I have a Bobcat 50 chinese ARF ready to test fly with a Wren 44 conversion.

I fitted a felt clunk to the Dubro fuel tank but when I run the engine I get tiny bubbles from the feed line.

I'm not using an air trap.

I went hunting for the problem today & traced the problem to the clunk itself, even when fully immersed in fuel it releases tiny air bubbles.

Is this just the 'nature of the beast' with felt clunks? The engine does not seem to worry about the tiny bubbles at all, it runs perfectly at all throttle settings. - John.

it's because the clunk is too restrictive. ditch it and add a UAT.

Yeah, it's full of trapped air. Yer gotta squeeze, and squeeze, and squeeze the air out in a shallow pan of fuel. I don't like 'em either. The fibers are said to break off and clog things down stream. []

... zak

Those clunks will cause the fuel to cavitate and form bubbles. David Gladwin wrote about it a few issues ago in RCJI....

Hi John,

I am glad you found the problem. After seeing yours, I have just ordered a little bobcat.


Regards

Hi Tom, yep, found the problem. Now all I have to do is fix it or learn to live with it.

Craig & I are off to Newcastle tomorrow. You? - John.

There are different types of felt clunks, ones with a plastic body and the heavier metal ones.
My Flash came supplied with two of the heavier felt clunks. One came with the wing tank and the other was meant to be used in the hopper tank. I used a hopper tank with a pleated paper filter so I used the extra felt clunk in the main tank which is flat and otherwise impossible to get most of the fuel out.

I think that even these heavy felt clunks are way too restrictive so I drilled them out considerably to do away with any cavitation. The opening is now larger than the tubing attached to it. There is absolutely no air in my hopper tank at the end of a flight. Still there is no way I would fly without a hopper or UAT tank.

Thanks for the clues, Mark. Mine has a cast body with a felt sleeve over the top, the felt is retained by a washer which is held on by the tygon tube attached to the clunk. The holes through the clunk are quite large & the feed rate is very modest, the Wren 44 runs about 8 minutes on 50 oz Dubro tank.

The clunk has only recently returned to service after being dry for months. Do they take a while to get thoroughly wet through? - John.

Hi John,

I was going to do some work on my FB L39 tomorrow. I have had it for two weeks and not touched it. Where in newcastle?.

Regards

The COMSOA club at Metford.

Craig has an MW44 gold powered Skyhawk & I have the Bobcat both to be certified. - John.

John,
I'll see if I can find a picture of mine tomorrow to see if it's the same as yours. Mine is on a Falcon and draws much more fuel than the 44 with no bubbles.
It's almost 4AM here so I'm off to bed.

I stopped using felt clunks many years ago.
Cavitations and felt break offs are the main reason.
On the 44 try using a Jetcat clunk (sometimes known as an Orbit) or go to a chainsaw supplier / service and buy a chainsaw clunk, these use the same membrane as the JetCat clunk but come in different sizes.

Paul

And strangely enough , I had air bubbles coming from an Orbit clunk and hundreds of flights with plain felt clunks with no problems..

Marc

Wren seems to swear by the felt clunks too.... [sm=confused.gif]

I have also heard of cavitation with the Orbit clunk. Here is a pic of the cast felt clunk taken from the Flash manual. There is plenty of flow at the felt portion but the bore of the hole is small and I drilled that out considerably using the largest bit that would still leave some material on the sides.

A clunk cannot "make" bubbles, can it? Sure, in the first run some will bleed from a felt clunk, but man, that would be a SERIOUS restriction to cause actual cavitation!!

I HAVE, though, seen other junctions pull air INTO the system if there is a restriction in say, the vent. (Wonder how I found that out!) Try it as a diagnostic tool if you like: Plug the vent and see where air enters the line. You just might find junctions you SWORE were tight will now leak air!!!


I'm running a Wren SS with TWO FELTS in series (series tank feed) and no bubbles from the felts. Not one. Not much time on the engine but no other problems with the clunks either.

Cavitation on an orbit clunk is possible on larger engines.
I have used them with total success on Wren 44's and 54's.
It is all about restriction, if the flow is restricted through the element (of any filter) then the pressure between the filter and the pump drops.
This causes trapped gas in the fuel to release into the fuel line in the form of micro bubbles.
So with any filter that cannot pass the flow rate required, it is possible to get de-gassing or cavitation.

Paul

I have sent Sara from Wren an email about the problem so I'll post the answer when I recieve it.

It's 6 AM & time to get ready for test flying! [X(]

Like I said the engine runs perfectly anyway so I'll sort it when I get back.

If the engine stops in flight I cannot see any problems, I always come home with a model & have never had one stuck up in the sky so far. - John.

LMAO

Well John,

How did the Little Bobcat fly?

Regards

Hi Tommy, it flew fine (no fuel feed problems!). It's very fast with even the early 44 which are supposed to be 8 pounds thrust but the vertical performance with a 9 pound plane plus fuel weight makes me think the engine thrust rating is very conservative.

Must get some colour on it, grey primer is a little too 'stealthy' for my liking.

Craig was not so lucky, his Skyhawk crashed on it's second flight with what appears to be radio problems. He was downwind, gear down, approaching to land when he lost control. In the wreckage the gear was up & the fuel valve off (both failsafe settings) - John.

Hi John,
I have ordered the bigger 55" Keecat, as well as the 51" Bobcat.
Perhaps the bigger one would be better for my 44 gold. I don't know how solid it is or if it will be suitable for turbine conversion.
They should be here next week.

The Keecat cost A$215 shipped. The bobcat was A$205.

Should be here soon.

Regards

Tom

I think Dave Wilde has the record for number of flights with his mini Bobcat with the MW44 Gold. He has had 120 flights now with no structural issues. The only mod he made to his was to put a timber dowel inside the aluminium. His is one of the first into the UK and has no retracts. It is extremely light and weighs about 8.5 lbs dry. The 44 Gold has in excess of 10- lbs thrust so performance is sparkling. Dave flies his pretty well flat out now and the airframe looks as good as new as he has looked after it. As John said it is pretty quick and amazing performance for the price. The only downside with the mini Bobcat is the lack of flaps. Dave has a special mix he uses for landings.

I put flaps and retracts on my mini Bobcat, but it is heavier than Daves and does not have the same performance as a result. However landings with flaps are very slow and under good control.

The Keecat is very flimsy in comparison.

One person I know has a 44 gold in his Falcon 120 and that is a nice flying combination and really great as a trainer.

John

Hmmmmmmmmmmmm.

Logged on and clicked this link to read the new posts about FELT CLUNKS, and somehow got re-directed to a BOBCAT/SKYHAWK/KEEKAT/FALCON 120 thread ???

Darn computers...........

Good point we have drifted off topic..

Dave like me uses pleated paper filters as a clunk. They seem the best solution if you want to avoid UAT and save some weight. They have a larger surface area than any of the alternatives and do not seem to clog up as readily. We did use felt clunks in a one tank system, but the pleated paper filter is a much better solution. Dave has only had 2 or 3 flameouts in the 120 flights and then when running low on fuel.

John