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Old 07-31-2016, 08:48 PM   #1
hitechflyer
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Tank and clunk


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Never thought the clunk was missing fuel.
Here's a link to a video I made of the tank in flight,
When the plane go's for a down line you can see the fuel drop to the front of the tank. I started with a quarter tank and you can see the clunk at the back of the tank, after the down line is had a dead stick.
https://youtu.be/FgP3vYvc6Gs
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Last edited by hitechflyer; 07-31-2016 at 09:10 PM.
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Old 08-01-2016, 06:56 AM   #2
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Re: Tank and clunk

That is pretty normal behavior. However, the length and rather narrow diameter would make it difficult for a clunk on a flexible enough line to flip forward to stay in the fuel. Besides, in your case that pickup line seems pretty rigid. Even on the down line the clunk barely moves.

There may be a couple of things to try. If flexible enough line is not available that would allow the clunk to flip to the stopper end wonder if a slightly longer line between the tank and carb inlet may hold enough residual fuel to keep it running.


There also is the option to find a small tank and build a header. This may be simply moving the issue to another tank as I cannot think of a 100% guarantee some maneuver would not uncover the clunk in that tank and the possible shorter line to the carb could actually make it worse.
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Old 08-01-2016, 07:43 AM   #3
Mike Sebastien
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Re: Tank and clunk

Header tank is a good idea except how do u keep it full? Used a header tank on a helicopter and I had to use muffler pressure to pressurize the main tank which keep the header tank full. Don't have that option with a gas engine.

The issue I believe is the geometry of your tank. It is long and narrow letting the gas uncover the clunk for longer periods. Going to a short and wide tank will keep the clunk in the fuel for a longer period of time. Also adding more fuel line or larger diameter fuel line will create a bigger reservoir of fuel. Shorten your clunk line in the tank may help also.

These are just ideas and could be a combination of things that solves the issue.
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Old 08-01-2016, 08:14 AM   #4
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Re: Tank and clunk

We use to be able to get tanks with a balloon type unit attached to the vent line inside the tank. You had to bleed the air from the fuel side, then there was no chance of getting an air bubble. If you get one reasonable size air bubble in the fuel line the glow plug goes out.

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Old 08-01-2016, 03:26 PM   #5
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Re: Tank and clunk

Was that a normal down line say after a spin or was it loooooong?
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Old 08-01-2016, 03:48 PM   #6
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Re: Tank and clunk

The jet turbine guys use a UAT tank which is basically a header tank , some with a pleated paper filter similar to the older carbed vehicle engines. I believe a felt filter will do the same thing. When filling the main tank the fresh fuel goes thru the UAT or header tank , fills it and moves any captured air back into the main tank and out the vent so ending up with a full header tank. The small pleated filter is rigidly located in the middle of the UAT/ header tank and thus tank is held at a 45 degree angle. The purpose of this header tank is to filter out any air bubbles or air dissolved in the fuel. As fuel is sucked up from the main tank there is the odd occasion when the clunk may suck air and thus is held in the UAT/ header tank. Of course , by the end of the flight , the level of the fuel in the UAT tank may drop but at least you get good fuel to the engine with no air bubbles as that is a no go with most turbines and a flame-out results.

Don't know how well this will work with straight suction fuel carbs but most walbro's have an internal pulse pump and that should work and there us also muffler pressure may help a bit

Hope this might help.
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Old 08-01-2016, 08:04 PM   #7
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Re: Tank and clunk

Thanks for all the info guy's. I never had a dead stick for this reason until I did this test with the camera rolling.
But as Frank mentioned, I got some good hight and came straight down at idle.
But I will improve my setup because I don't like to take chances.
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Old 08-01-2016, 10:03 PM   #8
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Re: Tank and clunk

Using a 'felt' clunk would give you a reservoir of fuel from which the motor is able to draw from during those times when a normal clunk will be exposed to the air.

I know, I know many folks are dead set against them because of beliefs (and perhaps actual experiences) of fouling but I've used them for more years than I want to admit and haven't had a dead stick or any fouling issue.

Here's a link to a thread on FG where clunks were discussed with some interesting results http://www.flyinggiants.com/forums/s...d.php?t=193734
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Old 08-02-2016, 06:17 AM   #9
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Re: Tank and clunk

Hank, check out Rotoflow fuel systems.
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Old 08-02-2016, 08:40 AM   #10
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Re: Tank and clunk

That system will help for rolling, but not dives, the clunk is still fixed at the far end of the tank. it will not come forward in a dive, it will still starve as frank and paul have experienced.
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