Autos and Rotor Stall

[generalspecific]

Ok, so we know the horn comes on for low rotor rpm and we know (in some cases) the published limits for low rotor rpm operation. However I don't think any POH publishes graphs for weight / density alt and rotor stall (proably to stop potential Darwin award candidates from seeing if they are true).

So my questions are;

1. How much margin is there below the limit before the rotor stalls on say an R44 at 2,500 lbs on an ISA standard day at sea level?

2. When the rotor gives up does it cone and fold up, unless of course you are a few inches from the ground, and if the part in the subjunctive clause is correct has anyone ever seen one demonstrated :eek:

[lelebebbel]

1) Robinson says it's 80% RPM, +1% per 1,000ft DA (so 85% at 5,000ft DA). They won't say if that number includes any safety margin, whether it is in or out of ground effect, in forward flight, climb, descent or a hover. I suggest not to even think about trying it.

2) The blades may fold up, and the retreating blade can flap down hard enough to cut the tailboom followed by the upper half of the cabin, which contains the unfortunate occupants' heads.
Both has been recorded on video. The tell-tale signs, such as the cut tailboom behind the strobelight, and blades bent upward a couple of feet from the hub, can be seen on many Robinson wrecks.

[VfrpilotPB/2]

On one of my yearly tests to see if I still had what it takes, a very well respected and ultra high time instructor Ex RAF showed me that 80% limit in the Robinson R22

It SCARED THE POOP OUT OF ME :eek:
It is so slow you can nearly count the revs with your eyes, everything is at that point going wrong if any one gets near to or past that point you should not be flying in the first place, in fact if you go past that point you wont be flying you'll be dropping "Like a Stone"

Read about it and understand it, Dont go There EVER or you will die!!

Peter R-B
Vfr

[helimutt]

Was convinced that i saw just under 80% once when I was starting my PPL(H). One closing the throttle for a practice auto, I pulled collective instead of dumping it. Luckily my Instructor was lightning quick and covering the controls and he recovered it. I swore i saw just under the 80 (I was positive it was more like 75%) and he swore it was 80% because 75% wouldn't have been recoverable. Not that I care now, just that i wouldn't ever go there again.

In an S76 sim once, I had a dual engine failure and some other problem (cant remember what now) but I pushed right pedal, the collective came up, the Nr decayed and I watched as the blades stalled in front of me. Next stop? The ground, nose down, a sudden stop and a red screen. Very unpleasant even though I was just being shown something, but all the same something you really need to be aware of especially in low inertia rotor heads.

Look at the other thread on flying low level in bad vis, you slow down, then slow down some more, then you forget carb heat, the air is moist, you're at partial throttle, hmmm, not a good place to be.

[[email protected]]

I think the problem is that below the 80% level (or equivalent on other helos) there is insufficient autorotative force being generated by the blades - they won't stall immediately but will continue to slow down and, much below the 80% level, even lowering the lever and manoeuvering won't recover the Nr again if the engine has quit for real.

A teetering head helo can't cone up and clap hands, the aircraft just falls from the sky with ever slowing rotors meaking the AAIB investigators job messy but straightforward since the fuselage is squashed flat but the blades are intact.

[spinwing]

Mmmmm ...

I agree with 'crab' .... its all to do with those pesky vectors pointing in all the wrong directions ...


:eek:

[Gordy]

Agree with crab...

There was a low-rotor rpm demonstration that was shown to me at the Robinson factory years ago, which I would also demonstrate to students, (this was back in the day before the governor and when we were still required to demonstrate Low G).

In a hover, slowly roll off the throttle and try to maintain a stable hover over the ground. Depending upon weight, I have had the RPM at 78% and recovered--with throttle.

Please do not try this in flight---Perform the demonstration at about 3 foot hover, and it is more to demonstrate the effectiveness of the tail rotor, I doubt you could recover the RPM in an auto-rotation--although I could be wrong.

[rotaryman]

Many Years ago I recall seeing a video of an R22 Mariner in the U.K

The pilot was showing his wife his factory from the air and during the flight he began to suffer carby ice, the result was he began to settle with power and continued to raise the collective until he suffered blade stall.

The video clearly shows the blades slowing and then folding up vertically before the aircraft fell from the sky. both Pilot and Pax dead.:(

[generalspecific]

Very interesting, thanks all. I've also seen that carb ice rotor stall video... very nasty and led to the S/N from Robbo about "governor can mask carb icing"..

[Phoinix]

Gordy, I think low rotor rpm demonstration is vital to students, understanding the phenomena, but, operating outside of rotor limitations?

Crab, any comments on that topic are very welcome.

[Gordy]

Phoinix

Gordy, I think low rotor rpm demonstration is vital to students, understanding the phenomena, but, operating outside of rotor limitations?


Good point, however, are we not operating outside of rotor RPM limits when conducting autos and hover autos? Also getting SWP for demonstration, and back in the day doing Low G maneuvers, (although no longer permitted I hear) were operating outside of a "normal" flight envelope.

The demonstration I conducted was considered a demonstration of low rpm recovery and tail rotor authority and control.

[[email protected]]

I don't think drooping the Nr at the end of an EOL to cushion the touchdown should be considered as operating outside RRPM limits - you have to learn to do EOLs somehow and you have effectively landed anyway.

Drooping the Nr in a low hover to show overpitching or TR effectiveness might technically be considered outside RRPM limits but is essential training for students to understand what it looks and feels like so they can recognise the symptoms early and recover quickly.

I am sure all of these techniques are taught on the Robinson safety courses so the manufacturers must be happy with them.

[n5296s]

The pilot was showing his wife his factory from the air and during the flight he began to suffer carby ice, the result was he began to settle with power and continued to raise the collective until he suffered blade stall.
This is on the Robinson DVD that you are required to watch before you can solo (at least I was - it may be an insurance rather than a factory requirement).

On the Youtube video of the Australian R44 incident (don't have the link to hand but I'm sure people know which one I mean) the rotor RPM gets awful low, it certainly SOUNDS like less than 80%.

n5296s

[VeeAny]

For reference, without comment on the RPM

Griffin Helicopters | Video Player | An R44 suffers LTE over the sea. (http://www.griffin-helicopters.co.uk/videos/playonevideo.aspx?videokey=1661)

[bugdevheli]

If my addups and takeaways are somewhere near, a R22 running at 5 .digree pitch would lose approx 544 lbs of lift if the rotor rpm decayed from 520 to 416. { i have had a couple of wines though) .

[rotorfan]

This is on the Robinson DVD that you are required to watch before you can solo (at least I was - it may be an insurance rather than a factory requirement).Insurance? Factory? Maybe that pesky SFAR73 by those FAA guys, eh? You're supposed to view that and be suitably scared before even manipulating the controls, so you don't do anything stoopid. IIRC, the instructor is supposed to endorse the logbook to that effect before you handle the controls for the first time. My first viewing of that video was about 12 years ago. I remember well the impression it made. The helis made some impressions, too.:{

[Mungo5]

Is said video on youtoob or the like, unfortunate as it is?

Maybe that pesky SFAR73 by those FAA guys, eh? You're supposed to view that and be suitably scared before even manipulating the controls, so you don't do anything stoopid.

Quite right.. it is after all written in the blood of fallen robbo drivers. Mind you, that whole FAR is written in blood mostly.

[zlocko2002]

Mi-8 POH states 75%, I have never landed in auto-rotation with less than 85%, but friend of mine experienced 82% (flight engineer later sad that it doped to 79%) during auto-rotation on maintenance test flight. Luckily they had enough altitude to regain rotor RPM, rate of descend was around 6000ft/min.

[n5296s]

Maybe that pesky SFAR73 by those FAA guys, eh? You're supposed to view that and be suitably scared before even manipulating the controls,
No mention of DVDs in SFAR73 (I just checked), just a list of the things you're required to be trained on. Of course the DVD (referred to by my instructor as "the death video") is quite a good way to get your attention.

There's also nothing about requiring SFAR73 signoff before manipulating the controls, except in the case of "A person who holds a rotorcraft category and helicopter class rating", or to act as PIC - i.e. it is not a pre-solo requirement for students.

n5296s

[HueyLoach]

...1. How much margin is there below the limit before the rotor stalls on say an R44 at 2,500 lbs on an ISA standard day at sea level?...
General,

The manufacturer does not have to test the helicopter to any specific margin beyond what is published in the operator's manual or what is required by the certification authority.
"How much margin"?, I guess some of the test pilots and engineers may know but it will pure guess work from our part.

[RS82]

When drooping the rotor in a 3 foot hover even to the point that it touches down does nothing but show that the aircraft will settle to the ground from rapidly decreasing rotor effeciency because of the loss in lift associated with the slowing rotor RRPM like in a hovering auto. The rotor does not stall in that scenario. in order to create a full unrecoverable stall you would need a high up flow velocity through the disk like in a high descent and a decreasing rotor RRPM. The up flow can an primarilly does increase the AOA on the blades untill it reaches a stall . Pulling on the collective makes this happen faster only because critical angle of attack is reached sooner. The large angles of attack remain even at flat pitch and create huge amounts of drag wich you can not over power with the engine. Forget about the auto the rotor meets the up flow at a time when RRPM is below normal and the driving autorotation part of the disk has already been aerodynamicly destroyed . So there is no way to demostrate this due to that fact that it is fatal when encountered.

[waragee]

Just thinking about the loss of centripal strength in the blade due to running at low RPM in these demonstrations in an R22 (any type for that matter), the abnormal flight load placed upon the structure of the blade would be enormous. Also I am told that brinelling of the main rotor pitch change bearings is mostly caused through underspeeding. I dont reckon I would be to keen on flying a ship that had done a few of these demonstrations.

[Shawn Coyle]

The rotor can stall.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.

[VeeAny]

Warragee

A while ago I was told by a flying mate of a demo he had been shown on his R44 conversion with the RRPM down at 70% in the hover, I was horrified and emailed my tech man at Robinson whose reply was quite succinct and went along the lines of

Why did they do it ? and We recommend replacing the blades now.

GS

[Mungo5]

I'm always reminded of this very sad incident when thinking about rotor stall.. Although I think the AAIB are still investigating, so we can't draw conclusions, you can clearly see how the blades have clapped up.

BBC NEWS | UK | England | Lancashire | Helicopter crash pair identified (http://news.bbc.co.uk/2/hi/uk_news/england/lancashire/8271035.stm)

What worries me is that this seem quite possible with drooping Nr and fully artic head.

[Shawn Coyle]

Mungo:
Nearly all (new design) articulated heads have flap restrainers to prevent the blades from flapping up too high.
Even the S-61 had them, and it goes back quite a long time…

[chopjock]

A while ago I was told by a flying mate of a demo he had been shown on his R44 conversion with the RRPM down at 70% in the hover, I was horrified and emailed my tech man at Robinson whose reply was quite succinct and went along the lines of

Why did they do it ? and We recommend replacing the blades now.

GS I must be missing something here. Why would Robinson recommend replacing the blades just because they were run at 70%?

What's the difference between holding a 70% hover on the cushion with power and
when you perform a throttle chop in the hover, you hold the lever where it is then raise it to cushion the landing. Both maneuvers would probably see 70% rrpm before touch down.:confused:

[nigelh]

yes but i would guess the stresses on a blade going through 70% quickly ,and sinking , not under power would be very different to hanging there off the blades under power at 70%. I am not an engineer but would agree that there could be some real damage done to blades and hinges if operated like that . I certainly would never do that in mine ...and can see no good reason to do it !!!!

[waragee]

During fast and furious mustering operations in an R22 it is possible to experience a flight condition that I am not really sure how to explain, I guess it is Blade stalling but there is no loss of RPM, just an onset of shuddering and severe loss of lift. It is easily fixed by reducing the amount of control input but if allowed to continue will end up shuddering onto the ground with no loss of RPM although I have only heard of it happening in severe operations close to the ground so it’s all over pretty quickly, back off the control input and the problem is gone. To encounter it proceed at 60kts+ straight and level or in a dive, introduce the cyclic input first quite strongly so that the airframe is still travelling in the same plane but with the disc in a flared attitude obviously intending to throttle off and control RRPM with collective and if you are able to get a severe enough flare and you are fast enough (60+) it will happen. If more collective is input after the shuddering has started it gets worse. I learned to fly around it by introducing a slight amount of collective first before the cyclic and no problem, basically same manoeuvre, same result. I guess I am probably just a rough pilot but I have 18,000 hrs mustering in R22’s so plenty of time to practice. So the main requirements are
1/ Speed 60kts +
2/ Severe flare of the disc with the airframe still in the same plane
3/ Not much Collective pitch input
Would someone like to comment

[delta3]

Some data from my R44-I scientific simulator (quite a while since I used it..)

First the autorotation

at 80% rpm, fully loaded 60 knt auto

http://www.pks.be/files/public/80_8_100.jpg
http://www.pks.be/files/public/80_8_75.jpg
http://www.pks.be/files/public/80_AoA.jpg

8-curve shows the angle of attack at a certain radius versus the max angle, based on a 4th order viscous flow stall model. 8-curve should remain below the stall curve.

AoA show the effective angle of attack of the blade segments

So at 80% : acceptable stall zone



Now at 70% rpm, fully loaded, 60 knt auto

http://www.pks.be/files/public/70_8_100.jpg
http://www.pks.be/files/public/70_8_75.jpg
http://www.pks.be/files/public/70_AoA.jpg

so at 70% : fully stalled retreating blade



In between at at 75%

http://www.pks.be/files/public/75_8_93.jpg

so at 75% : very questionable

Still 93% of retreating blade is stalled


Secondly the flair

Don't have any figures yet, but my guess is that at 75% you will run out of collective. So not so much stall, but collective limited



Hope that helps, convinces me not to go below 80%. Of course different TOW etc may be considered to refine margins etc, but I'll keep the 80% in mind as a pilot.

d3

(edited to get img links correct)

[[email protected]]

Nigel and chopjock - I would say it is all about the bending stresses at the root with high pitch angles (very high considering the low Nr). The amount the blades can cone on an EOL is limited by the Nr decaying rapidly - no Vsquared = no lift = no coning. But in powered flight at low Nr the engine keeps them going at high coning angles - much more bending at the root I think.

Wargee - I think your rotor is ingesting its own vortices, not unlike VRS - hence why it get worse if you raise the lever. A mixture of BVI (Blade Vortex Interaction) and incipient VRS.

[handbag]

The amount the blades can cone on an EOL is limited by the Nr decaying rapidly - no Vsquared = no lift = no coning. But in powered flight at low Nr the engine keeps them going at high coning angles - much more bending at the root I think.

So consider the B206, why should touchdowns be made before 70% Nr?

[delta3]

By looking at data and running more simulations I got convinced that Shawns explanation is right on (that in it self could have been stated without doing the calculation and just trusting Shawns experience..)

What I see is that depending on load say between 75% and 50% usefull load the retreating blade stall will set in rapidily at any significant forward speed at rpm's below 80%. As Shawn explained this will create a blow back, and even moving cyclic fully forward the heli will slow down getting into a vertical autorotation.

I see that at low speeds (say 10 knots) RPM of 70% down to 65% are reachable before main rotor stall sets in. Up to that point coning remains reasonable (below 10°) but of course we run out of steam very rapidly beyond that point and even before the stall will hit the ground with significant vertical speed.

m2c

[AnFI]

(also not good with star flex helicopters ... probably.... ?)

[delta3]

Teathering versus Star flex

My simulator is R44, so fully teathering and coning hinges.
But as per Shawns remarks (he referred to the 407 correction: MD520 if I recall correctly) and my personal remarks, I do not thing this makes a big difference before main rotor stall. The only possible problem with teathering is that when retreating blade stall sets in brutally, tail cone contact will be made easier than with the star flex or other more rigid systems.

d3

[AnFI]

that was to answer handbag's question (and a little for nigelh and crab)

stress free with coning hinges. (at zero airspeed - other stresses then....)

[waragee]

Thanks Crab, I think something like what you describe was happening.

When I introduced cyclic first before collective it obviously flared the disc with low angles of attack on the blades but low disc loading giving rise to the shuddering and loss of lift.

By introducing some collective first loaded the disc a bit more allowing the severe flare of the disc to take place without causing any problems.

It was sort of like the entire disc was stalling rather than the blades singularly.

[RS82]

During fast and furious mustering operations in an R22 it is possible to experience a flight condition that I am not really sure how to explain, I guess it is Blade stalling but there is no loss of RPM, just an onset of shuddering and severe loss of lift. It is easily fixed by reducing the amount of control input but if allowed to continue will end up shuddering onto the ground with no loss of RPM although I have only heard of it happening in severe operations close to the ground so it’s all over pretty quickly, back off the control input and the problem is gone. To encounter it proceed at 60kts+ straight and level or in a dive, introduce the cyclic input first quite strongly so that the airframe is still travelling in the same plane but with the disc in a flared attitude obviously intending to throttle off and control RRPM with collective and if you are able to get a severe enough flare and you are fast enough (60+) it will happen. If more collective is input after the shuddering has started it gets worse. I learned to fly around it by introducing a slight amount of collective first before the cyclic and no problem, basically same manoeuvre, same result. I guess I am probably just a rough pilot but I have 18,000 hrs mustering in R22’s so plenty of time to practice. So the main requirements are
1/ Speed 60kts +
2/ Severe flare of the disc with the airframe still in the same plane
3/ Not much Collective pitch input
Would someone like to comment




ITS TIPS STALL

[RS82]

The rotor can stall.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.

I completely agree that rotor can experience retreating blade stall at any IAS under the right conditions .I was commenting on full rotor stall as a result of low rotor RPM not tip stall. The main difference to me is the disk is stalled in other areas accept the tip of the retreating side in this condition. large portions of the disk are stalled including the root not as a result of reverse flow ,with high coning angles on all blades and rapidly increasing descent rate which is not recoverable to my understanding and control over the disk is gone . This fatal scenerio is much different then tip stall which is recoverable in most cases I would think. Or after experiencing RBS you can prevent by flying at lower altitudes and airspeeds or not loading the disk hard from rough handling at high speed.
http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/statusicon/user_offline.gif http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/report.gif (http://www.pprune.org/report.php?p=5660986)

[fling-wing_1]

Reading this makes me shudder to think how naive we were back in the day. My boss and I were up doing patterns in the 269A-1 (TH-55,) talking about Robinsons and wondering how low you could let the rpm decay before recovering. We rolled throttle off, pitched for 60kts and counted to 5 before we got too nervous and bottomed collective and rolled throttle on. RPM was waaay down past bottom of the green. In retrospect it was a stupid thing to do. Proves to me just how forgiving the 269 really is. In the Robbi we'd probably be dead. :sad:

[Sulley]

In an r22 fling, you could remove the' probably ' from your post !:eek:

[chopjock]

We rolled throttle off, pitched for 60kts and counted to 5 before we got too nervous and bottomed collective and rolled throttle on. RPM was waaay down past bottom of the green. In retrospect it was a stupid thing to do. Proves to me just how forgiving the 269 really is.Yes but one day it could save your life. Imagine having to "stretch" an auto to reach a safe landing site (because of trees or rough terrain etc). There you are, engine has failed, set up in the auto and you know you will not reach the clear area. What do you do? keep it in the green and fall short, smashing to pieces in the trees or raise the lever a little, increase to max airspeed, loose rrpm to below the min, then when clear recover to minimum by flaring like a b@2tard? I suppose the more you practice, the more you will know the absolute limitations?.

[handbag]

loose rrpm to below the min

I have a feeling that's one for Mr Lappos' Urban Myths department. :}

[PIC]

es but one day it could save your life. Imagine having to "stretch" an auto to reach a safe landing site (because of trees or rough terrain etc). There you are, engine has failed, set up in the auto and you know you will not reach the clear area. What do you do? keep it in the green and fall short, smashing to pieces in the trees or raise the lever a little, increase to max airspeed, loose rrpm to below the min, then when clear recover to minimum by flaring like a b@2tard? I suppose the more you practice, the more you will know the absolute limitations?.

Chopjock

You seem to be advocating that we intentionally practice decaying RRPM below the power off minimum (the red line) to find the "absolute limitations". You might be playing devil's advocate, or you could be a half-wit, I'm not sure which. The reason the red line is there is that someone with access to a lot more data has already done something similar under controlled circumstances and decided that that is where the line should be drawn. Beyond the line is only more uncertainty (risk), not less.
http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/evil.gif

[Heli-Jock]

Chop Jock

Unfortunatelly,,,,the more you practice,, the closer you will get to meeting your maker! :=
Good luck,,, i think youve pushed your luck so far and lived to tell the tale.
Others have not:eek:

be warned!

HJ

[fling-wing_1]

Chopjock,

While I understand the importance of learning to manipulate rpm and airspeed to alter glide path there's another side to this.

Not too long back a friend of a friend ran an a/c out of fuel through an unfortunate situation. He was trying to make an open field when the engine flamed out. He did just what you describe and bled rpm down trying to make the open field. I understand the psychological draw of trying to make that field rather than put it in the trees. Unfortunately he made the field just in time to come straight down 100 ft with no useful rpm. He died in the impact. Had he chosen to settle into the trees, pulling collective to bleed off his rpm and speed as he settled into the canopy, he might have survived.

Had a boss do that once. Got pretty beat up but he survived to fly again.

[zlocko2002]

generally it is advisable to follow procedures written in POH/FCOM!!! :ok:
maybe one can get several % more from machine(if one is highly experienced/talented/lucky), but its dancing on the edge... :ouch:

[Heli-Ice]

This must have been one of the tricky questions.

My answer would be "why would you want to check this?"

Operate within the limits on your meter or PFM and your'e good, most of the time...

Leave it up to Shawn and his folks to experiment for our well being.