Vortex Ring / Settling with power (Merged)
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Just a quick invite to those posting to this thread. If you email me your postal address I will send you a comp copy of the next issue, out in two weeks, which has the LTE story in it. See what you think of it.
[email protected]
Regards.
Pacific Rotors Magazine
www.southerncrossimages.com
[email protected]
Regards.
Pacific Rotors Magazine
www.southerncrossimages.com
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Also known as recirculation vortex.
Onset tends to be easier/earlier on twin and three-bladed helos (than on four/five-bladed say) simply because of disk solidity producing a weaker TE vortex sheet. Blade loading and downwash on two-bladed is going to be higher, leading to easier formation of the recirculation characteristic of the vortex ring state.
Of interest is the early tilt-rotor paper wherein academics urged more blades than the tilt-rotor's present three-bladed arrangement. However, unfortunately the weight, inertia and complexity of hooking up four blades controlled that outcome - and we know the rest. Because of the 95 degree tilt capability that was included because of ground manoeuvre considerations, yet is usable airborne, the Osprey is able to arrange to get into a VR state quite a bit earlier. i.e. at 95 degree tilt it has essentially arranged for it's own downwash wake to earlier (and more closely) approximate its approach flight-path (think about it). Add an arcing-over tendency due to close-in pad-edge obstacles and a bit of tail-wind at height - and you're easily in that ball-park.
Unfortunately the asymmVR nature of the tilt-rotor's VR encounters precludes recovery once the condition is encountered. Instinctive application of differential collective (as if to pick up a dropped wing) exacerbates the problem - much as simply raising the collective does in a helo. But in the Osprey's Marana accident time-line, it is evident that the roll-rate/nose-drop was totally without warning and non-recoverable. That was borne out by the lead aircraft crew-chief who witnessed that final evolution. Those who say that an emergency "conversion" would be a recovery solution are simply not familiar with what the Patuxent River test-pilots have been writing about their experiences.
As Offshoreigor says above: "the biggest problem is that if you don't react it increases in intensity exponentially." This is true in a helo but unfortunately it's irrelevant in a tilt-rotor. But it's interesting to still read the Marine Corps General's briefings wherein he reassures everyone that VR is not a problem for the Osprey - because of the amount of excess power the MV-22 has. Right sir, as you say sir.
Onset tends to be easier/earlier on twin and three-bladed helos (than on four/five-bladed say) simply because of disk solidity producing a weaker TE vortex sheet. Blade loading and downwash on two-bladed is going to be higher, leading to easier formation of the recirculation characteristic of the vortex ring state.
Of interest is the early tilt-rotor paper wherein academics urged more blades than the tilt-rotor's present three-bladed arrangement. However, unfortunately the weight, inertia and complexity of hooking up four blades controlled that outcome - and we know the rest. Because of the 95 degree tilt capability that was included because of ground manoeuvre considerations, yet is usable airborne, the Osprey is able to arrange to get into a VR state quite a bit earlier. i.e. at 95 degree tilt it has essentially arranged for it's own downwash wake to earlier (and more closely) approximate its approach flight-path (think about it). Add an arcing-over tendency due to close-in pad-edge obstacles and a bit of tail-wind at height - and you're easily in that ball-park.
Unfortunately the asymmVR nature of the tilt-rotor's VR encounters precludes recovery once the condition is encountered. Instinctive application of differential collective (as if to pick up a dropped wing) exacerbates the problem - much as simply raising the collective does in a helo. But in the Osprey's Marana accident time-line, it is evident that the roll-rate/nose-drop was totally without warning and non-recoverable. That was borne out by the lead aircraft crew-chief who witnessed that final evolution. Those who say that an emergency "conversion" would be a recovery solution are simply not familiar with what the Patuxent River test-pilots have been writing about their experiences.
As Offshoreigor says above: "the biggest problem is that if you don't react it increases in intensity exponentially." This is true in a helo but unfortunately it's irrelevant in a tilt-rotor. But it's interesting to still read the Marine Corps General's briefings wherein he reassures everyone that VR is not a problem for the Osprey - because of the amount of excess power the MV-22 has. Right sir, as you say sir.
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RAF CFS taught me that the vortex ring state affects the rotor induced flow by increasing it (the air above the disc has already been through the disc before as it is now recirculating outwards, upwards, over and down!). From the standard velocities diagram, this has the effect of decreasing the angle of attack of the blades whilst increasing the rotor drag.
The clearest description I have come across is from R. W. Prouty in his book "Helicopter Aerodynamics".
He shows, in a series of diagrams, the airflow in vortex ring. This is shown as a "doughnut with a hole" shaped (my interpretation - SC) vortex encompassing the entire rotor disc tip path (similar to a huge smoke ring as blown by some smokers, but upside down here, as the airflow direction is downwards at the centre) with the blade tips in the centre of the "dough / smoke" itself. The downwards / induced flow within the vortex is inboard from the blade tips and the corresponding "up draught" is outside of the tip path plane. He goes on to describe how underneath the "doughnut" is a higher pressure air bubble, fed from the downdraught flow within the "doughnut". The bubble cannot be sustained and randomly bursts below the aircraft and then quickly re-forms, causing the random rolling and pitching moments associated with this phenomena.
Prouty states that "from flight and wind tunnel tests, unsteadiness starts at about one-quarter, peaks at three quarters,and disappears at 1 and a quarter times the hover induced velocity". (Beyond this it is theoretically possible for the disc to go into autorotation but don't try this at home as you will probably hit the ground first - SC).
Easier seen with the aid of the diagrams than described though - I hope this can be understood!
Acknowledged to R.W. Prouty - Thanks!
[This message has been edited by Skycop (edited 20 January 2001).]
The clearest description I have come across is from R. W. Prouty in his book "Helicopter Aerodynamics".
He shows, in a series of diagrams, the airflow in vortex ring. This is shown as a "doughnut with a hole" shaped (my interpretation - SC) vortex encompassing the entire rotor disc tip path (similar to a huge smoke ring as blown by some smokers, but upside down here, as the airflow direction is downwards at the centre) with the blade tips in the centre of the "dough / smoke" itself. The downwards / induced flow within the vortex is inboard from the blade tips and the corresponding "up draught" is outside of the tip path plane. He goes on to describe how underneath the "doughnut" is a higher pressure air bubble, fed from the downdraught flow within the "doughnut". The bubble cannot be sustained and randomly bursts below the aircraft and then quickly re-forms, causing the random rolling and pitching moments associated with this phenomena.
Prouty states that "from flight and wind tunnel tests, unsteadiness starts at about one-quarter, peaks at three quarters,and disappears at 1 and a quarter times the hover induced velocity". (Beyond this it is theoretically possible for the disc to go into autorotation but don't try this at home as you will probably hit the ground first - SC).
Easier seen with the aid of the diagrams than described though - I hope this can be understood!
Acknowledged to R.W. Prouty - Thanks!
[This message has been edited by Skycop (edited 20 January 2001).]
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Lmlanphere, I'll guess at this and someone with more knowledge can correct me.
I think the effect intensifies because the air currents have formed a vortex (in this case a "ring" vortex). Once a vortex is formed it's almost self-sustaining as long as the pressure differentials that created it, are still acting upon it.
When a vortex is formed it creates a very organized airflow pattern in response to the pressure differentials acting upon it. As long as the pressure differential remains, the velocity of the air flow increases within the vortex in an attempt to equalize the pressure. Anyone familiar with a tornado can readily understand the velocities that can develop in a vortex. Higher velocities mean more air mass is being moved in the attempt to equalize the pressure. It's this ever increasing velocity that plays havoc with the AOA and increased drag on the rotor blades.
I might add that even when the pressure differential is removed (such as a wing tip vortex on a FW aircraft) the vortex will sustain it's organized airflow for a time due to its rotational momentum. Without the energy input from the pressure differential, the vortex will dissipate. However with sustained energy input from the pressure differential, it intensifies.
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Safe flying to you...
[This message has been edited by Flight Safety (edited 21 January 2001).]
I think the effect intensifies because the air currents have formed a vortex (in this case a "ring" vortex). Once a vortex is formed it's almost self-sustaining as long as the pressure differentials that created it, are still acting upon it.
When a vortex is formed it creates a very organized airflow pattern in response to the pressure differentials acting upon it. As long as the pressure differential remains, the velocity of the air flow increases within the vortex in an attempt to equalize the pressure. Anyone familiar with a tornado can readily understand the velocities that can develop in a vortex. Higher velocities mean more air mass is being moved in the attempt to equalize the pressure. It's this ever increasing velocity that plays havoc with the AOA and increased drag on the rotor blades.
I might add that even when the pressure differential is removed (such as a wing tip vortex on a FW aircraft) the vortex will sustain it's organized airflow for a time due to its rotational momentum. Without the energy input from the pressure differential, the vortex will dissipate. However with sustained energy input from the pressure differential, it intensifies.
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Safe flying to you...
[This message has been edited by Flight Safety (edited 21 January 2001).]
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Pac Rotors will hopefully agree that I have fully explained why the effect increases
as I have fully described it in graphical terms in my book.
I firmly beleive I have the answer and although I have never had the opportunity to Read Ray Prouty's book (one day, hopefully!)
I have come up with a similar description as
Skycop describes (I used a doughnut simile thinking I had thought of it... Bah !)
I also used the triangle of ingredients that offshoreigor has mentioned, as taught to me by Mike Green of CFS.
Give me a day to get other stuff done (like work!) and I'll see if I can convince my computer to allow me to post the graphics on the thread...!
By the way, I too am not a fan of the description "settling with power'
It's to cuddly and friendly, it does not impart the seriousness of the situation.
'Settling' is what happens to the contents of your coffee jar during transit. Vortex Ring State as a title is a little more threatening, a little more violent, and so it IS!
as I have fully described it in graphical terms in my book.
I firmly beleive I have the answer and although I have never had the opportunity to Read Ray Prouty's book (one day, hopefully!)
I have come up with a similar description as
Skycop describes (I used a doughnut simile thinking I had thought of it... Bah !)
I also used the triangle of ingredients that offshoreigor has mentioned, as taught to me by Mike Green of CFS.
Give me a day to get other stuff done (like work!) and I'll see if I can convince my computer to allow me to post the graphics on the thread...!
By the way, I too am not a fan of the description "settling with power'
It's to cuddly and friendly, it does not impart the seriousness of the situation.
'Settling' is what happens to the contents of your coffee jar during transit. Vortex Ring State as a title is a little more threatening, a little more violent, and so it IS!
Guest
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And by the way implanere, ALL of the factors you mentioned in the beginning of your thread are involved. The intensification of the 'doughnut' effect is directly due to an upward incoming airflow (does not matter whether that is due to ROD or updraft, the effect is identical) which 'pushes' the doughnut up to disc level, when previously it would have been held below the disc (if a/s is low or zero)therfore increasing recirculation of air that has already been accelerated downward. All tht remains is to consider the effect of that upward incoming airflow on the ROOT area of the blade(s) (as you say, it increases a of a and brings that part of the blade to stall) and the anatomy of Vortex Ring and its exponential development is explained. and of course, the more it develops, the more ROD increases....
You can easily demonstrate the effect with a desktop fan. If you rig up some way of opposing the fan's induced flow with another (stronger) airflow it will begin to develop recirculation and its RPM should increase audibly as it takes less 'bite' at unaccelerated air. Also interesting is what happens when you ADD to its induced flow with another airflow, this time from behind. As the induced flow of the fan is increased RPM will again rise audibly. This shows the effect of recirculation in a different (but less accurate) way.
I may struggle to get graphics onto this post as my software is not easy to convert (any tips here would be welcome, but I'll have a go. If not I could post them on my website for a short time) Anyway, I'll try first !
SPS, still wondering how they managed to convince us that 'Nestles' were always called 'Nestle' (accent on the E)
because in the 'Milky Bar Kid' adverts it was always NESTLE'S milky bar.......
You can easily demonstrate the effect with a desktop fan. If you rig up some way of opposing the fan's induced flow with another (stronger) airflow it will begin to develop recirculation and its RPM should increase audibly as it takes less 'bite' at unaccelerated air. Also interesting is what happens when you ADD to its induced flow with another airflow, this time from behind. As the induced flow of the fan is increased RPM will again rise audibly. This shows the effect of recirculation in a different (but less accurate) way.
I may struggle to get graphics onto this post as my software is not easy to convert (any tips here would be welcome, but I'll have a go. If not I could post them on my website for a short time) Anyway, I'll try first !
SPS, still wondering how they managed to convince us that 'Nestles' were always called 'Nestle' (accent on the E)
because in the 'Milky Bar Kid' adverts it was always NESTLE'S milky bar.......
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The AoA along the length of the blade is increased by the rate of descent flow (or updraughting when hovering in the mountains). As the root has the highest AoA it is the first to stall whilst the increase in pressure on the underside of the tips (as the airmoving upwards meets the air moving down ) intensifies the ever present tip vortex.
The tip is encountering recirculation and the root is stalling, leaving the remainder of the blade to do the work of keeping the aircraft in the air. If no action is taken the overall loss of lift will cause an increase in RoD and make the situation worse. If the pilot tries to arrest the Rod with lever he increases the AoA - the recirculation at the tip gets worse and the stalled area at the root moves outboard. At the point where rotor thrust no longer balance weight, the aircraft accelerates rapidly downwards ( as much as 8,000 fpm on some types).
It is at the incipient stage of vortex ring where the aircraft first catches up with its own downwash, before the RoD begins to develop that the warning signs - an increase in vibration and or blade noise (slap) - can be felt. The American term Settling with Power seems more appropriate to this stage than the fully developed "Doughnut" of the Vortex Ring state.
Two factors affecting the susceptibility of an aircraft to vortex ring are
a. All Up Mass - for the same aircraft you will have a higher AoA at a higher AUM and therefore be closer to root stall.
b. Disc Loading - aircraft with a high disc loading have a higher downwash speed and therefore need more RoD to catch up with their downwash.
The generic "less than 500 fpm RoD below 30 kts IAS" is still the best way to avoid vortex ring - I have talked to people who have survived the fully developed state and it's not a place you really want to be.
SPS, in answer to Nestles v Nestle, it was the good old advertising whizz kids who decided the British Public would not buy something that sounded distinctly foreign (especially French/Swiss). These are the same tossers who have conned the Post Office into changing their name to Consignia. How can allegedly intelligent businessmen believe some of the bo**ocks the marketing consultants tell them.
The tip is encountering recirculation and the root is stalling, leaving the remainder of the blade to do the work of keeping the aircraft in the air. If no action is taken the overall loss of lift will cause an increase in RoD and make the situation worse. If the pilot tries to arrest the Rod with lever he increases the AoA - the recirculation at the tip gets worse and the stalled area at the root moves outboard. At the point where rotor thrust no longer balance weight, the aircraft accelerates rapidly downwards ( as much as 8,000 fpm on some types).
It is at the incipient stage of vortex ring where the aircraft first catches up with its own downwash, before the RoD begins to develop that the warning signs - an increase in vibration and or blade noise (slap) - can be felt. The American term Settling with Power seems more appropriate to this stage than the fully developed "Doughnut" of the Vortex Ring state.
Two factors affecting the susceptibility of an aircraft to vortex ring are
a. All Up Mass - for the same aircraft you will have a higher AoA at a higher AUM and therefore be closer to root stall.
b. Disc Loading - aircraft with a high disc loading have a higher downwash speed and therefore need more RoD to catch up with their downwash.
The generic "less than 500 fpm RoD below 30 kts IAS" is still the best way to avoid vortex ring - I have talked to people who have survived the fully developed state and it's not a place you really want to be.
SPS, in answer to Nestles v Nestle, it was the good old advertising whizz kids who decided the British Public would not buy something that sounded distinctly foreign (especially French/Swiss). These are the same tossers who have conned the Post Office into changing their name to Consignia. How can allegedly intelligent businessmen believe some of the bo**ocks the marketing consultants tell them.
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They changed the name of the post office to WHAT?!!
I don't know, I'm away for only 7 months this time and look what happens!
Anyway, it doesn't rhyme or even fit. 'NESTLE milky bar...' indeed.
Anyway, good post, looks like you have it boxed off. All we have to do now is recovery
methods and why they work.
You or me?
Anyway, better hurry and Consignia this...
I don't know, I'm away for only 7 months this time and look what happens!
Anyway, it doesn't rhyme or even fit. 'NESTLE milky bar...' indeed.
Anyway, good post, looks like you have it boxed off. All we have to do now is recovery
methods and why they work.
You or me?
Anyway, better hurry and Consignia this...
Guest
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SPS:
RE: Recovery.
It doesn't matter what type you fly, the only difference will be the intensity of the VRS.
The most effective way to recover is to reduce power, depending on the severity of the situation, up to autorotation. What this does is it reduces the intensity of the Vortex and re-establishes cyclic effectiveness allowing you to increase your airspeed above translational speed and thus leave the vortex behind the disc rather than back up through it.
Having said all this, hopefully you had enough altitude in the first place to recover! It's not the fall that will kill you, it's the sudden stop at the bottom.
Cheers, OffshoreIgor
RE: Recovery.
It doesn't matter what type you fly, the only difference will be the intensity of the VRS.
The most effective way to recover is to reduce power, depending on the severity of the situation, up to autorotation. What this does is it reduces the intensity of the Vortex and re-establishes cyclic effectiveness allowing you to increase your airspeed above translational speed and thus leave the vortex behind the disc rather than back up through it.
Having said all this, hopefully you had enough altitude in the first place to recover! It's not the fall that will kill you, it's the sudden stop at the bottom.
Cheers, OffshoreIgor
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Are the Westland Lynx and the EHI 101 suceptible to vortex ring state? It is my understanding that their "BURP" blades were designed to minimize tip vortices.
BERP/BURP they both sound the same.
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The Cat
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
BERP/BURP they both sound the same.
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The Cat
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
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Joe Pilot - reports and tests of Vortex Ring suggest that it is far from stable in the fully developed state. Apparently a bubble of higher pressure air forms and then bursts regularly but in unpredictable directions giving random pitching and rolling. Additionally, the disturbed airflow from the main rotor plays havoc with the flow through the tail rotor causing yaw fluctuations.
Stable? - No, A fast way to get down? - yessireeebob!
SPS - the only recoveries are 1. A lot of forward cyclic to gain speed then apply power once out of the dirty air, or 2. Lower the lever to enter autorotation - but you need a lot of height and may overspeed the Nr!
Lu - The BERP blades have a very powerful tip vortices, just like a swept fixed wing - the tip losses were so high in the hover due to Blade Vortex Interaction (BVI) that the had to add the anhedral "droop" tip to spit the vortex out of the way before the next blade came along.
Stable? - No, A fast way to get down? - yessireeebob!
SPS - the only recoveries are 1. A lot of forward cyclic to gain speed then apply power once out of the dirty air, or 2. Lower the lever to enter autorotation - but you need a lot of height and may overspeed the Nr!
Lu - The BERP blades have a very powerful tip vortices, just like a swept fixed wing - the tip losses were so high in the hover due to Blade Vortex Interaction (BVI) that the had to add the anhedral "droop" tip to spit the vortex out of the way before the next blade came along.
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Have been in "setteling with power" in a Hughes 269A (not a fun aircraft to teach it in as the vibration level puts the viewable world out of focus), a two bladed Bell and, a CH-47.
Have done it at 3000' and 13,500'. I'm not an expert but, my position differs with some.
If you are going down with no power applied and no forward airspeed and you don't feel too badley about it all, you are in a descent.
If in the descent, you apply up collective or forward TCL and stall the inner/slower portion of the rotor system where some air direction has already been up, you increase the stalled area where the second (at the inner portion of the rotor system) Vortex Ring has been for as long as the air has been going up through the rotor system instead of down and, you now feel very badley about it all, you are in setteling with power. More power/pitch, larger area where the air is going up rather than down meaning more of the blade is effectively stalled and not performing. Point being, you may be able to get the aircraft to go down fast enough to get a flow of air up through the rotor system near a the center/slower downward flow area. But, you are not "setteling with power, in my humble opinion till you make the mistake of increasing the power/pitch and increase all that has been only a potential bad thing to this point. When the pitch of the blade increases at the root, it increases at the tip. Which means it creates more/faster downflow of air until it exceeds design limit.
Recovery: correct what got you here. Forward airspeed and reduce power/pitch till the rate of descent slows noticably and the vibrations smoothe out. Then add power to stop the descent and fly out. Adjust as necessary for nearness to the ground. In a tandem rotor aircraft give thought to going to the side to find the clean air for the rotor system as forward or aft cyclic will only keep one of the rotor systems in setteling with power while the other has reverted to vortex ring state when the power/pitch was reduced. Two rotors fore and aft or lateral makes no no difference. The rotors don't know they have a twin, they stall/recover as individuals.
[This message has been edited by VLift (edited 22 January 2001).]
[This message has been edited by VLift (edited 22 January 2001).]
Have done it at 3000' and 13,500'. I'm not an expert but, my position differs with some.
If you are going down with no power applied and no forward airspeed and you don't feel too badley about it all, you are in a descent.
If in the descent, you apply up collective or forward TCL and stall the inner/slower portion of the rotor system where some air direction has already been up, you increase the stalled area where the second (at the inner portion of the rotor system) Vortex Ring has been for as long as the air has been going up through the rotor system instead of down and, you now feel very badley about it all, you are in setteling with power. More power/pitch, larger area where the air is going up rather than down meaning more of the blade is effectively stalled and not performing. Point being, you may be able to get the aircraft to go down fast enough to get a flow of air up through the rotor system near a the center/slower downward flow area. But, you are not "setteling with power, in my humble opinion till you make the mistake of increasing the power/pitch and increase all that has been only a potential bad thing to this point. When the pitch of the blade increases at the root, it increases at the tip. Which means it creates more/faster downflow of air until it exceeds design limit.
Recovery: correct what got you here. Forward airspeed and reduce power/pitch till the rate of descent slows noticably and the vibrations smoothe out. Then add power to stop the descent and fly out. Adjust as necessary for nearness to the ground. In a tandem rotor aircraft give thought to going to the side to find the clean air for the rotor system as forward or aft cyclic will only keep one of the rotor systems in setteling with power while the other has reverted to vortex ring state when the power/pitch was reduced. Two rotors fore and aft or lateral makes no no difference. The rotors don't know they have a twin, they stall/recover as individuals.
[This message has been edited by VLift (edited 22 January 2001).]
[This message has been edited by VLift (edited 22 January 2001).]
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NR:
As anyone will tell you, once you are in fully developed vortex ring state, not the inscipient stage, the cyclic tends to have little or no effect until you reduce power (ie. enter autorotation). So for my money, it would be reduce power followed very shortly thereafter by forward cyclic.
I also agree with the comment on the Chinook recovery with sideward cyclic.
Cheers, OffshoreIgor
As anyone will tell you, once you are in fully developed vortex ring state, not the inscipient stage, the cyclic tends to have little or no effect until you reduce power (ie. enter autorotation). So for my money, it would be reduce power followed very shortly thereafter by forward cyclic.
I also agree with the comment on the Chinook recovery with sideward cyclic.
Cheers, OffshoreIgor
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I personally don't like the term "settling with power". In some ways it is correct, but to be pedantic isn't that what we do when we land ?? 
We don't land in autorotation (at least I don't), so therefore when landing, we let the a/c settle using power.
During some re-current training (Hughes 500D), my instructor and I did some VR work. We went up to about 4,000 agl and slowed down to below trans lift, and began a descent. It was quite interesting to watch the vsi begin to rise, even though we were increasing power. We lowered the power, then eased the cyclic forward. Once the shaking eased, and airspeed started to recover, we flew out of it.
Cheers
Randy_G
If you can't stand the heat...
then turn up the air conditioner !!
[This message has been edited by Randy_g (edited 23 January 2001).]
We don't land in autorotation (at least I don't), so therefore when landing, we let the a/c settle using power. During some re-current training (Hughes 500D), my instructor and I did some VR work. We went up to about 4,000 agl and slowed down to below trans lift, and began a descent. It was quite interesting to watch the vsi begin to rise, even though we were increasing power. We lowered the power, then eased the cyclic forward. Once the shaking eased, and airspeed started to recover, we flew out of it.
Cheers
Randy_G
If you can't stand the heat...
then turn up the air conditioner !!
[This message has been edited by Randy_g (edited 23 January 2001).]