
I have experienced VRS in an R-22 at a ROD indicated of 400-500 FPM, but in theory, I should experience VRS until I achieve a ROD of over 900 FPM.
So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.
I gave an example of a helicopter that can experience VRS at above 35 knots.
One reason books explain it in this manner is that we don't use sophisticated equipment to determine airspeed or ROD. Vertical speed indicators and airspeed indicators are subject to many many errors. Therefore, in consideration of safety, most books overstate the regime in which you can experience VRS.
I have experienced VRS in an R-22 at a ROD indicated of 400-500 FPM, but in theory, I should experience VRS until I achieve a ROD of over 900 FPM.
So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.
CJ Eliassen:
I agree with Nick that helicopters will never see VRS at 35 knots. By definition, to get into VRS the rotor must be back below ETL. There is no vortex ring during translated flight. I've never understood why the books use such hard (and obviously wrong) numbers for defining VRS, which is why so many pilots seem to not understand it. I agree with you CJ, that the instruments we use in the cockpit are very imprecise. In my opinion, what the texts really ought to be saying is:
a) Speed below ETL
b) Some RoD
c) Some power applied (not autorotative descent).
The trouble with the above is that there is no Translational Lift Indicator- nothing to look at in the cockpit that would definitely tell us whether we are in translated flight or not. Ergo, the conservative "Look out below 40 knots!"
There have been many times in my career whilst making landings to offshore oil platforms in grossed-out BO105s on hot, calm days (or when the light wind was variable) when I've wondered if the shaking, shuddering and rivet-loosening was the normal BO translational event or the onset of VRS? Either it never was or I just got lucky.
Finally CJ, I got quite a chuckle out of trying to imagine an CH-53 in the flight conditions you described. A RoD of 4000+fpm with a forward speed of only 38 knots? While that might work in theory, I would imagine that such a manuevre would be impossible to perform with any sort of power applied (have to defer to real '53 pilots there though). But it sure would be exciting to watch, eh!
So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.
a) Speed below ETL
b) Some RoD
c) Some power applied (not autorotative descent).
The trouble with the above is that there is no Translational Lift Indicator- nothing to look at in the cockpit that would definitely tell us whether we are in translated flight or not. Ergo, the conservative "Look out below 40 knots!"
There have been many times in my career whilst making landings to offshore oil platforms in grossed-out BO105s on hot, calm days (or when the light wind was variable) when I've wondered if the shaking, shuddering and rivet-loosening was the normal BO translational event or the onset of VRS? Either it never was or I just got lucky.
Finally CJ, I got quite a chuckle out of trying to imagine an CH-53 in the flight conditions you described. A RoD of 4000+fpm with a forward speed of only 38 knots? While that might work in theory, I would imagine that such a manuevre would be impossible to perform with any sort of power applied (have to defer to real '53 pilots there though). But it sure would be exciting to watch, eh!
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Charlie asked:
".......how do you calculate what sort of airspeeds the aircraft is going to be susceptible to VRS (the airspeeds corresponding to the red area on your B206 graph)? I understand how to estimate the descent rates using the formula, just missing the link to figuring out the airspeeds"
Charlie, the forward speed scales on those plots are in ratios to the downwash speed, so 1.0 is simply 1x the downwash speed (which is calculated using that formula shown in the beginning) That formula is simply an attempt to approximate the speed of the downwash needed to produce the lift equal to weight. It assumes that the momentum transferred from the air to tha aircraft has produced a uniform stream of air.
Sorry that it takes a while to download. I can break the page up into chunks if that would help, or I can email the plots to you separately.
".......how do you calculate what sort of airspeeds the aircraft is going to be susceptible to VRS (the airspeeds corresponding to the red area on your B206 graph)? I understand how to estimate the descent rates using the formula, just missing the link to figuring out the airspeeds"
Charlie, the forward speed scales on those plots are in ratios to the downwash speed, so 1.0 is simply 1x the downwash speed (which is calculated using that formula shown in the beginning) That formula is simply an attempt to approximate the speed of the downwash needed to produce the lift equal to weight. It assumes that the momentum transferred from the air to tha aircraft has produced a uniform stream of air.
Sorry that it takes a while to download. I can break the page up into chunks if that would help, or I can email the plots to you separately.
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Charlie,
In the spreadsheet I sent you, Vi 1 is the airspeed that the helicopter experiences ETL. On the chart it is represented on the bottom as Vi/101.4.
Hope this helps.
In the spreadsheet I sent you, Vi 1 is the airspeed that the helicopter experiences ETL. On the chart it is represented on the bottom as Vi/101.4.
Hope this helps.
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charlie,
You ask ", why isn't the vertical band making up the red section of the chart more to the left, or more to the right? What is it that has fixed it at those points? I suspect "testing" is going to be the answer, but I was hoping there would be a mathematical model for it, or a rule of thumb like 50-75% for the descent rate"
Those plot points that are colored red and greenon my website are made from flight test points on a V22 test. They are not where VRS is truly developed, they mark the outer boundary where VRS just starts for a helicopter that is slowing down from forward flight. It could be possible to test from a steady zero knot hover outward, if you wish, and that would scatter some points from the left side toward the center. Those points are not VRS, they are associated with small torque variences (note the percent in the legend). The actual boundary for VRS is inside the oval considerably far from the test points. See the first plot on that page where the orange circle marks the real VRS boundary.
To calculate the downwash speed (which is the reference for VRS for a particular helo) just take the disk loading in pounds per square foot and multiply by 210, then take the square root. That will give you the feet per second for the 1.0 speed value.
To be conservative, if you take a descent at 50 to 75% of that descent rate, you can avoid VRS.
Here is an example For an R-22, disk area 498 sq ft, GW 1370 lbs, disk loading is 2.75 lbs/sq ft. 2.75 x 210= 577. The square root of 577 is 24 ft/sec, which is the average downwash of an R-22 (24 ft/sec is about 14 knots, or about 1440 ft/min).
This means that the R-22 can't get VRS above about 11 knots forward speed (75% of the downwash velocity), but also that it can't get VRS in a descent less than 700 ft/min (50% of the downwash velocity).
You ask ", why isn't the vertical band making up the red section of the chart more to the left, or more to the right? What is it that has fixed it at those points? I suspect "testing" is going to be the answer, but I was hoping there would be a mathematical model for it, or a rule of thumb like 50-75% for the descent rate"
Those plot points that are colored red and greenon my website are made from flight test points on a V22 test. They are not where VRS is truly developed, they mark the outer boundary where VRS just starts for a helicopter that is slowing down from forward flight. It could be possible to test from a steady zero knot hover outward, if you wish, and that would scatter some points from the left side toward the center. Those points are not VRS, they are associated with small torque variences (note the percent in the legend). The actual boundary for VRS is inside the oval considerably far from the test points. See the first plot on that page where the orange circle marks the real VRS boundary.
To calculate the downwash speed (which is the reference for VRS for a particular helo) just take the disk loading in pounds per square foot and multiply by 210, then take the square root. That will give you the feet per second for the 1.0 speed value.
To be conservative, if you take a descent at 50 to 75% of that descent rate, you can avoid VRS.
Here is an example For an R-22, disk area 498 sq ft, GW 1370 lbs, disk loading is 2.75 lbs/sq ft. 2.75 x 210= 577. The square root of 577 is 24 ft/sec, which is the average downwash of an R-22 (24 ft/sec is about 14 knots, or about 1440 ft/min).
This means that the R-22 can't get VRS above about 11 knots forward speed (75% of the downwash velocity), but also that it can't get VRS in a descent less than 700 ft/min (50% of the downwash velocity).
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went out with my instructor to get vortex ring experience , on return to airfield was on final and got us into it at 300ft ended up forward cyclic and whizzing along at 5 ft agl at 50knts for go round on next approach did exactly same again. i like a 50ft hover and lower it down gently. r22s can easily get into vortex, buy the time the vibrations have started its usually to late!
Davehearn - you are only getting incipient VRS experience not the fully developed variety or you would never recover from 300'. Frankly your instructor is a clown to even do the incipient stuff so low - what is he going to do if it goes a bit far and starts to get into VRS proper?
Avoid imitations
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A useful link:
http://naca.larc.nasa.gov/reports/19...ca-tn-4330.pdf
And to think I could be at the pub.....
http://naca.larc.nasa.gov/reports/19...ca-tn-4330.pdf
And to think I could be at the pub.....
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Vortex Ring
I hope there are some heli instructors out there that can help me on this one.
When demo-ing vortex ring, I find that it is quite hard to get the robbie (22) into this state.
Here's what I do:
I reduce power to about 13-14 inch MAP,
Reduce speed to about 20 kts and a ROD of about 500-800 feet/min.
But I just can't get the symptoms to show CLEARLY. Is it just that the robbie is designed this way or am I doing something wrong.
Advice and Input, Please.
DD:
When demo-ing vortex ring, I find that it is quite hard to get the robbie (22) into this state.
Here's what I do:
I reduce power to about 13-14 inch MAP,
Reduce speed to about 20 kts and a ROD of about 500-800 feet/min.
But I just can't get the symptoms to show CLEARLY. Is it just that the robbie is designed this way or am I doing something wrong.
Advice and Input, Please.
DD:
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DD,
I agree with Mr S; alternatively you could try entering the manoeuvre downwind. (Need lots of height and only let the ruddy thing enter the incipient stages!) However, there have been times when I have tried to demo it and it really won't go. It is usually when I have a newish Beta 2 and a light student.
Good luck.
BS
I agree with Mr S; alternatively you could try entering the manoeuvre downwind. (Need lots of height and only let the ruddy thing enter the incipient stages!) However, there have been times when I have tried to demo it and it really won't go. It is usually when I have a newish Beta 2 and a light student.
Good luck.
BS
Come to a complete hover at 2000'agl. You should be pulling 20+ inches, maybe 23. Ensure zero airspeed. Wait. Don't descend, don't move forward.
Then lower the lever a little to start a gentle descent.
Then the fun starts.
Then lower the lever a little to start a gentle descent.
Then the fun starts.
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Are you doing these with an instructor? (I hope).
If yes, find another instructor !!!
If no, find an instructor !!!
From your description of the maneuver, it doesn't seem you or your school has a "procedure" for demonstrating this.
For my students I use:
Establish Hover at an altitide that will ensure recovery by 1500ft AGL (read: Plenty high)
Slightly lower collective to achieve ~500fpm descent
When buffeting starts (or other such excitment) increase collective to show it will not arrest rate of descent
Pitch over to regain airspeed (remove one of the three conditions of VRS)
Establish Vy climb
And, this is what the Examiner in our area wants to see on check rides.
Fly Safe.
HOSS-1
If yes, find another instructor !!!
If no, find an instructor !!!
From your description of the maneuver, it doesn't seem you or your school has a "procedure" for demonstrating this.
For my students I use:
Establish Hover at an altitide that will ensure recovery by 1500ft AGL (read: Plenty high)
Slightly lower collective to achieve ~500fpm descent
When buffeting starts (or other such excitment) increase collective to show it will not arrest rate of descent
Pitch over to regain airspeed (remove one of the three conditions of VRS)
Establish Vy climb
And, this is what the Examiner in our area wants to see on check rides.
Fly Safe.
HOSS-1
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Thanks for all the replies, hope there are many to follow. I'd like tyo see all the various ways and opinions.
HOSS-1...
I AM the instructor. The way I described above was the way I was taught and the way it is done is SA. Thanks for the input though.
I am going to be trying these different ways, I think that Ascend Charlie has a good tecnique going.
Keep it coming
HOSS-1...
I AM the instructor. The way I described above was the way I was taught and the way it is done is SA. Thanks for the input though.
I am going to be trying these different ways, I think that Ascend Charlie has a good tecnique going.
Keep it coming
The two instructors that have demo'd it to me have used slightly different scenarios, but basically the same technique:
(Both start very high!)
John - set up a downwind approach to a field a long, long way off (normal approach profile, but we are very high) and patters the approach including "wash the speed off", "wash the speed off" until suddenly it all goes horribly wrong. His theory being that a downwind approach is where you're likely to get into the state anyway.
Stuart - Pretends to be a photographer: "left a bit, no, right, slow down slightly, come left, nono too fast, right slightly..." then suddenly you're doing the dance.
Both have an advantage in that they are 'real life' examples that show how easily you can be distracted and get into the situation.
Never had any trouble getting into it, maybe because of my non-too-sylph-like figure!
(Both start very high!)
John - set up a downwind approach to a field a long, long way off (normal approach profile, but we are very high) and patters the approach including "wash the speed off", "wash the speed off" until suddenly it all goes horribly wrong. His theory being that a downwind approach is where you're likely to get into the state anyway.
Stuart - Pretends to be a photographer: "left a bit, no, right, slow down slightly, come left, nono too fast, right slightly..." then suddenly you're doing the dance.
Both have an advantage in that they are 'real life' examples that show how easily you can be distracted and get into the situation.
Never had any trouble getting into it, maybe because of my non-too-sylph-like figure!
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try this
Start at atleast 1,500ft....2,000 better...The FAA test standards says you have to recover before 1,000...nothing about how high to enter, so high is good.
Pick a prominent landmark and fly towards it...lower the power down to around 15-18" mark....depends on a/c an weight, and keep altitude. This means you have to come back a bit on the cyclic...keep that vsi at 0...it shouldn't take to much input...
This will bleed off airspeed, while keeping altitude.
When you get lower than ETL, you will start to loose altitude...due to low airspeed, power and rate of descent you will know most likely enter vortex ring state...
Works for me anyway.
Timts
Pick a prominent landmark and fly towards it...lower the power down to around 15-18" mark....depends on a/c an weight, and keep altitude. This means you have to come back a bit on the cyclic...keep that vsi at 0...it shouldn't take to much input...
This will bleed off airspeed, while keeping altitude.
When you get lower than ETL, you will start to loose altitude...due to low airspeed, power and rate of descent you will know most likely enter vortex ring state...
Works for me anyway.
Timts
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Hello all,
I am from Canada, spend most of my time looking down a long line, and flying in the mountains. I find myself in the kind of work environment that frequently lends itself to vortex ring situations. I am not an instructor, but I can perhaps throw a few ideas on the table that I wish I knew when I was starting out. Some of this will be redundant for people, and I apologize for any inaccurate terminology that may be used in current instructor speak.
It's important that students have a thorough understanding of exactly what Vortex Ring is. They need to know:
-What 3 factors you need to get into it,
-How to recognize it,
-How to get out of it.
I just wanted to elaborate a bit on some of these points and add to the concepts that the basic exercise teaches.
To get into it we need:
-Little or no airspeed,
-Rate of decent,
-And using power.
If you don’t have all 3 present no worries right? Well not always!.....Up flowing air gives you the same situation as being in a descent. Hovering at the top of a ridge can be brutal if the air is screaming up the slope at you from below. First off you are surprised at how little power you need to hover, you feel the mushiness of the tail, pull a bit of power and all of a sudden it gets real ugly. Same mechanics as the practice scenario, but different causes and your not 2000' off the ground!
Next part would be on how to recognize it. Most will talk about shakes and shudders, tail is all over the place etc..All very important of course, but the reality is that if you are on very short final and it gets that bad, you are in big trouble unless you make immediate and drastic action to recover.
In production work your going as fast as practical (not possible), floating in on minimal power and pulling power on very short final. Unless your SURE that you have no V.R. concerns its always good to do a little check with the collective just before the final pull. This is the crucial part... when you do this your looking to see how the helicopter is reacting and how it feels under your ass. If all is good you should feel HEAVIER in your seat when you pull a bit of collective. If your anywhere close to entering V.R. you will not get that "heavier" feeling as your weight will stay neutral. If your closer to V.R. you will feel lighter in your seat...all this happens WELL before you begin to notice the usual signs of V.R. that everyone talks about.
Now put this into the hovering on the top of the slope scenario previously mentioned. Your hovering 40' off the trees (or your longline load is 40' off the trees) trying to decide if your going in or not. Your at a low power setting due to upflow air, make sure you have some air under you so you can abort if necessary and give the collective a bit of a pull, are you heavier or lighter in your seat?
Finally a few comments on how to get out of it. We are all well versed on that one, I just wanted to expend a bit on the practice scenario that we all learn with. The reality is that if it happens unexpectedly at 100' or less, there is not a lot of room to work with. Another reality is that unless your in full blown V.R., it doesn’t take much to get out of it. Generally just getting out of your downwash by moving a few rotor widths will do it. (try to go crosswind if you know your downwind as it started). What if your are already below treeline going into a pad and dont have the room to maneuver? One thing that can help is to stir the cyclic in a circle. This disperses your downwash a bit and can help a lot in the recovery.
Awareness that you are working in conditions conducive to V.R. , early detection followed by swift and accurate recovery techniques is what its all about concerning V.R.
The emphasis though should be on awareness and detection.....all the recovery techniqes in the world wont help a guy that repeatedly stumbles into in unaware.....he wont live long or he will be so scared after the first time he will have a hard time in a production work environment and will be quick to shut it down as he gets out of his comfort zone.
Conditions that are most conducive to V.R. are working in the bush with light and variable winds. You don’t always have the luxury of any form of wind indication (no water, leafy trees etc). If you want to be quick in these conditions you need tools to detect early V.R., and be comfortable with rapid recovery methods. In conditions like this the "collective check how does it feel in the seat thing" allows you to stay well ahead of the game. Always do the check early enough to get out of it if it doesn’t feel good. This kind of thinking allows you to work downwind, and in a variable wind condition with confidence and swiftness in a production environment.
Can you tell that I am sitting on a fire contract and its pissing rain? Too much time on my hands..
Thanks for reading my ratings....just wanted to offer suggestions on how to teach a new pilot to think about the whole concept and bring it a little closer to the ground so to speak.....
Signed;
Dammyneckhurts
I am from Canada, spend most of my time looking down a long line, and flying in the mountains. I find myself in the kind of work environment that frequently lends itself to vortex ring situations. I am not an instructor, but I can perhaps throw a few ideas on the table that I wish I knew when I was starting out. Some of this will be redundant for people, and I apologize for any inaccurate terminology that may be used in current instructor speak.
It's important that students have a thorough understanding of exactly what Vortex Ring is. They need to know:
-What 3 factors you need to get into it,
-How to recognize it,
-How to get out of it.
I just wanted to elaborate a bit on some of these points and add to the concepts that the basic exercise teaches.
To get into it we need:
-Little or no airspeed,
-Rate of decent,
-And using power.
If you don’t have all 3 present no worries right? Well not always!.....Up flowing air gives you the same situation as being in a descent. Hovering at the top of a ridge can be brutal if the air is screaming up the slope at you from below. First off you are surprised at how little power you need to hover, you feel the mushiness of the tail, pull a bit of power and all of a sudden it gets real ugly. Same mechanics as the practice scenario, but different causes and your not 2000' off the ground!
Next part would be on how to recognize it. Most will talk about shakes and shudders, tail is all over the place etc..All very important of course, but the reality is that if you are on very short final and it gets that bad, you are in big trouble unless you make immediate and drastic action to recover.
In production work your going as fast as practical (not possible), floating in on minimal power and pulling power on very short final. Unless your SURE that you have no V.R. concerns its always good to do a little check with the collective just before the final pull. This is the crucial part... when you do this your looking to see how the helicopter is reacting and how it feels under your ass. If all is good you should feel HEAVIER in your seat when you pull a bit of collective. If your anywhere close to entering V.R. you will not get that "heavier" feeling as your weight will stay neutral. If your closer to V.R. you will feel lighter in your seat...all this happens WELL before you begin to notice the usual signs of V.R. that everyone talks about.
Now put this into the hovering on the top of the slope scenario previously mentioned. Your hovering 40' off the trees (or your longline load is 40' off the trees) trying to decide if your going in or not. Your at a low power setting due to upflow air, make sure you have some air under you so you can abort if necessary and give the collective a bit of a pull, are you heavier or lighter in your seat?
Finally a few comments on how to get out of it. We are all well versed on that one, I just wanted to expend a bit on the practice scenario that we all learn with. The reality is that if it happens unexpectedly at 100' or less, there is not a lot of room to work with. Another reality is that unless your in full blown V.R., it doesn’t take much to get out of it. Generally just getting out of your downwash by moving a few rotor widths will do it. (try to go crosswind if you know your downwind as it started). What if your are already below treeline going into a pad and dont have the room to maneuver? One thing that can help is to stir the cyclic in a circle. This disperses your downwash a bit and can help a lot in the recovery.
Awareness that you are working in conditions conducive to V.R. , early detection followed by swift and accurate recovery techniques is what its all about concerning V.R.
The emphasis though should be on awareness and detection.....all the recovery techniqes in the world wont help a guy that repeatedly stumbles into in unaware.....he wont live long or he will be so scared after the first time he will have a hard time in a production work environment and will be quick to shut it down as he gets out of his comfort zone.
Conditions that are most conducive to V.R. are working in the bush with light and variable winds. You don’t always have the luxury of any form of wind indication (no water, leafy trees etc). If you want to be quick in these conditions you need tools to detect early V.R., and be comfortable with rapid recovery methods. In conditions like this the "collective check how does it feel in the seat thing" allows you to stay well ahead of the game. Always do the check early enough to get out of it if it doesn’t feel good. This kind of thinking allows you to work downwind, and in a variable wind condition with confidence and swiftness in a production environment.
Can you tell that I am sitting on a fire contract and its pissing rain? Too much time on my hands..
Thanks for reading my ratings....just wanted to offer suggestions on how to teach a new pilot to think about the whole concept and bring it a little closer to the ground so to speak.....
Signed;
Dammyneckhurts
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I'll second that...
dammyneckhurts, great stuff... it's good to hear about techniques from different operations! Nicely put and you relate your message well (easily to understand).