Standard vent vs Uni-flo

[Terry Caron]

What's the difference in the plumbing?

Terry

[Paul Smith]

With standard venting the vent is in the air space within the tank and the engine runs leaner as fuel is depleted.

With uniflow the air vent is submerged in the fuel and the setting remains constant until the fuel is depleted below the vent level.

[Terry Caron]

Can you supply a drawing of the internal tubing layout for the 2 types?

Thanks.

Terry

[Tim Wescott]

Terry: look at the bottom of this page http://www.aeromaniacs.com/Tips.html

Remember if you're building one from scratch that you want the uniflow to be separated from the pickup line by about half an inch, to keep bubbles from getting into the pickup.  If you're putting both along the ridge of the tank then put the pickup at the very back and the uniflow forward of that by half an inch or so.

[Terry Caron]

Excellent Tim - thanks! 

The diagram shows 4 tubes, with separate fill/U-vent lines - is the separate fill tube necessary?

Terry

[Gerald Arana]

Excellent Tim - thanks! 

The diagram shows 4 tubes, with separate fill/U-vent lines - is the separate fill tube necessary?

Terry

No. You can fill through the uniflow line. That's what I do.

Cheers, Jerry

[Terry Caron]

Made sense to me - thanks Gerry. 

Terry

[Steve Helmick]

Note also that you can leave the overflow uncapped and effectively have a standard vented tank. You can decide if you want to use uniflow or not, simply and quickly. Use something good for the overflow cap. Those cute little yellow caps are not trustworthy. Neither is a screw stuck into a short piece of fuel hose. A BB stuck into a short piece of hose is almost 100% sure.

Also, many of us plumb muffler pressure to the uniflow vent. Your engine may or may not like it, but it seems that a lot of engines like it. It becomes real important that your stuff doesn't often spring leaks or the muffler come loose. The nicest thing is that you get no bugs and seeds in the tank and filter, and no richening upwind/leaning downwind nonsense.   Steve

[Tim Wescott]

Excellent Tim - thanks! 

The diagram shows 4 tubes, with separate fill/U-vent lines - is the separate fill tube necessary?

Terry

That diagram was about how to convert an existing tank to uniflow.  If you're building from scratch just leave off old "fill" tube -- if I were converting one, I'd probably remove the fill tube and either put the uniflow in the same hole (while hoping not to confuse people) or cap it off with a scrap of sheet tin.

[Terry Caron]

I'll try it w & w/o muffler pressure Steve.

In fact Tim, I have a standard tank from an ARF Flight Streak w/a loose top tube, so I've converted it to Uniflo (if the new tube came out in the right place).

Terry

[Brett Buck]

I'll try it w & w/o muffler pressure Steve.

In fact Tim, I have a standard tank from an ARF Flight Streak w/a loose top tube, so I've converted it to Uniflo (if the new tube came out in the right place).

Terry

     I highly recommend soldering the end of the vent tube to the pickup tube, or to the tank, inside the tank. If you don't, the tube will be very prone to shaking loose. IF you do that you can also ensure it stays where it is supposed to be. It's pretty important because the end of the vent tube defines the pressure reference for the tank.

     Brett

[Dan McEntee]

  The late Jim Thomerson researched the theory to the "Nth" degree and if you search his name or just "uniflow" here and on Stuka Stunt, I think he had some great posts with diagrams. The concept goes all the way back to the invention of the steam engine, or maybe even further. Jim was a college professor and was very good at explaining things, besides just being a neat guy! This subject was one of his favorite things to talk about sometimes, I think.
  Type at you later,
  Dan McEntee

[Terry Caron]

Thanks for the heads-up Brett, I hadn't considered vibration and that rather long tube.
I'll redo it -  more practice of my soldering technique.

Thanks Dan - I'm still a bit fuzzy on the principles involved.

Terry

As a follow-on:
Does a chicken hopper accomplish much the same as a Uniflo by maintaining a constant head at the pickup tube?

[Tim Wescott]

Does a chicken hopper accomplish much the same as a Uniflo by maintaining a constant head at the pickup tube?

Two different things.  A chicken hopper tank is shaped to manage sloshing fuel and keep the pickup tube covered right down to the dregs when the tank is otherwise shaped wrong.  It can be either standard vent or uniflow.

[Terry Caron]

Two different things.  A chicken hopper tank is shaped to manage sloshing fuel and keep the pickup tube covered right down to the dregs when the tank is otherwise shaped wrong.  It can be either standard vent or uniflow.

Different mechanically, but it's been my understanding that the amount of fuel at the pickup tube, "regulated" by the hopper's relatively small volume, maintains ('til the hopper begins to empty) an essentially constant fuel head, that result being what the Uniflo does by a simpler design.

Terry

[Motorman]

There are 2 kinds of chicken hopper tanks, true chicken hopper and big tank little tank. The first maintains an airgap in the small tank and the other is basically a baffle.


MM

[Terry Caron]

OK, so this is a Uniflo hopper, the baffle-type having standard venting?

Terry

[Dan McEntee]

    That is a VERY good design for a tank, if you are able to fabricate it. One of my favorites and can be made to any size and works the same. You can even move the "dog house" aft for any extra clearance and plumbing issues. Great design for making a tank to fit is small places like a Ringmaster of Flight Streak. can be made with common K&S sheet stock also. And yes, it is a combination of the two.
   Jim Thomerson also used to do a great demo on what a true chicken hopper system is, and I believe that also had basis in the steam engine also, not necessarily in poultry farming!
  Type at you later,
     Dan McEntee

[Terry Caron]

So what makes this worth the extra construction difficulty over the simpler design to which Tim linked above?

And, while I don't remember where I found them, for anyone interested in a uniflo chicken hopper, here's the layout for it.

Terry

[Tim Wescott]

So what makes this worth the extra construction difficulty over the simpler design to which Tim linked above?

I don't think that page to which I linked has any designs, but the pages it links to may.

A standard tank can't be too high, or the fuel will be flung away from the pick-up tube in maneuvers.  So the standard tank is about twice as wide as it is high, and two or three times as long as it is high (i.e., 1 x 2 x 6 inches).  If you have room for it, it's great.

If you have a Ringmaster and you want to keep it in the air for a whole pattern, you don't have enough room for an inch-tall tank to do the job.  If you make the tank taller then you still can't do the pattern, because now the engine cuts out in some maneuver or another with fuel still in the tank.  With a chicken hopper tank, every time fuel sloshes by the hopper it runs in, and keeps the engine going during maneuvers.  Your plane stays in the air, and you are happy.  You can get the same effect with a clunk tank, but even then it's a shoe-horn job (you need about 2-1/2 ounces to do the pattern with a Ringmaster and a 20 FP).  So a chicken-hopper tank might look like a good deal.

[Brett Buck]

A standard tank can't be too high, or the fuel will be flung away from the pick-up tube in maneuvers. 

    What is critical is not the height (depth), it's the apex angle. As long as that is sufficiently sharp, you can make the tank any depth you want, and the only limitation is how the sharp angle limits the capacity.

    Brett

[Chris Wilson]

Hi Brett, I am assuming that the apex angle becomes non critical with a moveable 'clunk' or weighted pickup ?

And does the pickup normally move with the inlet pipe or is it fixed in place with the flexible feed setups?

Thanks.

[Tim Wescott]

    What is critical is not the height (depth), it's the apex angle. As long as that is sufficiently sharp, you can make the tank any depth you want, and the only limitation is how the sharp angle limits the capacity.

So what's the correct apex angle?  A 60 degree peak wasn't sharp enough -- that tank ended up getting converted to clunk, as such it worked well for years.

[Terry Caron]

Guys, this is great - the light you're bringing begins to shine through the muddy notions I've had about tank functions. 

Terry

[Chris Wilson]

Hi Brett, I am assuming that the apex angle becomes non critical with a moveable 'clunk' or weighted pickup ?

And does the pickup normally move with the inlet pipe or is it fixed in place with the flexible feed setups?

Thanks.

No one?

I see the 'uniflow' principle as equating to the Mariotte bottle principle where it is a solution to the problem of changing head pressure giving a similar changing outlet pressure in a stationary and virtually two dimensional environment.

For control line use that suffers G forces it works well with wedged tanks because that shaped area forces the relevant fuel space to become fixed and almost stationary, just like the lab tested Mariotte bottle does. The issue with wedged tanks is that they hold less than one that is not and to gain back the capacity in a conventional nacelle extra length is used with the trade off being a longer fuel draw.

So the evolution of a tin tank with no wedge and greater capacity came about with a mobile volume of the critical (between the air inlet and the fuel pickup) fuel space and the complimentary following pickup or 'clunk'  - and of course the RC style plastic cube that extends the space solving problem even further.

Now if we have a 3D fuel volume and a 3D fuel pickup does the air inlet necessarily have to follow that pickup in order to be a true 'uniflow?"

I have seen fixed air inlets and mobile ones that are tied to the fuel inlet clunk both working well.

The fixed inlet seems to have the advantage of being adjustable for evening out upright to inverted flight but does seem to break the rule of having a constant distance between the mobile pickup and the fixed inlet.

Perhaps the differences do not matter but it is something that I have pondered about.

Thanks.

[Terry Caron]

Clear and concise Chris, thanks. 

Terry

[Bill Johnson]

Chris,
Perhaps I'm not understanding something but it seems to me that whether you have a clunk type or a metal tank, the uniflow vent is still, in both cases, in a fixed position in relation to the pick-up.

[Brett Buck]

Hi Brett, I am assuming that the apex angle becomes non critical with a moveable 'clunk' or weighted pickup ?

     Yes. Clunks are a different story.

And does the pickup normally move with the inlet pipe or is it fixed in place with the flexible feed setups?

  I think you mean, does the vent tube move with the clunk, or not. I have done it three different ways - a rigid pipe, the vent connected to the clunk, or a second clunk. I had the best results with a rigid pipe. The dual clunk works OK most of the time. The single clunk with both the pickup and vent works OK, as long as you arrange it so the clunk can move freely, and not be spring-loaded to one side or the other.

   The nice thing about the rigid vent tube is that you can easily move it to adjust the upright/inverted times without moving the entire tank up and down.

    I still much prefer conventional tanks since I can make them the size I need, and, they usually cut off much more cleanly. But my first successful Fox 35 runs were achieved with a round Sullivan clunk tank.

    Brett

[Terry Caron]

 
The nice thing about the rigid vent tube is that you can easily move it to adjust the upright/inverted times without moving the entire tank up and down.
   Brett

Brett, would you please explain the adjustment process for the vent tube?

Thanks.

Terry

[Chris Wilson]

Chris,
Perhaps I'm not understanding something but it seems to me that whether you have a clunk type or a metal tank, the uniflow vent is still, in both cases, in a fixed position in relation to the pick-up.

Hi Bill,
            with clunk tanks its a given that the fuel pickup moves but .......the air inlet is only at a fixed position to it if its physically tied to it.
As in both move as one.

Sometimes the air inlet is a rigid brass tube and the fuel inlet is flexible silicon and as such the distance between them will vary.
   

[Terry Caron]

Sometimes the air inlet is a rigid brass tube and the fuel inlet is flexible silicon and as such the distance between them will vary.
   

Chris -
Is this then still a Uniflo?

Terry

[Tim Wescott]

Hi Bill,
            with clunk tanks its a given that the fuel pickup moves but .......the air inlet is only at a fixed position to it if its physically tied to it.
As in both move as one.

Sometimes the air inlet is a rigid brass tube and the fuel inlet is flexible silicon and as such the distance between them will vary.

What's important in a uniflow tank is the relationship between the uniflow vent and the hole in the spraybar.  I'm not sure what having a clunk on a uniflow does, other than make it move up and down when you're looping this way and that.

I see the 'uniflow' principle as equating to the Mariotte bottle principle where it is a solution to the problem of changing head pressure giving a similar changing outlet pressure in a stationary and virtually two dimensional environment.

Yup.  It figures that someone else has used that principle.

Brett, would you please explain the adjustment process for the vent tube?

I'm not Brett, but you move the tip of the vent tube the way you'd move the entire tank.  If the plane is faster upright than inverted then you need to richen it upright and lean it out inverted, so raise the tank or vent tube (turn the plane over to visualize what that does inverted.  If the plane is faster inverted than upright then you need to lean it while upright and richen it while inverted, so lower the tank or vent tube.

If this doesn't make your brain hurt, think of it this way: with a rigid tank that has the vent tube nailed down, you're accomplishing the goal of moving the vent tube by moving the whole tank.  In a uniflow setup, the head pressure (more accurately the amount of suction needed) at the spraybar hole is a function of the amount of drop from the spraybar hole to the opening in the vent tube.  How you move that opening around is largely immaterial to how the engine behaves, so you can either move the vent within the tank or you can move the entire tank.

[Brett Buck]

Brett, would you please explain the adjustment process for the vent tube?

  Usually, if you make it rigid, you can rotate the tube in the stopper, so the far end moves up and down. The position of the far end of the vent tube is the reference for the entire tank. So instead of shimming the entire tank up or down, you move just the end of the tube. Once you find the right spot, you can solder a piece of brass in between the 3 tubes to hold it in place permanently.

   Of course, the usual procedure for determining how to adjust it applies. Fly upright, get an accurate lap time (take off, fly level for several laps, then time 3 laps, then divide by 3), flip over inverted, repeat. If it is faster upright, move the vent tube (or entire tank) higher, if it's faster inverted, move the vent tube lower. It can be pretty sensitive, so I usually shim in 1/32" increments until it's close, then 1/64 or a few layers of masking tape.

     Note that this should be done, ideally, *entirely* on the upright/inverted level flight lap times. Many will recommend shimming it based on inside/outside loops, and setting it based on where in the arc of the loop it breaks into a 2-stroke. Like, it breaks at the 90 degree mark upright and at the 120 degree mark inverted, move the tank up. You can do that in a pinch, but recheck in level flight. If the level flight tank position needs to be significantly different from the maneuvering tank position (like more than 1/32"), there's some other issue with the engine.

    This effect has plagued my buddies and I for years because the effect seems to be grossly exaggerated in dry sea level air. I have had engines that ran 5 seconds upright and 6 seconds inverted, with 1/4" shim, that still ran faster on outsides. We (and really, Ted, before I even moved here) discovered this in the late 70's when schneurle engines were tried. This is why wre hung on with baffle-piston engines until the bitter end - none of could get acceptable run symmetry with schneurle engines, despite the fantastic increase in power, it still wasn't worth it. The effect can sometimes disappear completely or be greatly reduced in humid and relatively thin air and many of the midwest/southeast engine reworkers have claimed it's nonsense. Some engines are far more prone to it than others and appear hopeless, but any of the normal engines (PA or RO-Jett) are capable of a symmetrical run most of the time with changes to the intake and fuel supply system.

  Note that there are many other small details of engine run that matter a lot. Like this thread:

http://stunthanger.com/smf/index.php/topic,28567.msg275948.html#msg275948

    Paul and I were discussing this at Golden State, with regard to the lower bound for the Igor feedback control system. It can be made to overdo the airspeed control, and back off in spots you don't necessarily want. This is exactly the same effect I was discussing in the thread above about engine feedback based on the oil content. This is why it is so complex to get these engines, even the really excellent systems we have now, running exactly the right way. And also why most of what you hear about it is utter and complete bullshit.

     Brett

[Terry Caron]

OK, I understand the mechanics of adjustment, tho' I can't grasp the physics involved.
However, with a clunk tank, when I make up tubing I can set the vent opening anywhere up/down, fore/aft within the tank.
Is it that initial vent opening placement within the tank (and therefore exact tank position in/on the plane) isn't important as long as the vent tube opening is level with the spraybar and I have some adjustment room available?
Assuming MOL "normal" tank placement, of course - not out on one wing. 

Terry

[Brett Buck]

OK, I understand the mechanics of adjustment, tho' I can't grasp the physics involved.
However, with a clunk tank, when I make up tubing I can set the vent opening anywhere up/down, fore/aft within the tank.
Is it that initial vent opening placement within the tank (and therefore exact tank position in/on the plane) isn't important as long as the vent tube opening is level with the spraybar and I have some adjustment room available?
Assuming MOL "normal" tank placement, of course - not out on one wing. 

Terry

OK, we are clearly miscommunicating. The only thing that matters is where the end of the vent tube ends up *inside* the tank, which I would refer to as the "free" or "open" end.  The routing of the tube to get out of the stopper doesn't matter at all.

      Brett

[Tim Wescott]

OK, we are clearly miscommunicating. The only thing that matters is where the end of the vent tube ends up *inside* the tank, which I would refer to as the "free" or "open" end.  The routing of the tube to get out of the stopper doesn't matter at all.

And, for it to be a uniflow tank the vent has to terminate within the tank in such a manner that it stays submerged for nearly the entire run -- basically until there's nothing left but dregs.  This is why the conventional uniflow tank has the uniflow vent located close to the pickup.  If the vent comes out in a spot where the fuel is flung away from it immediately as the plane starts flying, then it isn't a uniflow.

[Terry Caron]

OK, we are clearly miscommunicating. The only thing that matters is where the end of the vent tube ends up *inside* the tank, which I would refer to as the "free" or "open" end.  The routing of the tube to get out of the stopper doesn't matter at all.

      Brett

Pardon the "quick & dirty" sketch, but this is what I'm trying to ask about - do they both fit your criterium?

Terry

[Terry Caron]

And, for it to be a uniflow tank the vent has to terminate within the tank in such a manner that it stays submerged for nearly the entire run -- basically until there's nothing left but dregs.  This is why the conventional uniflow tank has the uniflow vent located close to the pickup.  If the vent comes out in a spot where the fuel is flung away from it immediately as the plane starts flying, then it isn't a uniflow.

So the rigid brass tube set-up Chris mentioned previously and Brett noted as giving best results is not uniflo?

Terry

[Tim Wescott]

So the rigid brass tube set-up Chris mentioned previously and Brett noted as giving best results is not uniflo?

You bring the brass tube close to the rear of the tank, hard up against the outside (outside of the circle) surface, and maybe 1/2 inch or 3/4 inch forward from the back so that it doesn't foul the clunk.  Then it stays submerged, and you have a uniflow tank.  It's what might fit your bottom drawing in your "quick & dirty" sketch, but definitely not the top one.

[Brett Buck]

So the rigid brass tube set-up Chris mentioned previously and Brett noted as giving best results is not uniflo?

Terry

  No. Its still up against the outside wall of the tank, so it still works as uniflow. The total adjustment range is about 1/8" up or down, so rotating it in that range doesn't move it side to side to any great degree.

    I think these threads get greatly overcomplicated because of lack of good pictures, the idea is really pretty simple.

   Brett

[Terry Caron]

  I think these threads get greatly overcomplicated because of lack of good pictures, the idea is really pretty simple.

   Brett

I agree completely Brett, so pardon another Q&D sketch.
I'm sure the problem is my lack of comprehension of the basic physical principle involved.
But if this is what you're describing, the vent end is submerged for roughly only half the fuel load, which seems to conflict with Tim's comment on the uniflo principle.

Terry

[Brett Buck]

I agree completely Brett, so pardon another Q&D sketch.
I'm sure the problem is my lack of comprehension of the basic physical principle involved.
But if this is what you're describing, the vent end is submerged for roughly only half the fuel load, which seems to conflict with Tim's comment on the uniflo principle.

  Where do you think the fuel goes?  Because your sketch shows a mostly correct tube alignment that appears to keep the vent submerged for the entire run. The fuel all goes to the outboard edge of the tank due to centrifugal force, roughly 2.5G's worth.

   BTW, the one issue with your sketch is that the uniflow tube will interfere with clunk movement. Bend the uniflow tube into an flattened "s" curve so that it travels along the outside edge of the tank.

   Brett

[Mark Scarborough]

I agree completely Brett, so pardon another Q&D sketch.
I'm sure the problem is my lack of comprehension of the basic physical principle involved.
But if this is what you're describing, the vent end is submerged for roughly only half the fuel load, which seems to conflict with Tim's comment on the uniflo principle.

Terry

remember the fuel will be on the outboard face of the tank in flight,, not on the bottom of the tank

[Terry Caron]

OK - GOT IT!!!   
My apologies for Senior Dum*ssitude guys - my pea brain wasn't considering flight dynamics.

I owe all of you a cup of coffee. 

Terry

[Terry Caron]

Just to be sure I have it, in this admittedly exaggerated and unrealistic example, assuming the vent end is in proper relation to the spraybar, this would work as a uni-flo even inverted as long as there is sufficient fuel to immerse the vent end?

Terry

[Tim Wescott]

Just to be sure I have it, in this admittedly exaggerated and unrealistic example, assuming the vent end is in proper relation to the spraybar, this would work as a uni-flo even inverted as long as there is sufficient fuel to immerse the vent end?

If that's a side view, then there's two problems:

First: the uniflow tube will become uncovered while inverted after very little fuel has been used.

Second: your propeller is way too small  .

[Chris Wilson]

Chris -
Is this then still a Uniflo?

Terry

Strictly speaking, no as the distance will vary because the centre of the pickups arc is not the air inlet.

But what seems to matter most is steady level flight and that is what it is 'tuned' for.

Great thread guys, great!

[Terry Caron]

If that's a side view, then there's two problems:

First: the uniflow tube will become uncovered while inverted after very little fuel has been used.

Second: your propeller is way too small  .

Granted, but for some period, it works as uni-flo, right?
Point being, that with a clunk, tank position per se isn't critical, unlike a tank with fixed tubes.

Prop noted - no wonder my plane won't get off the ground!
I'll order some bigger ones.  

Terry

[Tim Wescott]

Granted, but for some period, it works as uni-flo, right?
Point being, that with a clunk, tank position per se isn't critical, unlike a tank with fixed tubes.

Yes, for a little while it acts as a uniflow -- I think you have the concept.

I'm not sure if this bit will clarify or confuse things for you, but in "normal" tank all of the fuel is at atmospheric pressure or higher, because of the weight of the fuel pressing toward the outside of the circle.  In a uniflow tank, the point at which the vent opens into the tank is at atmospheric pressure, and most of the tank is pulling a slight vacuum.  So you're actually losing some pressure at the spraybar (that's not the cool part) but in return you're getting constant pressure at the spraybar (that's the cool part).  This is both why uniflow works, and why a leak in a uniflow tank causes oddball run problems.

[Terry Caron]

Wow! That didn't confuse me at all Tim - I think I finally understand! 

But, at the risk of quickly getting in over my head with the physics, the partial tank vacuum comes from fuel draw and an on-going delay in the vent's attempt to equalize internal pressure with atmospheric?
(I can mount and adjust a tank now, thanks to the education I've received here, but it bugs me to not understand the why of things.)

Terry