Autorotation technique.

[alouette3]

Can anyone here direct me to material that clearly states the importance of moving cyclic back in an autorotation entry.I seem to recall somewhere a document/book that mentions that the cyclic needs to move back ,in case of an engine failure,to "restore Rrpm".Can't find it.
Seems to me,over the years, this aspect of autorotation training and technique has been lost in transalation.There are references to "adjust cyclic to attain Vy " and so on and so forth,but no publication really highlights the fact that a raising of the attitude (i.e a mini flare, if you will) helps restore the Rrpm and will increase your chances of a successful auto.There is a lot of emphasis on collective lowering and rightly so. However,IMHO, the cyclic moving back is critical too.But that is not emphasized nearly as much by OEMs,training manuals and even the FAA handbook.
Any assistance in this would be appreciated.
Alt3.

[paco]

Does this help?

As the advancing blade passes over the tailboom, it gets faster and produces more lift, which means that it climbs.

Because of the V2 part of the lift formula, the lift produced is way more than that of the retreating blade, and its maximum relative airspeed (and therefore lift) is gained at the front of the disc, because the climb starts 90° after it starts to increase its relative speed, and its maximum lift point is 90° after that, which is a shift of 180° overall. Thus there is a 90° difference between maximum flapping velocity (abeam) and displacement (at the front).

As well, there is more induced flow at the rear of the disc which increases the difference between the front and rear angles of attack (less at the rear)
.
Note: This is why the nose lifts when you apply collective pitch, or drops when you reduce it, so if the engine fails and you don’t apply rear cyclic to correct this tendency, you could get into an unrecoverable situation. The moral: don’t take your hand off the cyclic!

I know Shawn Coyle has his Little Red Book of Autorotations out now - perhaps he can help.

Phil

[alouette3]

Thanks, Phil,
I am completely clear on the reasons for pulling back on the cyclic. I am just looking for some authoritative publishing on the subject.While the currrent crop of pilots might understand the need to lower collective and also bringing the cyclic back to counteract the tuck, I am not sure there is much understanding on how the action actually causes Rrpm to increase back into the green.That needs more emphasis and hence, more published material sayin that.That is what I am looking for.
I hope Shawn chimes in.
Alt3

[Savoia]

Check out this thread also: http://www.pprune.org/rotorheads/435074-eols.html

[hihover]

I think it would be quite difficult for any publication to emphasise bringing the cyclic back on entry into autorotation because it depends very much on the airspeed at which the engine fails and rotor inertia. The amount and rate at which the cyclic is moved has to be judged by the pilot at that moment of entry.

TM

[Digital flight deck]

Also, by loading the disc the centre of gravity is moved inboard as the conning angle increases. This restores / increases the RRPM.

[paco]

That's in Wagtendonk and just about every other book, shirley?

Increased rotor RPM because:

Collective is lowered as you try not to climb

Coriolis effect - increased TRT causes coning angles to increase and the C of G of all blades to move inwards

Increased load factor

The lift vector being more forward (because the RAF is shifted downwards) produces an autorotative force and reduces drag

Phil

[[email protected]]

Lowering collective prevents further Nr decay - flaring and/or rolling into a turn loads the disc and helps recover lost rpm. In an ideal world lower the lever and flare.

But, as hihover says, moving the cyclic aft if you are only at 30 kts will just take you further away from your normal autorotation speed and reduce the likelihood of you making a safe EOL.

[Digital flight deck]

Thanks, wont bother in future.

[paco]

DFD - I see my answer ended up just after yours - I was responding to alouette3's post in case you think I was repeating what you said.

If you're below 30 kts you want a slight check back if you are going to attempt a vertical engine off.

Phil

[Aucky]

it would be worth trying to get hold of Mr Sanfords quote which I can't recite as I heard it second hand, but I believe he was quoted saying that at cruise speed and collective pitch the R22 rpm has been demonstrated to remain within limitations following engine throttle-close for up to 7 seconds using solely aft-cyclic. Not a method I suggest teaching :} but it illustrates the benefit of using aft-cyclic on entry...

Edit - I still find this very hard to believe and if this is vastly misguided information I apologise and would like to be corrected as I didn't hear it first hand and wouldn't like to spread rumour (or try it :eek:)...

[Shawn Coyle]

I'm becoming more convinced that regardless of the situation, the first thing should be to move the cyclic aft (at the same time as lowering the collective).
The few scenarios where this is not the best thing to do are pretty small - I can think of only a few - like at 40 knots or slower and less than 100 feet above the ground; (please feel free to add to the list).
Nearly anywhere else, it's a very good thing to do. There are way too many examples of helicopters hitting the ground with little or no rotor RPM, because the pilot tried to get airspeed but had no rotor RPM for control.
Remember that real engine failures have a huge element of surprise about them - way different than practice ones.

[Savoia]

Remember that real engine failures have a huge element of surprise about them - way different than practice ones.

Ain't that the truth!

Early 80's southbound of out of the fomer RAF airfield Hethel in Norfolk flying at 900ft agl when the donkey quit in a Bell 206. First manifestation was that the aircraft yawed, then the sound of the donk winding-down, then the caution pips.

What I witnessed is that in a real-life scenario dumping the collective occurs with a reactionary efficiency which I believe is driven by the knowledge that this, ultimately, is what will preserve rrpm.

A 'stick back' initial response may well be a positive refinement to entering an auto but .. I do wonder whether drivers will give this precedence over dumping the lever in an actual emergency not least of all because, depending on the degree of aft cyclic, it may interfere with one's field of view and, in a single when the donkey's resting and you're still airborne, identifying where you will alight is something you feel with an impressive keenness.

[[email protected]]

If you're below 30 kts you want a slight check back if you are going to attempt a vertical engine off.

Vertical EOL in a Robinson........no thanks:)

Better to pull the cyclic back than to shove it forward which reduces Nr and, on Robbie type helos, can lead to mast bumping or tail strike if combined with a rapid lowering of the lever.

Surely most crashes due to low Nr are due to the poor inertia characteristics of certain rotor systems and a lack of practise in PFLs - if you don't lower the lever you will never recover the Nr.

[Hughes500]

Sorry I cant see how any student cannot pull the cylic back after dumping the lever. In the types i teach on 300/500 if you dont pull the cylic back as the lever goes down you will be at about 40 degrees pitch down within a couple of seconds. So cant see how anyone can teach not to bring the cylic back, let alone a student not realising the ac is in an apparent vertical descent. Most low rrpm crashes I suspect are from people flaring way too high and pulling the lever up way to early. Most single engine machines ( unless 206 and R44) need to be flared at under 50 ft with a hard flare nice check and level followed by a cushion on to the ground

[fadecdegraded]

There was very good article written by a Pete Gilles in the Heli Ops mag a few years ago called Cyclic Back.
In the article he looked at 2 500 crashes where they both had engine failures in the climb out from Take off, so both where low and at high power settings.
He basically states that the only hope they had was to go aft on the cyclic.
He also states that when they do check rides at there company they put as much emphasis on pulling the cyclic back as they do on lowering the collective, if i knew how to post it on here i would, its well worth the read.
I agree with the cyclic back theory, especially in the hover such as longlineing etc, fwd cyclic will only lower the RRPM and the little bit of A/S that you build up will be of no use. It will also compound the nose dropping.
Someone tell me if i have got it wrong but in my opinion aft cyclic will help conserve RRPM and if you hit the ground going backwards a bit no big deal in fact better if over trees and as long as it hits the real estate skids/wheels first thats the main thing with hopefully a little bit more RRPM to cushion the impact than one would have had if he had gone fwd with the cyclic.

[Thomas coupling]

Assuming you get the lever down in time, I can't see ANY reason NOT to pull cyclic back other than wanting to go faster, because that's what she'll do if you do absolutely nothing with cyclic on entering auto:confused:

[oggers]

The topic raised by Alt3 here is the merit of advising pilots to pull back into a 'mini flare' when entering auto as distinct from merely applying enough aft cyclic to maintain the desired attitude/airspeed.

Can anyone here direct me to material that clearly states the importance of moving cyclic back in an autorotation entry...no publication really highlights the fact that a raising of the attitude (i.e a mini flare, if you will) helps restore the Rrpm and will increase your chances of a successful auto.

Certainly, I was taught the importance of maintaining Nr AND the effect of loading the disc.

I agree with hihover:

I think it would be quite difficult for any publication to emphasise bringing the cyclic back on entry into autorotation because it depends very much on the airspeed at which the engine fails and rotor inertia. The amount and rate at which the cyclic is moved has to be judged by the pilot at that moment of entry.

..and as Savoia points out:

A 'stick back' initial response may well be a positive refinement but... I do wonder whether drivers will give this precedence over dumping the lever... [and] it may interfere with one's field of view and, in a single when the donkey's resting, identifying where you will alight is something you feel with an impressive keenness.

I don't really see any merit in flaring the aircraft 'for the sake of restoring Rrpm' unless you've identified that Nr is low. In that case I would say fine get it back as a matter of priority by trading off some speed if necessary.

[Savoia]

I don't really see any merit in flaring the aircraft 'for the sake of restoring Rrpm' unless you've identified that Nr is low. In that case I would say fine get it back as a matter of priority by trading off some speed if necessary.

You've got my vote Oggers.

Sincerely speaking, in a real life scenario with a single, once you've implemented rrpm preservation (which is pretty much an instantaneous response), the only thing one is concerned with is identifying a suitable touchdown spot and lining-up for it.

In craft such as the Angry Bumble Bee (H500) rrpm would build rapidly and most D or F models I have flown would overspeed at the drop of a hat if one didn't re-introduce pitch on the way down.

[FH1100 Pilot]

I guess it depends on the make and model. In Recurrent Training at PHI when I was there, they taught a technique in which the *first* response to the engine-out was to come back on the cyclic. In an auto-entry from cruise airspeed in a 206, if you merely lower the lever without coming back on the cyclic the helicopter will immediately enter a descent. And if you're only at 500 feet to begin with, you've really screwed yourself if you subsequently have to turn into the wind. There's a lot of energy available as you decelerate from 120 mph to 60. Might as well not waste it.

[Tourist]

Wot crab said.

It is all about loading the disc.

[Hughes500]

FH1100 in a 500 if cruising at 120 kts and you shut the throttle you darent bottom the lever as you will have the haed off the machine. I teach people to put the lever down about 2/3 and maintain quite a lot of progressive rear cylic maintaining until you get airspeed to 80 or 60 kts depending on what range you are after. You can then lower the lever gently to the bottom with more rear cylic as you get to 60 kts. This technique will give you an initial rate of descent of 500fpm for about 10 seconds rising up to 2000 ft a min at 60 kts lever fully down. From 1000 ft in 15 kts of wind you will get nearly a km range

[alouette3]

Great discussion. Thank you all for the valuable inputs.A fine instructor once told me that the successful autorotation is determined by what you do at the entry to an auto.I am afraid that today in training, the emphasis on reflexive actions,to include aft cyclic, is just not there. The venerable AP3456, in it's extract for helicopter aerodynamics,talks about this and that was what I was taught as a baby pilot.In casual discussions with some of the younger blokes around me, I find the mindset is to lower collective and then look for a good spot to put the aircraft down.Blank stares when I ask how you would restore Rrpm in case it has edged into the lower part of the yellow arc.When I further explain what I am trying to get at then there is the shrug of the shoulders and the "Of course, the nose will pitch down and I will automatically bring the cyclic back" reponse.All answers are within the ball park.But, IMHO, not quite there.I am just hoping that someone someday does not run out of altitude,airspeed and a good idea(aerodynamics) at the same time,because the book did not at least add a note in passing.
Please keep the discussion going.
Alt3.

[Hedge36]

I suspect if you ever tucked a 269 you'd be hard pressed to do anything but draw the cyclic back instinctively as you dumped the collective.

[FH1100 Pilot]

alouette3:The venerable AP3456, in it's extract for helicopter aerodynamics,talks about this and that was what I was taught as a baby pilot.In casual discussions with some of the younger blokes around me, I find the mindset is to lower collective and then look for a good spot to put the aircraft down.Blank stares when I ask how you would restore Rrpm in case it has edged into the lower part of the yellow arc.When I further explain what I am trying to get at then there is the shrug of the shoulders and the "Of course, the nose will pitch down and I will automatically bring the cyclic back" reponse.

In a helicopter which does not cruise significantly faster than it autos (Bell 47 comes to mind), it's probably okay while dumping the lever to just maintain the current pitch attitude - which would probably be an instinctive response. "Did I input any aft cyclic? Uhh, I...guess so. The nose didn't drop so I must have."

But there's more to it than that.

In a helicopter that cruises substantially faster than it autos (e.g. Bell 206), lowering the collective lever while maintaining the same cruise flight pitch attitude WILL result in a descent, no question about it. If you're up at high altitude when this occurs, no big deal. But if you're down where helicopters fly, like at 500' agl or so it can be a very big deal.

So it's not merely a matter of making some automatic aft cyclic input to maintain attitude. The technique must be to consciously and deliberately raise the nose, get the disk up as you transition from powered flight to autorotative. And I think this technique is lost on a lot of pilots. Either that or they've never been taught it. I certainly had not until I'd been a professional pilot for a number of years.

It's not just a matter of maintaining pitch attitude.

[Shawn Coyle]

FH1100:
Well put - the aim must be to have air flowing up through the disk. That's the thing that we should be emphasizing.

[EN48]

In a helicopter that cruises substantially faster than it autos (e.g. Bell 206), lowering the collective lever while maintaining the same cruise flight pitch attitude WILL result in a descent, no question about it. If you're up at high altitude when this occurs, no big deal. But if you're down where helicopters fly, like at 500' agl or so it can be a very big deal.



In addition to the reasons given by FH1100 and some others here for using aft cyclic, it would seem to me that another reason to do so in a helo that cruises faster than it autos (B407: 140kt cruise, ~70kt auto) is to trade airspeed for altitude (not to only maintain altitude), especially if low. Of course, all of this will depend on specific circumstances.

[eagle 86]

Some very simplistic answers to a complex issue - briefly and sorry about sucking eggs - practice autorotations are not merely a flying exercise but an essential tool to save your life in the event of an engine failure.
At the hover - freeze lever, counter drift/yaw - cushion landing,
air taxi - freeze lever - counter drift/yaw - cushion landing,
air transit say 50 feet/30 kts - lower lever - counter drift/yaw - flare how much depends on height/airspeed - level - cushion landing,
transition to forward flight - lower lever - flare as required depending on height/airspeed - counter drift/yaw - level - cushion landing,
in climb - lower lever - counter drift/yaw - flare as required depending on height/airspeed - cushion landing,
cruise - lower lever - maintain attitude/airspeed even lower nose to gain best range airspeed (where was that forced landing area I saw a moment ago?) - from point of engine failure to the forced landing area is the most important and generally least practiced part of autorotative/engine failure practice,
in descent - lower lever - counter drift/yaw - airspeed range? - forced landing area?,
on finals - lower lever - counter drift/yaw - maintain attitude/airspeed - flare dependent on height/ROD/airspeed - cushion landing.
Bear in mind the helicopter rotor system takes about 1000 feet of descent to aerodynamically establish autorotation.
You will not necessarily get an engine failure at the best time - I know a pilot who had an engine failure due to the addition of lead - he forced landed backwards into the LZ he had just departed from.
GAGS
E86

[outofwhack]

Bear in mind the helicopter rotor system takes about 1000 feet of descent to aerodynamically establish autorotation.


Pardon a question from a low timer but why anything like 1000feet?

Won't an autorotative region of the blade be providing some force just as soon as the underside of the disk of the rotor is presented to the airflow. Ie within a few seconds after pulling aft cyclic to get say from 10degs nose down disk in forward powered flight to 10deg nose up disk while slowing to best auto speed.

Oow

[Shawn Coyle]

outofwhack:
Precisely!
It doesn't take 1,000' to re-establish autorotation. If it did, the HV curve would be a damn sight (dam site???) larger for every helicopter.

[Fareastdriver]

I am going to be shouted down over this.

Flying single-engined helicopters in Borneo in the sixties the choice of landing sites was somewhat limited if the engine packed it in. To reduce the percentage chance of arriving in 200 ft. high trees one went for the range autorotation. Should the engine stop than the Rrpm was allowed to decay to just above minimum controllable and the airspeed kept at the cruise. When (or if) you found a suitable landing area you flared at the appropriate time and took your chances with the undergrowth.

Otherwise you flared at the appropriate time and took your chances with the trees.

[FH1100 Pilot]

No Fareastdriver, I don't think anybody would shout you down for that; it was what worked for you guys in that environment and terrain. Plus, in the 1960's you guys probably were flying helicopters that weren't all that fast in the first place (S-58's?)

But as for this discussion, I think we're all operating under some general assumptions. First, I think we're talking about "plain vanilla" engine failures where it just stops with no warning and/or drama. I think we're talking about "normal" flying at between 300' and 1,000' agl over terrain which affords at least some decent forced-landing areas.

Higher than that and it really doesn't matter how you enter the auto as long as you do. Lower than 300' and it probably doesn't matter what you do as long as you put the pitch down, because you're most likely going to land/crash straight ahead regardless of the wind. (And if you're down below 300 feet you by God better be into the wind or have a damn good reason why not.)

"outofwhack" brings up an interesting point. If you're toodling along in your LongRanger in cruise with, say a 3 degree nose down cabin attitude (i.e. 8 degree nose-down disk), you will have to make a nose attitude change of 17 degrees to get the disk up ten degrees to the relative wind. That's a lot of pitch attitude change!

Which is why I sort of respectfully disagree with Eagle86 who, when talking about an engine failure in cruise says:cruise - lower lever - maintain attitude/airspeed even lower nose to gain best range airspeed...

Yeah well...maybe in a 47 or something. In cruise in a 206 you're already going faster than best-range-auto speed. And again, if you're down around 300-500 feet, you definitely don't want to descend any more, at least not until you've got a good landing site picked out, bearing in mind that it may be behind you and you have to turn around to find it again (assuming you remember flying over one a second or so ago).

I think that if the engine ever quits "for real" most pilots including myself will take a bit of time to recognize and react to it. During those few, hopefully brief seconds, the aircraft will have already started a descent. As you gather your wits and put the levers/pedals in the right places, you surely want to stop that descent to give yourself some time to start looking for a place to land.

Which is why "aft cyclic first" seems to me the proper technique in the event of an engine failure at any speed above 60.

[Ascend Charlie]

I think Eagle 86's ancient fingers trembled a bit on the keyboard and he punched an extra 0 onto 100'.

Poor old bugga.:8

[eagle 86]

I meant 1000 feet for all the aerodynamics to line up - most helos stabilise a ROD in auto of about 1800 feet/min and this takes around 1000 feet for this stability to be achieved. Rest assured you are not in auto should an engine failure occur inside the HV envelope.
GAGS
E86
PS If I had a dollar for every practice auto/PFL I carried out up to the age of forty I could have retired long ago!

[eagle 86]

Just to clarify further in order to keep AC back in his box - can't remember what his autos were like but do remember his nav was cr@p - I am talking about an abrupt total loss of power or an absolutely unannounced practice by the QHI. The first thing that is going to happen is a WTF - RRPM will drop out of the bottom of the green - a couple of hundred feet will be lost while the pilot computes and gets the lever down - ROD will be in the order of 3000 ft/min at this stage. While ever RRPM are below the optimum ROD will remain high. Only when RRPM/IAS/balance etc are all stabilised will the helo be in a stable auto at around something like a loss of 1000 ft.
GAGS
E86
PS Try any fancy rear cyclic movement in any of Bell's underslung teetering heads with low rotor RPM and you stand a good chance of separating that rotor head.

[eagle 86]

Another thing to throw into the mix - a fixed wing in a fully developed spin is in autorotation - after the stall and subsequent prospin control inputs a considerable amount of height is lost until stable autorotation is established.
GAGS
E86

[Hughes500]

E86

What rubbish you speak, have you ever had the donkey stop ? Can assure you that it does not involve a height loss of 1000 ft If that was the case I wouldnt be here. If you had stuck with your 300 ft I would say yes thats about right. Regularly do throttle chops from 500 ft downwind to full touchdown with students, whom quite often are not expecting it. Yes you have to be quick to get the aircraft round into wind ( loose 300 ft in a 300 from 70kts to a 50 kt auto). Granted the reaction time to it happening for real is a bit quicker, but as I said at the start when it happened to me the lever was punched down without me having to think about it.

[Fareastdriver]

I will go with Hughes500 on this. My first engine failure was on a Whirlwind 10 (turbine powered S55) on my second solo on type 300ft downwind. I had the lever on the floor and was turning into wind whilst I was going through the fire drill. Everything worked out fine and I arrived, somewhat surprised, just over a line of trees and just short of some power lines. I had yet again been blessed, as before and subsequently, with an aircraft with a very high instinct of self preservation.

Fortunately from 1971 and to my retirement I could always rely on one of the two decibel levers in the roof.

[oggers]

I would like to see a source for this 1000ft hypothesis.

[topendtorque]

I used always say 300'. but an auto always has three components if you start early enough.
autorotational flight;- that's the same from 5,000' or 300',
termination area;- that's from when the first flare happens, a rule of thumb can be about one foot of altitude per every knot of airspeed and,
touchdown;- that's the same as a hovering auto or close enough.

In an R22, one can easily find that a nice tidy autorotational flight profile of at least 50 knots can be set up say at about 150' depending on DA, after a snap throttle closure from the hover at 300', well before the termination area with 2 up and 50 litres of fuel at least.

same in a '47.

[Arm out the window]

I would like to see a source for this 1000ft hypothesis

All he's saying is that by the time you react, get the lever down and the aircraft settles into its fully developed auto with a stable rate of descent, it takes about 1000 ft.

Leading up to that, there will be various variables that take a while to settle down, as he's already described above.

That's not the same as saying you can't fly a proper auto from pretty much anywhere up the transition slope, or anywhere really - you just have to judge how much flare and how much lever lowering you can do.

Obviously from a decent height you can get the collective down, flare to achieve your desired speed, stabilise at a constant rate of descent and have a cup of tea on the way down, but from lower down you won't necessarily be able to do all those things. You may have speed that you can trade for extra potential, maybe not.

As you'll note, E86 has mentioned things like the delay involved in your brain processing what's happened, during which the Nr is decaying like a bastard. Hughes500, you obviously got the lever down quick and did what you had to do, but that's not to say everyone would. The other thing to keep in mind is that he's only talking about being established in full blown stable autorotation, not just in a position where you can pull off a landing, if you get my drift.

[Hughes500]

Arm

As an experiment today flew with a customer who had not flown for 14 months. Flew at 1000 ft agl over airfield ,70 kts in a 300C. I chopped the throttle with no prior warning. Customer ( tt 70 hours, not trained by me)dumped lever as ac yawed about 40 degrees to the left. RRPM down below red line about 380 rpm. He flared back with the cylic bringing rrpm back to about 410 rpm by 800 ft and 50 kts. By 600 ft the ac was rrpm of 480 rate of descent 1600 ft per minute, fully stabilised autorotation.
So I am afraid I do not understand where 1000 ft comes from, unless an R22 is the subject of 1000 ft . If that is the case I will eat humble pie and offer a sincere apology

[Arm out the window]

Fair enough mate, I haven't done any autos recently, although I've done heaps to touchdown in the past from many different setups.

When doing maintenance test flying in Hueys we would get set up 1000 ft above the test altitude to enter auto to ensure everything was stable to note the Nr when passing through that altitude. It was part of the test flight schedule so I guess there's something in it taking perhaps that much height to get totally stable to the nth degree.

Having said that, starting from say 1000 AGL on a straight-in auto, you're certainly comfortably established in autorotation and waiting for flare height to come up after two or three hundred feet or so, so yeah, who knows.

[topendtorque]

don't know where i got this 300' of airspace thing from been carrying it around without testing the idea for some time.
just now (one up R22 almost full fuel) experimented starting from Straight and level at about 150 feet using fence posts as measure markers. from cruise at 21" to established in auto descending flight it was 250 metres.

[Shawn Coyle]

eagle86
You will not have any risk of losing the rotor in a Bell with an aft cyclic movement, unless you know something the rest of us don't know.
Abrupt forward movement is a problem, but aft???
Please explain.
In a Bell 206 it takes less than 300' for the rotor RPM to get back into the green from a hovering engine failure (at a high enough altitude, obviously). Done it many times when teaching the H-V curve.

[Tourist]

1000ft is total rubish.

You can start an auto at 100kts/50ft and be fully stabilised in time to make a landing, equally from 1000ft using minimum height loss technique you can be stabilised after about 50ft of descent if you are smooth.

Autorotation can be established almost instantaneously by pulling back the cyclic. This can be shown by the need to contain Nr.

If Nr rises by itself, then you are in auto.
Rate of descent is totally unimportant, only AoA relative to the disc matters.

There is absolutely no need for any descent to occur to be in auto.

[eagle 86]

Correspondents appear to believe that I am saying you can't pull of a successful landing following an engine failure from any where - go back to my original post - I have carried out many thousands of engine off landings from any of those scenarios successfully. However, as an example, I can assure you that a loss of power from an IGE hover or an OGE hover almost anywhere inside the HV curve the rotor system will NOT be in fully developed autorotation before you need to pull pitch to cushion the landing. Let's not get too pedantic the height loss maybe say 800 feet for fully developed aerodynamically stable autorotation but stable aerodynamic autorotation is not instantaneous. As AOW stated when carrying out an auto rev check test flight the auto was commenced at least 1000 feet above test height to ensure fully developed stable autorotation was achieved by that test height.
A word on underslung teetering heads - years ago the service I was attached to suffered two fatal accidents, four pilots lost, as a result of main rotor separation - one flight was simulated IF which was to include UA recovery. The other was a two ship low level tactical formation. Somehow these pilots pulled the rotor heads off - there was no sign of a pre-existing failure. A senior Bell test pilot was invited to discuss the aerodynamics of this system. He emphasised that extreme care should be taken with cyclic movement with anything less than optimum RRPM particularly at less than 1G - exactly where you will be should you drop the lever after an unalerted power loss.
When constructing the HV curve manufacturers take into account many factors including pilot reaction time - low inertia/high inertia rotor system to name a couple. The result is the avoid curve - in other words avoid flying in this zone as we don't guarantee a successful landing following a sudden power loss.
GAGS
E86

[hillberg]

From what I read here,You all have a Big gap in what H/V tests are and how to use the controls and height /air speed to carry out a secessful power failure (autorotation), In Not just one but any of the different types of helicopters supplied to a pilot. Sad so sad.

[Tourist]

eagle86

What is this "aerodynamic autorotation" you are talking about?

Autorotation is an aerodynamic effect. We call it "autorotation"
There is no other form of autorotation.

I have flown Seaking/Wessex/Gazelle/Squirrel/UH1/AB212/S70B/Mi17 and none of them require anything like the height you talk about even from OGE hover.

Yes from a hover you will lose significant height if you try to regain speed, but that is nothing to do with autorotation, that is just trading height for speed. Autorotation is very quick to take effect. Try an over aggressive fast-stop and watch how quick the Nr gets excited.

[Arm out the window]

Could you explain what you just wrote a bit more clearly, hillberg? I don't get what you're on about.

Are you trying to say that no-one who's posted here knows how to do a successful auto? That's what it sounds like, and if so, you're way off the mark.

[eagle 86]

Tourist,
Aerodynamically stable autorotation.
GAGS
E86

[eagle 86]

Tourist again,
conservation of angular momentum ie the ice skater effect.
GAGS
E86

[hillberg]

Hit the books, Every helicopter handles a little differently than another, From Low Gross weight/High gross, Air speed and all the other stuff that you take in account for a safe flight. From the little R-22, and the Hughes 269 to the S-58. If you are flying with such a gap in knowlage required that you are asking a Forum a question "Why aft cyclic in the start of an autorotation" Get your money back from that "Instructor". What you don't know will kill you. Learn & live.

[Arm out the window]

I'm not asking that question, and before shooting from the hip you would be well advised to read the original post at the start of this thread.

There's a lot of knowledge and experience here, and I think we're all well aware of when to use cyclic in an auto.

Do you always need aft cyclic entering auto? Of course not, it depends on the situation, so go back and read what everyone's said before pontificating.

Ever flown a fixed float-equipped Bell 205 or 206? You need a bunch of forward cyclic entering auto to stop them tipping over backwards, which is what would happen if you applied aft cyclic because of all the extra surface area forward of the C of G that wants to pitch you up like crazy as the rate of descent builds up.

[Tourist]

eagle.

I have no idea what you are refering to.


Lower the lever until the aerodynamic forces have an accelerating effect on the NR.

You are now in auto.

End of.

You can now turn off the engines and pull wingovers and bunts and spot turns or accelerate to Vmax to your hearts content as long as you raise or lower the lever to maintain Nr in the correct bracket.


You are just plain wrong about needing many hundreds of feet to establish in a stable datum type auto.

[topendtorque]

Hi Hillberg,

Perhaps you could help explain procedures for a couple of scenarios from your books?

1. Peter pilot is loitering around at 180 to 250 feet agl at around 10 to 25 knots IAS, moderate timber being 35 to 45 feet. Winds light and variable.

Captain Peter is scanning, scanning for the elusive bovine or etc. Silly sod is concentrating on the bovines, the job, the local bosses daughter, the next pi**up or something - and not thinking about the inevitable - if the noise stops. It does, what does he do with the cyclic and collective?

2. Captain Peter is a silly sod, with the same wind, the very next day he is tooling along at about four feet AGL over an open clay pan at high cruise speed, despite being told many times as a newbie last year not to, and after a great yard up or a great night with the bosses daughter or a splendid night on the turps, inflated with his own ego, and all of the above, at about 1200 hours experience (the most dangerous for mustering pilots) and - not thinking about the inevitable - and dammit the noise stops. What should he do with the collective and cyclic?

This will be any light helicopter 12E, '47, 269 or the fabled 22. He might even be in a 44 or a 206.

Regards.
TET

[hillberg]

#1 Dump the collective and keep what RPM you have left , pull collective just above the tops of the trees and take the ride down through the branches. Keep the descent vertical as the nose of any helicopter will offer no protection,Let the structure do its job,Been there done this.(no trees)
#2 aft cyclic a little and let air speed become a little altitude and (bump) and then lower the collective a little to keep the tail rotor from hitting the ground (ARMY NOE TRICK) & keeping what RPM that will be needed for the tuochdown. Been there twice.
At cruise speed the 269 auto target is at your toes,The 206 is just above the lower windshild frame,The R-22 at 70 mph you can "ballon" and do a second auto to a new spot. The Brantley can finnish an auto with out adding collective at the bottom. and In a good stiff wind the Bell47 will float in the bottom for 300+ feet @.13" above the ground.
As for floats the tunaboat job as fun. As for the R-44 beter have the Bladders installed. Fire sucks.
A poor Peter pilot should allways fly with a way out,A keep his mind on flying. I like it use "What If?" & RPM is life.

[eagle 86]

Tourist,
I guess your public profile says a lot about you. Mate, I can't help you. Let me know where and when you intend to conduct your next autorotational display - I'd like to come and watch - from a safe distance!
GAGS
E86

[topendtorque]

Interesting hillberg, those books; I think they miss a bit of detail.

#1 Yep RPM is life no question.
Another one is knowing your winds even when you are sound asleep in bed. Getting down quickly and arresting the ROD into wind is far easier than doing it downwind. Even 2 to 3 knots is significant. No such thing as Light and Variable when your world is around the tree tops, I am sorry, but that was a planted clue.

Another clue was 'moderate' timber, reads “enough room to get the hull down straight and let the blades chew their way down”. Trunks up to five inches are OK even in a R22.

Conduct the auto to the ground, the collective control will be useless after the first decent strike but the blades will be absorbing energy like you wouldn’t believe and slowing everything down. The cross tubes may bend but you should be able to undo and step out, been there seen that.


I do not believe that to pull pitch and run out of RPM at the 25’ tree tops is a good idea, we have seen a couple of deaths from overpitching here lately, from about that height. You need that energy of RPM to use against the timber to slow down ROD.

Firstly as you say get that lever down pronto, at the same time point the cyclic directly at the spot you have already subconsciously been aware of. That sub conscious awareness as much into wind as possible, must be demonstrated prior to a low level and mustering sign off.

Get the cyclic back just before you go through the canopy. If it is into wind its forward airspeed will stop on a dime.

Forget the garbage about the low G danger to the head; the aircraft is only descending at the rate of one G. It is not in a low G push over manoeuvre, don’t forget that.

#2 There was a couple of clues here too. Notice how I changed the order of controls in the question and also a 1200 hour ego machine will not stop a machine descending at least one foot or more if it is a sudden and complete power loss like a free wheel slip.


So from anything under four foot skid height there will be no, y'll hear, NO REAR CYCLIC until the aircraft is cleared away from the ground to a safe T/R clearance height.
It is simple, a quick jerk on the collective, then start with your cyclic as you put the lever back down and then control RPM with the lever as you would in a quick stop.

Zoom it a bit if you like, maybe jump over an obstacle or some such, whatever you fancy, even a 180 can be accomplished with most types from twenty feet or so and a high cruise speed, to get back into wind. A G5 will do it from 65-70 knots in the width of an airstrip.

The thing is, in both instances the people that are usually going to be good at mustering are often a bit flamboyant, sometimes will take risks and can be counted on to forget, sometimes defy, instructions regardless in their early years. So, one solution to reduce risk is to teach the survival technique basics at an age when they usually remember the basics. Just like a young pup.

cheers tet

[hillberg]

1. An unpowered rotor will NOT Chew its way down, The ROD will be as low as you can get it before entry in the tree tops. If you have time for in the wind I'm all for it , But adding forward cyclic will droop rpm- with a minimum of time you need to arrest the fall before the rotors are lost,After the blades are gone all control is lost and you become the battering ram at the front if any forward speed is left.

2. aft cyclic and bumping collective will rotate the aircraft attitude and raise the tail rotor clear. No aft cyclic? Some helicopters tuck on a power failure, It all depends on the way some helicopters handle.

3. Ty a right 180 auto then a left 180 auto-Tell the results.
The #1 rule is get home alive.
You have some good answers.

[Matthew Parsons]

Some good thoughts here. Of course, the right answer is to not put yourself into a bad situation, if you can avoid it.

In general, I think of the autorotation in three phases: entry, sustained, landing. Each has its own peculiarities that we could all talk about for days. When you get into any low level situation, then you may have to omit the sustained portion.

For example, zero speed 500' AGL, lower collective and dive for airspeed, you should be able to attain your normal sustained parameters, so all is simple.

The entry and the landing both have airspeed changes and height changes. I think of the landing phase as a graph of speed and height, start high and fast, end at 0/0. If the entry speed and height approaches the landing speed/height line you just switch from entry phase to landing phase, allowing time and energy for the transition. Sounds complicated, but its really not.

Change the example to 200' and zero speed, you lower collective and dive. At 100' you have ~40knots (made up numbers), you haven't attained your sustained parameters, but your speed/height combination looks just like the middle of your normal landing phase, so you raise the nose to your decelerating attitude, allow the rpm to build a bit and continue with the landing you've trained.

topendtorque,

#1 I agree you don't want to lose all rpm or collective at the tree tops, but you also don't want to hit the trees at max speed. Seen the effects of that. I'd rather take my chances on a 35' foot fall, with a small cushion available than a 2000fpm descent into lumber. My plan if I don't think foliage will cushion me is to zero speed and minimize descent just above the tree tops, then dump collective to try and recover rpm on the way down, cushion at the bottom. Of course, that would be pointless if I thought the trees were going to destroy the blades so this is all situationally dependant.

This gets a bit more difficult on the West Coast of Canada. 200' trees that are greater than 5' diameter at the base, and with foliage not always dense enough to stop or slow your descent. Reliable engines and/or prayer is the best alternative we could come up with.

#2 Trade airspeed for altitude. Same as above, you start in the entry phase, but in this case you approach the landing phase from below. When you flare on entry, you made need to raise collective to control rpm, but that is type and situation dependant. When your altitude & height intercept the landing phase, it gets easy again.

[oggers]

Bear in mind the helicopter rotor system takes about 1000 feet of descent to aerodynamically establish autorotation

Interesting opinion but not one I share. I agree with Tourist: you don't need anything like that to establish a steady state auto in the types I've flown.

[Tourist]

Eagle 86

"Tourist,
I guess your public profile says a lot about you"


Nice.

Always good to play the man not the argument.:=

(anyway, what's wrong with being a wafu?!)

Come to Predannack any day of the week and you will see a variety of autos.

My personal favourite was always the
3000' 90kts into wind Seaking comedy auto invented by a certain very grumpy pilot who now flys for Loganair.
On top the target (15kts of wind was nice), enter auto and rapidly ramp it up to hit zero speed without height loss.
As soon as zero airspeed, level the cab and spot turn in auto right.
After one complete turn, bury the nose to get 70kts and continue the turn into a 360 auto right.
As you approach the into wind heading dump the nose further to get into a range auto up near Vne.
As the wind has drifted you backwards, the two stage flare should bring you nicely back to a spot landing on the H.:)

I still remember my large ginger instructor on 810sqn demonstrating that for a real double engine failure, you can do whatever necessary to get you back to the spot. His party trick was to fly over directly the spot into wind 90kts 2000' and get the student to say when he thought it was no longer possible to get back to the spot in auto. When you eventually said "now" some 20 seconds past the spot he would wait another 10 seconds before entering auto then straight into a wingover in auto, dive back towards the spot before another wingover to bring him back into wind for the flare/check/level. As he said. "if you control the Nr, then the rest is just normal flying"

[baobab72]

Hi guys
One thing i am having an hard time to understand and visualize is the relationship between cyclic movements and rrpms? Why rrpms change as the helicopter pitch is changed during the autorotation?

Many thanks

Baobab72

[Phoinix]

Think of it as a windmill. If you blow towards the windmill perpendicular to the windmill rotor plane (directly into the windmill rotor), you get a lot of rotation in the system. That same windmill, if you blow from the side, you won't get anything out of it.

Cyclic alters your attitude towards the airflow or if you will alters the relative direction of the airflow in relation to the rotor disc. Flare - increase of area-airflow, cyclic forward, decrease in area through which the airflow is going (just like blowing the windmill from the side.

[[email protected]]

Baobab

When you bring the cyclic back, you are raising the nose and changing the angle the air meets the blades - this will give more lift (and drag) but the extra lift will make the blades try to fly up (cone up). As the blades cone up, their centres of gravity will move towards the rotorhub (only a small amount) enough to increase the RRPM because of the conservation of angular momentum (think of a spinning ice skater who brings her arms in to spin faster and out to spin slower).

The opposite is true when you move the cyclic forward (this can be very dangerous in a teetering head helicopter if you do it quickly as it can lead to mast bumping).

In summary - bringing the cyclic back increases RRPM in auto (and reduces speed) and moving it forward decreases RRPM (and increases speed). It is the rate of change of the airflow that determines how quickly the RRPM rises or falls.

[AnFI]

Ice skaters can't keep moving their arms in forever - the coning explanation at best is only a (transient) minor contributory effect - likely to be the dominant effect is the Total Reaction becoming 'more forward' (causing more 'autorotative force') corresponding to the increased angle of attack derived from increased negative induced flow achieved by the flare.

You can see this by holding an increased loading through bank in auto where the transient coning effect does not manifest itself above the effect of the ongoing continuous bank.

[AnFI]

like Phoinix says but more simply:

the thing that makes the blades go around in auto rotation is the air passing up through the disc.
when you pull the nose up more air goes through the disc faster, so it'll get more of what makes the blades go around - that will make the blades go faster.

[Lonewolf_50]

Another thing to throw into the mix - a fixed wing in a fully developed spin is in autorotation - after the stall and subsequent prospin control inputs a considerable amount of height is lost until stable autorotation is established. GAGS E86
It's a different sort of autorotation, eagle. You are right, it takes a few turns to get steady state, to where you can let go of the controls and the aircraft spins in a predictable fashion, and stay that way until impact if no spin recovery inputs are made. In that case, the entire aircraft is rotating about the vertical axis ... you sort of create a rotating wing out of the wings and fuselage ... is that the point you are making?

I've taught spins in FW, and autorotations in helicopters, and I'd never take the position that they are the same sort of maneuver. Not sure what you are after there.

A helicopter autorotation is a different critter from a spin. The rotating wings are not stalled during the evolution, and the rest of the aircraft, once the torque is reduced/removed by reducing pitch, doesn't flop about. You remain in controlled flight.

About autorotations to check control rigging:

I agree that it takes some altitude to establish a steady state auto, steady state ROD, and your test airspeed right on the money to take the precise data point needed to check control rigging. I think it's a bit much to use such a requirement to assert that you aren't in a stable auto in less than a thousand feet. Depending on rotor inertia, you can get stable enough in light helicopters to make a complete auto in less altitude. In the big and heavy ones, maybe not ... haven't done on in a while, so I'll not go any further on that score.

eagle, I very much liked the list of cases you provided for reference, since engine out and auto fit into a lot of different conditions, and one needs to be prepared for any of them.

Great thread, by the way. :ok: Thanks to all who have contributed.

[[email protected]]

Ice skaters can't keep moving their arms in forever no, they reach a steady state - just like when you stop moving the cyclic and the Nr returns to a steady state.

increased angle of attack derived from increased negative induced flow achieved by the flare. still struggling with English

when you pull the nose up more air goes through the disc faster, so it'll get more of what makes the blades go around - that will make the blades go faster. and P of F;)