Vortex Ring / Settling with power (Merged)
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Won't be differing with any of that!
Will only add that recovery method has to be 'tailored' to height available. At 3000'
that's one thing, at 75' into a confined is something else. Poor speed/power management is the major cause but a downdraft would be very unfortunate at the latter height.
I stick firmly to the 30/30 rule, no more than 30(0) ft per minute if speed is less than 30 kt. You won't fall into any trap then. I have also heard that it cannot develop at approach angles of LESS than 30 degrees but have had no time to go into that.
Maybe someone else has?
Will only add that recovery method has to be 'tailored' to height available. At 3000'
that's one thing, at 75' into a confined is something else. Poor speed/power management is the major cause but a downdraft would be very unfortunate at the latter height.
I stick firmly to the 30/30 rule, no more than 30(0) ft per minute if speed is less than 30 kt. You won't fall into any trap then. I have also heard that it cannot develop at approach angles of LESS than 30 degrees but have had no time to go into that.
Maybe someone else has?
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Reduce power, forward cyclic; forward cyclic, reduce power. I agree.
It's much like the fellow who showed up to work in a new duty site at 6000' in the summer, having arrived from a long period at sea level.
I said " The book tells you that the 180 auto has several steps. Collective down, pedal adjust, cyclic into the turn, check rate of turn, etc, etc. Not correct. At this elevation it's one step or you won't finish the turn till your on the runway" He didn't believe that. Half way through the turn I caught sight of the man with no forhead that I've seen a few times before. From cheek bones to hairline he was nothing but eyeballs.
To take a position, I do agree that you won't get what you need from forward cyclic till you reduce the collective.
The worst aircraft for VR and S w/P for me has been the CH-47. He was flying, I was digging for a map (not doing my job of paying attention). There were no vibrations to alert me out of the map case. When I looked up we were falling like and anvil out of trim, so smooth. Not what anyone who has flown a few Chinooks would ever expect in terms of vibratory level. He was still operating well within the bliss that ignorance provides.
It's much like the fellow who showed up to work in a new duty site at 6000' in the summer, having arrived from a long period at sea level.
I said " The book tells you that the 180 auto has several steps. Collective down, pedal adjust, cyclic into the turn, check rate of turn, etc, etc. Not correct. At this elevation it's one step or you won't finish the turn till your on the runway" He didn't believe that. Half way through the turn I caught sight of the man with no forhead that I've seen a few times before. From cheek bones to hairline he was nothing but eyeballs.
To take a position, I do agree that you won't get what you need from forward cyclic till you reduce the collective.
The worst aircraft for VR and S w/P for me has been the CH-47. He was flying, I was digging for a map (not doing my job of paying attention). There were no vibrations to alert me out of the map case. When I looked up we were falling like and anvil out of trim, so smooth. Not what anyone who has flown a few Chinooks would ever expect in terms of vibratory level. He was still operating well within the bliss that ignorance provides.
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Lu Z,
As I understand it the BERP blade was designed to overcome problems with compressibility at the high speed end of the flight regime. They are in effect a swept tip but to reduce twisting moments the rear of the blade shape is modified too.
As I understand it the BERP blade was designed to overcome problems with compressibility at the high speed end of the flight regime. They are in effect a swept tip but to reduce twisting moments the rear of the blade shape is modified too.
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SPS/Randy_G:
I agree with you completely, settling with power is not the correct term for VRS (Power Settling).
A classic example of Settling with power is the young hotdog trying to impress his onlookers and starting a return to target into wind! The resultant course reversal into a downwind return sets you up for a classic settling with power situation. Too heavy for the ambient conditions without enough power to recover.
Cheers, OffshoreIgor
[This message has been edited by offshoreigor (edited 23 January 2001).]
[This message has been edited by offshoreigor (edited 23 January 2001).]
I agree with you completely, settling with power is not the correct term for VRS (Power Settling).
A classic example of Settling with power is the young hotdog trying to impress his onlookers and starting a return to target into wind! The resultant course reversal into a downwind return sets you up for a classic settling with power situation. Too heavy for the ambient conditions without enough power to recover.
Cheers, OffshoreIgor
[This message has been edited by offshoreigor (edited 23 January 2001).]
[This message has been edited by offshoreigor (edited 23 January 2001).]
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When I posted the earlier comments on why a VRS intensifies, I wanted to add the following comments but chose to wait to see if the comments would be anywhere near correct.
A vortex also has the property of being "semi-static". By this I mean the rotational energy and velocity of a vortex causes it to stablize its position in the airmass that it was created in. However it will move with the prevailing winds and air currents that move the airmass it exists in. That's why it's "semi-static", it remains positionally static within the surrounding airmass, but moves when the surrounding airmass moves.
I think a very good example of visualizing a vortex ring occurs when a prop driven aircraft is taking off in high humidity air. In these conditions you can "see" the prop tip vortexes being generated by the prop tip, by seeing the moisture inside those vortexes. I think this happens because the rotational energy of the vortex causes a low pressure area inside the vortex (like in a tornado or hurricane) to develop, and the low pressure area causes the inside air's "dew point" to be reached and the inside air "fogs". You also notice that the visible moisture soon dissipates as the vortexes dissipates.
Being able to "see" these circular "ring" shaped vortexes on the prop tips makes it easier to understand how vortex ring state develops in a helicopter. If you've ever seen this, you know that the visible circular vortexes move towards the rear of the aircraft in a spiral corkscrew shape as the aircraft accelerates down the runway. On occasion you can even see this corkscrew pattern when a prop driven aircraft has its engines under power but is stationary with its brakes applied.
The corkscrew pattern is being developed because the vortexes are stationary within an airmass that is being moved towards the rear of the aircraft by the force of the propellers. Now use your imagination and suppose that a rather hefty tug were put on the nosewheel of the aircraft. Imagine that a pilot in the aircraft pushed the throttles forward on a very humid day (so you could see the vortexes) and the tug operator began to push the aircraft backwards until the speed of the aircraft moving backwards began to equal the speed of the airmass being driven backwards by the propellers. The ring vortexes on the prop tips would no longer be stretched out in a corkscrew pattern, but all of the vortexes being generated would now accumulate in one spot around the arc of the propeller, within the propeller driven airmass that is now stationary with respect to the rearward moving airframe. I hope you can visualize this.
In a helicopter "settling with power", this is exactly what you have. The rotor is driving down an airmass around the helicopter's airframe, while the airframe itself is falling down at a rate that is at or near the airspeed of the decending air column generated by the rotors. The rotor tip vortexes are no longer being spread out in a corkscrew pattern, but are being allowed to accumulate at the rotor. This causes the effect of "vortex ring state" to intensify as the rotor tip vortexes have nowhere to go, and they just keep adding energy to the vortex ring that already exists.
I agree completely with the methods of escape described in the other posts on this topic. Since the vortex ring is "semi-static" within the descending air column, then the best method of escape is to cut collective and/or power and to push the cyclic forward, leaving the vortex ring behind. Side stepping the vortex ring with side cyclic could also work, which the Chinook pilots seem to find helpful. For a tiltrotor, forward cyclic would seem to work best as this would seem to remove both rotors from the vortex rings at the same time.
Please correct me if any of this sounds wrong.
------------------
Safe flying to you...
[This message has been edited by Flight Safety (edited 25 January 2001).]
A vortex also has the property of being "semi-static". By this I mean the rotational energy and velocity of a vortex causes it to stablize its position in the airmass that it was created in. However it will move with the prevailing winds and air currents that move the airmass it exists in. That's why it's "semi-static", it remains positionally static within the surrounding airmass, but moves when the surrounding airmass moves.
I think a very good example of visualizing a vortex ring occurs when a prop driven aircraft is taking off in high humidity air. In these conditions you can "see" the prop tip vortexes being generated by the prop tip, by seeing the moisture inside those vortexes. I think this happens because the rotational energy of the vortex causes a low pressure area inside the vortex (like in a tornado or hurricane) to develop, and the low pressure area causes the inside air's "dew point" to be reached and the inside air "fogs". You also notice that the visible moisture soon dissipates as the vortexes dissipates.
Being able to "see" these circular "ring" shaped vortexes on the prop tips makes it easier to understand how vortex ring state develops in a helicopter. If you've ever seen this, you know that the visible circular vortexes move towards the rear of the aircraft in a spiral corkscrew shape as the aircraft accelerates down the runway. On occasion you can even see this corkscrew pattern when a prop driven aircraft has its engines under power but is stationary with its brakes applied.
The corkscrew pattern is being developed because the vortexes are stationary within an airmass that is being moved towards the rear of the aircraft by the force of the propellers. Now use your imagination and suppose that a rather hefty tug were put on the nosewheel of the aircraft. Imagine that a pilot in the aircraft pushed the throttles forward on a very humid day (so you could see the vortexes) and the tug operator began to push the aircraft backwards until the speed of the aircraft moving backwards began to equal the speed of the airmass being driven backwards by the propellers. The ring vortexes on the prop tips would no longer be stretched out in a corkscrew pattern, but all of the vortexes being generated would now accumulate in one spot around the arc of the propeller, within the propeller driven airmass that is now stationary with respect to the rearward moving airframe. I hope you can visualize this.
In a helicopter "settling with power", this is exactly what you have. The rotor is driving down an airmass around the helicopter's airframe, while the airframe itself is falling down at a rate that is at or near the airspeed of the decending air column generated by the rotors. The rotor tip vortexes are no longer being spread out in a corkscrew pattern, but are being allowed to accumulate at the rotor. This causes the effect of "vortex ring state" to intensify as the rotor tip vortexes have nowhere to go, and they just keep adding energy to the vortex ring that already exists.
I agree completely with the methods of escape described in the other posts on this topic. Since the vortex ring is "semi-static" within the descending air column, then the best method of escape is to cut collective and/or power and to push the cyclic forward, leaving the vortex ring behind. Side stepping the vortex ring with side cyclic could also work, which the Chinook pilots seem to find helpful. For a tiltrotor, forward cyclic would seem to work best as this would seem to remove both rotors from the vortex rings at the same time.
Please correct me if any of this sounds wrong.
------------------
Safe flying to you...
[This message has been edited by Flight Safety (edited 25 January 2001).]
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Agreed. Although 'lever down' is part of the generally accepted recovery , IMO , it is more of an attempt to make clear in the mind-set of the 'victim' that 'normal' rules no longer apply and raising lever in response to increased sink rate is futile. Once fully-developed I have found a very stable elegant aerodynamic condition (v. high R.O.D.) and have still had good cylcic authority (Offshore; not also true in your types? inertesting.) to maintain or 'fly out' of V.R. - min hight loss recovery being Attitude (only) till VR eliminated then Power up and nose up to climb - recoveries which reach more than 30-40 kts are just energy (height)inefficient. IMO.
Auto recovery is nice (but does not minimise height loss).
Auto recovery is nice (but does not minimise height loss).
Unrecoverable Vortex Ring State
After reading the discussions on the Blackhawk accident threads, could it be possible to get into an unrecoverable VRS? Say a helo is in a full blown VRS with a huge ROD of 4000+ fpm. Could it be possible that:
1. The rotor is completely stalled and so cyclic pitch changes will have no effect on disc attitude, or
2. With such a huge ROD, even with the collective on the floor, the ROD airflow is large enough to give a resultant relative airflow and angle of attack that exceeds the stalling angle on the rotor blade?
I have been in many doozies in the training environment and have always effected the recovery before the ROD exceeded 3000 fpm, but I always presumed some parts of the rotor disc were still doing something right. Any ideas or test data to prove or disprove my theory?
1. The rotor is completely stalled and so cyclic pitch changes will have no effect on disc attitude, or
2. With such a huge ROD, even with the collective on the floor, the ROD airflow is large enough to give a resultant relative airflow and angle of attack that exceeds the stalling angle on the rotor blade?
I have been in many doozies in the training environment and have always effected the recovery before the ROD exceeded 3000 fpm, but I always presumed some parts of the rotor disc were still doing something right. Any ideas or test data to prove or disprove my theory?
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(Edited to reflect light disk loading helicopters)
The state where the rotor is in VRS is strictly defined by the ratio of the descent rate to the downwash velocity of the specific helicopter. Only within that boundary is VRS possible. One caviat is that we read RoD with a static instrument, which can go flakey when the wash around the fuselage is skewed beyond the normal certification envelope.
For modern helicopters, VRS is unlikely below vertical speed ratios of .5 and beyond ratios of 1.5. For an R-22. descent rates less than about 700 fpm are below the VRS threshold, and descent rates beyond 2100 fpm are above VRS. For Black Hawk, descent rates of about 1200 FPM are below VRS and greater than 4000 FPM are above VRS. The Vortex in VRS is produced when the downward velocity matches the rotor's downwash, so the exact match causes the air to simply wrap around the rotor tips. Lift is lost quickly, and the bottom falls out. If the power is left up and the condition is stable, the descent rate is constant and very high (about equal to the downwash velocity - maybe 1400 fpm (robbie) to 3000 fpm (Hawk)). The thrust variations might be 20 or 30% thrust (producing low frequency turbulence that is about .3 G's).
If you lower the collective in VRS (and you have lots and lots of altitude) you will simply get into windmill brake state and autorotate at about 5 to 6000 feet per min.
For data and photos, see my web site at:
http://mywebpages.comcast.net/llappos/
During VRS, typical modern helicopters retain some cyclic control. The rate of descent will make the horizontal tail try to pitch the nose down, which will help you recover. Reduced collective and nose down will produce a fast exit from VRS. Increased collective will only help if the aircraft has a great deal of excess power, not at all likely in anything but an empty machine with powerful turbine engines.
Note from the data on the web site that VRS is unlikely in a purely vertical descent, some forward speed is needed, maybe 8 knots. In practice, it is difficult to attain and hold VRS, as the condition is unsteady and tends to break of its own if any disturbence is induced. That does not mean it can't do harm, since the first 1000 feet of drop might be several hundred feet too many!
The state where the rotor is in VRS is strictly defined by the ratio of the descent rate to the downwash velocity of the specific helicopter. Only within that boundary is VRS possible. One caviat is that we read RoD with a static instrument, which can go flakey when the wash around the fuselage is skewed beyond the normal certification envelope.
For modern helicopters, VRS is unlikely below vertical speed ratios of .5 and beyond ratios of 1.5. For an R-22. descent rates less than about 700 fpm are below the VRS threshold, and descent rates beyond 2100 fpm are above VRS. For Black Hawk, descent rates of about 1200 FPM are below VRS and greater than 4000 FPM are above VRS. The Vortex in VRS is produced when the downward velocity matches the rotor's downwash, so the exact match causes the air to simply wrap around the rotor tips. Lift is lost quickly, and the bottom falls out. If the power is left up and the condition is stable, the descent rate is constant and very high (about equal to the downwash velocity - maybe 1400 fpm (robbie) to 3000 fpm (Hawk)). The thrust variations might be 20 or 30% thrust (producing low frequency turbulence that is about .3 G's).
If you lower the collective in VRS (and you have lots and lots of altitude) you will simply get into windmill brake state and autorotate at about 5 to 6000 feet per min.
For data and photos, see my web site at:
http://mywebpages.comcast.net/llappos/
During VRS, typical modern helicopters retain some cyclic control. The rate of descent will make the horizontal tail try to pitch the nose down, which will help you recover. Reduced collective and nose down will produce a fast exit from VRS. Increased collective will only help if the aircraft has a great deal of excess power, not at all likely in anything but an empty machine with powerful turbine engines.
Note from the data on the web site that VRS is unlikely in a purely vertical descent, some forward speed is needed, maybe 8 knots. In practice, it is difficult to attain and hold VRS, as the condition is unsteady and tends to break of its own if any disturbence is induced. That does not mean it can't do harm, since the first 1000 feet of drop might be several hundred feet too many!
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Great thread,
Nick, thanks for the charts and especially the picture of the little midget helo on your site.
Imagining myself as very small and inside the little helo in the picture I was wondering about the recovery at that point.
The state is fully developed,
Cyclic not making desired effect,
Collective has become excercise machine for left arm,
Disc not happy, pilot not happy.
But the tailrotor can still provide translating force to the fuselage. Tractoring it (and the disc) away from the bad ju-ju happening at the time.
I'd think that a fair amount of pedal input (not enough for a spin) should slowly edge your disc out of it's nasty state to the point where the other controls start to take effect again.
Holes in the idea?
Obviously pedal input topping the engines would be a concern.
Nick, thanks for the charts and especially the picture of the little midget helo on your site.
Imagining myself as very small and inside the little helo in the picture I was wondering about the recovery at that point.
The state is fully developed,
Cyclic not making desired effect,
Collective has become excercise machine for left arm,
Disc not happy, pilot not happy.
But the tailrotor can still provide translating force to the fuselage. Tractoring it (and the disc) away from the bad ju-ju happening at the time.
I'd think that a fair amount of pedal input (not enough for a spin) should slowly edge your disc out of it's nasty state to the point where the other controls start to take effect again.
Holes in the idea?
Obviously pedal input topping the engines would be a concern.
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tgrendl, for most modern aircraft, cyclic is still quite effective, because you do not need to be developing thrust to develop control. The exception is a teetering rotor, which needs thust to develop lateral or longitudinal control power.
In any case,reduce collective, push the nose forward, and fly out. It would be the rare case that everything fails to respond. If you get stuck, be sure the collective is fully down, and you should slide down into autorotation.
Here is a very good web site that has flow depiction of the rotor flow as a descent progresses from gentle descent to VRS and then to autorotation, it was put up by Dr. Gordon Leishman of University of Maryland (and a Brit, I believe, formerly of Westland). Scroll down to the single rotor cases which are well labeled, and run the movies. Note that he shows how little forward component is needed before the VRS is broken. For those on dialups, it might take a while to download each. I would be glad to post some stills from them if anyone needs that done.
http://www.enae.umd.edu/AGRC/Aero/vring.html
In any case,reduce collective, push the nose forward, and fly out. It would be the rare case that everything fails to respond. If you get stuck, be sure the collective is fully down, and you should slide down into autorotation.
Here is a very good web site that has flow depiction of the rotor flow as a descent progresses from gentle descent to VRS and then to autorotation, it was put up by Dr. Gordon Leishman of University of Maryland (and a Brit, I believe, formerly of Westland). Scroll down to the single rotor cases which are well labeled, and run the movies. Note that he shows how little forward component is needed before the VRS is broken. For those on dialups, it might take a while to download each. I would be glad to post some stills from them if anyone needs that done.
http://www.enae.umd.edu/AGRC/Aero/vring.html
Senis Semper Fidelis
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Recovery from High state VRS
Last year 2001, I was with a very experienced Ex mil Cfi who showed me what can happen in a very fast descentVRS(Heli was R22) we went very high(5000ft) and he showed me a rate of decent to 3500 fpm, recovery was achieved by simply completely closing the collective, after recovery to height again it was my turn, and the same sort of recovery was made, It was very fast, very intimidating but after the event I feel able to cope with this problem( if it arises) in the future, brilliant for the old ticker!!
Regards Peter RB
Regards Peter RB
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A simple question perhaps:
Why do we need a small amount of forward speed to induce VRS, why doesn't a purely vertical descent induce it. I can see from the data that it doesn't just wondered why ?
cheers all
Why do we need a small amount of forward speed to induce VRS, why doesn't a purely vertical descent induce it. I can see from the data that it doesn't just wondered why ?
cheers all
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I recently did an Enstrom Type conversion and the instructor could not get any sort of VRS, even when lifting the lever from the bottom in low speed auto to 38 inches MP. The aircraft just stopped decending and started to climb. No fuss. He had several goes all with the same result. This was NOT the case last time I did it in a R44. Anyone got any ideas why the difference? Articulated vs teetering? Rotor RPM? Conditions on the day?
I have no idea.
I have no idea.
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(Edited to accurately reflect Robinson and Black Hawk downwash velocities, based on the coyote's calculations)
VRS is only encountered when the downward speed of the helicopter matches the downwash velocity from the rotor. In the cases mentioned by vfrpilotpb it is likely that the VRS was passed through, and then a zero knot autorotation was flown, thus the 3500 ft/min descent rate.
The flow around the rotor is established by the balance between the rotor's push on the air, and the upwind matching that push. Tales of VRS sort of locking the rotor up aerodynamically, and causing tremendous fall rates are simply not true. Those descents are experienced, we can be shown them, but they are not VRS. The reason for VRS is that the rotor downwash is pushed upwards by the free stream and then recirculated back down through the rotor. If the ROD is very much higher than the downwash velocity, the free air just passes through the rotor, and you have to raise the collective to keep from overspeeding the rotor.
For Gaseous, the autorotation first could be a difficult way to experience VRS. If you are into auto, the rotor must be transitioned to powered state, and you must try to trim the descent rate at somewhere between the range of 50% to 150% of the downwash speed. The rate of descent varies a lot depending on how heavy the disk loading is. For a Robbie, the VRS range is 700 fpm to 2200 fpm, for a Black Hawk, it is 1400 fpm to 4000 fpm.
One of the difficulties comparing what your instructor shows you with the actual VRS is that there is no telling what your instructor knows about actual VRS, and there is not standardization of techniques for the demo. The texts are poor, and much pilot lore surrounds the maneuver. I am sure many well intentioned instructors show a descent, some vibration and then an awesome vertical autorotation, and call the whole thing VRS. Why not, mine did in flight school back in 1968.
For VRS to be established in a rotor, the rotor must be lowered to about half the downwash speed under powered conditions. By 75% of the downwash speed (R-22 = 1100 fpm, H-60 = 2000 fpm) the VRS will show its head, the thrust will oscillate (you will feel low frequency vibration like turbulence, with maybe 3/10 of a G of magnitude, really big) and the aircraft will pitch and roll somewhat, the cyclic will be sloppy, and raising the collective will not necessarily produce a reduction in ROD. If the descent is increased to about 150% of the downwash speed, VRS is gone, clean air passes through the rotor, and you are in a vertical descent.
Any nose down (or even lateral tilt to slide out sideways) will help break up the VRS, and a climb will probably start (or at least the rate of descent will reduce somewhat. If you are falling at 3000 or 6000 feet per minute, you are not in VRS, you have slid through it and are now into a vertical descent or autorotation.
VRS is only encountered when the downward speed of the helicopter matches the downwash velocity from the rotor. In the cases mentioned by vfrpilotpb it is likely that the VRS was passed through, and then a zero knot autorotation was flown, thus the 3500 ft/min descent rate.
The flow around the rotor is established by the balance between the rotor's push on the air, and the upwind matching that push. Tales of VRS sort of locking the rotor up aerodynamically, and causing tremendous fall rates are simply not true. Those descents are experienced, we can be shown them, but they are not VRS. The reason for VRS is that the rotor downwash is pushed upwards by the free stream and then recirculated back down through the rotor. If the ROD is very much higher than the downwash velocity, the free air just passes through the rotor, and you have to raise the collective to keep from overspeeding the rotor.
For Gaseous, the autorotation first could be a difficult way to experience VRS. If you are into auto, the rotor must be transitioned to powered state, and you must try to trim the descent rate at somewhere between the range of 50% to 150% of the downwash speed. The rate of descent varies a lot depending on how heavy the disk loading is. For a Robbie, the VRS range is 700 fpm to 2200 fpm, for a Black Hawk, it is 1400 fpm to 4000 fpm.
One of the difficulties comparing what your instructor shows you with the actual VRS is that there is no telling what your instructor knows about actual VRS, and there is not standardization of techniques for the demo. The texts are poor, and much pilot lore surrounds the maneuver. I am sure many well intentioned instructors show a descent, some vibration and then an awesome vertical autorotation, and call the whole thing VRS. Why not, mine did in flight school back in 1968.
For VRS to be established in a rotor, the rotor must be lowered to about half the downwash speed under powered conditions. By 75% of the downwash speed (R-22 = 1100 fpm, H-60 = 2000 fpm) the VRS will show its head, the thrust will oscillate (you will feel low frequency vibration like turbulence, with maybe 3/10 of a G of magnitude, really big) and the aircraft will pitch and roll somewhat, the cyclic will be sloppy, and raising the collective will not necessarily produce a reduction in ROD. If the descent is increased to about 150% of the downwash speed, VRS is gone, clean air passes through the rotor, and you are in a vertical descent.
Any nose down (or even lateral tilt to slide out sideways) will help break up the VRS, and a climb will probably start (or at least the rate of descent will reduce somewhat. If you are falling at 3000 or 6000 feet per minute, you are not in VRS, you have slid through it and are now into a vertical descent or autorotation.
Nick, based upon your formula to calculate the downwash velocity and the following data:
R22, at MAUW=1370lbs, rotor diam=7.7m/25.26ft, I calculate the downwash velocity to be 1440 fpm. Am I right? Is the 0.002378 number in your formula, is that specific to a particular rotor system or a constant for all helos?
When I used to demonstrate VRS to students, I would get it into the hover (obviously with at least 3000 feet to spare) and in the R22 this would usually require pretty much max power. I would then very slowly lower the collective. As the ROD got to around 500-900 fpm or so, (and sometimes you would have to gently search for the downwash with cyclic), bingo you would be into it and most times so quickly you would lose your stomach. All the normal control sloppyness, pitch and roll and vibrations just prior to it. On a windy day often you couldn't get into VRS no matter how hard you tried for obvious reasons. From your formula it seems like a higher ROD is necessary but experience tells me otherwise, maybe I am under-estimating the lag in the VSI? Or does the downwash velocity vary considerably depending upon the power being applied at the time?
Thanks for your input and a Disclaimer for students: DONT do the above solo.
R22, at MAUW=1370lbs, rotor diam=7.7m/25.26ft, I calculate the downwash velocity to be 1440 fpm. Am I right? Is the 0.002378 number in your formula, is that specific to a particular rotor system or a constant for all helos?
When I used to demonstrate VRS to students, I would get it into the hover (obviously with at least 3000 feet to spare) and in the R22 this would usually require pretty much max power. I would then very slowly lower the collective. As the ROD got to around 500-900 fpm or so, (and sometimes you would have to gently search for the downwash with cyclic), bingo you would be into it and most times so quickly you would lose your stomach. All the normal control sloppyness, pitch and roll and vibrations just prior to it. On a windy day often you couldn't get into VRS no matter how hard you tried for obvious reasons. From your formula it seems like a higher ROD is necessary but experience tells me otherwise, maybe I am under-estimating the lag in the VSI? Or does the downwash velocity vary considerably depending upon the power being applied at the time?
Thanks for your input and a Disclaimer for students: DONT do the above solo.
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the coyote,
You are exactly on. The disk loading of the R-22 is very light, so it does not need to push the air very hard to get its lift. The downwash velocity of 1440 is correct. The .002378 is the density of air at sea level. The density is important because the formula simply matches the downward air momentum change needed to make the upward lift for the rotor. With denser air, less velocity is needed, and vice versa.
OK, so we estimate that you get into the beginning of VRS at 50% of the downwash speed. That means by about 700 fpm ROD you should start to get VRS effects. You have observed that at 62% of that speed, 900 fpm, you are fully into the effect (that is the orange area on the plot on my website.) It all checks very nicely.
I think it all fits. The misleading thing about my post is that I have assumed downwash velocities more in the range where turbine helicopters are built, note the use of a Jetranger for the website chart. I will edit the post right now to add Robbie numbers! I will also put a Robbie scale on the chart right now.
Turbines have higher disk loading bcause they have more power, so they can afford to spend more making higher downwash velocity. I will eventually add that to the web site, too.
You are exactly on. The disk loading of the R-22 is very light, so it does not need to push the air very hard to get its lift. The downwash velocity of 1440 is correct. The .002378 is the density of air at sea level. The density is important because the formula simply matches the downward air momentum change needed to make the upward lift for the rotor. With denser air, less velocity is needed, and vice versa.
OK, so we estimate that you get into the beginning of VRS at 50% of the downwash speed. That means by about 700 fpm ROD you should start to get VRS effects. You have observed that at 62% of that speed, 900 fpm, you are fully into the effect (that is the orange area on the plot on my website.) It all checks very nicely.
I think it all fits. The misleading thing about my post is that I have assumed downwash velocities more in the range where turbine helicopters are built, note the use of a Jetranger for the website chart. I will edit the post right now to add Robbie numbers! I will also put a Robbie scale on the chart right now.
Turbines have higher disk loading bcause they have more power, so they can afford to spend more making higher downwash velocity. I will eventually add that to the web site, too.