Clunk Tank

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by Nils Norling
Clunk Tank
by Nils Norling

         I've been having very good luck running plastic R/C type fuel tanks on my stunters & have had several people contact me about how I set them up. As I was writing an email the other evening explaining how I'm doing them, it occurred to me that other people might benefit from this information as well. I'm using pretty much the same setup shown in one of Bob Hunts excellent videos with a few minor changes.  Here's how I set up my plastic tanks for uniflow:

       To begin with, I've used several types of plastic tanks & they all seem to work fine,  so I don't think it matters what type you start with. 


First, I poke out the third hole in the stopper for the vent & mount the stopper so that looking at it from the front, the vent hole is up top, the fuel line is outboard on the bottom & the uniflow is inboard on the bottom. I use copper for the vent line & have the inside end turn around & exit to the top inboard front corner. For the tanks with the stopper coming straight out the front I use
the hard plastic tubing that comes with the tank for the other two lines through the stopper. I don't trust brass, & the plastic tubing is much lighter than copper & seems to work fine as long as there's room for it at the front of the tank area. For limited space tank installations (like my U-KEY 35) I use the slanted front tanks like the one in the picture.

On these tanks I use copper for all three lines, since it's so much easier to bend than the hard plastic, (no heating). The fuel & uniflow copper tubes inside the tank must be bent so that they are parallel with the top & bottom of the tank. That way the clunk will be able to flop up & down the same in both directions.

I assemble the clunk line just like I would for an R/C tank, only just long enough to reach the outside rear of the tank without actually touching it. The uniflow comes off the other tube & ends right at the end of the other line where it stops on the clunk nipple. I have some copper eyelets about   5 / 8ths" long  (left over from a few Brodak kits), & I shove one in the end of the uniflow tube. There are two reasons for the eyelet. One, it can be pulled out a bit if you want to fine tune your break, though I haven't found that necessary so far, & two, it gives the shrink tubing something to grab without squeezing the uniflow line.

Next, I slide a piece of shrink tubing over both lines. Slide the shrink tubing on so it's flush with the end of the uniflow at the exposed front end of the clunk & hit it with the heat gun or a Bic to hold the lines together tightly. Now you can pull out on the uniflow line where it goes over the tubing
until the bias on the two lines hold the clunk next to the outer tank wall. Generally this takes a few trial runs, but if you hold it up to a light you can see it in there, so it's not hard to tell what needs to be done. It has to slide up & down that outer tank wall without hanging up. So far there's always been enough  flexible fuel tubing that comes with the tanks to make both lines.

Bob Reeves offers an alternate method of attaching the uniflow line.
Thought some of you might be interested in how I make a uniflow clunk for a plastic tank. Have tried all the other published methods and found this works the best. The main advantage is the clunk tubing remains flexible as possible because the uniflow tubing is allowed to move independent of the pickup tubing. If you hard connect the uniflow tube to the clunk as many how to's show the tubes tend to fight each other and you loose allot of flexibility.

Use a Dremel cut off wheel to grind a small slot in the side of the clunk and solder the wire guide in the slot. Bend the wire guide at a right angle about 3/4 inch behind the clunk and use a drill bit slightly larger than the uniflow tubing to form your loop. The uniflow tubing is 1/2A fuel line which is plenty large enough and more flexible than standard fuel line. After you get the fuel lines connected to the hard exit tubes just make sure the uniflow tubing is long enough so it won't come out of the loop as it moves inside the tank.

I mount the tanks with the centerline slightly above (3 / 16"-1 / 4") the NVA just like a metal tank. Since I've been using them primarily on profile airplanes,  I've been adjusting them for height just like a metal tank. However, you can grab the fuel & uniflow tubes from outside the tank & twist them to move the uniflow up & down in relation to the fuel pickup inside the tank to fine tune without moving the tank itself. I use a piece of firm white plastic foam that a door skin came wrapped in at
the 'Ol body shop for padding between the tank & fuse.
Set up this way my Oriental gives me about one lean lap & shuts down. After several hundred flights on the U-KEY & somewhere around 100 on the Oriental, I haven't found it necessary to silicone around the stoppers (yet). If I do have a problem I'll just spend a buck & buy a new stopper.
Basically this is how Bob Hunt sets up his tanks, only he's been using a soldered together brass "Y" rather than shrink tubing to hold the lines together. I prefer the shrink tubing because it's easier to get the clunk to move freely without the "Y" & the shrink tubing is lighter. I've tried both methods & they both work the same once they're set up, as near as I can tell.
I've noticed a few things about these two styles of tanks. For one, the   slant front tanks don't seem to require that the clunk swings all the way from the top to the bottom & vice-versa to work properly. Apparently the tanks are thin enough top to bottom that full travel of the clunk isn't necessary. For another, the "whiskey flask" type tanks with the stopper on the end can be considerably larger than need be & still work fine while being only partially filled. I have a six oz. tank
(I didn't know how much I'd need & got carried away), on my profile Oriental & I only put three &
three eighths ounces of fuel in it for the pattern. It doesn't seem to matter. Also, the "whiskey flask" type tanks are considerably easier to plumb since there's very little tube bending required.

           Of course, the vent must be plugged after fueling & the uniflow vent should face forward, just like a metal tank, though exact placement doesn't seem to matter as long as it's not blanketed by the fuse.

    Give these plastic tanks a whirl, they're very light, readily available, & can be heated & molded to fit available space, (within reason). Even if for some reason one doesn't work for you you're only out about three bucks, which is another thing I like about plastic tanks--they're cheap!