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Vortex Ring / Settling with power (Merged)

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Vortex Ring / Settling with power (Merged)

Old 2nd Aug 2009, 15:19
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With my respect for Shawn unabated, I must respectfully disagree with him. There is a meaningful difference between the two cases, light vs heavy.

But first we must clarify what we mean by Vortex Ring State (VRS) - and what is meant by a different problem called "Settling with Power (SWP)" or "over pitching" or "insufficient power to Hover OGE"

Most accidents where a hovering helicopter falls and crashes and VRS is blamed are actually cases of SWP or over pitching, where the hover performance is marginal, and insufficient reserve power (power margin) is available to allow moderate climbs and descents while OGE. The aircraft "falls through" the hover, hits hard (usually with just a bent helicopter and bruised ego) and then someone says "It was VRS." Sometimes the mistaken person is an official accident investigator!

In a helicopter at high MGW, with only slight or no margin between the power needed to HOGE and the power available from the engines, "over pitching" is more likely than in a lightly loaded helicopter where lots of power above hover power is available.

When lightly loaded, there is much extra power available above the hover power, so the lightly loaded helo is much less likely to experience "over pitching" and thus the lightly loaded helo is much less likely to be mistakenly labeled as a VRS accident.

Now the truth: Since true VRS involves the descent of the helicopter into its own downwash, and since in a light helicopter the downwash velocity is quite a bit less than in that same helicopter when heavy, a lightly loaded helicopter needs much less rate of descent to experience true VRS.

Thus, heavy helicopters require more descent rate to get true VRS, and so are less likely to enter that state, but heavily loaded helicopters have more over pitching power control accidents that are too often labeled "VRS", so the mistaken pilot lore says heavy helicopters are more likely to experience VRS.

For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.

Also for the record, most helicopters can experience SWP or overpitching at rates of descent near zero if they have little hover power margin.

Also, heavily loaded helicopters have less propensity to enter VRS because they need more vertical descent rate than lightly loaded helicopters, which need less descent rate to get into VRS.
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Old 2nd Aug 2009, 19:55
  #262 (permalink)  
 
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Maybe...................

For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.
For the record, no helicopter can experience true VRS unless it is descending nearly vertically, with respect to the relative airflow, at about 800 to 1000 feet per minute.

Starting to drift off topic a little, but what is wrong with this..................

If you fly level at a hill which has an upslope of 30 degrees (not that steep) with a 25 knot tailwind, the following should/could happen.

At 25 knots groundspeed as you approach the hill you would have zero airspeed. In fact at 50 knots groundspeed your ASI will probably read zero.

The vertical component of the wind would be ~ 11Knots or ~ 1100 FPM. Plenty right for VRS.

The VSI would read ZERO.

If the hill is 45 degrees for the same vertical component you would only need 15 knots of wind. 100 FPM is only ~ 1 Knot after all.

In this situation you are flying where the ASI won't tell you much and neither will the VSI. Where VRS can get you is where the wind blows parallel to the rising ground. It will get worse as you get closer to the hill (Up to a point where surface friction will start to affect things).It is true that VRS will not get you if you have relative flow across the disk. At these low speeds the ASI will not indicate much.

The indication which will also catch you is that you are not descending according the static instruments (ALT/VSI/IVSI) and you are using LESS power than you would be in a hover (Where hover = zero airspeed). You are descending at ~1100 FPM after all. If it feels too good to be true, that is probably right.

Also your escape route is blocked by rising ground. That is why this style of approach is very dangerous. Remember all these scenarios are for a LEVEL approach. Add in a little decent and it all happens a lot easier.

If you think these conditions don't happen, I have experienced a climb in a Bell 205 at 11,200lbs with the needles split!

Back to your subject - as pointed out, a lighter helicopter is more susceptible than a heavier helicopter, but I think the point should be a lower disk loading versus a higher disk loading. i.e. disk area/weight.

Go and talk to an experienced Cheetah/Chetak mountain pilot. Be quick as those aircraft will evidently be gone next year.
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Old 2nd Aug 2009, 20:04
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RVDT,
This is getting to be a fun thread. Let me weigh in a bit:
The theory that a wind against a slope would produce such a smooth flow that it would look like a steady descent is just that - theory. In reality, the typical turbulence and flow disturbance that the ground imparts on the air would make your case nothing more than a bumpy ride.
The flow that makes VRS is steady and smooth, with exact angles of descent (vertical will not do it, about 8 knots forward as you descend is the critical condition).

One can theorize a glass-smooth slope, and the perfect storm of slope, wind speed and flight condition, but in actuality, it is simply not possible.
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Old 2nd Aug 2009, 20:32
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VRS Verticality

Ramen,

Must agree with Nick and therefore disagree with you on the verticality question. In several test programs on different models, all have been consistent in that VRS was achieved only with vertical descents. Vertical means relative to the air mass, now, so if there is some wind, the descent must be at zero translational velocity relative to the air mass. Its a lot harder to find that perfect spot than a lot of people think.

Thanks,
John Dixson
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Old 2nd Aug 2009, 23:02
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Ramen,

Glad I don't have to fly in the mountains with you. I have found this sweet spot many times when a young and inexperienced sprog pilot. Luckily I never hit anything solid before I worked it out. The sweet spot in these conditions WILL find you, but maybe only 1 in 100. As JD says it is difficult to find the exact spot but when you do it will be pretty clear. As I pointed out 800-1100 FPM is only 8-11 Knots. Hence the methods used in the mountains are to avoid exactly this condition. Go ask a Llama pilot what makes his sphincter twitch.
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Old 3rd Aug 2009, 02:07
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I have the utmost respect for both Nick and John and am very glad Nick took the time to point out the difference between VRS and SWP as there is much confusion out there between the 2. So much so that Transport Canada has mandated that during PPC check rides, the examiner must question the applicant on these items ( as well as dynamic roll over).


I am glad John has qualified Nick's statement about vertical descents being relative to the air mass. As someone who has been in VRS a few times (both intentionally and NOT) I can tell you that you can get into it while definitely not APPEARING to be in a vertical descent ( downwind).

Also I would like to qualify Nick's statement:

"For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute."

I do believe he meant to say, "no MODERN helicopter....."

Again from experience, I can tell you an S61 can enter this at between 300 and 500 ROD, not sure of the exact number as I was kinda busy at this point

Also, thanks Nick for the explanation about the reason why a lightly loaded helicopter has more propensity to enter VRS than a heavy one......I knew that was the case, but never knew why.

Last edited by Outwest; 3rd Aug 2009 at 10:08. Reason: spelling
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Old 3rd Aug 2009, 02:23
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Now that it seems we have gotten Nick's attention back on this board ( good to see you back) I would like to ask you once again to explain the relation ship between the tail rotor disk and the fin area. I say once again as you explained this to me once over the phone, but I want be sure of the numbers.

For the record, I was asking Nick about the size of the tail fin and its effect on keeping the a/c sort of going in the right direction in a tail rotor failure. Nick explained a glaring difference that I had not considered. Tail rotor DRIVE failure (tail rotor stopped) and fixed pitch failure.

Nick, can I ask you to explain again, I seem to remember a number of 1.3 times the area.........
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Old 3rd Aug 2009, 07:14
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Hi Nick, Shawn, John,

Thanks for your comments.

I have heard and read a few different views with regards to removing yourself from VRS.

Will the aircraft react to cyclic inputs similarly to normal flight?

Would you pull maximum power or enter autorotation?

What are your views?

Regards,
bb
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Old 3rd Aug 2009, 16:43
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Respectfully to Nick:
A very light disk loaded helicopter can get into VRS at much less than 800 fpm.

And for bb:
A positive forward cyclic movement to get at least 20 degrees nose down will start you going into forward flight. No need to enter autorotation. But be positive with the cyclic input - it will work eventually.
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Old 3rd Aug 2009, 17:33
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A positive forward cyclic movement to get at least 20 degrees nose down will start you going into forward flight. No need to enter autorotation. But be positive with the cyclic input - it will work eventually.

All true, however if altitude allows (and it rarely does) entering autorotation would be a way out. A disk in autorotation can not, by definition, be in VRS.
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Old 4th Aug 2009, 15:42
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Question fENESTRON vS cONVENTIONAL TR (THRUST/POWER/SIZE)

To Nick, Shawn, John, RVDT, and all the Experts... Please enlighten with your comments on the subject...

Vis-a-vis a Conventional Tail Rotor (TR)...
1. ...Fenestron produces same thrust with a smaller size..??
2. ...Fenestron consumes same power for equivalent thrust output...??
3. ...Ring-fin is best amongst the three..??
4. ...NOTAR has better/same/poorer directional control availability in MR VRS..??

Thankyou all in anticipation...
-Praveen
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Old 5th Aug 2009, 21:42
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Outwest

You mention entering autorotation at a low altitude to get out of VRS.

If already low enough not to be interested in placing the nose attitude down a healthy bit to fly off the VRS state just how low an altitude would you actually choose to enter autorotation and what would be the outcome? I'm having a tough time with this autorotation solution due to the proximity of ground. When muddling with blade element theory of lift I really don't understand the flow transition from VRS to autorotation being very useful. My stomach says ouch, but in the practical there may be something to your idea I'm just not getting.

My 40+ years flying helicopters has always told me to push the cyclic slightly forward, and/or the collective down just a bit, at the slightest hint of VRS (far removed from SWP) and life remains good. This, to my mind, actually tends to save altitude and options for figuring to continue or overshoot an approach. If really wound up in a healthy VRS, which may have been the case a couple times for myself, I did place the nose attitude down a healthy fashion to fly off and away from the problem - thankfully altitude was sufficient. Experienced these situations in mountain flying.

Thanks.

WIII
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Old 6th Aug 2009, 01:14
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WWIII,

not sure if it is your english skills or mine, but please re-read my post and tell me where I advocated entering autorotation at a low altitude. On the contrary, I said rarely will you have the altitude to accomplish this......or at least that was what I intended to say.

I was responding to Shawn's post, hence the quote. If you do re-read my post, the first thing I said in response to him was "All true"........meaning his description of how to extricate oneself from VRS was correct. My comment was in response to his "no need to enter autorotation".......which again, with my first statement "All true" I agreed with.

However, just to clarify, that if by chance you were at a high enough altitude, say during a training exercise demonstrating VRS, by lowering the collective into autorotation, you would no longer be in VRS. Simply stating an aerodynamic fact.

Last edited by Outwest; 6th Aug 2009 at 12:12.
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Old 6th Aug 2009, 07:46
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Knock yourself out
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Old 6th Aug 2009, 13:27
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Outwest

Thanks, you're right. " All true, however if altitude allows (and it rarely does) entering autorotation would be a way out. A disk in autorotation can not, by definition, be in VRS. "

You indicate altitude rarely allows entering autorotation - my problem! Apologies.


RVDT

Thanks, the article probably will 'Knock me out'.

WIII
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Old 6th Aug 2009, 14:44
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No worries WIII, actually glad we clarified that........would not want any youngsters to think bottoming the pole at low altitude would be a way out
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Old 12th Aug 2009, 17:53
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Post NO Replies as yet..."fENESTRON vS cONVENTIONAL TR (THRUST/POWER/SIZE)"

Knock.... knock...
Anybody Home
Awaiting inputs on my earlier post dated 04 Aug:

Code:
 
To Nick, Shawn, John, RVDT, and all the Experts... Please enlighten with your comments on the subject...

Vis-a-vis a Conventional Tail Rotor (TR)...
1. ...Fenestron produces same thrust with a smaller size..??
2. ...Fenestron consumes same power for equivalent thrust output...??
3. ...Ring-fin is best amongst the three..??
4. ...NOTAR has better/same/poorer directional control availability in MR VRS..??

Thankyou all in anticipation...
-Praveen
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Old 12th Aug 2009, 18:37
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Question VRS---- Once again--- Light or Heavy that is the Question..!!

I am confused ,..yet again... but due seemingly contravening stands on this Issue by the best of oue bests...
Request the Old hands to clear the air for us... Can we have some inputs from undoubtedly our infallible seniors... Ray Prouty, Nick, Shawn, etc.

Quote:
Code:
 
For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.
 
Also for the record, most helicopters can experience SWP or overpitching at rates of descent near zero if they have little hover power margin.
 
Also, heavily loaded helicopters have less propensity to enter VRS because they need more vertical descent rate than lightly loaded helicopters, which need less descent rate to get into VRS.
I was of the same belief ... convinced even till date but for....
Rotor & Wing Magazine :: Vortex Ring State and Gross Weight
Code:
Tuesday, January 1, 2008 Vortex Ring State and Gross Weight Ray W. Prouty Maj. Devasish Mishra of the Indian Army writes to inquire about vortex ring state. He is a helicopter flight instructor at the Combat Army Aviation Training School at Gandhinagar Airfield, Nashik Road, India. He flies the Hindustan Aeronautics Ltd.-produced versions of Eurocopter ’s Alouette 3 (the Chetak) and Lama (the Cheetah), conducting ab initio training there. "I have a basic doubt regarding vortex ring state," he says, "which none of the publications seems to address. "It is commonly understood that a helicopter would enter vortex ring state when it settles down in its own wake or when the rate of descent approaches the value of its induced flow," he goes on. "If this be so, when two similar helicopters are operating under similar conditions but different all-up (gross) weights, the helicopter with a lesser weight should enter vortex ring state earlier (that is, at a lesser rate of descent) since the induced flow is less. This, however, appears to be opposite of commonly held beliefs. Kindly clarify." I presented an explanation of the vortex ring phenomenon in vertical descent for the main rotor in the July issue, which also discussed this problem with tail rotors. ("Tail-Rotor Vortex Ring State," July 2007, page 58). To recap that explanation, when a cigarette smoker launches a smoke ring, it propels itself through the air with a constant velocity that is proportional to the strength of its "circulation." A hovering rotor makes its own smoke rings in the form of tip vortices, which propel themselves downward at a constant velocity proportional to the strength of their circulation. That can be related to the disc loading of the helicopter. When the pilot chooses to descend vertically by reducing collective pitch and using partial power, he can reach a speed approaching that of the self-produced velocity of the vortices. This happens at descent rates of about 800 fpm for the Robinson Helicopter R22 with a disc loading of 2.5 psf to about 2,000 fpm for the Sikorsky Aircraft CH-53E, with a disc loading of 14. Since the vortices are not descending any faster than the helicopter, they hang around, entangle with each other, and produce erratic angles of attack at the blade elements, resulting in lift changes and buffeting. As far as I know, this explanation holds for vertical descent and should apply to two similar helicopters with different gross weights. The lighter one, with a smaller disc loading, should enter the vortex ring state at a lesser rate of descent than the heavier one. I, for one, do not agree with the "commonly held belief" to which the major refers. I would be interested in any experiences or measurements that would make me change my mind. This flight condition of vertical descent with partial power should not be confused with autorotation in forward flight that was discussed in the October issue ("Gross Weight’s Effect on Autorotation," October 2007, page 44). In that case, the
lighter helicopter will come down faster.
and

Code:
Sunday, June 1, 2008 Settling With Power, Redefined Ray Prouty The recent letter from CW4 Steven Kersting reminded me that the term "settling with power" is somewhat misleading ("VRS vs. Settling With Power, February 2008, page 7). Power has little to do with the phenomenon. As I discussed in the July 2007 issue, in a vertical descent, tip vortices are being generated that self-activate themselves to go down at a speed that is determined by the rotor disc loading. At some descent speed, the helicopter is coming down at the same speed as the tip vortices and so the rotor gets entangled with them. It is now in the vortex ring state. Not only does the local environment become chaotic, making the thrust fluctuate, but the average thrust decreases. This is due to the surrounding tip vortices inducing some of the wake to make a U-turn and come back down through the rotor, as shown in the photo (at right) taken from one frame of a smoke movie. At a constant pitch, this reduces the angles of attack on the blade elements. The magnitude of the loss is shown on the plot of thrust versus rate of vertical descent. This is based on model tests conducted on a "long track" where the air stands still and the model moves through it, unlike a wind tunnel in which the model stands still while air moves past it. (The results are the same.) Although the test results were presented in coefficient form, I have converted them to represent a helicopter that hovers at 4,000 lb with a collective pitch of 12 deg. The effects on the plot have also been observed in a wind tunnel test simulating vertical descent by using a remote control on collective pitch to hold thrust constant as the tunnel speed was increased. Initially, the required pitch went down, but at the speed for the vortex ring, it had to be increased. As the rate of descent slowly increases from hover, the pilot must initially reduce the collective to maintain thrust equal to weight. If, in this initial phase, the rate inadvertently increases, the thrust increases and the helicopter returns to the rate it had before the disturbance. Thus, we can say that initially the thrust is stable with descent rate. For this helicopter, that characteristic changes above 500 fpm as it enters the vortex ring state. Beyond this point, if there is an inadvertent increase in the rate, the thrust decreases and the helicopter comes down even faster. Thus, it is unstable. If the pilot just hangs on, the helicopter will all by itself increase its rate of descent until it reaches a region of stability above 1,500 fpm on its way toward vertical autorotation. Even if, while in the unstable region, the pilot increases collective, he will only get a transient increase in thrust that will do him little good since the lines for higher collectives are also unstable. Down he comes! In a wind tunnel or on a long track, it is possible to acquire steady test data in the unstable region because the model cannot respond to changes of thrust. That is not true in the flight of an actual helicopter. The instability makes it impossible to acquire steady data in the vortex ring state. The pilot would be moving his collective up and down in a vain chase of any steady condition at which to take data. What about power? As you can see, power did not enter into this discussion. It does come in, as a secondary consideration, in those wind tunnel tests where the thrust is held constant with collective. In the steady conditions for vortex ring, the downward inflow shown in the smoke-movie photo simulates a rate of climb and consequently the power required goes up. In flight, since no steady point can be obtained, this effect would generally not be observable. So our vocabulary should change. Instead of "power settling," I would rather refer to the phenomenon as "thrust instability." This situation involving the vortex ring state should not be confused with that other "power settling" condition illustrated by flying to the top of a mountain in forward flight but finding there isn’t enough engine power to hover and so settling as the rotor rpm drifts into the red zone. I would call this "running out of power" and so we have eliminated "power settling" from our vocabulary.
Besides vertical descent, the problem also occurs in low-speed forward flight, which will be discussed next time.
What would be the correct thing to propagate to the youngsters....???

Personally, I guess this one time, I would (most respectfully) differ with Ray... and agree with Nick,,, lest there more to it in between these two ideologies.... Is there still more to it than it seems till now ???
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Old 13th Aug 2009, 06:56
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My two bob's worth:
Conditions beyond which you MAY get into, at least, incipient VRS:
low ias <20 kts
highish ROD>500 ft/min
low power setting but then rapidly increasing power.
What's happening? At tips vortex ingestion reducing angle of attack=less lift - at root increasing collective pitch causes angle of attack to go beyond stalling angle=less lift therefore portion of blade producing lift is shrinking towards the middle=overall much less lift.
Situation to avoid:
Steep approach gone wrong - try to correct by dumping lever - hauling back on the stick - then "oh sh1t" now haul in lever to stop increasing ROD. Now low/slow and no room to carry out corrective action ie lower nose/lever.
Lesson: don't try and salvage a stuffed up approach - admit defeat early, swallow pride and go around.
SWP: engines have been "topped" - increasing lever beyond this point means more drag and no more engine power to sustain RRPM.
Solution:
check RFM and know where your helicopter runs out of puff IGE/OGE with respect to DA.
GAGS
E86
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Old 15th Aug 2009, 09:11
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Hi Guys, i have found this thread really interesting, and nearly all of it confirms what i had always understood, I have however found one major discrepency which regards the effect of gross weight on VRS. I had always believed the Hot/High/Heavy scenario, which i now understand is much more relevant to Settling with power than vortex ring itself, however from reading this I gather that at a lower gross weight/disc area loading, that VRS can be achieved at a lower ROD!? I understand the reasoning for this from the explanations above however I had always believed the opposite & I would much appreciate if someone could clarify.

I used to believe that vortex ring state developed because a relatively high ROD airflow caused the blade root to stall due to the angle of attack being above the stall angle, as the pilot attempts to recover this he may apply more collective in an attempt to reduce the ROD, however this pushes the stall area over a greater area along the blade as greater pitch angle is selected and further reduces lift, further increasing the ROD, all of this time the tip vortices are also strengthening and a lack of translational lift causes them to recirculate into the disc reducing lift at the tips from a reduced angle of attack due to the large induced flow. In this case i figure that an increase is gross weight/disc loading is likely to make the situation worse because the initial stall may develop at a smaller ROD due to the higher collective pitch settings however is what i have described not VRS, (but SWP??)

I would not call this settling with power because however much power you had available applying power would not get you out of this situation it would actually deepen the effect of the recirculation at the tips and not improve the stall at the root?

Could someone possible clarify where i am going wrong if the aforementioned scenario is not VRS.

Aucky
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