Vortex Ring / Settling with power (Merged)
Nick,
I know and respect your credentials, but at the end of the day, you are, just like the rest of us, human and not 'perfect'
I am afraid this thread could end up like the one we all had with you regarding "ground effect" and the endless arguments and counter arguments for and against. I didn't (like so many others) believe your explanation then and I most certainly don't believe your explanation of VRS now
I support Mostafa 100% here, and will defend my understanding of VRS to the hilt.
VRS is NOT RECOVERABLE, not now, not never. There is only ONE reason you can survive fully developed VRS and that is IF the helo falls out of the aerodynamic condition it finds itself in of its own accord (and provided it does that before you run out of bernoulli's
VRS is where most if not all of the blade is 'STALLED'. The pilot may have initiated an auto but the stalled areas along the blade (innermost section near the root and the tip) now grow to envelope most of the surface of the blade. When that happens, the helo will accelerate to excessive ROD's as the rotor tumbles inside its own dirty air.
I believe VRS requires some fwd airflow over the blades to initiate it (induced airflow etc), therefore your statement about VRS in hovers being impossible might carry some truth...I'd need to check my notes.
Let the show begin.
I know and respect your credentials, but at the end of the day, you are, just like the rest of us, human and not 'perfect'
I am afraid this thread could end up like the one we all had with you regarding "ground effect" and the endless arguments and counter arguments for and against. I didn't (like so many others) believe your explanation then and I most certainly don't believe your explanation of VRS now
I support Mostafa 100% here, and will defend my understanding of VRS to the hilt.
VRS is NOT RECOVERABLE, not now, not never. There is only ONE reason you can survive fully developed VRS and that is IF the helo falls out of the aerodynamic condition it finds itself in of its own accord (and provided it does that before you run out of bernoulli's
VRS is where most if not all of the blade is 'STALLED'. The pilot may have initiated an auto but the stalled areas along the blade (innermost section near the root and the tip) now grow to envelope most of the surface of the blade. When that happens, the helo will accelerate to excessive ROD's as the rotor tumbles inside its own dirty air.
I believe VRS requires some fwd airflow over the blades to initiate it (induced airflow etc), therefore your statement about VRS in hovers being impossible might carry some truth...I'd need to check my notes.
Let the show begin.
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Thank heaven it is Saturday and we can all sit around the cyber-stove and just jaw a bit.....
In real flight, VRS is not a steady state condition. It is actually very hard to find, and not at all steady. It has absolutely nothing to do with any stall event of any kind. The angle of attack of the airfol is very low, thank you, and the flow through the rotor is quite unstalled.
What VRS is is a recirculated flow where the rotor works quite nicely to push a package of air downward, but the outside air simply walks that air package around the outside of the tips so it can get sucked back into the rotor. In effect, the rotor is simply recirculating the same air again and again. If we stated it simply, the rotor is now behaving very selfishly, and not taking much new air, and not letting all the air it pushes mix with the free stream. This means it is not transferring much momentum with the outside and thus the thrust drops somewhat. Since the state is so very unsteady, the indications are torque or MP jumps, eratic behavior, pitching and rolling, and the like. On incipient vrs, just lower the nose, increase power to max (do not droop the rotor) and it will walk out.
I will dig up a great graphic simulation of vrs on a rotor and post it. You can see the downwash get slowed as it enters the free stream below the rotor, and as the rod is increased, you can see the vortex reenter the rotor and form a beautiful ring.
In real flight, VRS is not a steady state condition. It is actually very hard to find, and not at all steady. It has absolutely nothing to do with any stall event of any kind. The angle of attack of the airfol is very low, thank you, and the flow through the rotor is quite unstalled.
What VRS is is a recirculated flow where the rotor works quite nicely to push a package of air downward, but the outside air simply walks that air package around the outside of the tips so it can get sucked back into the rotor. In effect, the rotor is simply recirculating the same air again and again. If we stated it simply, the rotor is now behaving very selfishly, and not taking much new air, and not letting all the air it pushes mix with the free stream. This means it is not transferring much momentum with the outside and thus the thrust drops somewhat. Since the state is so very unsteady, the indications are torque or MP jumps, eratic behavior, pitching and rolling, and the like. On incipient vrs, just lower the nose, increase power to max (do not droop the rotor) and it will walk out.
I will dig up a great graphic simulation of vrs on a rotor and post it. You can see the downwash get slowed as it enters the free stream below the rotor, and as the rod is increased, you can see the vortex reenter the rotor and form a beautiful ring.
Admittedly I have only basic P of F knowledge and a bit of ‘A’ level physics, but surely, if the rotor aerofoil is stalled then it is producing next to no lift. Therefore it is not throwing any airflow downwards to be re-circulated, if there is no recirculation you are not in vortex ring!
My image of the vortex ring state consists of the rotor disc producing lots of downwards moving air, now introduce a rate of descent (or upward moving air) and the downwards moving air can’t dissipate, instead it moves outwards and upwards where it finds a nice low pressure area immediately above the rotor disc, gets sucked in and round it goes again, and again, etc. Now we have the rotor disc ‘flying’ quite happily in its own ring doughnut of air – unfortunately, the doughnut of air will go wherever the rotor disc goes i.e. downwards!!
Downward airflow through the disc will surely reduce the angle of attack on the blades. Sorry, I can't see how you can disagree with Nick's explanation.
TeeS
My image of the vortex ring state consists of the rotor disc producing lots of downwards moving air, now introduce a rate of descent (or upward moving air) and the downwards moving air can’t dissipate, instead it moves outwards and upwards where it finds a nice low pressure area immediately above the rotor disc, gets sucked in and round it goes again, and again, etc. Now we have the rotor disc ‘flying’ quite happily in its own ring doughnut of air – unfortunately, the doughnut of air will go wherever the rotor disc goes i.e. downwards!!
Downward airflow through the disc will surely reduce the angle of attack on the blades. Sorry, I can't see how you can disagree with Nick's explanation.
TeeS
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There is some great info out there, but the data is for engineers, so it might be way too pointy-headed for us drivers. I can try to translate it into english, however. The points the below is intended to show is
1) That true VRS does not occur at 300 fpm, it takes a lot more descent rate.2) that VRS is not a destruction of rotor lift as we know it (like "crossing the streams" in Ghostbusters!) but rather a place where the thrust is unsteady and the power required goes up a bunch.
The paper that says it all is here, done by Dr. Gordon Leishman, who is a real pro at the U of Maryland:
http://www.enae.umd.edu/AGRC/Aero/AH...hman.pdf<br />
Vortex ring is a state where the recirculation makes the rotor less efficient, so it chews up more power making the lift.
Rate of descent for vrs:
Here is a rotor in a steady hover. See the ring of tighter vortexes forming at the bottom, which seems to form "just because", and is probably the reason why we feel the puffs of downwash from a rotor, instead of a steady stream as it flys by:
Here it is as the rotor is descending at 0.15 times the downwash speed (225 fpm for a R-22). Note the downwash has been stopped and a ring forms below the rotor, at the place where the downwash (which is slowing down as it disburses) matches the upward wind:
Here is a sequence as we descend ever faster, all the way to incipient vrs:
at 675 fpm, the rings are less steady, and closer to the rotor:
at 900 fpm, the rings are more compact but still push away from the rotor:
at 1050 fpm, 0.7 times the downwash speed, the rings start to get closer to the rotor, and sometimes puff through it. This 3 shot sequence takes place in about a second (six rotor revolutions!) so these rings are fast. The pilot would feel a 1 per second torque variability, cyclic and collective control problems, and lots of yaw, due to the torque spikes. This is incipient vrs, for the first time in this sequence:
For such descents, the power rise is shown below, with the curve illustrating the amount of increase in induced power needed to trim at flight thrust (full lift). Induced power might be 30% of the total power, so you can estimate the total power increase as maybe 1/3 of the amount shown. Note that for a descent at half the downwash speed (that is the -0.5 on the horizontal scale), the induced power would increase by about 40%, so the total power might fluctuate at about 15% above normal. The curve is experimental data, cited in Dr. Leishman's paper.
Here is an animated file of a rotor seen from the side at Dr. Leishman's web site. The animation takes a rotor from steady hover, then has it go faster and faster downward until it gets into VRS and then past VRS into autorotation (windmill brake state). Each blade's tip vortex is plotted as a different color green, red, orange, yellow. As the descent rate goes from steady hover to VRS and then into autorotation, see how the ring forms below the rotor then is pushed into the rotor as the descent rate increases, then finally the ring drifts away upward in smooth autorotational flow:
If the animation does not work, here is the link so you can just go directly to the site:
http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif
Also, the thrust of the rotor does not change and make the rotor stop producing lift. here is a plot of the thrust for the experimental rotors as they are forced to go through descent, VRs and finally autorotation:
One place where I must admit I was wrong, even leishman says that some inboard sections of the rotor experience stall in vrs. As I have always said, we all learn on pprune!
The detailed discussion from Dr. Leishman, and dozens of animated gif files are shown at his fantastic web site:
http://www.enae.umd.edu/AGRC/Aero/vring.html#Case%201st
I should point out for pprune's benifit, Dr. Leishman is a transplanted Brit!
1) That true VRS does not occur at 300 fpm, it takes a lot more descent rate.2) that VRS is not a destruction of rotor lift as we know it (like "crossing the streams" in Ghostbusters!) but rather a place where the thrust is unsteady and the power required goes up a bunch.
The paper that says it all is here, done by Dr. Gordon Leishman, who is a real pro at the U of Maryland:
http://www.enae.umd.edu/AGRC/Aero/AH...hman.pdf<br />
Vortex ring is a state where the recirculation makes the rotor less efficient, so it chews up more power making the lift.
Rate of descent for vrs:
Here is a rotor in a steady hover. See the ring of tighter vortexes forming at the bottom, which seems to form "just because", and is probably the reason why we feel the puffs of downwash from a rotor, instead of a steady stream as it flys by:
Here it is as the rotor is descending at 0.15 times the downwash speed (225 fpm for a R-22). Note the downwash has been stopped and a ring forms below the rotor, at the place where the downwash (which is slowing down as it disburses) matches the upward wind:
Here is a sequence as we descend ever faster, all the way to incipient vrs:
at 675 fpm, the rings are less steady, and closer to the rotor:
at 900 fpm, the rings are more compact but still push away from the rotor:
at 1050 fpm, 0.7 times the downwash speed, the rings start to get closer to the rotor, and sometimes puff through it. This 3 shot sequence takes place in about a second (six rotor revolutions!) so these rings are fast. The pilot would feel a 1 per second torque variability, cyclic and collective control problems, and lots of yaw, due to the torque spikes. This is incipient vrs, for the first time in this sequence:
For such descents, the power rise is shown below, with the curve illustrating the amount of increase in induced power needed to trim at flight thrust (full lift). Induced power might be 30% of the total power, so you can estimate the total power increase as maybe 1/3 of the amount shown. Note that for a descent at half the downwash speed (that is the -0.5 on the horizontal scale), the induced power would increase by about 40%, so the total power might fluctuate at about 15% above normal. The curve is experimental data, cited in Dr. Leishman's paper.
Here is an animated file of a rotor seen from the side at Dr. Leishman's web site. The animation takes a rotor from steady hover, then has it go faster and faster downward until it gets into VRS and then past VRS into autorotation (windmill brake state). Each blade's tip vortex is plotted as a different color green, red, orange, yellow. As the descent rate goes from steady hover to VRS and then into autorotation, see how the ring forms below the rotor then is pushed into the rotor as the descent rate increases, then finally the ring drifts away upward in smooth autorotational flow:
If the animation does not work, here is the link so you can just go directly to the site:
http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif
Also, the thrust of the rotor does not change and make the rotor stop producing lift. here is a plot of the thrust for the experimental rotors as they are forced to go through descent, VRs and finally autorotation:
One place where I must admit I was wrong, even leishman says that some inboard sections of the rotor experience stall in vrs. As I have always said, we all learn on pprune!
The detailed discussion from Dr. Leishman, and dozens of animated gif files are shown at his fantastic web site:
http://www.enae.umd.edu/AGRC/Aero/vring.html#Case%201st
I should point out for pprune's benifit, Dr. Leishman is a transplanted Brit!
Last edited by NickLappos; 13th Jun 2004 at 15:16.
TeeS, you mention you A level physics so just spoke to my daughter, doesn't usually help!!! albeit, she gave me a wonderful sum once about slope landings and how helicopters could land on excessive slopes by compounding the slope. Anyway, you dont have an equation without various weights, gravity, various accelerations, chords, washout/shape/taper types of head and a few other things I can't even spell she's a Dr MPhys (Hons) MSC AMinstP DIP IPEM, so far but not an aerodynamicist. Not sure I am disagreeing with Nick or he with me. Other than one of terminology.
Nick, nice animation and very graphical, a good learning lesson but cant see how it proves your point I believe it generalises, pity the max rates of descent are only half what you quoted hope you sort of agree with my my last sentance to TeeS. Enjoy
Nick, nice animation and very graphical, a good learning lesson but cant see how it proves your point I believe it generalises, pity the max rates of descent are only half what you quoted hope you sort of agree with my my last sentance to TeeS. Enjoy
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good working diagram nick!
thomas, what would the rate of decent be to stall the blade (not just the root) at 100% rpm? pretty scary i would imagin, can your downwash be that fast? otherwise you would be in auto, wouldnt you?
when learning to fly, VRS had me scared so my instructor took me out to show me it. we had the needle peged at 2000 fpm and came out quite easily. we used a fair bit of collective too, dont think we got to 7000 fpm though.
thomas, what would the rate of decent be to stall the blade (not just the root) at 100% rpm? pretty scary i would imagin, can your downwash be that fast? otherwise you would be in auto, wouldnt you?
when learning to fly, VRS had me scared so my instructor took me out to show me it. we had the needle peged at 2000 fpm and came out quite easily. we used a fair bit of collective too, dont think we got to 7000 fpm though.
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MOSTAFA,
Thanks for the questions, I redid the post above to more clearly show the progression to more descent. I hope the new organization is helpful. If the group can follow all this, I will transplant it to my web site to add to the more general stuff there.
Your daughter sounds wonderful, when will she start making enough money so you can retire, buy that helicopter and just fly around? Unfortunately, the unsteady nature of the helo rotor wake make it a candidate for simulation, and not equations, as Dr. leishman has done. His work does include a family of helicopters all with different chords, radius, etc. They have different properties, but follow the general behavior I have extracted here. Pass the web link for Leishman's site to her, she will blast right through it, I am sure.
As a note, in flight test I have descended vertically through vrs to vertical autorotation several hundred times, so I can confirm that this sequence IS what rotors do.
Thanks for the questions, I redid the post above to more clearly show the progression to more descent. I hope the new organization is helpful. If the group can follow all this, I will transplant it to my web site to add to the more general stuff there.
Your daughter sounds wonderful, when will she start making enough money so you can retire, buy that helicopter and just fly around? Unfortunately, the unsteady nature of the helo rotor wake make it a candidate for simulation, and not equations, as Dr. leishman has done. His work does include a family of helicopters all with different chords, radius, etc. They have different properties, but follow the general behavior I have extracted here. Pass the web link for Leishman's site to her, she will blast right through it, I am sure.
As a note, in flight test I have descended vertically through vrs to vertical autorotation several hundred times, so I can confirm that this sequence IS what rotors do.
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I hate to sound like another "Nick Lappos fan", since I work for another company and spend most of my time trying to argue against him behind the scenes....
But- why must everyone argue with his discussion on VRS? What he is putting out is as close to gospel as it gets.
I can verify that what he is trying to teach us is the same information that is taught at the US Naval test pilot school (where there are just a few people knowledgeable in helicopter aerodynamics). His information is backed up by flight test data by BHTI and Boeing as well as Sikorsky. Recent flight testing on VRS has focused on the V-22 and its somewhat unique issues because of the potential effects of VRS on only one of the two prop rotors, but the V-22 data agrees with what Nick is teaching.
The arguments are entertaining, but don't confuse "my instructor says" with actual flight test data backed by aerodynamics lab work (wind tunnels, CFD, and other simulation.)
On the other hand, the arguing does help, because it prompts Nick to give his lessons again, which is a valuable refresher training...
But- why must everyone argue with his discussion on VRS? What he is putting out is as close to gospel as it gets.
I can verify that what he is trying to teach us is the same information that is taught at the US Naval test pilot school (where there are just a few people knowledgeable in helicopter aerodynamics). His information is backed up by flight test data by BHTI and Boeing as well as Sikorsky. Recent flight testing on VRS has focused on the V-22 and its somewhat unique issues because of the potential effects of VRS on only one of the two prop rotors, but the V-22 data agrees with what Nick is teaching.
The arguments are entertaining, but don't confuse "my instructor says" with actual flight test data backed by aerodynamics lab work (wind tunnels, CFD, and other simulation.)
On the other hand, the arguing does help, because it prompts Nick to give his lessons again, which is a valuable refresher training...
Gregg, I believe its just one of those topics where terminology Americanism/Britism dont compute. If you read the posts I have put on this thread I dont think we disagree on anything except the gap between you and the ground. I cannot spk for piston drivers, never been one and I also think the FTGs or the FCUs, depending on types or both make a difference. Continue enjoying but be sure nobody is trying to pull down Nick, quite the opposite his threads are very informative, always food for thought. His last animation even got me talking to ETPS again.
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Somehow I think I've written the following before...
Demonstrating the incipient stages of Vortex Ring State is the RW equivalent to stalling a FW airplane. You learn what the symptoms are in training, and then learn to avoid it for the rest of your flying career.
I have only ever been able to get the symptoms when entering at sufficient height AGL (more than 2,000' AGL, just like stalling), and from a downwind condition. Start at 60 knots, reduce the power slightly below the power to maintain level flight at 60 knots and decelerate maintaining altitude. At about 20 knots, the first symptom will appear (low frequency airframe vibrations). Next the aircraft may (and note the may) exhibit uncommanded pitch, roll and yaw (i.e. don't move the stick or pedals and the aircraft wanders around). Not always repeatable. Rate of descent is 500- 1,000 fpm. Adding collective may or may not increase the rate of descent (not always repeatable).
Recovery is by smoothly, but positively lowering the nose about 20 degrees below the horizon and flying out of it. Safe to do as described.
Learn the symptoms and recover at the first sign of the symptoms.
Any time the induced velocity equals the airflow coming into the the rotor disk, something has to give. The induced velocity (the downwash velocity if you will) is not uniform across the disk when hovering, so the effect will probably start at the hub and move outboard as power is increased.
Something that should be done on transition to a new type, and at a later stage of PPL training as well, just so you remember it. Doing it too early risks losing it in all the other stuff you have to remember.
Demonstrating the incipient stages of Vortex Ring State is the RW equivalent to stalling a FW airplane. You learn what the symptoms are in training, and then learn to avoid it for the rest of your flying career.
I have only ever been able to get the symptoms when entering at sufficient height AGL (more than 2,000' AGL, just like stalling), and from a downwind condition. Start at 60 knots, reduce the power slightly below the power to maintain level flight at 60 knots and decelerate maintaining altitude. At about 20 knots, the first symptom will appear (low frequency airframe vibrations). Next the aircraft may (and note the may) exhibit uncommanded pitch, roll and yaw (i.e. don't move the stick or pedals and the aircraft wanders around). Not always repeatable. Rate of descent is 500- 1,000 fpm. Adding collective may or may not increase the rate of descent (not always repeatable).
Recovery is by smoothly, but positively lowering the nose about 20 degrees below the horizon and flying out of it. Safe to do as described.
Learn the symptoms and recover at the first sign of the symptoms.
Any time the induced velocity equals the airflow coming into the the rotor disk, something has to give. The induced velocity (the downwash velocity if you will) is not uniform across the disk when hovering, so the effect will probably start at the hub and move outboard as power is increased.
Something that should be done on transition to a new type, and at a later stage of PPL training as well, just so you remember it. Doing it too early risks losing it in all the other stuff you have to remember.
B*gger me Shaun!!! thats another British'ism, nobody is disputing what you are saying about the onset of any type if incipient stall or your words about recovery. I for one have had enough, enjoy.
Anyone ex-brit mil should have been taught the same thing about VRS and it goes as follows:
1. Heli in the free air hover - root of blades at higher AoA than tips due to washout and rotational speed - total thrust of disc equals weight so hover maintained.
2. Rate of descent eg 500' - 1000' fpm allowed to develop (for whatever reason) - tips start to experience recirculation (loss of thrust) root of blade reaches Cl max and starts to lose thrust (stalled aerofoils still produce thrust) - net thrust of disc reduces and RoD increases. At this stage power available maybe sufficient to overcome RoD - if not then settling with power is occurring and step 3 is next if the ground doesn't get in the way.
3. As RoD is allowed to increase, more of the tip experiences recirculation and more of the root stalls -compounded by raising the lever which spreads the stall outboard from the root - further loss of thrust and acceleration downwards.
4. Eventually, only the central portion of the blade that is neither stalled or in recirculation is producing any useful thrust - nothing like enough to match the weight so the ac continues to accelerate downwards. The air trying to escape round or through the rotor gives random pitch, roll and or yaw and makes recovery from just forward cyclic very difficult.
Recovery - lower the lever, to auto if possible and try to achieve airflow across the disc with forward cyclic.
MOSTAFA - Nick is right, in fully developed VRS raising the lever will not make the ac fall faster as it is already going down as quickly as it can - however, in the incipient stages, raising the lever can exacerbate the problem and take you into fully developed VRS.
Nick's stuff is excellent, but for mere mortals a more than adequate explanation is in AP3456
1. Heli in the free air hover - root of blades at higher AoA than tips due to washout and rotational speed - total thrust of disc equals weight so hover maintained.
2. Rate of descent eg 500' - 1000' fpm allowed to develop (for whatever reason) - tips start to experience recirculation (loss of thrust) root of blade reaches Cl max and starts to lose thrust (stalled aerofoils still produce thrust) - net thrust of disc reduces and RoD increases. At this stage power available maybe sufficient to overcome RoD - if not then settling with power is occurring and step 3 is next if the ground doesn't get in the way.
3. As RoD is allowed to increase, more of the tip experiences recirculation and more of the root stalls -compounded by raising the lever which spreads the stall outboard from the root - further loss of thrust and acceleration downwards.
4. Eventually, only the central portion of the blade that is neither stalled or in recirculation is producing any useful thrust - nothing like enough to match the weight so the ac continues to accelerate downwards. The air trying to escape round or through the rotor gives random pitch, roll and or yaw and makes recovery from just forward cyclic very difficult.
Recovery - lower the lever, to auto if possible and try to achieve airflow across the disc with forward cyclic.
MOSTAFA - Nick is right, in fully developed VRS raising the lever will not make the ac fall faster as it is already going down as quickly as it can - however, in the incipient stages, raising the lever can exacerbate the problem and take you into fully developed VRS.
Nick's stuff is excellent, but for mere mortals a more than adequate explanation is in AP3456
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I attended an HAI Instructor Pilot Refresher clinic in the year 2000 where I sat through a very interesting presentation by a Bell test pilot. He told those in attendance that it is impossible to get Vortex Ring State (Settling With Power) in a Bell 206. He claimed to have the test data to prove it. I can't remember the fellows name but it was a very interesting discussion. I don't buy his theory, but many walked away from the presentation believing it was true.
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I think the way it could be said, Rich, is that the VRS you experience in a properly powered helo is not something that will cause it to descend unstoppably or lose control. The crane driving test pilots at Sikorsky used to describe vertical descents until deep into vrs, then lifting collective and climbing straight up out of it. Done by professionals on a closed course, do not try this at home!
I got a great case of vrs doing short field landing OEI in an S-76 once. Engine cut at 140 feet, passed thru 75 feet at 50 feet per second rate of descent. Hit at 13 fps, bounced. Makes you shake the hand of the airframe designer, vigorously!
I got a great case of vrs doing short field landing OEI in an S-76 once. Engine cut at 140 feet, passed thru 75 feet at 50 feet per second rate of descent. Hit at 13 fps, bounced. Makes you shake the hand of the airframe designer, vigorously!
MOSTAFA - I have re-read the thread and your posts - settling with power is not the same as overpitching - overpitching is when the Nr decays due to insufficient engine power to keep driving the rotors - settling with power can occur without Nr decay, you simply don't have enough power to arrest your RoD. Overpitching may rapidly follow settling with power or be combined with it as the pilot tries to stop the ground from smiting him but the 2 events can happen quite separately.
Get Alan Wiles on the subject and he will tell you about VRS in a Scout and losing 6000 plus in the recovery in NI.
All of Nicks other points are valid, as ever, and we have been through the terminology issue several times on these threads.
The civvy world teaches incipient VRS recognition and recovery, the military doesn't, although the Sea King force used to do it at 10,000 feet, it is now covered in the simulator (much safer).
I have been shown and carried out the demo in an R22 and a 206 and frankly it's a bit of a non-event. Taking it beyond the incipient stage is in my view practise bleeding and the risks outweigh the benefit - at the point at which pilots are blase about VRS, someone is going to get hurt.
The argument about updraughting air producing VRS is cobblers - in an updraught your pitch settings are much lower (sometimes in autorotation) so the recirculation at the tips is much less and the root AoA much lower. I have sat in more updraughting air than I can remember in several different aircraft types and the only time a descent occurs is when the wind gusts or drops and you are a bit slow to react. The only problems with approaches to mountain sites in these conditions are that even with the lever fully down, a big updraught will send you upwards and once you move out of the updraught your power required increases very quickly, especially if there is any turbulence caused by ledges/ridges which can produce a local downdraught (like rotor streaming). We teach SAR pilots to fly in the updraughting air in the mountains because it is so much safer/comfortable.
Get Alan Wiles on the subject and he will tell you about VRS in a Scout and losing 6000 plus in the recovery in NI.
All of Nicks other points are valid, as ever, and we have been through the terminology issue several times on these threads.
The civvy world teaches incipient VRS recognition and recovery, the military doesn't, although the Sea King force used to do it at 10,000 feet, it is now covered in the simulator (much safer).
I have been shown and carried out the demo in an R22 and a 206 and frankly it's a bit of a non-event. Taking it beyond the incipient stage is in my view practise bleeding and the risks outweigh the benefit - at the point at which pilots are blase about VRS, someone is going to get hurt.
The argument about updraughting air producing VRS is cobblers - in an updraught your pitch settings are much lower (sometimes in autorotation) so the recirculation at the tips is much less and the root AoA much lower. I have sat in more updraughting air than I can remember in several different aircraft types and the only time a descent occurs is when the wind gusts or drops and you are a bit slow to react. The only problems with approaches to mountain sites in these conditions are that even with the lever fully down, a big updraught will send you upwards and once you move out of the updraught your power required increases very quickly, especially if there is any turbulence caused by ledges/ridges which can produce a local downdraught (like rotor streaming). We teach SAR pilots to fly in the updraughting air in the mountains because it is so much safer/comfortable.
I think the only person missing in this discussion is: Prouty!!!
Great to get such a professional response.
However, in an attempt to clear the misunderstandings:
I think we all agree there is such a thing as incipient VRS and then there is VRS. [Let's forget about overpitching and settling with power for now!].
We all agree that symptoms for both, are subtly different.
[I have read Leischmanns report on VRS and several others, too.]
What I have yet to see in print, is that VRS is neither demonstrable , nor survivable (at low level).
If we all accept that VRS is a development of IVRS, where nearly all of the blade surface is stalled and that the controls are behaving erratically and randomly, how can one take control of the aircraft to be able to recover? The helo, in its 'uncontrollabe state' might decide to flip backwards or roll inverted??
IVRS is, I believe, misconstrued as the "full monty" that is to say, people think they are either being shown VRS or are experiencing VRS. Not true, they are experiencing IVRS.
No-one to my knowledge would demonstrate fully blown VRS with all the trimmings, surely
Crab: I am ex mil, and often demonstrated IVRS to ab initio's? Don't the mil do it any more?
Great to get such a professional response.
However, in an attempt to clear the misunderstandings:
I think we all agree there is such a thing as incipient VRS and then there is VRS. [Let's forget about overpitching and settling with power for now!].
We all agree that symptoms for both, are subtly different.
[I have read Leischmanns report on VRS and several others, too.]
What I have yet to see in print, is that VRS is neither demonstrable , nor survivable (at low level).
If we all accept that VRS is a development of IVRS, where nearly all of the blade surface is stalled and that the controls are behaving erratically and randomly, how can one take control of the aircraft to be able to recover? The helo, in its 'uncontrollabe state' might decide to flip backwards or roll inverted??
IVRS is, I believe, misconstrued as the "full monty" that is to say, people think they are either being shown VRS or are experiencing VRS. Not true, they are experiencing IVRS.
No-one to my knowledge would demonstrate fully blown VRS with all the trimmings, surely
Crab: I am ex mil, and often demonstrated IVRS to ab initio's? Don't the mil do it any more?
Last edited by Thomas coupling; 15th Jun 2004 at 08:35.
Last one, for what has been a most enlightening discussion. Crab@SAAvn thanks for your input both TC and I have entered into this debate and again I think added nothing but good old fashioned honest experience. Its a shame really that we all write under usernames because it doesn't really allow the reader to guage any experience. Perhaps its time to come out, perhaps its time to go back to school and take english lessons because like you, I have read all the posts again and cannot see anywhere, where anybody disagree's other than pilots having IVRS demo'ed to them. I advocated the use of a simulator and that people should be extremely wary of allowing the aircraft into a VRS. Like, as you quote "Get AW on the subject" I have on numerous occasions having served with and for him for the best part of 25 years. It would be impossible not too having a good 3000hrs on the Scout myself. The best aircraft I have ever instructed on to demonstrate overpitching in manual throttle. Perhaps that is why I am so reticent about playing around with IVRS, anyway enough of my ranting. I shall continue to enjoy reading in between lifts
At the risk of being shot down (or even entering irecoverable VRS!) could someone explain what difference being downwind will have when conducting this demonstration (IVRS) at a suitable altitude?
I fully understand the significance of being downwind close to the ground and allowing IAS to decay due to external cues, but surely if at N thousand ft plus,, and using the IAS as a reference for the entry conditions there should be no relevance to the wind direction?
Not wishing to get into a 'downwind turn' debate!
I fully understand the significance of being downwind close to the ground and allowing IAS to decay due to external cues, but surely if at N thousand ft plus,, and using the IAS as a reference for the entry conditions there should be no relevance to the wind direction?
Not wishing to get into a 'downwind turn' debate!