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How far into the fuel tank should the clunk go to?

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How far into the fuel tank should the clunk go to?

Old 03-13-2007, 09:31 AM
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skaap2k
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Default How far into the fuel tank should the clunk go to?

Hi,

I know this may be a silly question, however I am a beginner, I have my fuel tank assembly (it did not come assembled) and I was wondering how far into the tank the clunk should extend into... first thoughts were that it should go as far in as possible, then thought set in, and made me wonder if it should sit around halfway or 3/4 of the way into the tank as it would have more access to fuel while it is tilted up or down during flight?

I understand the clunk can move around inside, but in a fairly small fuel tank, how well does it actually move?? In my dry tank it doesnt seem to move enough if the tank is pointing down...

Could someone with some experience let me know what their views on this are?

Thanks!

RN
Old 03-13-2007, 09:51 AM
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piper_chuck
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Default RE: How far into the fuel tank should the clunk go to?

It needs to go as far back in the tank as it can, but not so far that it hits the back wall of the tank. With a small tank, it's important to be sure the clunk line isn't too stiff, this can prevent it from reaching the far corners of the tank. Also, it's common to use a short piece of brass tubing in the clunk line. This helps prevent the clunk from going to the front of the tank on a hard impact.
Old 03-13-2007, 09:55 AM
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RCKen
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Default RE: How far into the fuel tank should the clunk go to?

The clunk should go back to the bottom rear of the tank. But you need to be careful to not push it in too much. It should not contact the back of the tank. Once the clunk is installed you need to be able to turn the tank over and have the clunk fall freely to the top (now bottom after turning over) of the tank. This will ensure that you can pick up fuel no matter what orientation the tank is in. Another thing that I do when building a tank is to put a piece of stiff tubing (brass or aluminum) in the middle of the fuel pickup line. This will keep the clunk from falling to the front of the tank and getting stuck there.

Hope this helps

Ken
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Old 03-13-2007, 10:23 AM
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skaap2k
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Default RE: How far into the fuel tank should the clunk go to?

Thank you for your quick advice, it is very appreciated and I will do that with my fuel tank.

However, to be devils advocate, if the fuel tank was only 1/4 fuel, and the tank was angled down at 30-45degrees, I would not imagine that the clunk would fall, and therefore stop picking up fuel? perhaps in my situation the tubing may be a little stiff for the tank, but I am using the provided equipment. Perhaps with exposure to fuel over time the inner surface & fuel line gets more "slippery" & flexible respectively?
Old 03-13-2007, 10:29 AM
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Default RE: How far into the fuel tank should the clunk go to?

How long do you expect the tank to be angled down at 45 degrees? Usually not long enough for the fuel line to go dry. Your more concerned that the clunk not get sucked against the rear wall of the fuel tank in level flight or while on an up angle. You do not want the clunk to go to the front of the tank. It will stick there, and you will deadstick.

I usually set my tanks up to give between 1/4 and 1/8 inch clearance against the rear of the tank with the tank positioned vertically. That will allow for some stretching of the fuel line. I can usually suck the tanks dry with this configuration.

Brad
Old 03-13-2007, 10:59 AM
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RichD
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Default RE: How far into the fuel tank should the clunk go to?

now I feel like an idiot. I never thought about the brass tubing in the middle of the line to prevent "stuck clunk" syndrom. That would have save a lot of deadsticks on my magic and extra 260. Some of us not so beginners still learn a lot here ! thanks !!!!
Old 03-13-2007, 11:01 AM
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skaap2k
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Default RE: How far into the fuel tank should the clunk go to?

Interesting...

As I say, I have very little experience of this, and it was just a theoretical thought...

Thank you for your advice much appreciated!

RN
Old 07-24-2009, 05:45 PM
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Default RE: How far into the fuel tank should the clunk go to?

I'm glad I came back. I was having a problem with my engine (OS91) sucking air bubbles in the fuel, so I pulled the tank and am replacing all the fuel lines, even inside on the clunk. Looking at this configuration, how does the engine pull fuel if the plane is nose down and the clunk does not fall to the front of the tank?. Someone previously said there should be enough in the line until you level out, but hasn't there been air drwn into the line, which could cause the engine to quit? What am I missing?
Old 07-24-2009, 08:38 PM
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Default RE: How far into the fuel tank should the clunk go to?

skaap2k and golfdaddy720,
The momentun of the plane will force the fuel to be pushed to the rear of the tank. That's what keeps the clunk submerged in fuel.

However, I too, had never heard of the trick of putting a stiff section in the clunk line. Thanks, RCKen.

EJ
Old 07-24-2009, 08:42 PM
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Default RE: How far into the fuel tank should the clunk go to?


ORIGINAL: N429EM

However, I too, had never heard of the trick of putting a stiff section in the clunk line. Thanks, RCKen.

EJ
EJ,
You are welcome. But Ihave to give credit where credit is due.I got it from Minnflyer.

Ken
Old 07-25-2009, 01:10 AM
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jimmyjames213
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Default RE: How far into the fuel tank should the clunk go to?


ORIGINAL: N429EM

skaap2k and golfdaddy720,
The momentun of the plane will force the fuel to be pushed to the rear of the tank. That's what keeps the clunk submerged in fuel.

However, I too, had never heard of the trick of putting a stiff section in the clunk line. Thanks, RCKen.

EJ
nope momentum doesnt keep fuel at the top, their is a youtube video of someone flying while videotaping his tank (cant seem to find it). the clunk is in the fuel less than you think it would be.
it is the fuel in the line that keeps the engine going while in a dive. you would be surprised how long the engine would run on just the fuel tank line. if you feel like it, start the engine with less than a half tank, hold the plane down (dont chop off your legs) and see how long it will run before it dies. if you dont understand how the engine could run once the cluck was back in the tank because of the air in the line try this. suck all the fuel from the tank, but not to the point where you are also sucking air, then start the engine and leave it at idle. you should see fuel come through the line, but not in one contunious stream, it will be in sections. once the section burns off it quickly jumps to the next section of fuel in the line dispite the fact their is air inbetween.
i would guess at idle you have 20+sec of fuel in the line, your plane would have to be a dot in the sky to dive strait down for 20+ seconds. a few days ago while tuneing my ax55 the fuel line came off the engine at WOT. it ran for a good 5 seconds before i managed to get the fuel line back on and the engine kept running.
Old 07-25-2009, 05:16 AM
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Default RE: How far into the fuel tank should the clunk go to?


ORIGINAL: jimmyjames213


ORIGINAL: N429EM

skaap2k and golfdaddy720,
The momentun of the plane will force the fuel to be pushed to the rear of the tank. That's what keeps the clunk submerged in fuel.

However, I too, had never heard of the trick of putting a stiff section in the clunk line. Thanks, RCKen.

EJ
nope momentum doesnt keep fuel at the top, their is a youtube video of someone flying while videotaping his tank (cant seem to find it). the clunk is in the fuel less than you think it would be.
it is the fuel in the line that keeps the engine going while in a dive. you would be surprised how long the engine would run on just the fuel tank line. if you feel like it, start the engine with less than a half tank, hold the plane down (dont chop off your legs) and see how long it will run before it dies. if you dont understand how the engine could run once the cluck was back in the tank because of the air in the line try this. suck all the fuel from the tank, but not to the point where you are also sucking air, then start the engine and leave it at idle. you should see fuel come through the line, but not in one contunious stream, it will be in sections. once the section burns off it quickly jumps to the next section of fuel in the line dispite the fact their is air inbetween.
i would guess at idle you have 20+sec of fuel in the line, your plane would have to be a dot in the sky to dive strait down for 20+ seconds. a few days ago while tuneing my ax55 the fuel line came off the engine at WOT. it ran for a good 5 seconds before i managed to get the fuel line back on and the engine kept running.
Here is the video you are probably looking for.

http://richter-lackierung.de/aktuali...amefuntana.wmv

Old 07-25-2009, 10:26 AM
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Default RE: How far into the fuel tank should the clunk go to?


ORIGINAL: RichD
Some of us not so beginners still learn a lot here ! thanks !!!!
Ive been learning for the past 61 years. When I stop learning push me over and bury me. Im dead and dont know it.
Old 07-25-2009, 01:00 PM
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Default RE: How far into the fuel tank should the clunk go to?

I've had trouble with engine starving on the down leg maneuvers when the clunk was placed all the way at the back of the tank. So I started locating my clunks at about 2/3 to 3/4 of the way back.
Old 07-25-2009, 07:23 PM
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Default RE: How far into the fuel tank should the clunk go to?

For small size tanks with light weight clunks you may need a more flexible tube.

Try this http://www.readyheli.com/Hayes_clunk...p/hayq0249.htm
Old 07-25-2009, 07:48 PM
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Default RE: How far into the fuel tank should the clunk go to?

KORPS...Thats a great video! Looks like that engine runs on wine! [sm=tongue_smile.gif]
Old 07-26-2009, 07:57 AM
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jester_s1
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Default RE: How far into the fuel tank should the clunk go to?

Some of the more intense aerobatic planes do have trouble with fuel supply due to all the negative G's they experience. They use a hopper tank to fix it. But for normal flight the airplane doesn't decelerate in dives that much, so it's just not a problem.
Old 08-05-2009, 03:22 PM
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Default RE: How far into the fuel tank should the clunk go to?

Thanks people! Just what I needed. And that video is a real eye opener!
Old 08-05-2009, 04:04 PM
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Default RE: How far into the fuel tank should the clunk go to?


ORIGINAL: bkdavy


I usually set my tanks up to give between 1/4 and 1/8 inch clearance against the rear of the tank with the tank positioned vertically. That will allow for some stretching of the fuel line.

Brad
Yes VERY GOOD advice.

I did not allow for fuel line stretching on hard maneouvers and learned about it the hard way!

The clunk "gets heavy" during high "G"'s and it can stick to the back leaning out the mixture, long enough to kill the engine.

I went crazy looking for the cause, and solved it by keeping the clunk 1/4" off the rear.
Old 08-05-2009, 05:30 PM
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Default RE: How far into the fuel tank should the clunk go to?

ORIGINAL: RCKen


ORIGINAL: N429EM

However, I too, had never heard of the trick of putting a stiff section in the clunk line. Thanks, RCKen.

EJ
EJ,
You are welcome. But I have to give credit where credit is due. I got it from Minnflyer.



Like RCKen says, lets "give credit where credit is due".......I believe that Piper Chuck first mentioned putting the brass tubing in the clunk line (in this thread) Check post #2. Thanks Piper Chuck.....good idea

Lm


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