"free" turbines and the freewheeling unit
 

is this a mistake:
the N2 stage of a "free" turbine engine acts as the freewheeling unit in the event of autorotation....

reason I ask is because I read somewhere - probably here - that certain types of turbine helicopters when in practice autos have slightly improved performance because the engine, while reduced to idle, is still supplying a small amount of power to the N2 stage (making for a slightly unrealistic practice performance). I don't know why I'm now just asking, but, any help?

The "free turbine" is in reference to the engine and not the powertrain as a whole. What it means is the gas generator and power turbine are "gas" coupled and mechanically coupled.
The freewheel or overrunning clutch or sprague clutch is the unit which decouples the transmission from the engine. The decoupling is necessary so the engine will not be driven by the transmission. If the engine were driven by the transmission this would use up rotor system rpm and not allow sufficent rpm to complete the autorotation.

The Westland Scout is fitted with a Rolls Royce Nimbus Mk105 free-turbine engine.

A quote from the Pilots notes:- "Under standard conditions the rate of descent will be approximately 2200 ft/min at 5000Ibs and will reduce to 1500 ft/min at light weight, due to the increased power contribution required to maintain 420 rpm. A high weight flight-idle autorotation is very representative of an engine-off autorotation."

if I remember correctly what I read, it stated the N2 stage was still supporting Nr, even though throttle was reduced - if there were a seperate freewheeling unit this would not make sense, which led me to wonder if "free" turbines didn't need a seperate unit....evidently not the case..... can anyone figure what the reason for that unrealistic auto performance would be?

[This message has been edited by lmlanphere (edited 17 March 2001).]

Not quite sure of the terminology being used here. N2 is the term used to describe the rotational speed of the power turbine or free turbine as it is variously known. The power turbine is driven, as stated, by the exhaust gases from the gas generator (engine to you and me) and so has a fluid coupling but no mechanical coupling (hence the term, free turbine).

In the cockpit, N2 and Nr are normally on the same gauge, and in powered flight read the same value (normally a percentage figure). In true autorotational flight the needles will be split, ie N2 will be lower than Nr and the freewheeling mechanism will have disconnected the transmission from the engine (free turbine) input. In this instance, it wil make no difference whether the engine is running or stopped ie practice or for real. If, though, the needles are joined, then the engine will be contributing some power to the transmission and will be assisting the autorotative section of the blades with maintaining Nr, and hence the performance may be better than in the event of a real engine failure.

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Another day in paradise

A turbine powered helicopter with a "draggy" rotor may not fully enter autorotation at low weight because the engine/s may not back off enough to allow the freewheel/s to disengage. The ECU flight idle setting may be quite high to ensure good engine response when going from lever fully down to a subsequent collective lever power demand, especially on older types.

The same aircraft at a higher AUM will have a higher rotor RPM in the same autorotative conditions (more "potential" energy being dissipated)and this will allow the freewheel to disengage. The aircraft will then fly in full autorotation, i.e. as has already been said, with the Nr/N2 needles split.

maybe that was the case, that the needles weren't split and therefore the freewheeling unit wasn't doing anything, I don't actually recall that as part of the equation, though.... Thanks for the input

At the bottom of the auto the RPM decay is slowed down once you pass ground idle. The engine will supply whatever power it produces at that setting.

Thanks shytorque, I missed a bit. With some a/c such as the S76 it is possible to beep the N2 back to a value that allows autorotation with the "needles joined", but as stated it will be not entirely representative of the engine(s) out case.


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Another day in paradise

I remember reading an account of a accident in a H500 where the pilot, who was experienced in flying practice autorotations, had an engine failure and performed poorly in the ensuing auto. He basically landed very hard and in the summary he cited the difference in performance with the engine idling and completely off line. Could be a number of factors of course, including the 500 autos like a brick. Wish I could remember where I saw the article, any help?

Gov

[This message has been edited by The Governor (edited 18 March 2001).]

I think we should consider the difference between FIG and autorotation.

Firstly, the situation you refer to when you put the collective down and conduct a PFL is not an autorotation unless the engine (gas turbine type) is not running. You are in fact conducting a Flight Idle Glide (FIG) during which there may well be a significant contribution from the engine towards the rotation of the free turbine. During a genuine autorotation, the autorotaive force that turn the head is a result of aerodynamic forces only which results in a higher rate of descent during an autorotation thatn during a FIG. I have noted this on a number of aircraft ( when conducting a formal flight test)by selecting gound idle or shut down during steady FIGs and noting an increased rate of descent ( about 200 ft/min on some types). I do not suggest you try this unless your current regulations permit it as I do note promote DIY flight test.
Obviously it is only a problem where a large contribution in FIG means that PFLs are not representative of the real engine failure case. Any thoughts?

bit, is it not possible then for the freewheeling unit to work until the engine is completely shut down?

The engine, be it piston or turbine will only be contributing to the auto if the freewheel unit has not disengaged, ie the needles are still joined. Once the needles are split, it matters not one iota whether the engine is at 100% or 0% or any figure in between, it is physically disconected from the transmission. It IS possible to lower the collective and carry out a glide with the engine idling, but that is not autorotation.

It's a bit like saying that if you coast down a hill on a bycicle at 30 mph, slowly turning the pedals whilst in 1st gear is going to make you go faster.

It's not rocket science.

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Another day in paradise

The freewheel is not "freewheeling" unless the Nr is > Free turbine speed. The problem is in some ac where during steady FIG it is not possible to get a split off as it would require the collective to be lowered to a point where the rotor would exceed its Nrlimits. If some collective was applied during FIG to keep the rotor from overspeeding and that corresponded to say 5%Tq; that represents an engine contribution during FIG, therefore the ROD during a real autorotation would be higher. It is the magnitude of the engine contribution that is important if you have to do a real engine off as it will not be there to maintain Nr if the the donk has stopped, unlike during your training PFLs.

bit, isn't it possible to retard the throttle while collective is raised to control Nr? (at least throttle on the collective types?)

On the SA330 Puma, for example, the difference between autorotation with both engines throttled back to GROUND idle and both engines shut down is a reduction about 5 Nr for the same WAT conditions. A slightly increased Rate of Descent, but also, importantly, less flare effect and slightly faster rate of decay in Nr following the flare. (square laws, etc.)
In general, expect a small contribution to keeping everything turning from a 'gas clutch' aka free turbine engine.
On the other hand, aircraft like the Gazelle have a mechanical clutch, which offloads any power contribution when Ng are reduced to idle in autorotation to an engine-off landing. Thus practice and for real will be the same...apart from the adrenalin factor!

Nice one multp. Are you a graduate of the college of knowledge 7 part paragrapgh writing course?

If the rotor system is being turned by the action of the air passing through it and the clutched is disengaged, you are in autorotation. Engine at idle, at 20%, at 100%, or 0% you are in autorotation. I think the H500 fellow claiming that the difference in performance mentioned above contributed to the poor out come was looking for some help from a myth. I have done literally hundreds of touchdown autos in training and one for real. No difference I could see with the much narrowed perspective these kind of nearer-my-god-to-thee situations provide.

I think there are some misleading terms being used here. The air/fluid coupling in a free turbine engine is not the clutch. The free turbine is considered to be part of the engine although not mechanically connected (hence the fact that Bell refer to N2 as engine RPM). Therefore there is a mechanical clutch located between the free turbine and the transmission (may be actually in the transmission casing), ie a freewheel unit. If this is disengaged for any reason (not necessarily auto) the transmission will be oblivious to the fact that the engine is running, or indeed stopped. Any reference to engine assistance is only correct if the freewheel unit is engaged.

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Another day in paradise

212Man:
You are always a precise and well informed commentator! I take your points exactly. I'd just like to amplify my point, though.
Going back to the SA330, there is no N2 indicator so it is impossible to tell, with the engine running, whether or not there is sufficient residual thrust at 'idle' to prevent the freewheel being fully disengaged. The same is true for any other helo with no N2 indication, unless it has a centrifugal clutch such as on the Astazou and Artouste engines.
If the freewheel is not fully disengaged, the engine's residual thrust will provide a small power contribution, offloading the aerodynamic effort required to keep the rotor and gearbox train turning. For the SA330 Puma (one last time!) there is hard data to show that the engines make a power contribution in (flight/ground idle glide) autorotation.
On a piston-engined aircraft, the split between ERPM and RRPM will indicate whether the freewheel unit has functioned and the engine is not assisting. For turbine engines, it is the N2 / NR split which is vital to show the true status of the autorotation.
(One last point: the Super Puma AS332 has a different, though similar, power train to the SA330: but with N2 indications.)