Uniflow Tank & Stock Tube Muffler Setup

[Kafin Noe’man]

I’d like to ask another question. Currently, I always put a uniflow tank on my plane, and use a tongue muffler on its engine. My setup is like this:
— Filling/Uniflow Tube: is uncapped
— Fuel Pick-Up Tube: direct line to the needle-valve assembly
— Overflow Tube: is capped when flying

I don’t run muffler pressure, and I leave the nipple on the tongue muffler open.

Now, I wanna switch the tongue muffler into the stock tube muffler. Is the setup going to be the same? Or will it be different?

I have seen some people are capping both of the Filling/Uniflow & Overflow tubes when using the tube muffler, what is this now?


Best,
Kafin

[Colin McRae]

Kafkin, I typically use uniflow fuel tanks and run stock tube mufflers with pressure. I find the engine's run more consistent with pressure.

Fuel pickup tubed to NVA
Uniflow vent tubed to muffler nipple
Overflow is plugged during flight

Pics are of an Enya 29-5224 and OS 25LA

One thing to be aware of. If you decide to use muffler pressure, the needle will need to be reset a bit leaner for the same desired ground rpm.

And on the overflow plug, recommended to use a short piece of fuel tubing with a bolt to plug to ensure a good seal. Don't just use a cap. And do a simple pressure test on the fuel system.

You also mention that you have seen some plug both the uniflow vent and overflow when using a tube muffler. This does not make sense to me. If you plug both the tank vent and overflow, the tank has no way for air to enter. The uniflow vent needs to be unplugged (no muffler pressure), or tubed to the muffler for pressure.

[John Paris]

Kafin,
If you decide to use muffler pressure to the uniflow, be careful that the fuel does not siphon out into the muffler before you get a chance to start the engine.  I normally wait until I am ready to flip to hook up the pressure line to the uniflow vent.  This condition is easy to spot on the pavement, not so easy in the grass.

As far as using a tube muffler vs a tongue muffler, I do not see any particular need to add muffler pressure if you did not need it before.

John

[Ken Culbertson]

Fuel pickup tubed to NVA

Colin, I am curious how you fill the tank without flooding the engine.  I always use d a fill tube and an overflow tube.  Just curious.

Ken

[Colin McRae]

Colin, I am curious how you fill the tank without flooding the engine.  I always use d a fill tube and an overflow tube.  Just curious.

Ken

On my Brodak uniflow tanks (only 3 tube connections), the uniflow vent is also the fill tube. I remove the overflow plug and pull off the tube (muffler pressure) on the uniflow vent at the tank. Then I fill through the uniflow vent. Then just before engine start, I reconnect the tube from the muffler to the uniflow vent and plug the overflow.

[Steve Helmick]

A machine screw is a poor way to plug a short length of fuel hose to plug off a uniflow tank. It's not uncommon for air to leak past the threads. A BB in the tube is much better, with zero chance for leakage. I think it's DuBro that makes plastic plugs for the job...they also work fine. A leak in a uniflow system makes it not function as a uniflow system.

I rather like muffler pressure, but I always caution others that if you sometimes have a muffler come loose or fall off, it's NOT FOR YOU! I like it because it keeps a fair amount of tiny bugs and seeds out of the fuel tank and fuel filter. You can also make an air filter for the uniflow inlet from a lightweight fuel filter (Master Airscrew makes one) and a short section of fuel hose.

There is only a very slight flow of exhaust into the tank...only as much flow as the volume of fuel being sucked into the engine. Size of the pressure tap makes no difference. No, it doesn't clog up the tank. 

Do NOT use a gasket on the muffler/engine faying surface. If you think it's needed, lap both surfaces flat and seal with red silicone gasket maker stuff from the auto parts store, but remember that there is one very big 'leak' out the back of that muffler. What you need to avoid is a change in the amount of leakage. Socket head machine screws, tighten goot 'n tight. LocTite is optional.  Steve

[Dan McEntee]

A machine screw is a poor way to plug a short length of fuel hose to plug off a uniflow tank. It's not uncommon for air to leak past the threads. A BB in the tube is much better, with zero chance for leakage. I think it's DuBro that makes plastic plugs for the job...they also work fine. A leak in a uniflow system makes it not function as a uniflow system.

I rather like muffler pressure, but I always caution others that if you sometimes have a muffler come loose or fall off, it's NOT FOR YOU! I like it because it keeps a fair amount of tiny bugs and seeds out of the fuel tank and fuel filter. You can also make an air filter for the uniflow inlet from a lightweight fuel filter (Master Airscrew makes one) and a short section of fuel hose.

There is only a very slight flow of exhaust into the tank...only as much flow as the volume of fuel being sucked into the engine. Size of the pressure tap makes no difference. No, it doesn't clog up the tank. 

Do NOT use a gasket on the muffler/engine faying surface. If you think it's needed, lap both surfaces flat and seal with red silicone gasket maker stuff from the auto parts store, but remember that there is one very big 'leak' out the back of that muffler. What you need to avoid is a change in the amount of leakage. Socket head machine screws, tighten goot 'n tight. LocTite is optional.  Steve

      i am in favor of BBs as a plug for a fuel tank cap also. years ago, I got turned onto a Windy suggestion of using the the colored plastic pegs from Lit-Brite toys. They are translucent and almost glow in the sunshine if you drop it in the grass. Some have longer stubs that works very well and hold fuel line nice and tight. the shorter ones you just need to stake a knife and scribe some spiral grooves around it to give the fuel line something to grab. I keep some of these in my flight box along with a small container of BBs, and when I use BBs I usually use two or three, and that gives you a more substantial grip to pull it off the tank or tube. If I have to use an old screw to plug a piece of fuel line, I put it in head first and it creates a better seal.

    On muffler screws, I have learned that you do not want to crank on then too hard. The screws are plenty hard, but the aluminum in the muffler isn't I think that is more important to have the screws go into the muffler as far as allowed to grab as many threads as possible. if you only thread in 1/4" or so, you start pulling threads from the first time you tighten them down. If you have to remove those screws frequently, you will find that from the heat/cool cycles that the actually get tighter, and if you have allen wrenches with rounded off business ends, you will strip the hex in a heartbeat.  If you use screws that are as long as possible, and you clean the screws and the muffler threads and apply a dot of loctite, you will find that you don't have to tighten them like a lug nut on your car. Just make them good and snug, and the same heat/cool cycles will keep them where they belong. And always check your allen wrench ends. Touch them on a grinder lightly from time to time to make them square and sharp. Refrain from using a ball type allen wrench, as they don't fit as well. In the wrenches I have measured, I find that the tiny surface that makes contact is several thousandths undersize from the manufacturing process. I use the to run down screws that are at a difficult angle until they are snug, but figure out a way to use a standard wrench to tighten things down.
  Type at you later,
   Dan McEntee

[Steve Dwyer]

When pressurizing a uniflow tank (which many do) I have wondered why someone would connect the muffler pressure to the uniflow line and not the overflow. It would seem to me this would add to the disturbance of the fuel in the tank (bubbles) possibly resulting in air reaching the fuel pick up. Venting the muffler to the overflow simply adds pressure to the space above the fuel level.

Steve

[Brett Buck]

When pressurizing a uniflow tank (which many do) I have wondered why someone would connect the muffler pressure to the uniflow line and not the overflow. It would seem to me this would add to the disturbance of the fuel in the tank (bubbles) possibly resulting in air reaching the fuel pick up. Venting the muffler to the overflow simply adds pressure to the space above the fuel level.

Steve

     It also makes it not uniflow any more - you add pressure in the ullage, you have to plug the uniflow vent , so you are back to running leaner as the fuel runs out. It does slightly reduce the leaning out effect over using atmospheric pressure, but it still gets leaner as the fuel runs out.

   Note that this is how I ran the tank on the Skyray.

   Brett

[Lauri Malila]

When pressurizing a uniflow tank (which many do) I have wondered why someone would connect the muffler pressure to the uniflow line and not the overflow. It would seem to me this would add to the disturbance of the fuel in the tank (bubbles) possibly resulting in air reaching the fuel pick up. Venting the muffler to the overflow simply adds pressure to the space above the fuel level.

Steve

What Brett said, and also why would using muffler pressure introduce more bubbles in the tank than when not using pressure? If fuel consumption stays the same, also the amount of replacement air stays the same.
But, pressure or not, you are kind of right. Incoming bubbles can disturb the fuel pickup if uniflow tube end is too close to pickup tube end. Usually, if the distance is less than 1/2", you can expect problems. Not during the flight, but when adjusting needle valve. L

[Steve Dwyer]

Yes I understand, pressuring a uniflow tank eliminates the uniflow properties and returns its function to the "liquid head effect" potentially causing variations in engine mixture in the venturi inlet.      I have never opened a metal uniflow tank to see what the dimensional position of the uniflow line relative to the pickup is. I have, however, been positioning the UF line centered midway laterally along the outboard edge of a tank when "building a poly clunk" tank and have not had a problem of leaning out except for the last moment when the residual amount of fuel migrates to the back outer corner.

Steve

[Brett Buck]

What Brett said, and also why would using muffler pressure introduce more bubbles in the tank than when not using pressure? If fuel consumption stays the same, also the amount of replacement air stays the same.
But, pressure or not, you are kind of right. Incoming bubbles can disturb the fuel pickup if uniflow tube end is too close to pickup tube end. Usually, if the distance is less than 1/2", you can expect problems. Not during the flight, but when adjusting needle valve. L

  Mine is about 1/8", no issues.

       Brett

[Dan McEntee]

Yes I understand, pressuring a uniflow tank eliminates the uniflow properties and returns its function to the "liquid head effect" potentially causing variations in engine mixture in the venturi inlet.      I have never opened a metal uniflow tank to see what the dimensional position of the uniflow line relative to the pickup is. I have, however, been positioning the UF line centered midway laterally along the outboard edge of a tank when "building a poly clunk" tank and have not had a problem of leaning out except for the last moment when the residual amount of fuel migrates to the back outer corner.

Steve

   " Traditional" or most common placement for the uniflow line in a metal tank is soldered to the pick up tub, at the same level, and about 1/2" or more towards the front of the tank. I don't know how that got started, maybe one of those things that "It's the way it's always been done" things. But it doesn't have to be installed like that. Of the tank has a wedge or pointed outside edge, it's kind of important to get the YF tube at the same level as the pick up, just for reference same when mounting the tank. Mount it above or below the pick up and the tank has to be adjusted accordingly up or down. Just for the sake of education, you ought to open up a couple of old metal tanks just to see what's going on inside. Just make sure you don't have any fuel in them or flush them with a nonflammable liquid, even water,  to flush any fuel out. Then use a propane torch to gently heat the end cap, and when you see solder flowing, use a screw driver or ice pick to try and pop the cap off.  A burner on a hot plate will get it hot enough for this also. With a gloved hand, grab the tank while still hot and wipe off the still flowing excess solder if you plan on putting the cap back on and re-soldering. Tank repair and building really is a very handy skill to know. I had trouble finding a tank that would work with the two ARF Noblers I had been trying to get finished up, so I carved a wooden blank that was the dimensions I needed to get the most fuel on board I could, then determined where I wanted the various tubes to exit the tank to make plumbing the fuel lines as easy as possible, and an hour or two later I had the first tank finished. I tested it in the first model, and it worked right off the bat, so i made another for the second airplane. With the way the prices of everything is climbing these days, metal tanks can be made pretty cheaply once you are set up and tooled up for it.
  The late Jim Thomerson once did a great explanation of how a uniflow tank works, and the history behind the concept. It goes all the way back to the invention of the steam engine, if not further, and I think it had something  to do with regulation of boiler pressures and feed them with fresh water to help keep a certain level and temperature. It may be on the forums here some where , but I'm pretty sure it was on Stuka Stunt also.
 Type at you later,
  Dan McEntee

[Steve Dwyer]

Dan,

Tank building is definitely a skillset to develop that could easily fall into my bucket list. I did a ton of silver soldering using map gas one summer during college for a refrigeration company and plumbed all the copper water pipe in my house using 50/50 years later. Today I find my determination to fabricate my own tank tends to wane by the time I'm ready to wrap up a new model. A simple call to Brodak, parting with some cash and its a done deal. I must say I've had two tanks from B leaking at a seam and at the brass tubing. Bringing out the Weller electric gun doesn't qualify for soldering a tank but I can correct the leak and pass a water submerge pressure test. Hey, it brings up another thought, Aircraft Spruce sells Proseal used for building/sealing aluminum aircraft fuel tanks in the experimental area...you'd have to sell your first born to buy it but I wonder if anyone has tried using it on our smaller version application?

Steve