While replacing the clutch on my Raptor, I am trying to understand the principle on which this works.
When the RPM of the engine is below a certain level ( idling ) the clutch disengages
but when the RPM increases, the clutch engages the clutch bell.

How does this happen ? Also, wouldnt there always be energy loss, due to slipping, and generate heat ?

Thanks guys for helping me understand this.
-Gaurav

The clutch shoes (both sides of it) expands out when the rpm increases, and begins to "hang on" to the liner.  Above idle rpm, they rotate together, so there is no slipping and little heat in the normal operation.

Ok, so ideally there shouldnt be any slipping at any time be it Idle or Full Throttle.

On my Raptor, the clutch bell was getting hot from the first day/flight. That indicates slipping. I guess i will need to get a Dial gauge to check if the clutch bell is true or not, and then replace the clutch.. which now has failed.

Hi,

Being honest, I have no experience with Heli clutches. But I am going by the principle of the functioning of a clutch.

The clutch shoes are held together with a circular spring. (you may notice this)
The shoes are pivoted at one end each (opposite to each other) (in case of a 2 shoe clutch)
As the RPM increases, because of the centrifugal force, the non-pivoted side of the shoe starts expanding and begins to hang on to the liner, inside the clutch bell. At this time, there may be some heat being produced and some splipping as well, but this duration is small and is usually negligible. As the RPM increases more, the shoes clings on to the liner more strongly and hence preventing any further slippage and generation of heat. At this time, it is said that the clutch is engaged fully.

In an ideal situation, at throttle idle, the clutch shoes should not be touching the liner of the bell. You would realize that this is due to the tension in the spring that holds the shoes together.

Now, coming to your questions.
1.) you are correct in saying that there should not be any slippages at idle and full throttle. (No slipping at idle - because of the tension in the spring) and (no slipping at full throttle, because of massive centrifugal force)
2.) As indicated by you, since your clutch bell was getting hot from day 1. If it is getting hot at idle speed (chances of which are less, as at this speed, the engine would rather die if loaded), then it indicates that the tension in the spring is way too less. But if it is the reverse case, then there is too much tension in the spring because of which the shoes are not able to expand and cling to the liner properly even at high RPM, hence causing slippage and building heat.

However, ther may be more reasons for heat generation, which may not be ruled out unless experts have their say.

Hope this helps.

Thanks Saurabh, that was a great explanation !

You are welcome dear

Haven't seen springs in the heli clutch shoe assemblies.  They look like this :

http://www.rcheliresource.com/new-lynx-heli-innovations-t-rex-600700-clutch-upgrades/

Both sides are connected to the middle of the mass using a small area, allowing it to flex and expand.

Yeah, but I guess the small area does the work of a spring ... Keeps the arms together when idling and let them expand when above a certain RPM. ...

Yes... what one needs to watch out for is the thickness of the liner, as they wear out, the shoes tend to "stay expanded", and when the liner gets too thin, the shoes break off.

Aah, now I see... from the pic, there are no springs, but because of the shape of the clutch shoe, it is apparent that the tension is due to the shape of the shoe rather than a spring (which is found in R/C car clutches).

The centrifugal force would throw the arms away, expanding the shoe and clinging to the liner.

Thanks Anwar !!!

Here is a good article explaing the working and tuning of an RC clutch. BTW, I was referring to this clutch....
http://www.nitrorc.com/articles/clutch.html