Just thinking out loud. A higher ROD would imply more energy is required to maintain RRPM with the engine out vs at idle. At the idle detent you're getting 50-60% Ng which is providing a fair amount of drive to overcome some of the inherent drag and friction in the rotor system/drive train. With no engine you would need to overcome 100% of the forces via autorotation, which would require a higher ROD.

Some strange ideas cropping up on this thread. Maybe some clarity would help:

1. Autorotation (AUTO) - The rotors are driven only by the airflow up through the disc.

2. EOL - the thing you do at the bottom of an autorotation. (If the engine is running with zero TQ (typically at idle) it is a"Simulated" EOL.

3. During autorotation, if the engine is producing ZERO Tq, the only friction difference between that and the engine shutdown, is the increased rotational Delta speed of the inner and outer race of the freewheel. Compared to the Autorotational forces acting on the rotor I suggest this difference is inconsiderable. Assuming of course the freewheel is serviceable and the TQ during autos with engine at idle is zero.

4. Having spouted this, I have no idea how a H300 works. Saw one once from about 300 yards.

Sad for this guy but I would say if he planned a deliberate entry into auto, something we all practise a lot in our initial and other training, it's difficult to assume the pilot screwed this bit up. Maybe something else, like seized freewheel, intervened.

The 300 engine is mounted horizontally to a belt system at the back.

TC says ..If you committ to an EOL at the top of the slope, you physically switch the engine off. In an Auto - you don't.
Also "So he apparently opts for an EOL (not an auto as some state - an auto would drive the Nr off the clock and damage the blades
Is this correct ? It appears he believes an auto is done with the engine on . I may be wrong ( again !!) but I thought autorotation was just an aerodynamic state regardless of engine state ? So you can have engine on autorotation or engine off autorotation ....just as 212 said .
Lastly I also thought EOL was relevant just to the pull at the bottom being with engine OFF with no reference to the descent part .
If in idle it is actually a simulated EOL .
Please no TC type of replies ....as I genuinely would like to know if I have had a 35 year misconception!!!

Many of TC's EOLs, like mine, will have been in a Gazelle where you enter auto and, when sure of making the landing area, retard the throttle to ground idle.

It is an EOL in that there is no assistance possible from the engine, since we were actively discouraged from advancing the throttle again in case of surge or damage to the clutch ( have done it once though to avoid a mid air).

Back to the drag issue - I don't know what sort of freewheel arrangement the 300 has, I assume it has some sort of centrifugal clutch/ramp and roller gizmo that does the job.

With the engine running, but at idle, there is a small rotational speed difference between driving and driven parts so drag will be minimal - with the engine stopped there will maximum speed differential so there must be more friction and drag. Engine ancillaries (pumps, generators, alternators etc) may also contribute to the 'dead load' depending on the layout.

Out of interest, the Brit Mil still do some EOLs on the Squirrel, they call them EOLs in that again the engine is back at idle but the throttle can be quickly opened to save embarrassment.:ok:

Crab ....Thanks . It would be interesting to get an actual figure on the % extra friction and therefore rod . As I said in the ones I have done I didn't really notice any change . I was slated by TC for saying that an auto with engine in idle is the same as off ...!! By that I just meant there was no connection due to freewheel. I accept I was wrong with regard to some extra friction..ok ?
Now the same man who slammed me for my " oversight " is calling engine ON landings ...engine OFF landings !!!!! Are you certain landing with engine in idle is IDENTICAL to landing with engine OFF ? So how many genuine EOL,s has our resident expert really done ??!!!
If you want to be pedantic and aggressive here you had better make sure you never make a mistake !!

Nige, I think there are so many variations of transmission across helos that might be difficult - I am also transferring what I know from turbine aircraft across to piston but there may be differences - eg on the R22 with that weird squirrel cage and drive belt arrangement, I have no idea what drag there might be through that system.

Crab, you appear to be one of the few that know what they are writing about(also double bogey). Please look up the definition of both ''friction" and "drag". If there is any of either when ANY type of freewheel device is activated, then there is a fault with the device.

I do not know about turbines, but with the R22 the squirrel cage sprag clutch most definitely does not have any contact when disengaged. No contact, no friction, no drag. I doubt that any Helicopter would be certified if there was not an approved freewheel device that completely disconnects the power input source from the rotor system.

The poster that has concerns about the MR over speeding when in auto rotation has either not been trained correctly, or has never been in auto rotation! Cheers.

edited to add the following: http://www.rotorandwing.com/2010/02/...a-sprag-thing/

Old Farang, I don't claim to be a scientist or an engineer, I am just passing on what was taught to me many years ago by people who were; if it is wrong then so be it.

I know how a freewheel works and I also know they don't always operate perfectly - some friction drag is quite possible if the sprags don't completely disengage.

Perhaps you can come up with an alternative answer as to why some have experienced higher RoD in auto without the engine running compared to with it at idle.

Well, I did make suggestion in a previous post. But it may depend on what else is still rotating such as the cooling fan or other engine driven attachments rather than driven from the gearbox. I do not know, but would be inclined to think that a turbine may also show a diffence for the same reason, especially a H500 with the engine at an angle.
Better ask Nick Lappos!

Also the orientation of the cylinders in a piston engine. It is too long ago to recall just what the H300 did, but I have more time in an old Hiller, and I just do not remember if there was any difference. The point being here the difference in the engine layout between the two examples. I do know that it was quite easy to stretch the glide in the Hiller, but then the rotor system was more akin to a Huey.

The first R22 that I owned was before the tip weighted blades were introduced, and that required immediate and judicious use of the collective to the extent that it would not have been noticeable either way! Never did one with the engine stopped, just at idle.

TC seems to have gone very quiet since having very strong views on my comment about autos without engine ( EOL) . See below ...

"Nihelh - there you go again egg on face in public talking rubbish, transmitting and then thinking about it afterwards.
You'll never learn will you (well not in all the years you persistently continue to make these silly naiive remarks on a public forum)."

Now it appears that many others here have the same view that there should NOT be a large increase in rod with engine off as against idle . ( I think we all agree there will be some difference due to hydraulic pump still being turned but not enough to put 20% onto RoD ). It now appears that his million EOL,s have been with engine on and not Off as he implied .
Does anyone else have any idea ?? Maybe a member could actually do both , one after the other , and compare ?!!
Ps . Hyd pump still works on 350 in auto I think ....but maybe I'm being naive ...

Correct me if I'm wrong, but I was under the impression the ROD was controlled by the pitch on the main rotor blades? If the engine is at idle or stopped, surely the rotor blades and weight of the a/c will still determine ROD?
Is someone telling me if the engine stops on the way down during a practice autorotation the ROD suddenly increases? Or because a possible increase in friction on the spragg will slow the rotor rpm thus requiring more down lever which then becomes the reason for increased ROD?
Having said that, slowing the rotor RPM a little during the descent would reduce ROD, no?

Friction in freewheeling unit
 

The story of high friction in the freewheeling unit can't be true.
Some, low friction yes of course.

Long time, but I still remember the MR continue to turn forever after engine is shut down and you walk away from the 300. It would'nt ever stop. If there was anything more than 'almost no friction' the stopped engine would effectively act as a rotorbrake, and stop the MR.

For twin engine helos with NG-oriented idle, when selecting one engine to idle( the other engine in flight) the free turbine of the idle-ing one would be driven backwards via the freewheeling unit and show a lot higher NF when MR at 100% than when both engines at idle. But it doesnt, on the types i flown. When selecting idle on one, the NF reduces for example to around 70%, when the other engine is selected to idle, and NR decays, it still show the same number until rotor is driven by both eng on idle(a small reduction of nf caused bu the load of the drive line of course).

Was you guys ever shown a freewheeling unit bu the flight engineers in basic traning? Was allowed to turn it both ways? Remember any significant amount of torque when turning it the frewheeling direction? I don't.

Autorotation with engine(-s) in idle vs flight
 

Something that Nigelh and Reely340 also mentions and is very inportant:
If you do an autorotation with engines in flight you arent performing a true auto before the nf shows lower rpm/ disconnected to rotor rpm on the two-/threeneedle gauge. Most turbines continue to drive the rotor until somwhere in the 102-105% ng mark. Rotor rpm has to be higher than this, otherwise the engine(-s) still are driving the rotor, very much affecting the rate of descent and glide angle.

In an auto with engine(-s) at idle, the rotor drives itself and the freewheeling unit already put the said drag( but very,very low) on the system.

I fore sure hope tou guys train autos with engines at idle. I did training somewhere where the pilots newer did this. They actually also only did the simulator autos with engines in flight( :ooh: ). If you ever should be in the need for a real auto I think tou would feel very fooled, seeing a much much higher rate of descent and a lot shorter glide, and efter this tou die :-(

When you perform an autorotation, the first thing is: "RPM rotor"... So when you put the pitch down, you "normally " desynchronize RPM/NG UP or DOWN....And so, you check
the freewheel...Better if your RPM rotor are higher than your RPM NG ;) In turbine engine, when you perform an autorotation descent with engine runing but desynchronized ( free wheel runing normally ), and then you put the turbine on idle, i admit you feel a little increase of the rate of descent. What else ,

I think we have got a bit over the top on this freewheel question - the amount of friction I was suggesting would be very low unless there was some problem with the freewheel disengaging properly.

AAKEE - I agree with what you say about needing an NF/Nr split to be in true auto and maybe this is where the higher RoD issue has come from - it is always difficult on these forums comparing apples with apples without detailed explanations of exact circumstances.

Chopjock, you are correct about the RoD in auto and the only way a freewheel would affect that following an engine failure, would be if the freewheel had not fully disengaged - it can happen.

Out of interest, how many people actually check the functioning of their freewheels on a regular basis - we had specific checks for it on the Sea King but on a single you would need to enter auto and flare to split the Nr from the Nf (or equivalent).

H300 run up checks include a throttle chop to check disengagement of sprag clutch.

That's good for pistons as a throttle chop splits the needles quickly. Different on a 500 though as the turbine takes a while to wind down and the light inertia blades slow down with it and you don't get much of a split.

Gentleman, your discussion on freewheel drag has truly been entertaining... but it is a waste of time. Rate of descent is a function of density altitude, gross weight, and rotor RPM. anyone who has shut down a helicopter at the end of a flight knows that those blades continue to spin quite freely (as in "freewheeling"). The primary difference in the autorotation with and without the engine running, is the touchdown. With the engine at idle, the helicopter will fly the same down to the touchdown point, but... when you apply the collective and droop the rotor rpm for the landing, it will eventually meet back up with that idling engine. Most of the turbines I fly will idle around 90-92% power turbine speed. Once the rotor slows to that 90% speed, it couples with the idling engine and the droop will be drastically slower(0.5 - 2 seconds) in terms of how long we can cushion that landing. With the engine off, the rotor droop is faster and if you have never attempted this, it will feel as if the aircraft drops much faster during the touchdown since the rotor reaches the stall much faster. My speculation is that this professional entered autorotation and misjudged the approach, Stretched the glide, or got slow and pulled the cushion too early. Either way, the NTSB has video of the incident and I'm sure they will use sound analysis, video analysis, and all GPS data to rebuild exactly what occured.

In the R66 it's part of the shutdown checklist. Look for positive split of needles when you pull the fuel.

IIRC, our 206 daily preflight required pilots to hand pull the blades backwards to check the clutch operation. If you could hear/see the turbine rotating, you were good to go.

That depends on your RFM - if it says you can then it should specify a figure (on the Gazelle it was 330 Nr down from 380).

If it doesn't say you can do it then it is vital that you stay within your prescribed Nr limits. as you raise the lever, you move the 'driving' section outboard and increase rotor drag at the root (and possibly at the tip as well).

Overall you could see an increase in rotor drag which will need an increase in RoD to maintain the Nr.

Most people like to have as much Nr as they can for the EOL at the bottom and, even if you do droop the Nr for range you must recover it to normal for the EOL or run the risk of it being very exciting as there is not enough kinetic energy in the rotor to adequately cushion the touchdown.

As postulated earlier, it may well be what this chap did at Lumberton.

Armyav8r ....I completely agree with all you say and this was my point when i got shot down by TC !
I believe the auto is near as damn it exactly the same with engine in idle ...but the EOL ( Landing ) bit will be different as you will notice that even at idle if you pull collective the machine will at least go light on the skids ....so the engine will be producing some lift right at the last bit when the rrpm droops .
Apparently you and i are wrong !!!!!!

To be fair Nige, I think it depends on where the idle sits in terms of holding Nr - what does it give you when you sit on the ground at idle?

It is, I suspect, way below useful Nr that would show a difference on an EOL - and that is based on my experience on a Gazelle.:ok:

As part of my previous life I was fortunate enough to spend 6 years as an instructor at a fairly well known OEM training facility in Fort Worth.
Whilst doing in house recurrency in a 206 with a colleague who had been working there for 35 or so years I was asked if I had ever suffered an engine failure.
Upon my negative response the throttle was retarded and then placed in the off position. We did ten or so engine off landings, and, as I recall there is no noticeable difference performance wise between having the engine off or at idle. The biggest difference was having the engine out horn going off and AW Brown telling me I'd better not screw it up as a go around was now no longer an option.
I have had a partial sprag clutch in a 206. We were trying to adjust the autorotational rpm and no matter what we did the rpm wouldn't change, but would drop to the bottom of the green with the Nr and Np needles staying stuck together. It wasn't until after about the third engine shutdown that we noticed that there was no needle split. You couldn't turn the blades backwards, but by putting your hand down the exhaust, with very little pressure you could hold the Np wheels in place while turning the blades. What would have happened to the Nr with an engine failure is an unknown, but it probably wouldn't have been pretty.

Crab .. do you think that we all agree now , apart from your mate TC , that there is not a fundamental difference between the two states of autorotation?
IF there is one , it will show itself right at the end on the collective pull at low rrpm ? One would hope that is the case as I don't know many people who teach with shutting the engine down !!!!
I'm glad we finally got there .......

Crikey Nigelh, I've realy gotten under your skin haven't I:rolleyes:
Sorry I haven't responded earlier, but I have a life to lead outside Pprune too you know.
In fact I'm spending less and less time on Pprune now having spent the last 17yrs since joining it repeating myself. So you can take succour in that perhaps.
I will say this once again but for the last time................
Many posts ago, there was another thread on engines off etc - someone asked how many EOL's I had done. I checked and rechecked my log books and can confirm I have done in excess of 2500 EOL's. From nose bleed height to downwind, to going backwards, to 100' and 120kts.
ALL of them were initiated with the throttle lever being retarded and locked in the off position, fuel to shut off, ignitions off. Engine(s) out. In fact I have the dubious honour of switching all the engines off in a Seaking -to a landing - before the RN decided that this was another silly practice we could all do without....and banned this.
I'm not sure where Crab did his EOL instructing but mine was in the FAA and the technique was an E.O. (as above) - with or without warnings.
[Not just to FI].
An auto on the other hand - is a descent to the hover/land/overshoot with the engine(s) running at FI until just prior to the final decision to hover / overshoot where the engine(s) is brought back online.
Flying along minding your own business and an engine(s) stopping without warning is a totally different ball game compared to entering an "auto" for obvious reasons.
The delay caused due to the jesus factor (in an EOL) will already have cost you an increase in RoD which will prove difficult to make up, as you sort out the entry. In an auto, friction in the drive train and the engine itself sitting at FI will all have a play in preventing you from drooping the Nr too low before a warning is sounded reminding you things aren't looking good. In an EO, there are no safety nets.
Friction, engine running and confidence knowing you have a safety net when you need it, all add up to a lower RoD in reality.
Whereas in an EO: no friction, no engine(s) lack of practice/manhandling and a rapid onset of RoD during the initial entry will exacerbate the RoD. If you are practiced/well rehearsed/lightning reflexes - you might be able to recover this RoD difference. In reality you simply have to manage it/live with it.
That's me done on this particular matter.
Don't respond Nigelh. I'm really tired of your wisecracks. Go find someone else to annoy. [I seem to recall asking you to stop stalking me a few times now - enough is enough]. Chill out FFS. :*

Any chance this thread could return to the subject matter?

TC,
Sounds like you are saying it's impossible to auto rotate with the engine stopped?

Yep, in every damned shut down. Cause it was written in our flight manuals ( "Throttle to stop/off - needle split") on the three first types i flew. Its written in the spine, so eyes go there on later types that dont say this( except for when in the maintanence check flights, where there can be explicit checks for needle split, both in flight/auto or on ground run).

Thanks TC .....you now say ..."Whereas in an EO: no friction,"
That's all I wanted you to admit 👍
Well done !:D

disclaimer - not a helicopter pilot - but still technically adept ;)

now on a piston helicopter if the rpm of the engine decreases there can't be any torque transmitted from the engine to the rotor system / gear box. also it does not matter which accessories are driven before or after the sprag clutch. it will be the same no matter how far the engine is below output shaft rpm if it is minus 100 or minus 10 percent.

on a turbine helicopter with a gas generator and a power turbine on the other hand... you can reduce the rpm of the gas generator only. that running at idle still makes air go through the power turbine which i believe will still turn at sufficient rpm although with relatively little torque to transmit some power to the rotor system.

so we see we have a difference regarding autorotation:
piston helicopter there is no difference between idle and engine off
turbine helicopter there a difference between idle and engine off

now add to that: i'm not so sure a turbine helicopter needs a sprag clutch. you could well say the power turbine as a load is not significant enough to warrant one. but i guess they put one in in case turbine or compressor blades decide to rampage through the engine .... which could be a significant load.

still idle power should impart enough torque onto the power turbine wheel so that it makes a noticeable difference compared to engine out even if there is a sprag clutch.

so nigel: tc might have said no friction but might have meant no residual torque being transmitted.
but i guess that's the different viewpoint piston vs turbine.

anyway there haven't been any concise explanations on the issue so maybe my train of thought will help with that maybe not. at least you got my brain working that's always nice so thanks for the discussion :)

wiedehopf - very useful, thank you, it concurs with what I was taught (on turbines) and shows that you can't automatically transfer knowledge broad brush to pistons:ok:

AAKEE - checking the freewheel on shutdown is all well and good but what about checking it before you go flying?

It is a simple check on a twin since you just retard each engine in turn to check for Nf split.

On a gas turbine engine there are two governing parameters - Ng and Nf/N2. With the throttle open the engine will match its Nf/N2 to the Nr and the needles will be joined and torque applied to the rotor system. With the throttle at idle the Ng will be governed only, there will be no regard for Nf/N2 and the needles will be split with no torque applied to the rotor system. Ergo, in autorotation the rotor will not benefit from the engine when the throttle is at idle.

No, but many of the accessories may.

I'm all ears....

I think we might be closing up to OT.
Anyway, needlesplit check is to comfirm it is releasing now, to be checked safe for next flight. When running up you have one engage for the freewheel before taking of. If doing your check you still have one engage to do before flying. I do not think you add a lot of safety doing so.
The system with multiple engage-disengages during ground run after install of new freewheel unit ( and check for disengage during checkflight /autorotation on some types) and to check during each shutdown should be good enough, I think.

it might well be that the gas generator on idle does not turn the power turbine fast enough to torque the rotor. then there is no difference to the piston case.

but i'd say it all depends on the gas generator and its idle setting doesn't it?

@crab
the accessories either get driven by the gas generator and never are an additional load during the autorotation regardless of idle or off. or they are driven from a point after the sprag clutch in which case if the power turbine does not torque the rotor it also does not torque the accessories and they are drag for the rotor.
so talking about the accessories really does not help. they should make no difference.

I'm clearly wrong then - not the first time and I'm sure won't be the last. I have looked back through various tech notes but can't see anything to support the idea, even the Sea King where the TR drives the MRGB accessories in auto.

Hey ho:ok: It's just the sort of rabbithole you can disappear down with a throwaway remark on PPrune - far too many knowledgeable people on here:)

For what it’s worth, here’s my two pennyworth.
In 28 years of instructing I have probably done only hundreds of Simulated EOL (SEOL, to use EASA-speak) in piston helicopters, but thousands in turbine helicopters.
And just to clarify my own terminology a bit more, an autorotation is where the rate of descent airflow, rather than engine power, is driving the blades, and may end in a re-engagement of power, either at height, or in a flare recovery (ie the SEOL profile, but power used to recover to low speed IGE environment, no ground contact) or in a SEOL (engine running but disengaged and ground contact) or an EOL (engine not functioning, ground contact). OK, some purists may remind me that engine to MGB drive may have failed but the engine is left running to drive the TRGB…point taken!
My take is that in a piston, with the engine throttled back to idle, there would be a negligible difference, though I have never done a shut-down, thus actual, EOL in a piston.
In the turbines, it depends….
In the Gazelle, when you throttle back, the engine is completely disengaged because it has a mechanical centrifugal clutch. (BTW, 1000+ hrs as a Gaz military QHI)
In free-turbine installations there may be residual thrust at the flight idle (FI) throttle position. This is true for a couple of other light Airbus SET types, where there is a drop in NR at min collective pitch when the throttle is closed to the ground idle position. Less observable in the 206 variants, though I am open to contradiction: and only recently have I access to a 206 instrumented for flight data, so I may well have a scientific look when I next can. This may also be a function in the 206 of the high inertia rotor, so that a throttle closure to GI at committal height doesn’t manifest itself in NR drop before the flare commences.
In certification trials on a light twin with Allison engines, we found that in 4 minute OEI climbs, in identical WAT and geographical conditions, there was no discernible difference between a climb with one engine at GI and the same engine actually shut down. So conclusion: no residual thrust at GI. So with both engines at GI in autorotation in that type one might expect that to simulate faithfully the engines-out conditions.
I also had a little experience of autorotation and EOL trials in a 7-tonne class aircraft. Here with both engines shut down there was a 2-5% drop in NR at min collective pitch vs NR at GI…so definitely some residual thrust.
It is, perhaps, safest to assume that most SET may have some residual turbine thrust during practice autos, either at FI or GI throttle setting. But it's probable that for an unexpected engine failure the descent profile will result in potentially shorter range, due to the initial loss in NR before pilot intervention.
So, putting my FI(H) hat on again, two golden ‘Lessons Learned’ rules:
• In Autorotation, NR is King
• Never expect to go as far as you first think you might in auto; losing range is easy; stretching it is not!