Centrifugal clutch in a helicopter transmission

[SPS]

Hello to you all.

I'm new to this forum although I've been reading for a week or two with great interest.

I teach PPL H, ground school P of F and
Helicopter tech.

An examination question has been giving myself and my students trouble for some time now, and I'd like to see if anyone has an opinion to share on it.

The question appears in both P of F and tech.
papers (CPL) here in NZ. It goes like so;

What is the main function of the Centrifugal Clutch(CC)in a Helicopter?

Aside from the two silly choices (it disconnects the tail rotor or ejects the passenger(s)and spins the Xmas tree round)your choice is down to two others, (A) To off-load the engine at low RPM or (B) To disengage the engine from the main rotor should the engine fail.

Easy eh?

I have problems with it.

Firstly it seems hardly even relevant in this day and age (I only know of the Bell 47
having such a system although my knowledge of older types is limited) but I accept that it is within mechanical understanding of the Heli so I leave that aside.

Secondly, it's the word 'MAIN' That I don't like. On the ground (A) is clearly the function of any type of clutch (and it does not have to be a piston engined Heli. either, eg.Gazelle)

On the ground we have no primary interest in disengagement, hopefully our engine is to get further than that!

Of course, the CC has the virtue of disengaging the engine from the rotor system
if engine RPM become less than RRPM and in that way it can aslo perform the task more usually trusted to a freewheel device (be that a sprag clutch or another type)so it seems that the CC has TWO definate functions. (Especially if no other type of freewheel device has been fitted)

Going back to that word 'MAIN'.....

Which is it?

For me, on the ground, it has to be (A) as (B) is of little interest until the aircraft is flying.

I think that the designer chose to fit this type of clutch to the B47 (and others before it) when at the early stage, eg. 'How to start the engine without X inertia of rotor system mass stopping it doing so".

That choice must have been an easy one back then. It was easier to manufacture (small , high powered electric motors were not as easy to produce)no control levers or similar to be fitted in the pilot's reach, etc. etc.
The CC is automatic, reliable and has a wonderful extra benefit- It can disconnect the rotor if the engine stops.

However, in the air none of us would want to fly a Helicopter with no freewheel device included in the drivetrain and (B) must be awarded 'MAIN'.

So when is the main main? On the ground (where the Heli must spend most of its existence) or in the air (when the greatest need for engine failure management is plain)?

When I was going through my written exams, and now I watch students do the same, (sometimes struggling like I did now and then) I have to say that I become pretty fed up with
exams being more about comprehending or understanding one person's use of English than knowing your subject back to front.

I beleive that questions such as this are ambiguous and unhelpful to either side. What we should be interested in is whether the candidate knows what must be know. Trickery proves little than they may become better at crosswords after being forced to think in this manner. It has nothing to do with what
Bloggs (the candidate) will do if trouble looms one day (and I hope it does not).

If a Pilot has demonstrated an ability to guess correctly (passing the exam) has his problem solving ability been improved by this process? It certainly has not performed the original purpose (if I have this right)
which is to inssit that he should know 70% or 80% of a finite amount of material and prove this by passing an exam.

I can't help thinking that the question would be fairer and morev effective if it were worded slightly different, eg "What is the main function of the CC IN THE AIR"

Anybody have any comments to make?

I'd be very pleased to read them.

Thanks for your time.

SPS <A HREF="http://www.helicopterpilotsguide.com" TARGET="_blank">www.helicopterpilotsguide.com</A>

[Lu Zuckerman]

To: SPS

Re Centrifugal clutch.

Answer A as applied to Bell clutches would be incorrect. As applied to the centrifugal clutches used by Sikorsky answer A would be correct. Answer B is incorrect for both applications

There are several types. On the original design of what is now known as the Model 47 the clutch was made up of three spring loaded shoes that looked very much like brake shoes. The shoes were spring loaded to the disconnected position. When the engine was started, and it was below X rpm the shoes remained retracted and the rotor was not rotating. As the pilot raised the rpm to a level where the centrifugal force overcame the spring load the shoes would pivot outward and make contact with the clutch drum which resembled a brake drum.

Since this entire assembly was bathed in oil the engine really had to be reved up to make solid contact. As the rotor was spun up the amount of inertia that had to be overcome decreased and the clutch was in a solid drive lock. I don't remember the exact reason but back around the early fifties they took out the springs and the clutch was engaged but slipping just as the engine was fired up and the rotor started turning immediatly. The last Bell model that had this type of clutch was the J2 Ranger. After that, with the advent of the free turbine engines they no longer needed the centrifugal clutch.

On Sikorsky helicopters (HO3-S and HO4-S (S-55 A Model they had a different centrifugal clutch design that had multiple spring loaded shoes that were thrown outward against the spring load making contact with the clutch drum when the engine exceeded 1300 RPM +/-. After that the Sikorsky clutch and the Bell clutch worked the same.

With the advent of the S55-B model Sikorsky went to a new design or, the Hydro Mechanical Clutch. This was used on the S-55, S-56 and the S-58. After that, they went free turbines and the mechanical and hydromechanical clutches were no longer used.

The unit that allowed disconnection of the power line (engine to transmission) is called by any of several names. Overrunning clutch, Sprague Clutch or a Free Wheeling unit. If you want a description on how this unit works just make a post requesting it.

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The Cat

[This message has been edited by Lu Zuckerman (edited 15 December 2000).]

[This message has been edited by Lu Zuckerman (edited 15 December 2000).]

[mrfish]

the question is truely *****.

sadly asl exam questions are full of ambiguous questions such as this!

i would, however, have to say that 'main' would bear some relation to flight safety, in this case to autorotate....and therefore go with (b).

the question would be better phrased "what is the primary purpose of a free-wheel unit, in flight"

what part of nz are you in?

[Lu Zuckerman]

To: mrfish

In the case of the Bell centrifugal clutch the engine must be stopped in order for the clutch to stop powering the rotor. However in the case of autorotation there is a freewheeling unit between the clutch and the gear box. So even if the engine is still running and the clutch is still engaged the freewheeling unit will disengage if the rotor rpm exceeds the engine rpm.

Getting back to the question, the MAIN purpose of the centrifugal clutch is to allow the engine to idle in an unloaded condition. This is true for a Sikorsky helicopter but not true for the Bell helicopter. It was true for the Bell at one time when the clutch shoes were spring loaded but when they removed the springs answer A would be incorrect for the Bell.

I WOULD SUGGEST THAT THE QUESTION BE MADE HELICOPTER SPECIFIC OR IT BE REMOVED FROM THE TEST.

------------------
The Cat

[This message has been edited by Lu Zuckerman (edited 15 December 2000).]

[Skycop]

Surely answer B is not strictly correct.

Once a centrifugal clutch is fully engaged by accelerating the engine there is no relative movement between the input and output. For the engine to subsequently disengage in flight there must be a reduction in clutch RPM so that the springs can overcome the centrifugal force and allow the two halves to disengage. If the two are locked together by friction, as advertised, there must also be a separate freewheel device for this to occur. If there is no freewheel device then the rotor rpm would also have to decrease markedly before the springs could "pull in" the shoes.

[SPS]

Well I'm very pleased with the quick response and help that I'm getting!
Thank you to all.


The more info I read the worse this question becomes it seems. Did the '47 have a freewheel device included (of a sort) as well as the CC? If that is so the whole question is ermmm...questionable!

I think some clarity must be applied
when setting a question.

Maybe these two issues should be separated, something which can be done quite simply if both devices are fitted. If the freewheel device is fitted, allowing 'freewheeling'in the event of ERPM being less than RRPM whilst the CC allows the engine to be started without the load of rotor mass against it, then why should the two be mixed up? They are nothing to do with each other save being related in a drivetrain.


Mrfish, I'm UK but now a JAFA
(just another friendly Aucklander!)I'm at North shore.

Regards SPS

[Lu Zuckerman]

To: SPS

If the question is what is the main purpose of a centrifugal clutch in a helicopter the answer could be said that it serves the same purpose as the centrifugal clutch in a Cushman Motor Scooter or for the European types the Vespa. It allows the engine to idle and not be engaged to the driveline.

But from there on it dims. In the original design of the Bell 47 the case stated above was true and the same for the Sikorsky designs. It allowed the engine to run at a specified idle RPM without being engaged to the driveline. However later on they removed the springs from the B-47 clutch and as a result the engine was always engaged to the transmission via the clutch. The drive may not have been a hard friction lock but it was still in a lock condition.

In both helicopter designs there was a free wheeling unit that allowed disengagement if the engine RPM dropped lower that the rotor RPM.

In an earlier post I suggested that the question be helicopter specific or it should be eliminated. It seems from the other responses there should be two questions.

1) What is the purpose of the centrifugal clutch in early Sikorsky and very early Bell helicopters

And,

2) What is the purpose of the free wheeling unit in a helicopter transmission?


------------------
The Cat

[sprocket]

SPS I'd put my money on "(A) To off-load the engine at low RPM". You probably would not be able to start the engine without one. That is where I think the word Main comes into play in your question.
I believe when Mr Young was developing the Bell47 the clutch was introduced for that very reason. Another helicopter with a 'CC" is the Llama and it has a turbine engine. I think it is also for ease of starting too because the engine does not have a "Free power turbine".


[This message has been edited by sprocket (edited 15 December 2000).]

[Lu Zuckerman]

To: Sproket

On the early Bell 47 models when they took out the springs from the centrifugal clutch as soon as the engine fired the rotor started turning. This caused a problem when the helicopter was operating off of a ship.. In those days, they did not have centrifugal locks on the rotor head so when the ship rolled or pitched the blades would start to move on the teeter bearing.

I served aboard a Coast Guard Icebreaker with a Bell HTL-1 which was the very first military Bell. Ours was mounted on floats and when the pilot was getting ready to start the engine I had to hold the blade steady with the tips of my fingers. If I didn't pull my hand back at the instant the engine started I would get a very hard slap as the second blade came around. It was that fast.

We also had a Sikorsky S-51 (HO3-S) which had tricycle gear. When the ship was rolling and pitching the blades would flap up and down (Flexing) and when the engine was started and power was applied the blades would start rotating very slowly and we always feared ground resonnance.

------------------
The Cat

[This message has been edited by Lu Zuckerman (edited 15 December 2000).]

[skidbiter]

TO: MrFish

I am sorry to burst your bubble, but JAFA does not stand for 'Just another friendly Aucklander', try 'Just another f**ken Aucklander'

Kindest Regards

Skidbiter

[SPS]

To skibbiter

HA! I wonderered if the real meaning of JAFA
would come out!

I haven't been here long but I'm learning...

For the uninitiated skidbiter is correct, it's a term used by many non-Aucklanders.
With that, you do not hear it within Auckland realms very often (but I've lived elsewhere in NZ).

Much more common within Auckland is the term
"JAFWe"......

OK, fun over, back to the thread.

I'll be writing very soon but for now the exam question is not looking as easy or well
founded as it might first have seemed...?

[mrfish]

To the cat:

very informative.....thanks

[SPS]

To all,

OK, it seems pretty conclusive.

Taking Lu's input and advice on the two different types of CC for a start, (no pun!) and the way they differ in functon the question appears to be ill founded.

The question has not been made type specific and 'A' cannot be held to be correct in such a generalised way.

Moving on, it also seems clear that it was normal for a freewheel device to be fitted
IN ADDITION to the CC. Again, as has been said, 'B' cannot therefore be correct.

If that is so (and I do not doubt it) the CC has no part to play in disconnection of a failed engine. If it has any part to play at all it still cannot be labelled a 'MAIN' functon of the CC.

This was roughly my train of thought when I took my own CPL (H) Tech exam last month.
I followed a similar train of thought expressed by Mrfish.

I spend too much time thinking it through and finally went for 'B' as I felt the question was telling me the CC had two functions and 'B' was the MAIN as disconnection in the air would be paramount.

Guess what ?

The answer was marked WRONG!]

But then 'A' cannot be correct either....

Maybe I should have opted for 'disocnnects the tail rotor' or 'ejects passengers and spins the Xmas tree round' after all!

Thanks and have a nice Holiday.

SPS

[RW-1]

It should be changed. The US tests have two separate questions, one covering the clutch, the other covering a freewheeling unit.

No confusion imposed for either.

------------------
Marc

[This message has been edited by RW-1 (edited 17 December 2000).]

[sprocket]

It does appear to be an outdated question.

The 47G5 MM says there are two uses for the CC.1: To unload eng/rotor during starting. 2: To enable free turning of the rotor by hand when shut down (obviously due to piston eng. compression)

This question was probably written when the 47 was the only helicopter around.
But I still don't understand how you can relate the FWU to the Centrifugal Clutch though. The FWU's I've come across dont rely on or use centrifugal force to engage.
They use direction of rotation,spring pressure and friction.



[This message has been edited by sprocket (edited 17 December 2000).]

[Lu Zuckerman]

To: Sprocket

I hate to rain on your parade but the Sikorsky Hydromechanical Clutches I spoke of in a previous post on this thread use centrifugal force to engage the free wheeling unit. It's a bit difficult to explain but centrifugal force on a set of flyweights provides the mechanical force to engage the free wheeling unit.

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The Cat

[This message has been edited by Lu Zuckerman (edited 18 December 2000).]

[sprocket]

:)...I knew as I posted that this would happen.
Thanks for the enlightenment Lu.

[sparecrew]

As SPS mentioned at the beginning, the Gazelle has a centrifugal clutch using a ramp and roller arrangement that also funtions as a freewheel so both A and B would be correct. As with the CAA it seems that knowledege of the subject runs a poor second to being able to RTFQ!

[SPS]

Sparecrew (and all)

Thanks for that. It helps me to be more accurate (I should be always striving for that)and through that it helps others.

I will be challenging the validity of that question, (amongst others, see "which height is right"). I invite the authority to read this thread.

It should not be a guessing game or depend on which book you read. It shoud be down to FACT.

Mind you, in the case of the Q. on the CC,
I doubt if any but the deepest stuff would have helped,It would only serve the question unanswerable.

And maybe it would have put of the 'guesser' who managed to drop his pen on what the regard as the correct choice! (Not ME!)

And I have to ask......No, maybe not....
got the FQ part......!

[ShyTorque]

Sparecrew,

In fact the Gazelle has a separate "dry shoe" centrifugal clutch. It sits between the torquemeter and the "ramp and roller" freewheel unit.

They have adjacent housings but are totally independent units.