"free" turbines and the freewheeling unit

[lmlanphere]

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?

[Ancient Pelican]

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.

[avlerx]

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."

[lmlanphere]

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).]

[212man]

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

[ShyTorque]

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.

[lmlanphere]

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

[Grisoni]

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.

[212man]

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

[The Governor]

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).]

[BIT]

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?

[lmlanphere]

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

[212man]

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

[BIT]

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.

[lmlanphere]

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

[Multp]

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!

[BIT]

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

[VLift]

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.

[212man]

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

[Multp]

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.)

[ShyTorque]

Multp,

If you are referring to the composite bladed AS330 Puma rather than the metal bladed version, the autorotational Nr at anything other than very light weights tends to increase above the normal governed (power applied) Nr, even in straight flight. Once the Nr is above the normal governed band (265 +7 = 272? Sorry, it's 7 years since I flew it)I would be surprised if there is little, if any, effect because the freewheels WILL probably be disengaged. I certainly didn't notice any great difference in my time on the beast, throttles forward or retarded.

BTW, the centrifugal clutch on a Gazelle or the like does NOT disengage in a PFL / autorotation unless the throttle is retarded towards the ground idle position. The separate freewheel does disengage though; it's situated between the gearbox and the centrifugal clutch.

Just to expand the discussion further, who agrees that rate of descent is LESS in autorotation for an aircraft at higher AUM, not MORE as might be expected?

[212man]

Multip, thanks for your comments, and I was not aware that a 330 had no N2 indications (strange the French, they don't believe in torque either). I was simply trying to clarify what I felt might be misleading comments for the person who originaly asked the question. Glad we agree.

Shy Torque, are we talking about RoD for a given Nr? ie with the collective raised for the heavier a/c?

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Another day in paradise

[ShyTorque]

212,

Yes, the Nr within its normal limits for autorotation.

ShyT.

[rotorque]

I am interested in where this will lead, so I will be the one to bite.

The rate of descent DOES increase with an increase in weight. It is the same as putting water in the wings of gliders to make them go faster. The glide distance will stay the same but the speed increases.

Another way of saying it is, the heavier the machine the higher the collective (pitch) has to be to stop the RRPM from going above it's limits in an Auto. Rate of descent is increased, more airflow through the disc, higher RPM, etc etc etc.

It is also the reason that RRPM graphs take into account, not only ambient conditions, but the weight of the machine as well.

[Multp]

ShyTorque:
Thanks for your comments. I was, however, referring to the composite bladed SA330 and can assure you that my comments are based on experience: the NR will be lower with both engines shut down. If you want further detail, perhaps you may like to email me. (I wouldn't want to compromise my identity any further in this forum!)
I do take your point about the freewheel and mechanical clutch in the Gazelle. If memory serves me correctly, I did say in my earlier post 'in an auto to EOL with the Ng reduced to idle' (which means that the throttle must have been retarded: sorry, should have made this clear)the (centrifugal) clutch would be disengaged.
Incidentally, general question,how long do people think you have to be in a power off descent to be fully established in autorotation?

[This message has been edited by Multp (edited 20 March 2001).]

[far cue]

Guys,

In most normal turbine helicopters (I mean those which show N2/Np), IF THE Nr is > Np or N2, then what is going on within the transmission is irrelevant, because the aircraft is autorotating, ie airflow, and nothing else is driving the rotor.

Make sense?

Cheers

[ShyTorque]

Rotorque,

Well, the steady autorotative ROD (for the same constant Nr and airspeed) will DECREASE for an increase in weight. It sounds highly unlikely but it's true.

The power required to drive the rotor system in autorotation comes from the potential energy of the aircraft, ie. Mass x Altitude, or the energy expended to get it up to altitude against the force of gravity. For a higher aircraft weight at the same altitude there is more potential energy in the system.

For example, an aircraft that is loaded to be twice as heavy will have twice as much potential energy as before at the same height, but the power required to fly will be less than twice as much as before. This is because the extra power required is an increase in induced power only, profile, parasite and "installation" power will not change significantly. The increase in potential energy will support the aircraft for longer than before so that the steady ROD will be less than for the lighter aircraft.

This would cease to be true if the weight increase overloaded the rotor to the point where blade stall began to occur. Also, the heavier aircraft has more inertia and so will require more flare to stop it at the bottom of the auto!

(Edit: Whoops! There's an "r" in gravity).

[This message has been edited by ShyTorque (edited 21 March 2001).]

[ShyTorque]

Multp,

I take it you are ex Boscombe Handling Sqn (or French exchange) and took part in the EOL trials?

If what you say is true about the aircraft autorotating with a higher ROD with the engines shut down, it could only be because the Turmo 3ClV engines have such a high residual thrust at ground idle that the freewheels don't disengage. I certainly accept that this is possible at FLIGHT idle due to the appalling acceleration response of this type of engine - they always were set a bit high! With both throttles back to ground idle I never noticed any great difference and I didn't do engine offs for a valid comparison as they are obviously prohibited - even the RAF test pilot took the tail rotor off on the runway! Hey wait a minute! You're not ____ are you?

Difficult to prove one way or the other without N2 needles to give a clue.

[Multp]

Shy:
You are obviuosly as your name implies! Why don't you get yourself a PPrune e-mail address?
Your supposition in para 2 is correct, I think. But even at Ground Idle there is sufficient residual thrust to make a noticeable difference to NR between that and the NR with engines shut down.
Incidentally, for the benefit of those not in the know, quite a few successul EOL's were made, I believe, in the course of the Boscombe Down trials on the composite blades. The EOLs were part of a wider investigation of autorotational handling, which involved upper-air twin engine shutdowns, handling qualities investigations, then relights prior to landing.(All within safe distance of the airfield!)
Of the unfortunate incident you mention, it was the then american exchange test pilot who was the Pilot Flying , although the aircraft commander was a Brit.
As for the rest of your post, I could not possibly comment. But I do know the individual you are thinking of.

[ShyTorque]

Multp,

Sorry, don't have an email address available for reasons of once bitten twice shy! (Pun intended).

We will know each other, I am sure and probably flew together. I think I have narrowed your good self down to one of 2 people.

[rotorque]

ShyTorque,

Believe it or not, I followed what you posted reasonably well. Not only that but it makes sense, although I have a lot of trouble convincing myself that it is true.

Going back to the glider analogy, when the all up weight of the aircraft is icreased the potential energy gained after getting it to altitude is used as an increase in airspeed (kinetic). The aircraft will travel faster towards the ground, therefore it has a higher rate of descent for the same glide profile (same angle).

I have used this thinking for years, so I am fairly pi55ed that you have just ruined it for me.

What if I said .............

I am pretty sure that as a result of an increase in weight we find that there is an increase in Nr. This, I am assuming, is a given. We have Auto RPM graphs that tell us when we are heavy we are allowed higher Nr, for rigging purposes.

To get the increase in Nr we must have an increase in Rate of Descent in order to get the increase in the airflow coming up through the disc. It can't be done any other way for the same airspeed.

Sounded good hey?

cheers

[ShyTorque]

Rotorque,

Yes, the Nr would tend to increase so as 212Man asked, the Nr would need to be kept constant by raising the lever and the airspeed would need to be kept the same for "all else to be equal" as it were.

Increased Nr would cause more energy to be dissipated because of an increase in drag (increase in rotational airflow squared). That would mean that the ROD would increase.

Complicated, isn't it?

[Weight and Balance]

An interesting conversation! For what it's worth, when collecting data to build flight manual data on autorotation, it is essential that NR and N2 be split. As several posters pointed out, this insures that no engine power is making it to the rotor system.

This usually required milking the power lever(s) back towards Ground Idle, particularly at low weights. This was one maneuver where being a left handed FTE paid off. Sitting in the left seat, I could keep the right hand on the levers, and still make notes with my left. I never removed that right hand till NR and N2 matched up again, the torque meter started to rise, and the rate of descent decreased.

By the way, this was all done on experimental flight permits, and is probably outside of normal ops for certified machines. Kids, don't try this at home: unless, like me, you are lucky enough to be sitting beside the best helicopter pilots in the world.