Autos and Rotor Stall
 

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:

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.

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

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.

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.

Mmmmm ...

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


:eek:

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.

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.:(

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"..

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.

Phoinix

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.

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.

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

For reference, without comment on the RPM

Griffin Helicopters | Video Player | An R44 suffers LTE over the sea.

autos and rotor stall
 

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) .

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.:{

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

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

Mi-8
 

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.

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

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.