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Author Topic: uniflow clunk tank  (Read 2064 times)

Offline Chris Belcher

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uniflow clunk tank
« on: February 07, 2016, 10:22:38 AM »
I have built a tank for experimentation. This is a plastic Sullivan RST6. What I have done is run a piece of brass tubing down the outer wall and centered vertically about a 1/4" from the back wall. then I have my clunk free swinging as usual and the vent to the top bubble. So it is a fixed uniflow with free floating clunk. I have made as sure as possible that the clunk doesnt hang up on the brass tubing. Is there any reason the uniflow HAS to move with the pick up is my question. Trying to keep the uniflow in fuel a long as possible. WIththe uniflow fixed to the pick up ot uncovers way to soon...

Offline Dan McEntee

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Re: uniflow clunk tank
« Reply #1 on: February 07, 2016, 10:47:27 AM »
      I have plumbed plastic tanks in a similar manor and they worked fine. On some, I used the plastic tube and just ran it back to the back of the tank about 1/4" shy of the tank. The fuel is thrown to the outside of the tank during flight and if the model is flown smooth enough, the clunk barely moves, so in my opinion, this should function. Think about a hard tank, the tubes are in a fixed position on those and they work fine. I usually run muffler pressure, and this may make a difference. Mount the tank on a test plane and see how it works, and report back, please.
  Thanks a lot and type at you later,
   Dan McEntee
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Offline Chris Belcher

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Re: uniflow clunk tank
« Reply #2 on: February 07, 2016, 11:33:28 AM »
after typing the first post I remembered plumbing tanks with the uniflow half way down the outside wall but I never had much luck with those, though I know some have no issues. What I am getting with my uniflow fixed to the pick up is screaming rich laps at the end of the pattern....bout 8-10 of em. And a gradual increase in leaness throughout the flight...almost like its vented normally..not uniflow. My present tank has the uniflow tube freely moving in a loop of wire soldered to the pick up. THis puts the uniflow inlet a full 1/4" inboard of the pick up so I believe it is uncovering when there is still lots of fuel left in the tank.

Offline Randy Cuberly

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Re: uniflow clunk tank
« Reply #3 on: February 07, 2016, 03:41:00 PM »
One thing to consider is that if the uniflo tube is too close to the pickup clunk air bubbles may pass directly from the uniflo to the pickup and create a lean condition.  Personally I think 1/4 inch is not nearly enough distance between the two terminations.  I never use less than 1/2 inch and sometimes a little more.

Also consider that depending on the weight of the clunk centrifugal force may keep the clunk centered near your fixed uniflo tube!

Randy Cuberly
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Offline Brian Hampton

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Re: uniflow clunk tank
« Reply #4 on: February 07, 2016, 03:59:32 PM »
That's how I've been doing it for maybe 30 years or so and I've never tried any other method. I use a bend in the line where it comes through the stopper to act as a lever so I can twist the end of the uniflow line up or down from the centre line of the tank wall to get an even run upright and inverted. With muffler pressure a small movement makes a big difference but with no muffler pressure it takes a lot more. I angle the back of the tank outwards as much as possible to keep the uniflow line submerged until the tank is just about empty and that gives me maybe a lap or so with the engine leaning out somewhat as a warning it's about to cut.

Offline Tim Wescott

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Re: uniflow clunk tank
« Reply #5 on: February 07, 2016, 04:42:16 PM »
Leak-check the @#$% out of it.  I never had success with uniflow plastic tanks, although I've done pretty well with straight vented plastic tanks on muffler pressure and uniflow metal tanks.  All this without ever really getting a satisfying run from a straight metal uniflow tank.

Go figure.  I'm sure it's me, persisting in doing something wrong.  I just don't know what that "wrong" is.
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Offline Chris Belcher

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Re: uniflow clunk tank
« Reply #6 on: February 07, 2016, 08:03:25 PM »
Went flying today and found that the uniflow was not the issue after all. I have an adjustable "plate" that the tank is tywrapped to. It's made of 2 pieces of lite ply with spacers to pass tywraps through and I noticed a gap between the plate/tank and the fuse. It seems when I tywrap the tank to the plate it bows. The slotted tabs are on the ends so it bows away form the fuse and I believe this was causing a foaming issue. I used some foam to protect the profile fuse and tywrapped the ENTIRE assembly to the fuse just for a test and voila...steady run until the last two laps and then a little lean and clean shut off. The tank part of it seemed fine as far as the uniflow being run to the rear corner just short of where the clunk slides up and down. Got the tank height right the first time too..that's NEVER happened!

Online Brett Buck

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Re: uniflow clunk tank
« Reply #7 on: February 07, 2016, 09:01:10 PM »
One thing to consider is that if the uniflo tube is too close to the pickup clunk air bubbles may pass directly from the uniflo to the pickup and create a lean condition.  Personally I think 1/4 inch is not nearly enough distance between the two terminations.  I never use less than 1/2 inch and sometimes a little more.

  Mine are typically within 1/8" with no issues. I solder the vent tube right into the wedge of the tank, to within about 3/16" from the rear, then run the pickup along it, with a bend in the very end of it to get it as far back and as far into the wedge as possible, so I have to put a little 1/4" or so "kick" in the end of the tube. That's why they run the same to nearly the last drop everything, with no changes in the mixture until it is ready to run out.

   In the air, the last place the air wants to go is sideways (relative to the local acceleration) into the pickup. It might make some difference on the ground but I haven't had any issues with that, either.

    I have used a rigid uniflow pickup in a clunk tank many times in the past. The vent has to end up along the outside edge of the tank, but the fore-aft position doesn't seem to matter, as long as it doesn't interfere with the clunk movement. If you try to make the vent move with the clunk, you need a slip/sliding fit for the vent so the vent and clunk lines can move relative to each other. Otherwise the clunk will not move freely enough. I have tried separate clunks, but the chances of them interfering with each other is pretty high.

   I don't like clunk tanks very much because the cutoff is usually terrible. The round or flattened-ellipse type (similar to Billy's shampoo bottle tank) are generally better but I doubt that the clunk actually moves very much, with makes it more reliable. I haven't seen the elliptical type in a long time, and I don't know if they make them any more.


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