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Author Topic: Plastic clunk tank uniflow set up  (Read 2231 times)

Offline John Skukalek

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Plastic clunk tank uniflow set up
« on: November 14, 2023, 07:51:07 AM »
For those of you who use a plastic clunk tank uniflow set up where the uniflow line is connected to the fuel pick up line, where does the uniflow line reside in relationship to the pick up line? Inboard of the pick up? Outboard of the pick up? 

Offline realSteveSmith

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Re: Plastic clunk tank uniflow set up
« Reply #1 on: November 14, 2023, 10:12:26 AM »
**EDIT**  Sorry John, I misread your question.  The *pdf attached is NOT for the 'set up were the uniflow line is connected to the fuel pickup line', as you asked.  It's for a fixed uniflow line with a moving clunk.  I'll leave it here in case it's helpful for someone anyway. 
« Last Edit: November 14, 2023, 02:06:37 PM by realSteveSmith »
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Offline Dan McEntee

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Re: Plastic clunk tank uniflow set up
« Reply #2 on: November 14, 2023, 12:00:02 PM »
For those of you who use a plastic clunk tank uniflow set up where the uniflow line is connected to the fuel pick up line, where does the uniflow line reside in relationship to the pick up line? Inboard of the pick up? Outboard of the pick up?

   If you want the uniflow line to follow the clunk, use a length of copper tube, put a little dog leg bend in it, and attach the fuel line to that and secure the tube to the pick up line about 3/4" or so in front of the clink and outboard of the pick up line, and use soft wire. Don't pinch anything off when you wrap it. Make sure the clunk still moves freely. It used to be common that you would solder the tube directly to the clunk but that us too close to the pick up end. Be prepared to experiment and alter that positioning if you need it. Sometimes what works for one model doesn't work for another. Since it's a profile, making changes is easy.

  Type at you later,
   Dan McEntee
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Offline Steve Glass

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Re: Plastic clunk tank uniflow set up
« Reply #3 on: November 15, 2023, 06:03:04 AM »
I'm having success with double clunk tanks on profiles. The uniflow is about 3/4" shorter than the feed.

I suppose that a double clunk tank is not really uniflow and is counter-intuitive. But it works for me.

On a profile, I mount the tank on a solid shelf and shim up or down for trim.  Once the tank height is dialled in, you don't have to retrim after tank maintenance.

Steve

Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #4 on: November 15, 2023, 11:22:19 AM »
I first discovered the double clunk in the "control line aero modeller" publication mid seventies. It was an article written by Glen Alison of UK F2B fame.
I have used this principle in the past, but I stopped using the double clunk after  a while in favor of the fixed uniflow.

The double clunk is in fact a uniflow tank, but the uniflow reference is changing with the uniflow clunk vertical position inside the tank.

It is essential that the uniflow clunk can move freely: This requires super flexible silicone tubing and no obstructions inside the tank.
Connecting the feed and uniflow clunk together makes the system less free moving. Especially in small tanks.
Equalizing speed upright / inverted has to be done by moving the tank up or down.

When using a fixed uniflow , you don't have to worry about the free moving of the uniflow clunk and you can  move the uniflow inside the tank for adjusting upright vertsus inverted, without having to move the tank itself.

My 2 cents

Offline John Skukalek

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Re: Plastic clunk tank uniflow set up
« Reply #5 on: November 15, 2023, 03:22:14 PM »
I first discovered the double clunk in the "control line aero modeller" publication mid seventies. It was an article written by Glen Alison of UK F2B fame.
I have used this principle in the past, but I stopped using the double clunk after  a while in favor of the fixed uniflow.

The double clunk is in fact a uniflow tank, but the uniflow reference is changing with the uniflow clunk vertical position inside the tank.

It is essential that the uniflow clunk can move freely: This requires super flexible silicone tubing and no obstructions inside the tank.
Connecting the feed and uniflow clunk together makes the system less free moving. Especially in small tanks.
Equalizing speed upright / inverted has to be done by moving the tank up or down.

When using a fixed uniflow , you don't have to worry about the free moving of the uniflow clunk and you can  move the uniflow inside the tank for adjusting upright vertsus inverted, without having to move the tank itself.

My 2 cents

I also use the fixed uniflow set up Paul. It works well. I love not having to move the tank to change it's effective height.
The reason I started this topic is because I have heard others talk about attaching the uniflow line to the fuel pickup line and wonder if I am missing anything.

Offline Brett Buck

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Re: Plastic clunk tank uniflow set up
« Reply #6 on: November 15, 2023, 11:22:37 PM »
For what little it is worth, when I was running clunk tanks I had much better luck with a fixed uniflow vent.

    For any clunk to work correctly you (obviously) need it to move freely. Most current silicone tubing is rather stiff and you have to install it so that whatever natural curve it has helps it get to the outboard side. Either use the original thin-wall tubing that comes with it, or I the new Sullivan ProFlex tubing which is almost as flexible as the old gum rubber, but completely immune to fuel.

    The tank on the Skyray is almost identical to the drawing posted above, but it is rotated 90 degrees so that the flat side goes up against the fuselage. I ran it with the uniflow vent blocked, and the "overflow" as the only air inlet, fed by muffler pressure. It leans out slightly but the narrow tank and the pressure make that very minimal.

    Brett

Offline Steve Glass

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Re: Plastic clunk tank uniflow set up
« Reply #7 on: November 16, 2023, 05:53:28 AM »
Just saying. If you are struggling to get a usable engine run then try a double-clunk. Worked for me, as I was chasing my tail trying every type of tank known to man. Now I can focus on the flying 

This is the silicone tubing that I use for the clunks in small (4oz.) plastic tanks.   UK vendor I'm afraid.

https://www.slecuk.com/fuel-tanks-accessories/2mm-332-clunk-tubing-x-500mm-19quot

I always carry a spare ready-to-use tank in the tool box and swap it out at the first sign of trouble.

Steve 

Offline Colin McRae

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Re: Plastic clunk tank uniflow set up
« Reply #8 on: November 16, 2023, 08:53:23 AM »
It is my understanding that medical grade surgical tubing can also be used for the clunk pickup. It is much more flexible. Any reason not to use surgical tubing for typical glow fuels?

I am currently trying it on a very small 2 oz Sillivan tank where the Sullivan supplied tubing was way too stiff (being so short in the tank). The clunk moves freely with the surgical tubing. Seems to work quite well. But will glow fuel eventually cause a problem? I know surgical tubing was common long ago but has pretty much been replaced with silicone tubing for glow fuels.
« Last Edit: November 16, 2023, 11:35:29 AM by Colin McRae »

Offline Dennis Toth

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Re: Plastic clunk tank uniflow set up
« Reply #9 on: November 16, 2023, 09:49:28 AM »
Colin,
The surgical tubing does work but you will need to check it every 6 months or so as it can get a little wimpy when submerged in raw fuel for long term, just check it and it will be fine.

Best,    DennisT

Offline Brett Buck

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Re: Plastic clunk tank uniflow set up
« Reply #10 on: November 16, 2023, 10:35:41 AM »
It is my understanding that surgical tubing can also be used for the clunk pickup. It is much more flexible. Any reason not to use surgical tubing for typical glow fuels?

I am currently trying it on a very small 2 oz Sillivan tank where the Sullivan supplied tubing was way too stiff (being so short in the tank). The clunk moves freely with the surgical tubing. Seems to work quite well. But will glow fuel eventually cause a problem? I know surgical tubing was common long ago but has pretty much been replaced with silicone tubing for glow fuels.

  Do you mean gum rubber tubing? Definitely not OK, it degenerates in a few months.  Get some of the ProFlex tubing. For the Sullivan clunks, you will need to solder a short 1/8" od tubing into the bore, because ProFlex will not go over the nipple on the clunk.

    Brett

Offline Colin McRae

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Re: Plastic clunk tank uniflow set up
« Reply #11 on: November 16, 2023, 11:26:33 AM »
  Do you mean gum rubber tubing? Definitely not OK, it degenerates in a few months.  Get some of the ProFlex tubing. For the Sullivan clunks, you will need to solder a short 1/8" od tubing into the bore, because ProFlex will not go over the nipple on the clunk.

    Brett

Brett, it is medical grade surgical tubing I purchased at a local medical supply store. I even tried Dubro silicon tubing (which is more flexible compared to the Sullivan tubing that came with the tank), but it was still too stiff on a 2 oz size tank due the short length. The clunk would not move freely. But with the 1/4" surgical tubing, the clunk moves freely as it needs to do.

I believe medical surgical tubing is a latex rubber material.

Brodak also sells it, but it is a SIG product.

https://brodak.com/fuel/fuel-line/sig-surgical-fuel-line-small.html

Offline Brett Buck

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Re: Plastic clunk tank uniflow set up
« Reply #12 on: November 16, 2023, 04:44:32 PM »
Brett, it is medical grade surgical tubing I purchased at a local medical supply store. I even tried Dubro silicon tubing (which is more flexible compared to the Sullivan tubing that came with the tank), but it was still too stiff on a 2 oz size tank due the short length. The clunk would not move freely. But with the 1/4" surgical tubing, the clunk moves freely as it needs to do.

I believe medical surgical tubing is a latex rubber material.

Brodak also sells it, but it is a SIG product.

https://brodak.com/fuel/fuel-line/sig-surgical-fuel-line-small.html

  The latex/gum rubber will not last very long. We used to use that for fuel tubing and it was well-known for getting gummy and degenerating in a few weeks/months.

    I suggest this:

   https://sullivanproducts.com/product/s211-pro-flex-universal-fuel-line/

     I have two big rolls of this, you can have all you want within reason, if you are willing to wait, otherwise, that's a link to buy it.

      Brett

   p.s. you can bend the brass tube that goes through the stopper over towards the outside of the circle, so that the clunk tubing is aimed in the right direction.

Offline Dave_Trible

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Re: Plastic clunk tank uniflow set up
« Reply #13 on: November 19, 2023, 12:00:40 PM »
Surgical tubing is/was widely used on free flight airplanes because it was pretty soft and so the cut off timers could easily pinch it off.  However we found fuel could degrade it into jelly pretty fast so we just stocked up and changed it very regularly.  That would be more of a chore where you had to disassemble the tank to replace it.  I'd try something else...or go metal tank.

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Offline Jim Kraft

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Re: Plastic clunk tank uniflow set up
« Reply #14 on: November 19, 2023, 04:14:39 PM »
  The Sullivan tubing Brett is talking about is super flexible and is impervious to both fuel and gasoline. I would use a length of brass or copper tubing as part of the clunk as it is possible that the tubing is so flexible it can end up in the front of the tank on a hard landing. It really is the best tubing I have found. The only down side is it is opaque. You cannot see bubbles in it.
Jim Kraft

Offline Colin McRae

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Re: Plastic clunk tank uniflow set up
« Reply #15 on: November 19, 2023, 04:21:59 PM »
Thanks to all on the pros and cons of surgical fuel tubing use. I don't mind the effort and will go back inside the tank and switch back to silicon tubing.

The only place I was trying to use surgical tubing was inside a Sullivan tank for the clunk. The tubing from the tank to the NVA is silicon.

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #16 on: November 26, 2023, 10:21:46 AM »
Paul, relative to your earlier post above,

Referring to a fixed uniflow and adjusting for upright vs inverted speed I'd like a clarification. Are you saying to achieve this with a fixed uniflow you move the uniflow line height location inside the tank without adjusting the height of the actual tank?  I have heard to make the height/speed adjustment we must actually rotate the tank cap which results in a change of the actual outlet location of the clunk pick up relative to the needle valve inlet. I'm unclear here, how would moving the location of the uniflow vent inside the tank have any affect? I was lead to believe it is the fuel head adjusted by the tank outlet point relative the the NVA that affects engine flow to the engine altering the lean or rich depending on the model position.

Steve 



Equalizing speed upright / inverted has to be done by moving the tank up or down.

When using a fixed uniflow , you don't have to worry about the free moving of the uniflow clunk and you can  move the uniflow inside the tank for adjusting upright versus inverted, without having to move the tank itself.


Offline Ken Culbertson

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Re: Plastic clunk tank uniflow set up
« Reply #17 on: November 26, 2023, 11:45:10 AM »
When I was still IC I avoided uniflow on clunk tanks.  I never had issues using muffler pressure to the front of the tank outside of the fuel mass.  I also used tanks 1-2oz larger than needed so that they were never full and as such, the pressure was never in the fuel.  My last adventure used an LA46 with a 6oz rectangle Sullivan tank. It needed roughly 3.75 to 4.5 oz for a pattern depending on the weather.  I ran the muffler pressure line to the top of the tank near the front inside.  It also doubled as the vent/overflow but it rarely saw any fuel.  The fill tube went to the bottom front and was capped off after it was fueled.  I measured the fuel and, on my profiles, I also scribed markings to show how full it was.

It is difficult to set an engine on the ground when the pressure is pumping into the fuel on the ground and out of it in the air.  This duplicates the old "over/under" tubes we all used for airflow pressure back in the day.  It worked, then we got fancy....

Ken

This link is an OSLA46 without uniflow using the tank setup above.  Just listen to it.  The pattern itself sucks.  I only had about 20 flights under my belt, most of them with an ARF Nobler, since returning from a 30 year absence when this was filmed.
 

« Last Edit: December 01, 2023, 08:42:40 PM by Ken Culbertson »
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Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #18 on: November 27, 2023, 01:00:22 PM »
Paul, relative to your earlier post above,

Referring to a fixed uniflow and adjusting for upright vs inverted speed I'd like a clarification. Are you saying to achieve this with a fixed uniflow you move the uniflow line height location inside the tank without adjusting the height of the actual tank?  I have heard to make the height/speed adjustment we must actually rotate the tank cap which results in a change of the actual outlet location of the clunk pick up relative to the needle valve inlet. I'm unclear here, how would moving the location of the uniflow vent inside the tank have any affect? I was lead to believe it is the fuel head adjusted by the tank outlet point relative the the NVA that affects engine flow to the engine altering the lean or rich depending on the model position.

Steve 


[/i]

Hi Steve, indeed, the only reference for a uniflow tank w.r.t. upright/ inverted is where the uniflow tube exits in the tank. Moving it up, is like moving the complete tank up. The reverse is also true.  I use this adjustment for 20 years now and it is super easy to do. I add a small piece of copper to the tube, so I can rotate it in a more controlled way. See attached picture.
The pick up tube must be submerged in the fuel, but that is the only requirement. And keep the uniflow and Pick up at least 1 inch separated from each other. Picking up airbubbles will always give erratic engine runs

Offline Dennis Toth

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Re: Plastic clunk tank uniflow set up
« Reply #19 on: November 27, 2023, 03:25:25 PM »
Something is not right with only adjusting the exit point of the uniflow vent line to get equal upright or inverted lap times. This only works if you have a solid tube for the uniflow not if it is a free floating uniflow clunk. For the free floating UF vent you need to move the whole tank (just as you would a metal tank) up or down to equalize lap times and the clunk flex tubing needs to be able to flop to the top and bottom of the tank or it will give very inconsistent runs. 

Best,   DennisT

Offline Ken Culbertson

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Re: Plastic clunk tank uniflow set up
« Reply #20 on: November 27, 2023, 04:13:23 PM »
Something is not right with only adjusting the exit point of the uniflow vent line to get equal upright or inverted lap times. This only works if you have a solid tube for the uniflow not if it is a free floating uniflow clunk. For the free floating UF vent you need to move the whole tank (just as you would a metal tank) up or down to equalize lap times and the clunk flex tubing needs to be able to flop to the top and bottom of the tank or it will give very inconsistent runs. 

Best,   DennisT
True Dat. 

Ken
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Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #21 on: November 27, 2023, 04:31:34 PM »
Something is not right with only adjusting the exit point of the uniflow vent line to get equal upright or inverted lap times. This only works if you have a solid tube for the uniflow not if it is a free floating uniflow clunk. For the free floating UF vent you need to move the whole tank (just as you would a metal tank) up or down to equalize lap times and the clunk flex tubing needs to be able to flop to the top and bottom of the tank or it will give very inconsistent runs. 

Best,   DennisT

Absolutely!

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #22 on: November 28, 2023, 06:26:44 AM »
Something is not right with only adjusting the exit point of the uniflow vent line to get equal upright or inverted lap times. This only works if you have a solid tube for the uniflow not if it is a free floating uniflow clunk. For the free floating UF vent you need to move the whole tank (just as you would a metal tank) up or down to equalize lap times and the clunk flex tubing needs to be able to flop to the top and bottom of the tank or it will give very inconsistent runs. 

Best,   DennisT

Dennis,

Aren't we really being affected here by P=pgh where p is the constant fuel density and h is the head (height variation) in fractions of an inch achieved by rotating the cap position of the outlet point of the feed line from the clunk pick up relative to the NVA? I have changed the engine run from upright to inverted by rotating (changing) the cap outlet feed supply position and repositioning (holding) the hard uniflow line and tank overflow lines to their same positions respectively. That is keeping the hard uniflow vent position constant at the mid point outside of the tank as shown in the sketch above. The addition of a positioning tab on the uniflow line would help in assuring the position of the vent if there is enough room for a tab behind the engine. The tab idea is beneficial since the rubber bands holding the tank usually hide the actual position of the vent.

Steve


Offline Dennis Toth

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Re: Plastic clunk tank uniflow set up
« Reply #23 on: November 28, 2023, 06:51:34 AM »
Steve,
If I understand correctly, you hold the midpoint of the uniflow hard tube constant more or less like a pivot point and rotate the cap to equalize lap time. Or if by having the whole tube move up/down as you rotate the cap. If this is so then yes, I agree it works since the end of the hard uniflow line will move either up or down depending on the way you rotate the cap. It is the fuel end of the uniflow line that is setting the pressure reference that the engine sees.

There are many articles detailing how uniflow works if you google it or search this forum but the simple explanation is the head above and below the engine spraybar is what controls either slightly more head or less head pushing on the fuel feeding the engine and moving that reference point is what equalizes fuel feed and engine consistency upright/inverted. In a free flopping uniflow clunk as long as the clunk is free to flop to the top and bottom of the tank the pressure reference point can be equalized for upright/inverted by adjusting the position of the whole tank relative to the spraybar. If the clunk is to stiff it will be very challenging to find a height that consistently balances the pressure point.

Look at some of the hard tanks that have the uniflow vent coming out the top of the wedge, they still wind up with the end of the uniflow line (that is at the wedge point) near inline with the spraybar. They are adjusted up/down by moving the whole tank.

Best,   DennisT 
« Last Edit: November 29, 2023, 06:14:00 AM by Dennis Toth »

Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #24 on: November 29, 2023, 12:30:46 AM »
Steve,
If I understand correctly you hold the mid point of the uniflow hard tube constant more or less like a pivot point and rotate the cap to equalize lap time. If this is so then yes I agree it works since the end of the hard uniflow line will move either up or down depending on the way you rotate the cap. It is the fuel end of the uniflow line that is setting the pressure reference that the engine sees.



Hi Dennis, why rotating the whole cap? Rotating the uniflow tube is sufficient. The cap itself is very difficult to rotate I think.

Offline John Skukalek

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Re: Plastic clunk tank uniflow set up
« Reply #25 on: November 29, 2023, 07:22:52 AM »
I just rotate the uniflow line so that the end inside the tank moves up or down without rotating the cap or moving the tank. This changes the effective height of the tank. As a result its easy to achieve an engine run that is the same upright as inverted.   

Offline John Miller

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Re: Plastic clunk tank uniflow set up
« Reply #26 on: November 29, 2023, 03:55:50 PM »
Yep, what John & Paul said. That's why the diagram posted shows a bit of a "dog leg offset" on the outside part of the uniflow line. It makes it easier to rotate.

If it bothers you having the metal tubing so close to the tank sidewall, make the inside end a little short, and use some fuel line to extend the line up close, but not touching, the inside of the tank.

I've used the sintered bronze clunk pickup since the mid-80s. I only had a problem when a plane sat for 12 years and I tried to ruin the engine without servicing the fuel system. I never had a problem with the ones I was running each season.
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Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #27 on: November 30, 2023, 05:39:29 AM »
Yep, what John & Paul said. That's why the diagram posted shows a bit of a "dog leg offset" on the outside part of the uniflow line. It makes it easier to rotate.

If it bothers you having the metal tubing so close to the tank sidewall, make the inside end a little short, and use some fuel line to extend the line up close, but not touching, the inside of the tank.

I've used the sintered bronze clunk pickup since the mid-80s. I only had a problem when a plane sat for 12 years and I tried to ruin the engine without servicing the fuel system. I never had a problem with the ones I was running each season.

I keep a distance of about 1cm or 3/8 " from the wall. It gives a small amount of leaning out at the end of the flight . This does not matter too much when the tank is 2 inches wide

Offline Ken Culbertson

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Re: Plastic clunk tank uniflow set up
« Reply #28 on: November 30, 2023, 07:33:56 AM »
Yep, what John & Paul said. That's why the diagram posted shows a bit of a "dog leg offset" on the outside part of the uniflow line. It makes it easier to rotate.
This is the only uniflow setup that has ever worked for me on a clunk, but two things still bother me about uniflow.
First, *why* does the position of the uniflow vent override the position of the fuel pickup?  I just know that it does.
Second, if the position of the pickup relative to the needle valve determines equal upright and inverted settings without uniflow, *why* do you get a consistent run with only the slight normal (and desirable) speed up in maneuvers when using muffler pressure outside of the fuel?  And if this is the normal case, why do we need uniflow?

It always bothers me when I know something works but not why.  It is only a tool in your trim box if you know why it works.

Ken
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Offline Lauri Malila

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Re: Plastic clunk tank uniflow set up
« Reply #29 on: November 30, 2023, 08:56:16 AM »
[quote author=Ken Culbertson
Second, if the position of the pickup relative to the needle valve determines equal upright and inverted settings without uniflow, *why* do you get a consistent run with only the slight normal (and desirable) speed up in maneuvers when using muffler pressure outside of the fuel?  And if this is the normal case, why do we need uniflow?
[/quote]

Because the purpose of Uniflow is to guarantee a constant hydrostatic feed pressure from beginning to end of flight, thatís all.
Same hydrostatic laws apply to both pickup- and replacement air tubes when they both are submerged to fuel.
The position of pressure/replacement air vent inside tank does not really matter if it is not inside the fuel mass.
How the engine runs in level flight and maneouvres, has more to do with other parameters. L
« Last Edit: November 30, 2023, 10:10:31 AM by Lauri Malila »

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #30 on: December 01, 2023, 02:40:19 PM »
I still believe the static head of the liquid relative to the NV inlet is what affects the pressure the engine sees changing its speed (lean or rich) when going to inverted or vise versa. There are two things going on here, first the uniflow pick up line when located midway along the side of the tank but away from the tank clunk outlet is what provides the effective height of the liquid preventing the lean out as the tank empties, thus the fluid flow principal of a uniflow tank set up.  The second separate issue not to be confused with uniflow is the height variation of the "column" liquid that cannot be ignored as the liquid head varies during regular and inverted flight.

I have found that rotating the cap when I was unable to adjust the height of the tank in order to position the fuel outlet line in the cap level with the NV inlet did change the run from regular to inverted. When turning the cap I did in addition adjust the uniflow line keeping it in its mid position inside the tank to assure the leaning out would not occur.

Steve

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #31 on: December 01, 2023, 02:47:18 PM »
Ken,

I'm going to say muffler pressure or a sealed tank defeats or eliminates principal of uniflow. The effects of liquid column head during straight up of inverted flight cab be adjusted out by changing tank height or rotating the cap to change the relative position of the tank outlet (not the internal pick up) to the inlet of the needle valve.

Steve

Offline Ken Culbertson

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Re: Plastic clunk tank uniflow set up
« Reply #32 on: December 01, 2023, 06:44:23 PM »
Ken,

I'm going to say muffler pressure or a sealed tank defeats or eliminates principal of uniflow. The effects of liquid column head during straight up of inverted flight cab be adjusted out by changing tank height or rotating the cap to change the relative position of the tank outlet (not the internal pick up) to the inlet of the needle valve.

Steve
I am not arguing the principal of uniflow or it's use in a metal tank, only that with a clunk tank you can get the same or, in my case better results with muffler pressure outside of the fuel. 

Ken
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Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #33 on: December 01, 2023, 07:57:05 PM »
Ken,

I'm going to say muffler pressure or a sealed tank defeats or eliminates principal of uniflow. The effects of liquid column head during straight up of inverted flight cab be adjusted out by changing tank height or rotating the cap to change the relative position of the tank outlet (not the internal pick up) to the inlet of the needle valve.

Steve
Muffler pressure or atmospherics . It does not alter the uniflow principle. Muffler pressure can make the needle more sensitive. I only would use it to cure an issue . What do you mean with "sealed tank"?

Offline Brett Buck

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Re: Plastic clunk tank uniflow set up
« Reply #34 on: December 01, 2023, 08:52:34 PM »
I'm going to say muffler pressure or a sealed tank defeats or eliminates principal of uniflow.

   How so?   I think all you are doing is changing the reference pressure - instead of atmospheric pressure at the inlet, you get a (presumably higher) muffler pressure. It will still maintain this pressure regardless of the fuel depth.

  There are other, more subtle effects, like the premise that the muffler pressure will increase with RPM (which it might not...) but to first approximation it works just the same way with regard to hydrostatic pressure.

     Brett

Offline Lauri Malila

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Re: Plastic clunk tank uniflow set up
« Reply #35 on: December 02, 2023, 01:17:45 AM »
[quote author=Steve Dwyer
 The effects of liquid column head during straight up of inverted flight cab be adjusted out by changing tank height or rotating the cap to change the relative position of the tank outlet (not the internal pick up) to the inlet of the needle valve.

Steve
[/quote]

Once again🥱. What happens between pick-up point and needle valve makes no difference whatsoever. The *only* thing that affects feed pressure between +/- G is the difference in height between pick up point and spraybar.
For example, you can place a remote needle valve basically anywhere (heightwise) between these 2 points, without disturbing the symmetry. L

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #36 on: December 02, 2023, 10:08:56 AM »
Lauri,
I wish my old physics professor was around but he has left us many years ago. Aren't we really dealing with a differential manometer here where we are only concerned with the height differences between the tank outlet (not the internal pick up) and the spray bay?  For simplification ignoring absolute pressure or atmospheric pressure we can solve for delta gage pressure variations by inverting the liquid head. The system is from the tank outlet to the venturi spray bar. Agree you can place a remote NV anywhere as on an OS LA backplate with no effect because the remote NV is only a meter adjusting the flow to the venturi spray bar forward of the cylinder head. 

Ken,
Sealed muffler pressure system.

Brett,
Hum...Interesting but I had thought the subtleness of an open system at one atmosphere completely changes when we introduce exhaust pressure changing the hydrostatic pressure seen at the spray bar?? Thus overriding a constant reference pressure with respect to the height of the liquid.

Steve

Offline Ken Culbertson

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Re: Plastic clunk tank uniflow set up
« Reply #37 on: December 02, 2023, 11:38:06 AM »
I can't see why the tank outlet has anything to do with it.  If that were true you could never design a tank for a short-nosed profile.  Some of the best tanks I have had for profiles were the elliptical ones with the outlet on the top of the tank.  I don't know how the engine could tell if the fuel line is surgical tubing or brass pipe.  I have used the clunk tanks with the beveled front in full bodied stunters with the input plug pointing down for access.  As long as the end of the pickup tube in the tank was at the level of the spray bar there was a happy camper powering the plane.

Ken
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Offline Lauri Malila

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Re: Plastic clunk tank uniflow set up
« Reply #38 on: December 02, 2023, 03:36:31 PM »
Quote from: Steve Dwyer
Lauri,
I wish my old physics professor was around but he has left us many years ago.
[/quote

Why? He would just tell the same what I did. L

Offline Brett Buck

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Re: Plastic clunk tank uniflow set up
« Reply #39 on: December 02, 2023, 05:47:31 PM »
Brett,
Hum...Interesting but I had thought the subtleness of an open system at one atmosphere completely changes when we introduce exhaust pressure changing the hydrostatic pressure seen at the spray bar?? Thus overriding a constant reference pressure with respect to the height of the liquid.

   It doesn't override it, it adds to it. So, instead of getting atmospheric pressure at the internal end of the vent, you get the pressure supplied by the muffler. Presuming that is constant*, the fuel depth still doesn't matter, and it runs at a constant pressure throughout the run. The reason you run uniflow is to keep the fuel supply pressure constant as the fuel runs out, it doesn't have to be at the pressure at an open vent.

   Brett

*of course, it isn't constant, it pulses at the engine RPM (and probably a bunch of harmonics like 2x and 4x). But any sort of ullage has the effect of filtering out high-frequency components.

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #40 on: December 03, 2023, 01:52:09 PM »
Following is a response from Ken Cook back in 2012 on uniflow tanks. The point he makes here as we know, a properly set up uniflow tank will provide an even engine run by creating a constant tank pressure. Except of course at the very end. But in the last paragraph he goes on to say rotating the cap enables us to change the engine run. By doing so doesn't this impact the gravitational static liquid head (L) we see when inverting the model or tipping up the nose similar to a standard vented tank. This "head change" of course is completely unrelated to the position of the uniflow line but more so the position of the tank outlet relative to the spray bar.  Aren't we seeing two different conditions here?

Steve

Unfilow tanks have a additional pipe which travels alongside of the fuel pickup. It stops about 5/16" from the rear of the pickup. Uniflow tanks MUST BE FILLED through the uniflow pipe or else when the fuel level rises in the tank it will exit the feed pipe resulting in a flooding the engine. The unfilow pipe provides a magical quality to the engine run. The uniflow pipe itself exits the tank and is pointed directly into the wind. This pipe stays uncapped and the other pipes,(filler and overflow are capped). Using unfilow on profiles presents vibration to the tank and also turbulent air over the cylinder now in front of the tank. Getting your unfilow pipe extended to clean air (typically the inboard side of the plane) is important by adding flexible tubing and placing a piece of hard tubing mounted out of the way of obstructions is importantl A standard vented tank, and we can use the wedge that most are familiar with. As the tank is full it has a lot of head pressure. The engine run is rich, as the tank empties, the head pressure is less but the engine leans and continues to lean throughout the run until empty which usually unless properly set, results in a screaming lean run cutting in and out all the to the end. This type of run maybe ok for the sport flyer, but when flying stunt, this isn't ideal. We like the smoothest steadiest run we can achieve with a clean cut off at the end.

Uniflow tanks provide terrfic runs but they also lean out slightly just not to the level a standard vented tank does.
If the plane is vibrating and the fuel is foaming, UNIFLOW WILL NOT WORK. Uniflow generally needs to be set with the engine on the ground rich, the uniflow pipe is bent at a 90 deg angle into the atmosphere and as the air pressure increases into the pipe the engine leans out usually settling into a nice 4 stroke on the verge of a 2. Sounds great I bet, I can't ever get it to work for me and this is usually on profiles which are subject to vibrations. Sometimes the uniflow pipe needs to be necked down to work properly by inserting a small piece of tubing over the 1/8" tubing. Armor All in the fuel sometimes and most of the time works to settle down the fuel foaming internally.


I like uniflow tanks for 3 reasons, I can use it as a uniflow tank. If uniflow isn't successfully working, I can cap the uniflow and overflow leaving the fill vent open and it now is a standard vented tank. I can also cap the uniflow and overflow running muffler pressure to the fill making it a standard tank on muffler pressure. It just gives you many options.

I personally hook my muffler pressure directly to the uniflow to what I refer to as uniflow on muffler pressure which seems to work the best for me. I've seen many guys use the uniflow tanks just like I stated above and have perfect results. The idea of uniflow is just to enable the engine to run at a constant speed without leaning out other than when required which would be pointing the nose up.

Unfilow has a very unique characteristic, the uniflow pipe is what dictates your tank height and NOT your fuel pickup like a standard tank.
This can be extremely helpful in setting your tank height on a plastic tank as all you need to do is loosen the screw on the front and rotate the rubber stopper and piping and this will change your engine run. Metal tanks need to be shimmed up or down. I make adjustable bracketing for my hard tanks. I make a piece of ply with slotted holes and I attach the hard tank to the 1/8" ply with nylon zip ties. Screws then go into to the slotted holes of the ply and are tightened to affix them at that height on the plane. If needed, I loosen the screws and slide the tank up or down if needed.


Offline Brett Buck

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Re: Plastic clunk tank uniflow set up
« Reply #41 on: December 03, 2023, 08:50:04 PM »
Following is a response from Ken Cook back in 2012 on uniflow tanks. The point he makes here as we know, a properly set up uniflow tank will provide an even engine run by creating a constant tank pressure. Except of course at the very end. But in the last paragraph he goes on to say rotating the cap enables us to change the engine run. By doing so doesn't this impact the gravitational static liquid head (L) we see when inverting the model or tipping up the nose similar to a standard vented tank. This "head change" of course is completely unrelated to the position of the uniflow line but more so the position of the tank outlet relative to the spray bar.  Aren't we seeing two different conditions here?

    I am not sure what problem you are seeing with this. The position of the vent defines the pressure, you you want more pressure upright than inverted, you move the vent up, if you want less pressure upright, move it down. Where the rest of the tank volume is doesn't matter at all. I have made 100's a fuel tanks, and almost none of them are actually physically symmetrical.


    Brett

Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #42 on: December 04, 2023, 05:11:41 AM »

Once again🥱. What happens between pick-up point and needle valve makes no difference whatsoever. The *only* thing that affects feed pressure between +/- G is the difference in height between pick up point and spraybar.
For example, you can place a remote needle valve basically anywhere (heightwise) between these 2 points, without disturbing the symmetry. L

To avoid confusion I would change the statement 'The *only* thing that affects feed pressure between +/- G is the difference in height between pick up point and spraybar" to 'The *only* thing that affects feed pressure between +/- G is the difference in height between uniflow outlet in the tank and spraybar'

Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #43 on: December 04, 2023, 07:19:13 AM »
So are you saying in the case where some here promote attaching the vent and the fuel pick up at the same clunk point going inverted the engine run will remain the same? On a standard tank what in your opinion causes the leaning of the engine when you lift the nose, it's certainly not internal tank pressure but more so the liquid head change of the fuel relative to the spray bar. I still feel the placement of the vent inside the tank simply assures constant pressure so long as it sees fluid due the the available column in the tube to compensate for atmospheric pressure. This occurs anywhere in the tank regardless where the height or amount of fuel that exists. But the "head effect" in the pressure equation L alters engine performance as when we go inverted and is separate from the vent position. Tap a water 200' water tower at the base and at the 100' point do you see the same pressure? It's my thinking the outlet from the tank in the cap relative to the spray bar is a separate component we have to consider.

Steve


Offline Dennis Toth

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Re: Plastic clunk tank uniflow set up
« Reply #44 on: December 04, 2023, 08:03:09 AM »
Bill Lee wrote a detailed description of how the uniflow tank compared to a convention vent and chicken hopper works and why the pressure reference is at the submerged end of the uniflow vent line.

Best,   DennisT

Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #45 on: December 04, 2023, 04:21:43 PM »
So are you saying in the case where some here promote attaching the vent and the fuel pick up at the same clunk point going inverted the engine run will remain the same? On a standard tank what in your opinion causes the leaning of the engine when you lift the nose, it's certainly not internal tank pressure but more so the liquid head change of the fuel relative to the spray bar. I still feel the placement of the vent inside the tank simply assures constant pressure so long as it sees fluid due the the available column in the tube to compensate for atmospheric pressure. This occurs anywhere in the tank regardless where the height or amount of fuel that exists. But the "head effect" in the pressure equation L alters engine performance as when we go inverted and is separate from the vent position. Tap a water 200' water tower at the base and at the 100' point do you see the same pressure? It's my thinking the outlet from the tank in the cap relative to the spray bar is a separate component we have to consider.

Steve

Hi Steve,

2 important rules: Pressure in a gas is constant. Pressure in a liquid depends on the "head", relative to a reference point.

Now in a uniflow tank this reference point is the exit of the uniflow in the tank. If you apply muffler pressure, this pressure will be present at uniflow exit. if you apply atmospheric presure, atmospheric pressure will be at the uniflow exit.

So we know where we have a constant pressure in the tank: At the uniflow exit. From there fuel head will take over , in a direction based on earth gravity and centrifugal G's.
Above the uniflow exit there will be a reduced pressure, Below the exit, the pressure will increase.  Like the 200' water tower. But in the watertower, the reference point is the atmospheric pressure at the top of the water column , not a reference at the bottom like in our uniflow concept. 

For our use there are  2 references: Uniflow exit and spraybar location. The fact that the  flexible feed tube from tank towards the engine goes up and down, really does not  matter, nor a remote NVA as pointed out by Lauri.


Offline Steve Dwyer

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Re: Plastic clunk tank uniflow set up
« Reply #46 on: December 05, 2023, 10:12:17 AM »
Hi Steve,

2 important rules: Pressure in a gas is constant. Pressure in a liquid depends on the "head", relative to a reference point.

Now in a uniflow tank this reference point is the exit of the uniflow in the tank. If you apply muffler pressure, this pressure will be present at uniflow exit. if you apply atmospheric presure, atmospheric pressure will be at the uniflow exit.

So we know where we have a constant pressure in the tank: At the uniflow exit. From there fuel head will take over , in a direction based on earth gravity and centrifugal G's.
Above the uniflow exit there will be a reduced pressure, Below the exit, the pressure will increase.  Like the 200' water tower. But in the watertower, the reference point is the atmospheric pressure at the top of the water column , not a reference at the bottom like in our uniflow concept. 

For our use there are  2 references: Uniflow exit and spraybar location. The fact that the  flexible feed tube from tank towards the engine goes up and down, really does not  matter, nor a remote NVA as pointed out by Lauri.

Paul,

As much as I enjoy the discussion here I hope we can get to the bottom of this to resolve in my mind the fluid mechanics affecting a uniflow tank.
I take no exception to what you are saying here except for the reference affect of head on the Unilfow vent.

We know the atmospheric pressure or muffler pressure has no affect on the constant flow from a tank because of the partial vacuum created in the space above the top of the liquid. We know we can move the Uniflow vent line anywhere in the tank and get the same constant flow until we run out of fuel or the level drops below the vent breaking the vacuum supporting the column of liquid.

We know we can invert the tank and as long at the vent remains submerged the flow will remain constant. We do see a short slight moment of rich running when we change the configuration until the partial vacuum is restablished offsetting the (weight) height of liquid.

On the water tower, the pump supplied flowing water is open to atmospheric pressure at the top of the tank. If the water level in the tank was changed the outlet pressure would change as well.  Similarly houses located lower on the main from the tank will see a higher pressure at their spigot. The reference level from the top of the tank to the house spigot remains the same so long as the water level doesn't change.

So going back to the original question does rotating the cap on a Uniflow tank affect the engine run? I say yes because the reference point of the tank feed outlet relative to the spray bar is changed altering the static head while the movement of the internal vent continues to maintain constant flow by offsetting the atmospheric pressure against the vacuum being created. Two separate affects taking place. Inverting the plane changes the head pressure at the spray bar and rotating the position of the feed outlet through the cap alters the reference dimension and liquid head.

I wish I could go back to the field the repeat what I experienced this past summer but the winter has arrived.

Your thoughts?
Steve

Offline Paul Van Dort

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Re: Plastic clunk tank uniflow set up
« Reply #47 on: December 05, 2023, 11:23:14 AM »


So going back to the original question does rotating the cap on a Uniflow tank affect the engine run? I say yes because the reference point of the tank feed outlet relative to the spray bar is changed altering the static head while the movement of the internal vent continues to maintain constant flow by offsetting the atmospheric pressure against the vacuum being created. Two separate affects taking place. Inverting the plane changes the head pressure at the spray bar and rotating the position of the feed outlet through the cap alters the reference dimension and liquid head.

I wish I could go back to the field the repeat what I experienced this past summer but the winter has arrived.

Your thoughts?
Steve

To fully understand your question: Please confirm: You are basically using 2 clunks .. One for feed and 1 for uniflow. Those clunks are connected so the uniflow follows the feed and vice versa.

This setup demands for super flexible tubes inside the tank. I assume that turning the cap does not alter the freedom of movement of the clunks in the tank. They should remain free to move. from top to bottom, and mainly to the righthand side of the tank. This condition is difficult to verify at the field. But assuming all stays equal and only the position of feed and uniflow changes in the cap, I don't expect any change in engine run. The uniflow tube contains gas, so the pressure is equal everywhere in that tube.. If the feedpipe changes  position from the bottom of the cap to the top, the fuel will first go up and then go down, resulting in the same pressure at the spraybar as before.. The rising and dropping of the fuel will cancel out any extra effect on pressure at the spraybar.

Offline Lauri Malila

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Re: Plastic clunk tank uniflow set up
« Reply #48 on: December 05, 2023, 03:14:59 PM »
[quote author=Steve Dwyer
So going back to the original question does rotating the cap on a Uniflow tank affect the engine run? I say yes because the reference point of the tank feed outlet relative to the spray bar is changed altering the static head while the movement of the internal vent continues to maintain constant flow by offsetting the atmospheric pressure against the vacuum being created. Two separate affects taking place. Inverting the plane changes the head pressure at the spray bar and rotating the position of the feed outlet through the cap alters the reference dimension and liquid head.
Your thoughts?
Steve
[/quote]

Is it possible that this imaginary force in the equation that you believe in, is a misunderstanding coming from those using a clunk tank with a single clunk for pickup and a stiff inflow tube? with those, you can raise/lower the end of inflow tube end by turning the rubber cap and have some sort of adjustment for level/inverted speed. I've never tried it, maybe it works. The whole issue is somehow uncertain and un-elegant for me, so I believe in symmetry. Less surprises that way. L

Offline John Skukalek

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Re: Plastic clunk tank uniflow set up
« Reply #49 on: December 06, 2023, 09:02:20 AM »
When I rotate the uniflow line from outside the tank, it moves the fixed uniflow line inside the tank up or down because its a hard line bent so that the end is close to the tank wall about half way back. This changes the effective height of the tank without moving the tank or rotating the tank cap. Very convenient. this is on profile stunters.   


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