Vortex Ring / Settling with power (Merged)

[EN48]

Have been reviewing this thread from the beginning and finding it useful. I am trying to locate the website(s) frequently referred to in Nick Lappos's posts, but none of these links now work. Alternatively, looking for the formula mentioned for calculating downwash velocity. Any pointers?

[VeeAny]

From the momentum theory the induced flow can be calculated using the formula

Vi = √[ T / 2 ρ A ]
where

Vi Induced Flow Velocity [m s^-1]
T Thrust (approximately equal to weight in a hover). [N]
ρ Air Density [kg m^-3]
A Disc Area [m^2]

This is only an approximation, but is a good starting point for calculations involving the rotor system.

If you want any more PM me.

[MightyGem]

EN48, try here:

http://webpages.charter.net/nlappos/VRS.pdf

[EN48]

Vany:

Thanks. Trying to estimate downwash velocity for my Enstrom 480. After struggling with unit conversions for a bit, finally got it!:ok:

Mighty Gem:

Thanks, but this links to different info than the one Nick gave in earlier posts.

[DennisK]

Hi to all VRS fans out there. So what's a nice guy like me doing here on a warm Easter Sunday morning ... I logged on to the thread at 08.00 hrs and two hours later seem to have covered more than 280 or so posts.

That's something I love about pprune ... being able to read the studies and experiences of the likes of Shawn, Nick, and the other highly experienced and knowledgable guys so there's no question of any egg sucking lessons coming from me. But as a 14,000 hr rotary guy - can I put in my two pennorth and relate some adjacent VRS experiences.

First I'm a trifle concerned to know I have been teaching what apparently is an incorrect VRS recovery technique in advocating zero use of collective lever ... my reasoning being that whilst it may assist recovery due to the reduction in downflow, the associated loss of height is unacceptable in a real situation.

So at grass roots level in training for the VRS exercise, I demonstrate the well established, One-Two-Three technique.

Having completed the pre-exercise checks, 'Number One' is to select a suitable LS picture albeit from altitude with power set to provide a suitable ROD (circa 60 knots.) Once settled in a steady descent ...the establishment of the 'Number One' condition for VR is emphasised.

VR requirement 'Number Two' is introduced with aft cyclic to progressively reduce speed at some 5 knots per second to below translational lift speed.
I like to point out the change in airframe attitude and increase in ROD as translational lift is lost. As has been said here, it is important that aircraft attitude remains steady if we are to find that elusive VR tube of air! With the VSI indication showing an increase, I invite my pilot to increase ROD with a further lowering of lever.

Now we need to wait for say 5 to 10 seconds as the airframe settles in a steady but increased ROD. The establishment of the 'Number Two' condition is emphasised.

With a solid vocal warning, condition 'Number Three' is introduced with a firm,(possible panic mode) raising of lever. In the case of the Enstrom/SH300/MD500 series, the usual indications are: airframe buffet, random yaw and roll, disc flapping and RRPM variation invariably accompanied by an increase ROD. The right yaw can be quite severe but recovers .. see later.

To achieve the condition, which for the first 20 or so seconds is incipient, it is important the pilot uses minimum control inputs, especially with cyclic. Recovery is the usual forward cyclic to obtain a positive ASI reading before applying power.

The highly experienced FE who does my client's skills tests prefers the combined cyclic AND collective lever recovery, so this method is also shown, while I emphasise the extra loss of height that may not be adviseable in a real situation.

Now here is where I need help.

I explain to my pilots that the random yaw, (always right in the above mentioned types) is due to the loss of T/R effectiveness in the disturbed air, but the self-recovery is due to the T/R blades finding smoother air at the edges of the disc. On one occasion I experienced a rapid 90 degree disc roll to the right which I decided was due to the M/R blade tips doing a ditto. Not a good explanation perhaps so the whiz kids invited to explain please.

Following the rapid roll experience, I do recall my gentlemanly Irish 500 pilot saying at the time ... 'Dennis - please can we not practice that particular exercise again.'

Just a closing note and as I have written elsewhere - being intrigued by the off-quoted airflow reversal as the upgoing air punches through the centre of the disc - I decided to investigate by super-glueing a couple of dozen wool tufts to the fixed mast on a SH 300 and then running through the VR exercise. The 300 has excellent overhead vision so I was delighted to find my tufts flapping nicely in the turbulent airflow with their tails now obediently pointing skywards! A Eureka moment for me. I'd found the airflow reversal!

As has been said, let's keep this subject going for ever for all to read.

Take care all ppruners. Dennis K.

[Shawn Coyle]

Dennis:
Sadly won't be coming to UK for 20th May - some other time perhaps?
My preferred method of demonstration for VRS incipient stage is - start in level flt at 60 knots, downwind (and this only works downwind). Reduce power by about 20% of power for level flight (not % torque, % of power being used) and decelerate maintaining altitude.
The point is to show the symptoms - and they start as the airspeed reduces through about 15-10 knots - low frequency airframe buffeting, followed by uncommanded pitch, roll and yaw, and if power is increased, a possible increase in rate of descent.
The aim is to make the student aware of the symptoms they'll see and feel and hear in the cockpit and like stalls in a FW airplane, stay away from them, or recover at the first sign of them.

[Skycop]

As has been said, let's keep this subject going for ever for all to read.

Glad to see it's been going for almost ten years now since I first posted in July 1999.

[topendtorque]

Regardless of different type characteristics, the main sympton that must be recognised is always, the beginnings of a very unpalatable sinking feeling in the pit of the stomach.
The student that grasps the mettle at that point and demonstrates effective recovery is on a winner.
Those who gravitate, (ahem) to the feelings, being a tightening of a lower muscle in the body form will inevitably be sh@t on from a great height.

[pcpahari_IAF]

:confused:I am amidst a discussion with some of our fellas.:ugh:.... Need some expert opinions:8

For a RW pilot flying (rather descending from a peak to the valley bottom) with feel, visual perspective,sensory cues and probably with some cursory reference to instruments- ASI, VSI and Alti ("seat of the pants".. as most of us: ..pilot's..; would tend to, during such routine shuttles of short flight-legs in the mountains) is it more probable to enter/ risk entry into Vortex Ring state (Main Rotor) in...
(a) Lightly loaded (AUW) heptr (Say empty with minimal fuel)
OR
(b) Same ac Loaded to Max Capacity (MAUW)

Some brief explanation for the same...perhaps with specific cases of effect of winds:D.

I believe:ok:, contrary to popular opinion:=, its the lighter heptr that would catch the pilot early into vortex ring than the heavier one..??!!!

[Shawn Coyle]

Vortex ring state happens when the downwash velocity from the main rotor equals the speed of the up-flowing air.
It's impossible to say whether lighter or heavier weight will make a difference - it depends on the amount of power being used in the descent (lower power setting equals less downwash velocity which means you can get vortex ring state at a lower rate of descent)

[NickLappos]

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.

[RVDT]

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. :cool:

[ramen noodles]

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.

[JohnDixson]

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

[RVDT]

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.

[Outwest]

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.

[Outwest]

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.........

[bb in ca]

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

[Shawn Coyle]

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.

[Outwest]

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.

[pcpahari_IAF]

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

[WhirlwindIII]

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

[Outwest]

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.

[RVDT]

Knock yourself out
(http://terpconnect.umd.edu/%7Eleishman/Aero/vring.html)

[WhirlwindIII]

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

[Outwest]

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 :eek:

[pcpahari_IAF]

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

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

[pcpahari_IAF]

:confused:I am confused ,..: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:

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 (http://www.aviationtoday.com/rw/issue/departments/engineering/18168.html)

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


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 ???

[eagle 86]

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

[Aucky]

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!? :ooh: 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

[MightyGem]

pcpahari_IAF, could you use the "quote" tags rather than the "code" tags to display quoted text? It makes for easier reading.

The "code" button is the # symbol. The "quotes" button is the one to the left. Thanks.

[dammyneckhurts]

Ramen Noodles,

I can tell you that it is possible to get into V.R. with zero rate of descent if the wind is flowing upslope, actually incipient VR is probably a more accurate term.
The conditions have to be just right and it a very rare occurrence, but it does happen.
There are many a longline pilot here in Canada that has spent years and literally thousands of hours hovering at 100-200 feet over steep hill/cliffs. On a rare occasion when all the planets align if you come into a hover on the edge of a cliff on windy days, it has the potential to get nasty in a hurry.
When I am hovering over a spot that I know is going to give me VRS problems, if I can get the empty hook into the spot and pick up the load I never have a problem flying away with it. This relates back to the Nicks statements on the light vrs heavy theory, he is correct.

[stinkychicken]

Not to sure about all the aerodynamics but I am sure that if you have much low level experience its hard to confuse SWP and VRS.
Pre R22 govenor days it was not unusual to encounter SWP when low level hot and heavy so much so that it was standard (in Oz) to teach students to milk the Col and throttle to restor RRPM

My experience is that SWP will involve reducing RRPM and increasing everything else, whereas VRS will have increasing everything else (apart from IAS) and RRPM OK

Also in my experience it is very easy to induce VRS in light ac with light AUW. Just gain some Alt lose your IAS turn down wind initiate a rate of descent and pull some pwr then watch the ROD go off the clock.

Of course in real life we should know when we are potentially at risk and be ready to correct if we feel the onset of VRS, prior to it becoming fully developed.

I would hope this is still being covered in basic training

No idea about the heavies most of you seem to fly

Cheers

[[email protected]]

Aucky - I don't think there is anything wrong with your understanding of VRS but you are forgetting that at high AUM and therefore higher pitch and AoA settings, you are displacing air faster downwards than at lightweights - therefore you must have to descend faster to catch your downwash. This is what Nick has always tried to emphasise, it is all about downwash velocity - a helo of a given AuM with a high disc loading will have a higher downwash velocity than one with a lower disc loading.

You have to move enough air fast enough downwards to balance the mass of the helo and you can do it with a big disc with a large input area imparting a modest acceleration or a small disc working harder with a smaller input area and imparting a larger acceleration.

[Aucky]

Loud and clear. Thanks Crab.

[pcpahari_IAF]

Apologies to everybody for the ill-formatted post. Am new to forum participation... Still learning.

To MightyGEM
Thankyou for the inputs But..
I wanted to do the same Didn't know how?
Still not clear.
Req more details as to how to paste quoted text as quotes.)
-Thanx in anticipation.

[topendtorque]

I wanted to do the same Didn't know how


copy text from wherever, paste same on your reply, highlight again, click on the icon, it comes up with the word quote in brackets before and after, don't worry, just tab down, say the rest of your message.
magic.
cheers mate I had to ask heliport how to not so long ago.

[pcpahari_IAF]

Thankyou Topendtorque...

Wiser today ... feels better than yesterday...

Happy landings...:ok:

[pcpahari_IAF]

As you can see... its Light or Heavy again... :ugh:(Please refer my query earlier and Auckys post dated 15 Aug).

The RAF Manual on rotordynamics also explains Vortex Ring as outlined by Aucky's 15 Aug Post:confused:.

While I am personally convinced about Vortex ring being a purely Aerodynamic phenomenon resultin from interaction of the tip vortices to intensify with increasing ROD (Opposing Relative Flow) and induced flow produced by the rotors (powered rotor motion) ultimately settling the helicopter into this wake such that increasing power (coll) only intensifies the vortices at the tips causing the hepter to descend at an even increasing rate. I am in agreement with the finer aspects of winds and the fwd speed (relative) vividly brought out all throughout the posts in this forum however, I request the old hands to re-read Aucky's post... (with emphasis on the aspect of root end being stalled and insifying vortices at tips...&.. this state occuring earlier in Heavy hepters). Request point out where and how such an analogy would be wrong.... Thats the question my folks here have been arguing...:ugh:

Thanks yet again

[[email protected]]

Pcpahari - I think what you are asking is :

Does the heavier helicopter operating at higher pitch angles to hover reach a blade stall at the root earlier in a descent than a lighter helicopter?

I think the simplistic explanation of the flow around the tips and roots of the blades given in the RAF manual and others may be an outdated version of events which have subsequently been shown in more detail by better modelling and testing.

Nick's sources imply that VRS can be overcome with power if there is sufficient available which would indicate that intensification of the tip votices and extensive stalling of the roots doesn't happen.

Remember that the older manuals and explanations were created in the days of underpowered helos which had low disc loadings because of the low engine power. It may be that the 'intensification' of VRS was just overpitching as the aircraft ran out of power or that the older aerofoil sections on wooden or metal blades were not as efficient as modern, composite blades with cambered sections.

I am sure the TPs on this forum will have more to add.:ok:

[Shawn Coyle]

I'm reminded that a test pilot for one of the manufacturers told me (many many years ago):
"You can't get into vortex ring state in this helicopter because if you pull maximum power you can get out it..."

[MightyGem]

I seem to recall Nick also saying that you could power out of vrs if you had enough power to do it. I think he was referring to the UH60.

[kev_laline]

Shawn, also many many years ago, I was pax in a Bell 214 B, lightly loaded, steep downwind approach. Pilot looked at me and said "Don't worry about it, this thing has got so much power, it just doesn't make any difference."

[topendtorque]

I think that if you look at this thread (below) and some other related ones that you will get the impression that the statement "using heaps of power will pull them out of VRS", is Fallacious.

http://www.pprune.org/rotorheads/280474-video-released-aus-blackhawk-crash.html?highlight=blackhawk+vortex+ring

I have no doubt that many pilots who had never got right into VRS thought that it was power that pulled them out anytime, and maybe the story has mistakenly gone the rounds or many briefing rooms.

I know also that elswhere on pprune that we have discussed the phenomina of being able able to "flick it out" with a quick pull on the collective before VRS actually develops to the 'sphinter (spelling?)tightening' stage.

That techniques has nothing to do with power, but everything to do with awareness, reactions, and plenty of pre-emptive training, and just quietly, being able to get the damm collective down again real quick like.

I believe the technique carried strong endorsement from many experienced types here at the time. I just want to impress on newbies that it is fraught with disaster if you think that you will rely upon it without the right training.

It reminds me of an article in one our CASA safety digests many years ago, headlined; 'Mustering is for the highly trained'.

So don't do this sort of thing with your family car I say. seek help to explain, it's out there.
cheers tet

[[email protected]]

Tet - your link has far less to do with VRS than it does about mishandling when approaching downwind. It would appear that he simply ran out of power and didn't have enough height, speed or power to go around.

No-one is suggesting that trying to power out of VRS is the way forward - simply saying that according to the windtunnel and airflow tests, this can be achieved if you have enough power available and use it - chances are you won't have enough and will only apply full power too late to save you hitting the ground/sea.

[RVDT]

don't fly over Burning Man!

kHpBComsngQ

or anywhere in Japan!

7bvtvWxtI78
:cool:

[birrddog]

I suppose a "simple" explanation for "powering out of Vortex Ring State" is that if you accept VR to be the state when RoD ~= Induced Flow,

and,

RoD less than Induced Flow or significantly greater than Induced Flow (so vortices are lifted above and clear of the disc) to not be states of VRS,

then

It is conceivable for the scenario where you are in VRS (or early stages there of) that if you have the power (and rpm and angle of attack (until blade stall)) margin then you could create the induced flow to the point where it is greater than your RoD and put you back on top of the vortices (as apposed to keeping you in them).

The disclaimer being there are a lot of "ifs" in the above statement.

The Wagtendonk book has a pretty good section on the physics of VRS, including an interesting graph (Figure 19-6) of Effect of Airspeed and RoD on conditions required for VRS.

[Aucky]

Sorry to dig this back out again, but I was pondering on it again today.

Would it be reasonable to generalise this in the following way (differentiating between 'settling with power' & VRS):

'Settling with power' is more likely to occur when hot, high, heavy. These factors are more likely to lead to a situation whereby on an approach, with loss of translational lift, you develop an increasing ROD (if you don't regain TX lift), with power applied (probably over-pitch & low rpm) and low airspeed. This perhaps turns into fully developed VRS if there is sufficient height for ROD to develop before ground contact, but even without VRS developing this could be misinterpreted as actual VRS :ouch: (therefore the heavier the more likely)

However, to enter VRS without the cause being a lack of power induced ROD (perhaps just a bad out of wind approach etc), one needs a higher ROD when heavier because the speed of the downwash from the disc is greater and therefore you need a higher ROD to catch up with it... (going against the blade root stall etc... explanations that I was taught, but apparently true according to the real test data given by Nick, Shawn etc....) This is therefore more likely to occur accidentally at a lower AUM, given the required ROD may be less...

Does that sound reasonable as I often get asked on this because there is so much confusion between VRS and 'Settling with power'. Or is it me thats confused :confused:

Aucky

[NickLappos]

Aucky, I think you've got it. In fact, I would bet that virtually all VRS events are SWP events, since very few involve rates of descent anywhere near that needed to induce flow reingestion.

[delta3]

Aucky,

I kind of like the summary, only would refine :

Quote: "This perhaps turns into fully developed VRS if there is sufficient height for ROD to develop before ground contact, but even without VRS developing this could be misinterpreted as actual VRS (therefore the heavier the more likely)"

Into: One of the outcomes of settling with power can be VRS if an important increase of ROD develops. In most cases due to close proximity of the ground, this will not be the case.


wow, other effects of loss of RPM, ranging from LTE up to main rotor stall can also be expected.

m2c

d3

[Aucky]

:ok: glad to have a believable answer that highlights the differences between the two.

Delta - Agreed. My way with words has never great been :oh:

[topendtorque]

Easy way to learn SWP, is attempt to hover with less power than it takes, result, aircraft settles.
VRS, the difference between the two maneuvres is as startling as is the feelings of VRS. We used to call it shooting the tube. quite easy to figure that out. tears to the eyes and the wax outa yer ears.

crab how are you doin?
your quote.


Tet - your link has far less to do with VRS than it does about mishandling when approaching downwind. It would appear that he simply ran out of power and didn't have enough height, speed or power to go around.




Had another look at the numbers to refresh my memory, he went from 100 feet to 70 feet in 30 secs, not much descent rate there.

Very little descent next 11.4 secs as flare at 20 degrees nose up turning to a bank 0f 43 degrees, washing off airspeed.

At that point descent increased dramatically as did his realisation as within 1.1 secs he had got the nose to pitch to slightly nose down from 16.5 degrees up and impact was at between 1320 to 1800 fpm, another 1.5 seconds later. That is much too high a ROD to have built up just from his entry maneuvre or a simple overshot, as it happened from at or below 70 feet.

I feel that I can say again that he got himelf into VRS at about the time he caught up (slowed his ground speed down to) the downwind airspeed and reacted brilliantly to attempt recovery, except that as you say there was just not enough room to recover in the manner which he initiated, which to me was correct. a bit more height and the disc would have cut clean air.

Entering the maneuvre was the problem in the first place, and the reasons why,as we have discussed before, but perhaps a systemic breakdown of the command chain's understanding of the known, practised and aircraft capable maneuvres.

Check a previous post up the page a bit for a cue on this, you will see reference to the billy goat idea that if you have enough power you can pull it through VRS. Hah, the bigger they are the harder they fall

There is another one i'll dig out one day that demonstrates quite neatly the rotor system entering VRS.

[waragee]

A very effective tecnique to deal with VRS I have used and taught in light helicopters in mustering operations for a long time now is to fly the disc rather than the airframe. When you become used to recognizing the early stages of VRS onset it is quite simple to take the disc into clean air and avoid the problem, in most siuations it usually means you just have to move a couple of feet into wind regardless of which way the airframe is looking.
Also it is important to to keep your manuvering in a relatively flat plane so as to reduce your opportunitys to encounter VRS. Lots of height changes high and low are no good.

[NickLappos]

waragee,
You are absolutely right,That is great advice. I find that to perform low speed maneuvers successfully, one must sense the attitude, speed and power state of the rotor, and not concentrate so much on aircraft attitude or stick positions.

An exercise that I've used to sharpen the skills with understanding how the rotor flies goes like this:

Set the aircraft in a 5 foot IGE hover on a long runway on a very still morning. Use enough collective friction to keep the collective from moving and then relax entirely on it. This is your fixed power point, from now on you will use only speed to govern altitude and approach angle. From this steady hover lower the nose slightly, accelerate forward a few knots, 2 or 3, and note that you begin to climb very slowly. Carefully hold the nose position and climbed slowly to 100 to 200 feet. Now carefully pull back on the stick and raise the nose just enough to begin a slight descent. You will note that this slow gentle descent looks very similar to the very bottom of that approach. Remember you will not move the collective at all. Using only fine adjustments of speed set an approach angle that allows you to descend back to the runway. If your angle is too steep lower the nose very slightly to gain a few knots (two or three) and flatten out the angle. If your angle is too shallow pull back very slightly on the stick raise the nose (loose one or two knots) and steepen the descent angle up. Having started from 200 feet you should find plenty of room along the runway to carefully exercise that area of rotor angle, speed and power right at the edge of translational lift. This is the key place for power management and safety on approaches.

On a long runway on a still morning I have flown with students four approaches (take off ,climbed 200 feet, descent, approach to steady hover) without ever moving the collective pitch. If you could do that you really understand how to fly the rotor's angle and stay out of trouble.

[topendtorque]

Ah yes the disc always the disc, use it to save yourself, if you have climbed up not too high to have a look around leave it pointing at the ground, for airpseeed potential, it doesn't matter which way the A/C is pointing, same as a VOR just follow the disc if things become silent.

A corollary to your excercise Nick in disc awareness was one i enjoyed in the '47J. Simply just lock the cyclic and fly around for quite a while with just the smallest movements of throttle and pedals and sometimes collective, land it it like that also. I think the further forward seating position gave one the better feel for the torque, power on, power off. Surely that's stuff which you must have done in those slippery things we saw on the video. geat stuff thanks for showing it.

Flying like that gets to be a drug, on a beautiful crystal clear cool morning that you could cut with a knife, sky of azure blue, golden grass waving in the light breeze, fat cattle waddling along in front, big cattleman sitting just behind me on the side under his ten gallon hat, with a slight smile of pride on his face as he watches his men and horses trotting along around the cattle, bit like being addicted to this here turnout sometimes.

I'd have thought you'd be on hols on some sunny beach, it's about twelve months isn't it since you disappeared? Good to see you anyrate.
tet

[[email protected]]

TeT - in the example accident - he is already running short of power, then he sets 20 degrees nose up and rolls to 43 degrees!!! where is the vertical component of rotor thrust coming from to balance the aircraft weight? No wonder he descends quickly - then he compounds it by stuffing the nose down to 16 degrees!!

Not VRS in any way shape or form just mishandling and poor awareness.


Nick, I have always found that a fixed power transition from the hover in still air gives a descent just before ETL is achieved as the rotor tries to battle its way through the tip vortices to clear air - are you saying this doesn't happen?

[NickLappos]

Crab,
Absolutely, there is a very definite settling that occurs during acceleration in ground effect as you get around 8 kn where you catch up with the vortex from the outflowing air. This causes a slight increase in power required. It is far more pronounced if you are a low hover perhaps 2 feet or less, and non existent when OGE. From low IGE conditions, this settling can cause you to momentarily touch the ground.

During exercise that I described you will note some settling but by being a slightly higher IGE altitude usually is not enough to touch the ground.

[Aucky]

I was always under the impression that the sink you described was result of 'displacing the ground cushion behind you'. Is that just a simplified urban myth or is that a factor also?

[helmet fire]

Hi crab and TET, at the risk of thread creep, the BlackHawk accident was not a VRS event, and is very unlikely to have been "insufficient power". It is most likely to have been the S70A's tendency to suffer significant RRPM droop following a high NR manoeuvre (flare, dropping lever, etc) followed immediately by a rapid application of collective. The donks do not respond quickly and the result is significant RRPM droop until they catch up. This droop can be induced to the extent of dropping the generators offline, and has caused several other incidents in the S70A, though those others impacted the ground and became airborne again. Regardless of OGE hover power margin!
This one unfortunately impacted the ship's edge and was thus unable to become airborne again. Bingers (the pilot) came very, very close to lining up with the ship and recovering the situation, and this proximity no doubt enabled enough of the airframe to stay together and make it survivable to most on board.
Another example: US Army Demo (http://www.youtube.com/watch?v=tsDSvcEDCgg)
Of course, as I know all too well, I was not there and may have missed significant elements of the sequence, but I am concerned that we would point at VRS or SWP whilst excluding what I consider is the real lesson for UH60 pilots.

[NickLappos]

Aucky,
The myth that ground cushion is a pressure bubble that supports you is picturesque and actually helpful, but really physically inaccurate. At some point, it is useful to transition from the basic explanations that tend to simplify (but may work) to the actual physical relationship, which is deeper and more meaningful.
Power goes down as speed goes up beacuse the power "savings" of IGE acts only on the induced power, which drops to insignificance at mid speeds, so the effect of IGE at speed is small. This is not because the bubble is somehow left behind.
As some kind of proof, look at an airplane achieving significant ground effect at 150 knots, where the bubble is far behind the aircraft, as seen by the disturbance on the water behind the aircraft. The ground effect works even so, because the induced drag at that speed is high, so the 15% savings is significant.

http://www.free-online-private-pilot-ground-school.com/images/ground_effect.gif
An Ekranoplane showing the "bubble" behind the aircraft.

http://i35.photobucket.com/albums/d196/pattenicus/battleships/pic_1145.jpg

[Thomas coupling]

Nick doesn't like my resistance to his take on ground effect, but I still feel it is relevant:

In Ground effect - the air approaching the aeroplane (which is close to the surface) is forced up over the aircraft and has little or no effect...and it is also forced between the fuselage and the surface. The latter causes the air pressure to build which induces a ground effect...similar to the Ekranoplane in the picture. This 'effect' is utilised by the designer to assist the motion of the vehicle over the surface.

In addition there is Ground cushion. This develops when the downwash from rotors impacts the surface and bounces outwards and inwards. The inwards vectors induce addtional air pressure, or a 'bubble' which to some extent, supports the airframe, reduces the induced airflow and increases lift which allows the operator to reduce the collective to maintain position.
It also causes the static tube to register a downward indication in the VSI.

There are 2 effects consequently, each being sourced differently.

WRT VRS Vs SWP:

My take for what it's worth:
SWP: you arrive into the hover at your listed weight and respective approach speed and these two factors are too much for the remaining power margin you have left in the collective. Hence you run out of arresting power to stop the momentum of the arriving vehicle. I liken it to driving too fast and the brakes being too ineffective to stop you.

VRS: Is disc related and is to do with placing the vehicle in an aerodynamically unstable situation w.r.t.: RoD / Speed and disc angle.

Once again, my observations suggest that the North Americans tend to confuse the two, whereas the Europenas gravitate more towards VRS and its effects and barely talk about SWP.

two cnets worth ;)

[[email protected]]

TC - on the ekranoplane, the air between the fuselage and the ground/sea will speed up under the wing and reduce pressure rather than being squeezed to increase pressure. Once it has exhausted from under the wing it will slow down again and increase pressure back to static which will provide a barrier to the accelerated air coming off the top of the wing and reduce the downwash - if the downwash is reduced there is less drag and therefore more power available for thrust.

You can't 'squash' air unless it is constrained on all sides.

The CFS explanation of the ground cushion has always been a convenient way to explain what happens but never scientifically accurate - all that boll8cks about how a rough surface 'scatters' the air and reduces the 'cushion' never made sense.

[Thomas coupling]

Crab: nooooooo:eek:.

The pressure between the wing and the sea actually INCREASES:ugh:

Try this:

Air apporaching a normal aerofoil goes up and over, speeds up and the pressure reduces. The air going underneath carries on at normal pressure. Hence lift is produced overall.
However with Wing in Ground Effect (WIG) vehicles what happens in reality is that the sea/ground partially blocks the trailing vortices and decreases the amount of downwash generated by the wing. This reduction in downwash increases the effective angle of attack of the wing (up to 75% in some examples) so that it creates more lift and less drag than it would otherwise. This is ground effect.
An additional bonus is ram pressure. As the distance between the wing and sea/ground decreases, the incoming air is "rammed" in between the two surfaces and becomes more compressed. This effect increases the pressure on the lower surface of the wing to create additional lift.

A2 brutus?

[rotorfossil]

Crab. My understanding is that a rough surface/long grass etc slows the outflow which is then picked up and enhances the tip vortices. This leads to loss of rotor thrust at the outboard parts of the blade thus reducing the benefit of ground effect.

[Thomas coupling]

Rotorfossil:
Correct - for the same reasons as above. Anything which obstructs the exit path of tip edge vortices, causes the vortices to retain their integrity as they move onto the upper surface. Lift is lost outboard and the ground cushion weakens.

[RVDT]

Gentlemen,

Pick up your coats on the way out!

The answers are here (http://www.aviation-history.com/theory/lift.htm)

[NickLappos]

RVDT,
Brilliant site - were I smart enough, I would have written it. I have argued for decades that if pressure where the thing, the balsa gliders that I built as a kid didn't fly, since their wings were flat pieces of wood. Newton Si, Bernouilli Non.

As K.D. Wood said, "If it looks like a wing, it will fly almost as good as the best wing."

[[email protected]]

TC - that link would suggest some more bedtime reading required - and not just AP3456:) It's all about downwash not high pressure:ok:

Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.

[RVDT]

Leave your money on the fridge! :ugh:

[Thomas coupling]

RVDT: Bernoulli Vs Newton - each describes principles of equal transit and also conservation of energy, both allude to there being a pressure reduction on the aerofoils upper surface. This forms PART of the resultant lift. Coanda effect and airflow deflection also plays a big part. However there is definitely a speed increase over the upper surface.
-----------------------------------------
When air flows past an airplane wing, it breaks into two airstreams. The one that goes under the wing encounters the wing's surface, which acts as a ramp and pushes the air downward and forward. The air slows somewhat and its pressure increases. Forces between this lower airstream and the wing's undersurface provide some of the lift that supports the wing.

But the airstream that goes over the wing has a complicated trip. First it encounters the leading edge of the wing and is pushed upward and forward. This air slows somewhat and its pressure increases. So far, this upper airstream isn't helpful to the plane because it pushes the plane backward. But the airstream then follows the curving upper surface of the wing because of a phenomenon known as the Coanda effect. The Coanda effect is a common behavior in fluids—viscosity and friction keep them flowing along surfaces as long as they don't have to turn too quickly. (The next time your coffee dribbles down the side of the pitcher when you poured too slowly, blame it on the Coanda effect.)

Because of the Coanda effect, the upper airstream now has to bend inward to follow the wing's upper surface. This inward bending involves an inward acceleration that requires an inward force. That force appears as the result of a pressure imbalance between the ambient pressure far above the wing and a reduced pressure at the top surface of the wing. The Coanda effect is the result (i.e. air follows the wing's top surface) but air pressure is the means to achieve that result (i.e. a low pressure region must form above the wing in order for the airstream to arc inward and follow the plane's top surface).

The low pressure region above the wing helps to support the plane because it allows air pressure below the wing to be more effective at lifting the wing. But this low pressure also causes the upper airstream to accelerate. With more pressure behind it than in front of it, the airstream accelerates—it's pushed forward by the pressure imbalance. Of course, the low pressure region doesn't last forever and the upper airstream has to decelerate as it approaches the wing's trailing edge—a complicated process that produces a small amount of turbulence on even the most carefully designed wing.

In short, the curvature of the upper airstream gives rise to a drop in air pressure above the wing and the drop in air pressure above the wing causes a temporary increase in the speed of the upper airstream as it passes over much of the wing.

--------------------------------------------
Crab: Look into WIG more closely, old boy. There is definitely a pressure increase caused by ram effect of incoming air between hull and sea surface. And because the outriders slug the wing tip vortices, it prevents the IAF from increasing.
Thats how WIG's work.................:ok:

[RVDT]

Hmmm,

The hecklers are still here!

The air slows somewhat and its pressure increases. Forces between this lower airstream and the wing's undersurface provide some of the lift that supports the wing.

Oh really? I thought the air was already stationary. Air does not flow past a wing, a wing moves through the air.

What is containing the miraculous increase in the pressure of the "static" air?

If there was an increase in pressure it would need to be contained by something and then react against the lower surface of the wing to lift it.

Part of that container would have to be the surface. The surface in the above photograph of the Ekranoplane is water which you would think would react in some way to the increase in "pressure". How heavy do you think this aircraft is and your theory expounds that it is somehow supported by the surface. Clearly it doesn't.

[[email protected]]

TC - as RVDT asks, what is the mechanism by which the 'ram effect' increases pressure under the wing when there is no choke point, just a slightly convergent duct?

I don't think you can ram air into a tube with 2 open ends and hope to create dynamic pressure..

If your reading on WIGs has been on bernoulli based articles then I am sure it does talk about increase of pressure but said theory would appear to be incorrect.

Decrease of downwash - Yes

Increase of pressure and ram effect - No

Apparently the earth is no longer flat!!!!:ok:

[Thomas coupling]

RVDT: I think you misunderstand me. I'd like to summarise, if I may as I don't want to lose my money and my keys!

Bernoulli and Newton (pressure and AoA) are BOTH right , but for different reasons.
Bernoulli is the populous explanation, but may have been 'modified' en route:

The air hits the aerofoil and is deflected upwards and downwards.
The down deflection pushes UP on the +ve AoA aerofoil and produces lift.
The up deflection initially causes a +ve pressure increase at the front of the section and has the effect of trying to slow the aerofoil, but this is minor compared to the remainder of the surface of the aerofoil which experiences a pressure drop due to the fact that the AoA offers a blind spot behind the leading edge. This zone of reduced pressure causes the air to rush in to fill the imbalance and as a concsequence of this, it accelerates.

So as I have said all along - on the upper surface, two things happen:
Pressure (overall) reduces.
Air speed increases.

[What Newtonians and Bernoulians dont say is that the path that the air has to travel is longer on the upper surface and hence the air has to speed up to join its counterparts flowing under the aerofoil].

Newton bases his argument on AoA:
The air hits the aerofoil and is deflected upwards and downwards.
The downwards air will hit a +ve AoA aerofoil and push the wing upwards. BUT it will also deflect down on a neutral AoA plank of wood flying thru the air.
The upward air will experience the coanda effect which causes the air to go down in +ve AoA chords, but even with neutral chords it still goes down due to the coanda effect AND vortices coming off the trailing edge. Ironically Newton also accepts that there is a pressure reduction on the upper surface.

So: BOTH are right! What is wrong is the longer route theory on the upper surface.
------------------------
WIG:

Rostislav Evgenievich Alexeyev would be turning in his grave:=

what is the mechanism by which the 'ram effect' increases pressure under the wing when there is no choke point, just a slightly convergent duct?



What????

The air entering the underside of his plane WILL be compressed (by the slightly convergent duct) and WILL push on the hull of his baby. It is compressed between the boat hull and the sea surface.

I challenge you to slightly converge the end of a hose pipe and NOT experience an increase in pressure:ugh:

[RVDT]

I challenge you to slightly converge the end of a hose pipe and NOT experience an increase in pressure

And if the hose pipe was square, open at both ends, and two sides where missing and only the top surface is moving and the fluid in it is completely static - what is going to happen again? That's a new one!

It is compressed between the boat hull and the sea surface.

Another doozy - compressibility at less than .3 Mach!

My money is with the physicist and the Boeing dude -

https://secure.steenaero.com/Store/images/site_images/book_understanding_flight_640x801.jpg

[[email protected]]

TC - convergent duct gives increase in velocity and decrease in pressure and temperature - remember intake icing?

Week one day one at pilot school:)

[What Newtonians and Bernoulians dont say is that the path that the air has to travel is longer on the upper surface and hence the air has to speed up to join its counterparts flowing under the aerofoil].


I don't think you have read that article by Anderson and Eberhart:)

[Aucky]

I don't want to mess with the heavyweights here, but if we're talking ground effect with respect to helicopters below translational lift speed the air is not stationary, there is an obvious downwards flow of air, that undoubtedly builds an area of higer pressure beneath the disc in proximity to the ground, because it's flow is restricted, and it's direction changes, meaning a force needs to act upon it (newton), which must be due to the fact the pressure in that area is greater than the surrounding ambient pressure (pressure gradient).

I don't get this mention of the requirement to be sealed on all sides etc... because just looking at wx pressure systems, high's low's etc... they are not sealed and there are obvious changes in surface pressure due to expansion/compression upwards/downwards movement of air (like above/under our disc). If everything needed to be sealed wouldn't the surface pressure be equal around the globe (perhaps with some fluctuations due to temperature)

I think that the principle of a reduced induced flow in close proximity to the ground below tx lift speed must hold some truth as a result of higher pressure under the disc... having said that I haven't noticed the altimeter drop on take-off before

[[email protected]]

Aucky - surface pressure is due to the weight of air above it squashing it down against the surface of the earth. Variations in surface pressure are due to variations in surface temperature which cause pressure differentials at altitude leading to air moving from high to low (pressure and temp) at altitude which redistributes the air giving variations in the quantity of air above differewnt points on the surface. This is why the tropopause is higher at the equator and lower at the poles and is where all thermal winds originate.


there is an obvious downwards flow of air, that undoubtedly builds an area of higer pressure beneath the disc in proximity to the ground, because it's flow is restricted this is exactly the urban myth that Nick and others have tried to dispel - it is a popular notion because it seems intuitive and is easy to understand - doesn't make it right though.

[Aucky]

crab - surface pressure is due to the weight of air above it squashing it down against the surface of the earth

Isn't that what our helicopter is doing? it sounds like the same thing to me only on a micro scale as opposed to a macro scale. There is more weight (atmosphere + helicopter) over that small area under the disc than there is over the area next to you (atmosphere) in the hover....

I'd also like to put this question out there. F=M.A if the air is changing direction under the disc (diverging), by definition it's accelerating (change of direction/velocity). So a force is being applied to that air. If this is not due to a pressure gradient, what causes it's change in direction?

[loaysa]

Having read the "Newton vs Bernouilli"´s article, it´s assumed that the change of direction is due to air´s viscosity forcing the inner molecules of air flowing through the airfoil to slow down, actually until it reaches 0, thus changing direction of the free flow.

Really interesting article.Thanks :D

[[email protected]]

The weight of the aircraft does not impinge on the surface so only the weight of air above that point is acting on the surface.

The rotor downwash is not constrained by anything and so flows outwards - you can see that occur when you hover over grass.

The rotor imparts an acceleration to the air in order to create thrust to balance the aircraft weight - the air accelerates downwards because of the rotor and outwards because of the ground - still no requirement for a pressure increase below the rotor.

[407 too]

Bernoullii's principle states that when you slow an air mass, it increases in pressure, when the speed of an airmass increases, the pressure drops.

It is only a principle or a statement of physical activity, wrongly applied to why lift is generated. The principle DOES happen on an airfoil, but it is the laws of physics (Newton) that actually make it fly.

I would gesture that the only reason you have a wing in the typical airfoil shape instead of flat, is to more closley follow the natural flow of the airmass as to allow a clean laminar flow

[Aucky]

Crab - you may be right, but I'm not convinced

The weight of the aircraft does not impinge on the surface so only the weight of air above that point is acting on the surface

Are you suggesting that the downwash holding us up (newton) has no effect of 'pressing down' on the surface before it's forced to diverge?

The rotor downwash is not constrained by anything

Except the weight of the rest of the surrounding atmosphere (ambient pressure), and the mass of air it must displace...(quite significant)

The rotor imparts an acceleration to the air in order to create thrust to balance the aircraft weight

Agreed

the air accelerates downwards because of the rotor and outwards because of the ground - still no requirement for a pressure increase below the rotor

So how does the stationary ground cause this air to change direction if it's not a build up of pressure?

[helisphere]

Let's keep this thread alive until 2100!
Nick might just get his wish the way this is going though it seems to be wandering a little bit off topic.

I have to substantiate what Lama Bear and RVDT have said about longline and mountain flying. Nick's explainations are very good and very thorough but he does not account for any vertical air movement. And in mountainous terrain with any significant wind this is a given.

I can personally say that I have experienced symptoms of a loss of lift and power increase did not reduce the descent and the control of attitude became more difficult, and I was NOT overpitching, felt just like the VRS that you do in training at higher altitudes and descent rates, and it almost always happens with an empty hook or water bucket (meaning at lighter weights) but not always. I know my rate of descent (pressure altitude rate of change) was not high enough for VRS but that doesn't mean I didn't have enough vertical airflow to get into it. I've had it happen in Astars, 500s, and hueys.

Like Lama Bear said, talk to any experienced longline pilot, you will hear the same story from every one of them...

[helisphere]

Also, it looks like Nick's example of the R22 for vertical and horizontal velocities assumes a level rotor disc which of course is not always the case.

So here is a scenario: I'll use a disc load of 4 which is close for light to medium turbine helicopters at reduced weights of pilot fuel and longline. This would give a downwash velocity, according to Nick's rule of thumb, sqrt(4*210)=29ft/sec = 1740 ft/min

At the 75% horizontal velocity this would be about 13 kts fwd and translates to a descent angle of about 37 degrees from the vertical or 53 deg below level and a total velocity on this path of 1740/cos37=2175 or about 22 kts.

So if we put the helicopter in a 15 deg nose up attitude descending with longline to pick up a load, the new flight path that would provide the same disc angle of attack and velocity would be 52 deg from the vertical or 38 deg below horizontal with vertical velocity at 2175*cos52 = 1340fpm and horizontal velocity of 2175*sin52=1714fpm or about 17kts. Then lets say that he is coming in cross slope and crosswind to the sling site of 14 kts blowing up a 45 deg slope, this is about 1000fpm of vertical airflow. Since this 1000 fpm flow is not moving atmospheric pressure gradient lines past the helicopter then it will NOT show up on the VSI. So here we have a situation where the velocity component normal to the rotor disc is 1740fpm and a VSI that will only read about 340 fpm and this is a 100% downwash velocity descent rate and Nick says you only need 50% to start seeing VRS.

Yes I know I neglected to factor in the horizontal wind component but I think the point is still valid.

[[email protected]]

With a 100% of your theoretical downwash speed coming up at the rotor from underneath, you are in autorotation or as near as dammit, not VRS.

I have sat in autorotation in an updraught, as have many others, but suddenly getting a descent is due to variations in the strength of that updraught not VRS - although allowing the aircraft to descend in that configuration then pulling a handful of power to arrest the RoD might get you a lot closer.

[NickLappos]

Crab,
Actually, at 100% Vi you are in fully developed VRS. The Vi ratio tells all. Autorotation (windmill brake state) is from about Vi ratio of 1.2 and upward.

[[email protected]]

I take your point Nick but does that mean that helisphere's figures are correct and you could in theory be in fully developed VRS with less than 500 ft/min RoD on the VSI?

[helisphere]

I know I kind of threw those numbers together but here is a spreadsheet I made that lets you change all the parameters and see a plot on a VRS chart.

The white blocks are for user input and everything else is calculated. It lets you enter wind velocity and flight velocity in 3 dimensional vector form. Also, air density, nose up attitude, gross weight and rotor diameter. All velocities are in feet/min but its easy to see knots by just moving the decimal a couple spots in your head. The plot is on the old chart out of the Army Field Manual.

It's kind of interesting to play with the numbers.

Let me know if anyone catches any errors in it.

http://dl.dropbox.com/u/19282077/VRS.xls

[NickLappos]

Crab and helisphere,
I can't subscribe to the updraft theory of VRS, it is just not that clean. The precise disk angle needed to enter and sustain VRS is not something that an armchair calculation is likely to prove. True VRS is a flaky thing, often needs the pilot to work at staying in the condition. I would be interested to chat with a long line pilot to see what he/she experienced in the event.
I would bet dollars to donuts that power issues are much more the issue, and I must say that few "experts" know much about the distinction between VRS and SWP to differentiate, including experienced and well intentioned government safety experts and investigators.
To my knowledge, no tests have shown anything close to a VRS case where updrafts and such have triggered any events. I believe the conditions that would create the updraft would also create enough turbulent flow to disrupt all those calculations (but that is a belief, not based on data.)
The situation is perpetuated with a thousand "VRS" demonstrations by a thousand instructors who show SWP to their sutdents. I searched youtube for VRS demos on video, and found NONE that were VRS, all were SWP.
To recap, to get VRS, you must descend at about 70% of the induced velocity, about 700 fpm, and also have about 8 knots forward velocity (straight down will not do). If you enter from lower descent rates, it is not VRS, it is SWP, and if you do nothing to correct the SWP, it can degrade to SWP.
None of the above means there is no danger, just explains what the physics of the situation is. The cure for either is about the same, once in the situation, but the cure to prevent SWP is to retain some power margin, enough to allow some vertical maneuvering while OGE. THAT is not mentioned in the typical VRS discussion, making it somewhat misleading to spin on and on about VRS, descent rates and updrafts, when most cases can be solved with adequate attention to power and weight.

[helisphere]

Nick, I'll agree with you that the situations I am suggesting are not likely to be able to sustain VRS but I think they can create the condition for a short time which probably isn't going to hurt you but it can sure scare the wee out of you.

It would also exlplain why they always use such a low number as 300 fpm as a rule of thumb for avoiding SWP or VRS because you can definitly have a velocity normal the rotor plane of higher value than the VSI would tell. It may also be leftover from the days of low disc loadings.

I have a question: Is there any kind of testing requirement for VRS in regard to FAA certification?

I wonder how the Kmax with its angled rotors handles VRS. Can it get into it? If so what would it's descent angle chart look like? Different I would think.

[NickLappos]

Helisphere,
Good thoughts, I really agree that the updraft could make things worse. Maybe not VRS, because SWP is strongly influenced by descents. I think you might be on to something.

FAA does VRS testing - no, not required, maybe not necessary.

KMax? Great question! Different flow patterns might yield different/no VRS.

BTW nice excel spread sheet, good work!

[helisphere]

Well the spreadsheet certainly makes you think... I wouldn't have thought to make it if I wasn't reading through this thread.

A little background, I've got about 4000 hours and about 700 longline mostly in BHT205/210, AS350 and 369/500. In any type of normal flying I have never had any issues with VRS. But longlining in mountainous terrain has definitely givin me a few thoughful moments. I think with that experience I have a pretty good feel/sense for when it's an issue and I have to agree that I don't think it's near as big an issue as many instructors teach, and yes SWP without VRS is a more common problem. But I have to say that mountain longlining really makes me fly more respectfully regarding VRS or SWP than when I'm doing any other type of mission.

[helisphere]

Nick, I have a question for you. When you are taking data with an instrumented aircraft specifically on VRS, does the aircraft have angle of attack or vertical airspeed (besides VSI/ASI) instruments? And what parameters do you then look at and analyze from the data?

I think it would be cool to instrument a logging helicopter at work. Might scare you manufacturing types though, I mean for the stresses your strain guages would record, I know it scares me. I don't know, maybe it's already been done but for airflow data and to see if they are getting closer to VRS than one might think.

[pcpahari_IAF]

copy text from wherever, paste same on your reply, highlight again, click on the icon, it comes up with the word quote in brackets before and after, don't worry, just tab down, say the rest of your message.
magic.
cheers mate I had to ask heliport how to not so long ago.


.... GOTCHA.... aint' I ???

Thankyou MightyGEM

[MightyGem]

That's ok. Errrr....thanks for what?

[Boslandew]

Going back to the original question, I heard about a chap in Aden back in the 70's who thought his rotary experience should include VR and recovery. He apparently put a Scout into VR at 10000' and finally came out of it, a very chastened aviator, somewhere below 2000'

[jimf671]

https://www.youtube.com/watch?v=ehV9vLnBICE

https://www.youtube.com/watch?v=HjeRSDsy-nE

https://www.youtube.com/watch?v=pTJ1qfhrM40

[[email protected]]

So many half-truths in those Jim - at least Transport Canada highlights there is a difference between VRS and SWP, even if that guy doesn't understand it doesn't have to be a vertical descent for VRS, just a steep one.

The Vuichard video shows recovery action taken immediately which is really an incipient VRS condition as he never lets it develop. If he did, his technique wouldn't work any better than the standard recovery. Going sideways out of the dirty air or going forwards - the rotor doesn't know which way it is going - clean air is clean air.

[SLFMS]

I enjoyed the first video and found it refreshing for someone to differentiate SWP and VRS but he lost me when he started talking about the tips stalling during VRS.
My understanding is it's the centre of the disk that stalls and propergates outward whereas the tips are at very low angles of attacks due to blade washout and increased tip vorticies increasing induced airflow as well as the amount of the blade tips exposed to it.
Perhaps I'm hopelessly wrong but its logical to me and explained well in Wagtendonk.

[pilotmike]

The Vuichard video shows recovery action taken immediately which is really an incipient VRS condition as he never lets it develop. If he did, his technique wouldn't work any better than the standard recovery. Going sideways out of the dirty air or going forwards - the rotor doesn't know which way it is going - clean air is clean air.
... except for use of pedal which is a required ingredient of the technique, which makes a difference to whether you are poling forwards or sideways with the cyclic. Probably that's why the technique is careful to specify WHICH pedal is to be used, with opposing cyclic.

[[email protected]]

except for use of pedal which is a required ingredient of the technique, which makes a difference to whether you are poling forwards or sideways with the cyclic. Probably that's why the technique is careful to specify WHICH pedal is to be used, with opposing cyclic. That just induces a sideslip which doesn't get you into clear air any sooner than just applying forward cyclic.

Note also that he grabs a handful of power which, in developed VRS would aggravate the condition but, since he only ever does it in the incipient stage, just powers out of the IVRS condition.

Exactly the same result would be achieved if you just pulled power at the incipient stage without any of the sideslip

[pilotmike]

That just induces a sideslip which doesn't get you into clear air any sooner than just applying forward cyclic....
Exactly the same result would be achieved if you just pulled power at the incipient stage without any of the sideslip

Though I'd have to defer to your doubtless greater knowledge and experience, I believe that the pedal opposing the cyclic allows the additional translating force of the tail rotor to assist with moving out of the VRS sideways, whereas forward cyclic alone has no such benefit, and would rely solely on the result of cyclic, ie. main rotor vectored thrust only.

As for sideslip. I'm not sure I understand your point, as that would only apply in flight with significant forward (air)speed, whereas in this case, by definition, there's negligible horizontal speed. So it seems the technique is designed to use every means possible in order to translate out of the VRS ASAP, which undoubtedly is swifter with the assistance of the tail rotor thrust.

But I'll be happy to be corrected if I've missed something obvious in your reasoning.

[[email protected]]

Though I'd have to defer to your doubtless greater knowledge and experience, I believe that the pedal opposing the cyclic allows the additional translating force of the tail rotor to assist with moving out of the VRS sideways, whereas forward cyclic alone has no such benefit, and would rely solely on the result of cyclic, ie. main rotor vectored thrust only. Which control do you use to fly the helicopter in pretty much any direction? The cyclic - it has far more authority than the TR.

If the IAS is so low then full pedal will create yaw but not much else - the main movement will always come from the cyclic input.

Far more experienced people than me dismiss the Vuichard technique as window-dressing for IVRS recovery which can be achieved just by raising the lever.

[pilotmike]

Which control do you use to fly the helicopter in pretty much any direction? The cyclic - it has far more authority than the TR..
Far more authority, yes. However the tail rotor can still add some more. This isn't an either / or call, it is all about adding more. Even if the TR can only add 10% more, that's a whole lot better than 0% more.

Did you ever turn down a 10% pay rise on the basis that you already earned far more than the pay rise being offered?
If the IAS is so low then full pedal will create yaw but not much else - the main movement will always come from the cyclic input..
The TR is the sole cause of translating tendency. Yes, it is INTENDED for yaw, but without question, it causes translation too. Are we saying the cyclic alone is so very powerful by itself that there is no need for any further assistance from wherever that help might come? If that was true, we wouldn't even be discussing escaping VRS ASAP with minimum height loss as by definition, the only time nil further assistance is required is when we can already do it with zero height loss using cyclic alone. That clearly isn't the case.
Far more experienced people than me dismiss the Vuichard technique as window-dressing for IVRS recovery which can be achieved just by raising the lever.
I didn't even consider that to be a possibility!

[[email protected]]

How much angle of bank do you use in the hover to compensate for TR Drift/translating tendency? 2, 3, 4 degrees? Maybe 5 max in some aircraft - so a very small amount of cyclic easily overcomes the TR power - I think you are greatly over-egging the effect of the yaw - if you want more lateral movement just use more bank.

Back to the standard recovery of pushing the cyclic forward - you are far more aerodynamic moving forward since that is the way the aircraft is designed to fly, unlike trying to drag it sideways with opposing cyclic and pedal.

My point about the use of collective to power out of IVRS is that with enough power available you can do exactly that so why mess about with the roll and yaw. The problem is that not rasing it to max immediatley runs the severe rsik of aggravating the condition and putting you into VRS proper. You could also just take yourself into overpitching and Nr decay

If Vuichard can produce a video where he can go past the stage he normally recovers by just raising the lever enough to worsen the stall and enter fully developed VRS and then recover from full VRS using his technique in 50', I for one will be gobsmacked but I doubt it will happen.

He also needs to demonstrate his technique on bigger and heavier aircraft for it to be valid.

[pilotmike]

Far more experienced people than me
That was the bit I dismissed as being beyond the bounds of possibility....!:ok:
Returning to the technical discussion, you haven't taken on board my points, and you have argued against things which I have NOT said - I was not saying yaw had any particular purpose in this, nor was I advocating yaw for yaw's sake, if you read what I actually said. My point was in favour of adding the translating effect of TR in ADDITION to cyclic, which can only help the cause. However, your mind appears made up and closed to other ideas, therefore further discussion appears pointless, unfortunately.

[[email protected]]

Mike you misunderstand my point - why use a combination of yaw and lateral cyclic when simple forward cyclic will do the job just as well if not better?

People have got hung up on the 'magic' of this technique and, to be fair, it has been sold well with the glossy videos. If he just used the same amount of power and pushed the cyclic forward instead of laterally, he would fly out just as easily because it is IVRS and not VRS.

People can believe what they want but my concern over the publicity this 'technique' has gained, is that people might really think they can mishandle the aircraft sufficiently to get into VRS in the misguided belief that the Vuichard technique will guarantee recovery in 50'.

Just avoid the conditions in the first place.

[Vertical Freedom]

Mike you misunderstand my point - why use a combination of yaw and lateral cyclic when simple forward cyclic will do the job just as well if not better?

People have got hung up on the 'magic' of this technique and, to be fair, it has been sold well with the glossy videos. If he just used the same amount of power and pushed the cyclic forward instead of laterally, he would fly out just as easily because it is IVRS and not VRS.

People can believe what they want but my concern over the publicity this 'technique' has gained, is that people might really think they can mishandle the aircraft sufficiently to get into VRS in the misguided belief that the Vuichard technique will guarantee recovery in 50'.

Just avoid the conditions in the first place.

You're bang on Crab....I've pushed the envelope on VRS many times (training) in the 350, 125, 130, 206, 407 & G2; & I can tell ya once Your fully established in full blown VRS the Vuichard technique does diddly squat, absolutely nuttin, the machine continues accelerating like a greased anvil towards Mother Earth. Sure it works ok (just) in IVRS, but only for very mild IVRS as the demo vid shows! But, nothing works as well as poling forward which is the way the machine is designed to go (not side-ways).....it's no surprise then how the Robinson company is now pushing this flawed technique as the new saviour for VRS?

As You stated Mate; VRS is best avoided with good airmanship, unless deliberate for the purpose of training.

Happy landings

[SASless]

We can move two controls simultaneously can we not?

Key is to do something....and that which best resolves the problem.

It might just be you need to turn to gain an advantage afforded by terrain....so let's not get locked into exactly one response can we?

[[email protected]]

VF - :ok:

Sasless - getting locked into exactly one response is exactly what Vuichard is selling. I agree that there needs to be flexibility but based around good techniques.

[Vertical Freedom]

We can move two controls simultaneously can we not?

Key is to do something....and that which best resolves the problem.

It might just be you need to turn to gain an advantage afforded by terrain....so let's not get locked into exactly one response can we?

Hmmmmm SASless using a flawed technique doesn't sound like a good tool to have in Your kit! As I said Mate & proven on 6 types, more times than I can count; this thingymebob called the Vuichard recovery method, doesn't afford a recovery at ALL from deeply developed VRS.....so the point clinging to such a method is dangerous at Best! Probably why Robinson is fully into it!

Happy landings

[SASless]

Add terrain into the VRS situation and sometimes going straight ahead might not be the best choice is what I am suggesting.

What is important is understanding VRS and how it affects aircraft performance....the risk it poses....and the quickest way to identify the onset and perform immediate effective recovery techniques.

Knowing how the flight control's effectiveness is affected is part of that.

[Bell_ringer]

this thingymebob called the Vuichard recovery method, doesn't afford a recovery at ALL from deeply developed VRS....

This accident, and most others, have nothing to do with deeply developed VRS or anything that could be attributed to it.
Vuichard's technique was to make a recovery easier for situations where there was little altitude or suitable terrain to get the nose down.
No technique will make any difference to accidents like this where the cause is PPP.

[Vertical Freedom]

Add terrain into the VRS situation and sometimes going straight ahead might not be the best choice is what I am suggesting.

What is important is understanding VRS and how it affects aircraft performance....the risk it poses....and the quickest way to identify the onset and perform immediate effective recovery techniques.

Knowing how the flight control's effectiveness is affected is part of that.

Your ignoring the fact the Vuichard technique doesn't work in a real VRS situation! (maybe in IVRS; if Your damn lucky) Obviously be aware of surrounds & conditions & do everything You can to avoid VRS; then no recovery technique is required!

Happy landings

VF

[Outwest]

Wes Prouty explained this in an issue of Rotortales many many years ago......VRS requires 3 things, relatively high rate of decent ( most modern helicopters about 500') partial power ( can't be in VRS in an autorotation) and low airspeed ( less than translation). Only 2 ways out, moving out of the "bubble" or full down collective and entering autorotation (usually not an option). What you do with the flight controls to get you out of the bubble will depend on many factors, but in my experience lowering the collective as much as possible and forward cyclic is the best muscle memory method.

SWP is simply not enough power ( lift) available for the flight condition, hot/ high for example.

[Vertical Freedom]

This accident, and most others, have nothing to do with deeply developed VRS or anything that could be attributed to it.
Vuichard's technique was to make a recovery easier for situations where there was little altitude or suitable terrain to get the nose down.
No technique will make any difference to accidents like this where the cause is PPP.

So true Bell Ringer, nothing will save Your arse when in PPP. But You are ignoring the fact that Vuichard technique DOESN'T work in VRS; please don't take my word for it...go prove it for Yourself! & be sure You are fully established in VRS when testing this concept!

Happy landings

[Bell_ringer]

So true Bell Ringer, nothing will save Your arse when in PPP. But You are ignoring the fact that Vuichard technique DOESN'T work in VRS; please don't take my word for it...go prove it for Yourself! & be sure You are fully established in VRS when testing this concept!

Happy landings

I am happy to take your word (you're not alone in your view), I use the conventional methods taught but have always preferred to stick to the basics and to rather plan properly.
Recovery is a poor substitute for prevention.

[Vertical Freedom]

Wes Prouty explained this in an issue of Rotortales many many years ago......VRS requires 3 things, relatively high rate of decent ( most modern helicopters about 500') partial power ( can't be in VRS in an autorotation) and low airspeed ( less than translation). Only 2 ways out, moving out of the "bubble" or full down collective and entering autorotation (usually not an option). What you do with the flight controls to get you out of the bubble will depend on many factors, but in my experience lowering the collective as much as possible and forward cyclic is the best muscle memory method.

SWP is simply not enough power ( lift) available for the flight condition, hot/ high for example.

Bang on Mate.....If my old memory serves me it's 300fpm. But, from experience You need to have way higher ROD, more like +1,000fpm to go VRS! I don't go with lowering the club....just pole forward with power on. That's one less thing to do & I feel You get into cleaner air much faster, powers already on so away You go! The key is tilting the disc, with power on Your pulling clean air into the problem the moment You pole fwd!

SWP; that's a totally different story...pull more power (if You have it without cooking the donk) otherwise same as above pole fwd & fly away!

Happy landings

[Vertical Freedom]

I am happy to take your word (you're not alone in your view), I use the conventional methods taught but have always preferred to stick to the basics and to rather plan properly.
Recovery is a poor substitute for prevention.

Gudonya Mate I've been twice in VRS unintended, both times due to environmental factors (localised massive updrafts) which You can't be always planned for, so being ready with a plan B recovery method is always wise!

Cheers

[Outwest]

Bang on Mate.....If my old memory serves me it's 300fpm. But, from experience You need to have way higher ROD, more like +1,000fpm to go VRS! I don't go with lowering the club....just pole forward with power on. That's one less thing to do & I feel You get into cleaner air much faster, powers already on so away You go! The key is tilting the disc, with power on Your pulling clean air into the problem the moment You pole fwd!

SWP; that's a totally different story...pull more power (if You have it without cooking the donk) otherwise same as above pole fwd & fly away!

Happy landings

Yes, I said "modern" helicopters.....the S61 can easily do it at 300 ROD. Of course lowering the pole may not be practical, but if altitude allows it will actually get you out sooner and with less RRPM loss. As to SWP, that's the problem, there is no more power. Think loading your Jetbox up to gross at SL and then trying to land at 10,000 PA at +30C. Pulling more is not an option.

[[email protected]]

To get into VRS you need to be travelling downwards at over half the speed of your downwash and an S61 has far more than 600'/min downwash, more like 2000'/min.

It is all about disc loading - older helicopters had low downwash speeds because engine power was lower and the rotor had to be bigger.

Modern helos have very powerful engines and smaller rotors for the same or increased AUM.

Nick Lappos published a RoD vs Fwd speed diagram for VRS produced, I think, by the US Army. The VRS boundary was related to downwash speed - you needed 0.6 to 0.8 of your downwash speed to catch it up enough to start to ingest it.

https://www.rotorandwing.com/2011/11/29/calculating-rotor-downwash-velocity/

[Hughes500]

well as a demo to students in an old heli ( S 300C ) fly downwind at 17"map trying to hold height She looses translational lift at around 20 kts, ROD now about 500 ft plus.Now in IVRS, Pull power up to 24 inches and guess what aircraft climbs back up again. I have to have more than about 800 ft a min for the aircraft not to be able to pull herself out of the situation

[Hughes500]

Should have added even pulling full power with nose forward I cant stop the 800ft min ROD in 50 ft or Tim Tuckers 30 ft Where am I going wrong ?

[SASless]

VF,

Where did I ever say I endorse the Vulchard Method?

What I have stated is an opposition to limiting VRS recovery to exactly one single method as you seem to be saying.

That kind of mindset....one size fits all kind of thinking.....is not the way life works in flying helicopters.

At altitude well clear of terrain you might be technically correct but when close to the ground you might just want to go towards a particular direction that might just in fact be not straight ahead.

You fly in some mountains....and thus probably understand there are times there might be lots of air under you to one side and lots of rock ahead and on the opposite side to the valley floor.

Would you not want to go towards the valley floor rather than the Ridgeline?

[Outwest]

To get into VRS you need to be travelling downwards at over half the speed of your downwash and an S61 has far more than 600'/min downwash, more like 2000'/min.

It is all about disc loading - older helicopters had low downwash speeds because engine power was lower and the rotor had to be bigger.

Modern helos have very powerful engines and smaller rotors for the same or increased AUM.

Nick Lappos published a RoD vs Fwd speed diagram for VRS produced, I think, by the US Army. The VRS boundary was related to downwash speed - you needed 0.6 to 0.8 of your downwash speed to catch it up enough to start to ingest it.

https://www.rotorandwing.com/2011/11/29/calculating-rotor-downwash-velocity/
Well I can tell you from experience that the 61 can and will get into VRS at WAY less than 1000 ROD. I was shown this during my initial endorsement over 30 years ago. The instructor simply said climb up to 6000 ft, this near a coastal city and a hazy day so no real discernible horizon. Then he said come to a hover. I brought the nose up lowered the collective just as you normally would. As I was coming to but not quite to zero AS I noticed a slight ROD maybe between 300 to 500' ROD and I just added a bit of collective. Well in a split second we were coming out of the sky like the preverbal greased manhole cover. The instructor pushed the cyclic full forward so we were then looking outside thru the skylights. He said he hates doing that but wanted me to see just how easy it was to get the old girl into VRS. So, as Mr. Prouty said 300' ROD because the S61 is NOT a modern helicopter. Size and gross weight have no bearing on VRS, actually the 2 times I have been in it it was both times in an empty a/c.

Anyway, this has been beat to death......let it die.

[Skycop]

Beat to death? It's now 20 years since I originally began it by asking for VRS stories. Long may the discussion continue. :cool:

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Size and gross weight have no bearing on VRS, actually the 2 times I have been in it it was both times in an empty a/c.my point exactly - in a light aircraft with a big rotor, the downwash speed is much slower as you need far less power to hover. Therefore much easier to get into VRS.

It's not about the size and gross weight of the helicopter, it is disc loading which is AUM vs size of the rotor.

I am just passing on information from the likes of Mr Lappos who generally seems to know his way round a helicopter:ok: If you don't like or believe it, that is up to you but that doesn't make it not true.

[chopjock]

I think some of you are forgetting that a roll command would respond faster than a pitch (nose down) command. Therefore jumping sideways with pedal assist would be quicker than jumping forward when you have no airspeed!

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Unless you encounter VRS from the hover in zero wind in a vertical descent or are downwind, chances are you will have some forward airspeed, even if it doesn't register on the ASI so forward will be better and quicker.

The lateral control input might change the disc attitude slightly quicker than forward but you still have the same amount of inertia in the aircraft to move and you are trying to drag it sideways through the air.

[Vertical Freedom]

I think some of you are forgetting that a roll command would respond faster than a pitch (nose down) command. Therefore jumping sideways with pedal assist would be quicker than jumping forward when you have no airspeed!
Hey CJ.....have you actually tried & tested this method? It doesn't work in deeply developed VRS! Others here have stated the same concerns! if Your just getting into it (IVRS) then You've a chance of it saving you (fingers crossed). Please read my detailed comments above on this flawed method!

Good luck Mate & Happy landings

VF

[Outwest]

my point exactly - in a light aircraft with a big rotor, the downwash speed is much slower as you need far less power to hover. Therefore much easier to get into VRS.

It's not about the size and gross weight of the helicopter, it is disc loading which is AUM vs size of the rotor.

I am just passing on information from the likes of Mr Lappos who generally seems to know his way round a helicopter:ok: If you don't like or believe it, that is up to you but that doesn't make it not true.

I have great respect for Nick and don't for a minute think I know more than he does about aerodynamics, but all I'm saying is that the S61 can and does get into VRS with WAY less than 1000 ROD than you suggested, that is from personal experience and I will not attempt to explain that in aerodynamical terms, only underwear changing ones ;)

[Mast Bumper]

crab is correct. Disc loading has a significant impact on establishing at what ROD the rotor system enters VRS. It takes a lesser ROD with an empty helicopter at the end of the fuel cycle vs. a helicopter at max gross weight.

To make a slight correction to VF's contention that the Vulchard method doesn't work in fully developed VRS: in a tandem rotor helicopter it is the smartest method, no matter on your ROD. Also, an S-64 Aircrane responds very quickly to a lateral control input to exit VRS vs. a longitudinal recovery. I've seen RODs larger than 5,000'/min and the Vulchard method works like a charm. Loggers have been doing this since the '70s.

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I've seen RODs larger than 5,000'/min and the Vulchard method works like a charm. Loggers have been doing this since the '70s. with 50' recovery as advertised?????

Sideways recovery on a tandem rotor makes sense - both rotors in clean air

[Vertical Freedom]

crab is correct. Disc loading has a significant impact on establishing at what ROD the rotor system enters VRS. It takes a lesser ROD with an empty helicopter at the end of the fuel cycle vs. a helicopter at max gross weight.

To make a slight correction to VF's contention that the Vulchard method doesn't work in fully developed VRS: in a tandem rotor helicopter it is the smartest method, no matter on your ROD. Also, an S-64 Aircrane responds very quickly to a lateral control input to exit VRS vs. a longitudinal recovery. I've seen RODs larger than 5,000'/min and the Vulchard method works like a charm. Loggers have been doing this since the '70s.

Thanks Mast Bumper (I hope You're not bumping to often) I've never flown twins let alone a Chinook, so no comment......it is not my contention alone, many others have come to the same conclusion as I! I have many times tried & tested this method on a 350, 125, 130, 206, 407 & G2 & it didn't work at all in fully developed VRS, it did have some positive effect in IVRS. The 70's aye; then Vulchard didn't invent this at all, stole an old idea claiming it was his own invention, hmmmmm :=

Have You tried this method in a light single?

Happy landings

VF

[Vertical Freedom]

with 50' recovery as advertised?????

Sideways recovery on a tandem rotor makes sense - both rotors in clean air

that makes a ton of sense!

[SASless]

But with two sets of Rotors...how many possibilities of VRS are there....Forward head only, Aft Head only, both Heads involved?

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Outwest - out of interest, were the two occasions you experienced VFRs in the S61 in the same flight profile ie flaring to the high hover?

[Kiwi500]

May I ask the knowledgeable, what are the figures you consider it to be IVRS versus “full blown” VRS?

[ApolloHeli]

But with two sets of Rotors...how many possibilities of VRS are there....Forward head only, Aft Head only, both Heads involved?
It's certainly possible. I know an RAF Chinook pilot who had a student get both fore/aft into VRS on final for a pinnacle landing in Afghanistan.

I have many times tried & tested this method on a 350, 125, 130, 206, 407 & G2 & it didn't work at all in fully developed VRS,
VF - not to dispute your experiences, but I personally have had good results from sideways recoveries, even at late VRS stages - that's in a Squirrel and G2 - as you say, only testing, no real-world brown underwear scenarios. Side note; I know a few instructors here in Switzerland that also have played around with both techniques over time and found the Vuichard recovery to be the better option assuming no obstacles.

[nigelh]

I have great regard for V F but I can’t completely agree with him on this one !! I have just come back from Bell Academy and those instructors really do know a thing or two and they swear by it . I think they usually only go to 3000 ft min or a bit more but it does work very effectively ( whether that is regarded as full VRS I don’t know ) . I agree with Sasless that we need more than one method . Obviously if you have an obstruction coming up then pole forward is not very clever !!! I believe all the Bell pilots think it is a useful tool in the box . My last comment would be that in 99% of occasions ( VFs one of a huge updraft is in the 1%) you get so so much warning of it you would have to be asleep not to notice ...
yaw , shake , more yaw , more shake , falling feeling in stomach and after all that you are still in early stages !!!!! ( I can only talk about light / medium machines )

[SASless]

In the Chinook....clever use of the yaw pedals during takeoff can produce additional lift as you accelerate the aft rotor into clean air not disturbed by the forward rotor.

I wonder how that would work in dealing with a VRS situation.

In all of the VRS demo's I ever performed during conversion training for. new pilots....I do not recall it being all that easy to get into a full blown VRS descent.

I know it was not one of the demo's that caused much excitement as I recall it being a rather benign maneuver.....but always done at 3,000 feet AGL.

In the early A model...turning off the SAS in a steepish turn was not for the faint of heart....feeling the aircraft trying to swap ends at 100 Knots was far more exciting.

[Outwest]

Outwest - out of interest, were the two occasions you experienced VFRs in the S61 in the same flight profile ie flaring to the high hover?

I may have been misleading, I have been in it twice, once in the 61 and once in a 205. The 61 was not what you would call a flare, a simple slowing of airspeed to come to a hover at altitude, and upon noticing that what I would call a small ROD I used a small application of collective to arrest the decent and that fast we were coming out of the sky. The strange thing was the VERY experienced instructor never told me why he wanted me to come to a hover at that altitude, but he knew ( I guess from all the others he checked out) what would happen.

The other time was on a seismic longline job. I was going down the cutline ( downwind) looking for the drill and realized I had passed it. I lowered the collective and pitched up to slow to make my turn into wind and approach the drill. We had no flight instruments ( AS) on the left side or a means to jettison the belly hook from the left side so ( luckily for me) we were flying 2 crew. I obviously came to a very low AS but since I was going downwind I didn't realize this. The next thing I knew we the greased manhole cover and when I got my head back in from the bubble window both low rotor lights ( Yellow and red) were on. By this time the turn was completed and we were into wind. VERY luckily the other highly experienced pilot was pumping the collective to regain RRPM. We were now with the skids almost going thru the tree tops, and yes the longline was still attached ( remember I could not kick it off) how it did not wrap around a tree and drive us into the ground still haunts me to this day.

I have a very LARGE respect for VRS.......

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Outwest - very brown trousers on that last event I am sure:ok:

NigelH - did they do a comparison of the Vuichard vs 'standard' recovery? When they used the Vuichard, did they recover in 50'?

Kiwi500 - I don't know, other than using Nick Lappos' graph, if you can put numbers on it.

However, you could liken it to a FW stall in as much as there is light buffet, which gradually increases if you don't recognise it - it gradually worsens and then finally gets to the nose drop/wing drop stage which would be roughly the equivalent of IVRS going into full VRS.

The 30 kts 500'/min RoD figures have been taught for many years as a way to avoid even IVRS but they were based on low disc loading helicopters. Each aircraft will have different figures based on its disc loading and even they will vary with AUM and DA.