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General control line discussion => Open Forum => Topic started by: Terry Caron on October 25, 2015, 11:39:28 AM
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What's the difference in the plumbing?
Terry
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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.
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Can you supply a drawing of the internal tubing layout for the 2 types?
Thanks.
Terry
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Terry: look at the bottom of this page http://www.aeromaniacs.com/Tips.html (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.
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Excellent Tim - thanks! H^^
The diagram shows 4 tubes, with separate fill/U-vent lines - is the separate fill tube necessary?
Terry
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Excellent Tim - thanks! H^^
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
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Made sense to me - thanks Gerry. H^^
Terry
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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. ;D Steve
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Excellent Tim - thanks! H^^
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.
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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
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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
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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
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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?
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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.
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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
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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
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OK, so this is a Uniflo hopper, the baffle-type having standard venting?
Terry
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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
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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
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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.
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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
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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.
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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.
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Guys, this is great - the light you're bringing begins to shine through the muddy notions I've had about tank functions. H^^
Terry
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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.
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Clear and concise Chris, thanks. H^^
Terry
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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.
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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
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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
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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.
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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
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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.
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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
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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. ;D
Terry
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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. ;D
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
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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.
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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
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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
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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.
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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
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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
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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
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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
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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. ;D
Terry
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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
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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 >:D.
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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!
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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 >:D.
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. ;D
Terry
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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.
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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
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"OK, I understand the mechanics of adjustment, tho' I can't grasp the physics involved."
Terry, if its any help here the uniflow's guiding principle is that the negative pressure above the fuel level ALWAYS equals the positive pressure below it.
The point of adjusting the air inlet's position away from symmetrical is to compensate for the asymmetry of the engines run eg. - the engine may need more fuel pressure when up right so its hardly beneficial having the tanks supplying a constant pressure in any orientation.
Cheers.
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With a bit of clarification I'm sure it'll be helpful Chris.
Tim says in a previous response "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."
Is his + pressure point your "below" and his - pressure point your "above"?
And while I'd considered asking in a separate thread, since you've conveniently brought it up ( ;D), why does the engine's fuel demand vary with flight position?
EDIT: Upon reflection, I think I should be asking why, since constant pressure is the point of uni-flo, does the engine's fuel supply pressure vary?
Gravitational/centrifugal forces upon the remaining fuel?
Terry
BTW, from the ratio of viewers to responders, I hope many others are having their unasked questions answered here.
And I'm happy to report that now, after several previous unsuccessful attempts, I understand Nigel's explanation here:
http://www.fraserker.com/heli/uniflow/how_uniflow_works.htm
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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.
Thanks for the explanation, Chris. I had researched on building clunk uniflows but didn't realize some were using a fixed uniflow tube. I guess I got as far as the Tulsa Gluedobber's site, which has all you need to make one, and stopped there. Because the uniflow vent location in relationship to the fuel pick-up is somewhat critical, it never occurred to me to have one fixed and the other flexible.
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Excellent Tim - thanks! H^^
The diagram shows 4 tubes, with separate fill/U-vent lines - is the separate fill tube necessary?
Terry
In addition to the other comments about the extra, unnecessary "4th" tube for filling it is "essential" to note that any vent other than the uniflow vent "MUST" be capped for the uniflow to function properly. The uniflow vent must be the only opening in the tank as it is the action of fuel being drawn from the tank that requires that air be brought in thru the vent that is instrumental to the constant fuel delivery and consistent engine run until the uniflow vent is uncovered as the tank nears empty.
Ted
Ted
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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.)
Delay has nothing to do with it (sorry). This is one of those places where it may help to think backwards: the partial tank vacuum must happen, because if it doesn't the fuel above the uniflow vent would just flow out the vent.
With a bit of clarification I'm sure it'll be helpful Chris.
Tim says in a previous response "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."
Is his + pressure point your "below" and his - pressure point your "above"?
Yes
BTW, from the ratio of viewers to responders, I hope many others are having their unasked questions answered here.
You ask good questions...
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RSM sells well made chicken hoppers. $18. Worth it. Brodak also. They work well. Muffler pressure to uniflo vent is sweet. Usually it works. Engine runs same from beginning to end. Vibration issues, common in profiles, can sabotage the uniflo causing engine weirdness. Foam and bubbles make run erratic, uniflo exacerbates this condition. Usually capping the uniflo and running pressure to the overflow solves this. Engine leans up slightly during the flight. Not a problem.
I have read many different theories on how uniflo works. Explanations vary.
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I have read many different theories on how uniflo works. Explanations vary.
Only one is correct, and it is not at all mysterious, It's a fundamental physical principle that is taught in junior high science - the pressure of a fluid is proportional to the depth.
Brett
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Right.
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Brett is right,and the "depth" or head pressure perceived by the fuel inlet is exactly where the air inlet (or equaliser) comes in.
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How do you account for engine breaking? Among other variables. If the uniflo hitch up created a constant pressure that accurately countered the momentum shift of the burnt up fuel, fuel head perceived by the engine would be constant even when model nose was pointed up or down. A well setup uniflo does not preclude tuning in an engine break.
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How do you account for engine breaking? Among other variables. If the uniflo hitch up created a constant pressure that accurately countered the momentum shift of the burnt up fuel, fuel head perceived by the engine would be constant even when model nose was pointed up or down. A well setup uniflo does not preclude tuning in an engine break.
What the heck are you talking about? It sets a constant pressure WRT fuel burn at the point of the open end of the vent. It certainly doesn't ensure that the fuel delivery pressure is constant at the needle end in all conditions. The exact same principle applies to the rest of the system, so, on the ground, point the nose up, and the fuel delivery at the needle is atmospheric pressure at the vent minus the pressure head difference related to the vertical distance from the end of the vent to the needle. Point it nose down, same thing, except it adds to the pressure.
It makes the fuel pressure independent of the fuel remaining, and nothing else. All the other pressure variation effects still exist more-or-less unchanged.
Moreover, the "break" has less to do with fuel delivery pressure than it does with load on the engine. Compare from ground to air. Start a Fox, and you set it just barely into a constant 2-stroke. With a suction tank, when you take off, the fuel deliver pressure goes up. With a uniflow tank, it goes down (again from the same physical principle about pressure head and depth). In either case, after about a lap or so, its running in a 4-stroke. That's because the *load* went down, not because the pressure went up. Same with maneuvering - when you first start a corner, the pressure goes up, dramatically, as the airplane rapidly decelerates. You might think it would go dead rich because, effectively, it's the same as pointing the nose down. But in fact, it breaks into a 2-stroke. Plus all the other counter-examples (like 45 degree circling flight). Sometimes the pressure matters and the engine response follows the pressure, but many times, it goes in the opposite direction to the pressure. You can even model it on the bench, by changing prop load without moving the needle. It also shows, dramatically, how much unload there is, when you have to change your 12" 3-blade into an 8-4 2-blade to get the in-flight RPM.
In fact, many problems with engine runways can also be attributed to the same issue, when the fuel draw is minimal. With suction, once it takes off, the pressure is higher than static on the ground for the first part of the run, and everything is OK. As the fuel runs out, the pressure drops, leading to the well-known speeding up as the flight goes on. How much it changes depends on the width of the tank. Up to a point, the system is predictable, but if the tank it too wide, the pressure drops excessively towards the end and the system goes, effectively, unstable, as the engine can no longer suck the fuel that far "uphill", the speed increased, the pressure drop increases, etc, in a wild runaway that only stops when the engine goes so lean it starts to sag.
Convert the exact same tank to uniflow, change nothing else - and it happens almost immediately at launch, because the fuel pressure in a uniflow system from start to finish is the same as it is in a suction system at the end of the flight. There is a theory amongst nitwits that "uniflow causes runaways", because they don't notice that their suction tank does exactly the same thing at the end of the run. The real cause is a combination of inadequate fuel draw and an excessively wide tank.
That's the only issue I have with Terry's drawing. I would put the "bottom" of the tank up against the fuselage, instead of the "side" to get less pressure head drop on suction and an overall higher fuel pressure on uniflow.
Brett
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As I feared Brett, I'm immediately beyond the remnants of 50 yo high school physics left in my head, so I'll content myself with a basic understanding of how to make/modify/adjust a uni-flo tank.
One point of confusion though: your comment on too-wide tanks and the drawings.
I should have noted at the time, but I think you're aware they represent profile side-views.
I think of width as spanwise, so that rotating the tank would make it wider.
Would you clarify please?
Terry
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Hey Terry:
The whole act of spinning an airplane around on strings tends to make the terminology confusing.
If all else could be made equal, you'd want a tank that doesn't extend much to the outside edge of the circle from the spraybar. When you're flying, going from outside in is pulling fuel "uphill", and that causes the problems that Brett was talking about.
You'd like to make a tank that is short up and down (with reference to the ground), because that helps with sloshing. But then the tank gets too wide (from inside the circle to outside), which affects the motor run if it's got a fixed pickup.
So if you're running a clunk tank, you want to mount it so that the wide edge goes up and down, and the short edge goes from inside the circle to outside. You couldn't do this with a fixed pickup unless you used a chicken-hopper, but with a clunk it works just fine.
(As an aside, you can mount the tank to the inside face of the fuselage, with the motor on the outside. Then the centripetal effect is always slinging fuel "downhill" to the engine from the tank. This has some advantages, but people look at you oddly when you show up at the flying field.)
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Hey Terry, the basic junior-high physics to remember can be condensed into a fairly small package:
1: If you've got a hole in a tank that's open to the atmosphere, and there's no liquid gushing in or out, then the pressure in the tank at the level of the hole is atmospheric pressure. This is what a uniflow vent does -- as long as the vent tube is filled with air, the "hole" in the tank is the opening of the uniflow vent inside the tank.
2: The pressure difference between any two points in any vessel filled with liquid is proportional to the amount of drop between those two points. How much it is gets a bit confusing in our toy airplanes because the airplane is maneuvering which means that the 'g' forces aren't pointing down and they're almost always greater than one g, but that doesn't change the pressure difference being dependent on drop.
So if you have a tank that has a hole in it right next to the fuel pickup (i.e., a uniflow tank) then the pressure isn't going to vary with fuel load.
But remember that the tube from the tank to the spraybar hole is also a vessel filled with liquid. This means that if the pickup is to the outside of the circle from the spraybar, then the centripetal acceleration of the plane will tend to fling fuel away from the spraybar, which creates a vacuum that the engine has to overcome to feed fuel (this is the advantage of mounting your tank to the inside of the engine). This also means that if the pickup is below the spraybar (because you're climbing) then there will be a vacuum at the spraybar.
Just keep those two facts in mind, and think hard about them, and you can work your way through most of the nonsense that people throw at you about tanks. The rest mostly has to do with fluid dynamics (tubes too small and fuel changing viscosity with oil mix and temperature), things that don't have anything to do with fuel flow (some engines seem to run differently depending on where they're pointed, and all engines run differently depending on how they're loaded, sometimes in surprising ways), and practical considerations (like pinhole leaks or things clogging your needle valve).
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So if you're running a clunk tank, you want to mount it so that the wide edge goes up and down, and the short edge goes from inside the circle to outside.
That's what I intended to represent in the drawings.
(As an aside, you can mount the tank to the inside face of the fuselage, with the motor on the outside. Then the centripetal effect is always slinging fuel "downhill" to the engine from the tank. This has some advantages, but people look at you oddly when you show up at the flying field.)
Actually, I ran that set-up on a now-defunct Flite Streak.
People look at me oddly as soon as I begin to stumble around the in the circle anyhoo, so if it's advantageous I'm for it. ;D
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2: The pressure difference between any two points in any vessel filled with liquid is proportional to the amount of drop between those two points.
Drop = distance?
How much it is gets a bit confusing in our toy airplanes because the airplane is maneuvering which means that the 'g' forces aren't pointing down and they're almost always greater than one g, but that doesn't change the pressure difference being dependent on drop.
Because those forces act equally upon every point in the fluid?
But remember that the tube from the tank to the spraybar hole is also a vessel filled with liquid. This means that if the pickup is to the outside of the circle from the spraybar, then the centripetal acceleration of the plane will tend to fling fuel away from the spraybar, which creates a vacuum that the engine has to overcome to feed fuel (this is the advantage of mounting your tank to the inside of the engine). This also means that if the pickup is below the spraybar (because you're climbing) then there will be a vacuum at the spraybar.
I thought the vacuum at the spraybar was created by Venturi effect from piston movement, sucking air into the crankcase past the spraybar hole(s), thereby drawing fuel.
I suspect that notion is wrong, as I don't see acceleration forces on the fuel increasing that vacuum.
Terry
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Drop = distance?
Drop = distance, but only in the direction of the local "down" (basically whatever distance a plumb bob would point at any given moment).
Because those forces act equally upon every point in the fluid?
Yes.
I thought the vacuum at the spraybar was created by Venturi effect from piston movement, sucking air into the crankcase past the spraybar hole(s), thereby drawing fuel.
I suspect that notion is wrong, as I don't see acceleration forces on the fuel increasing that vacuum.
When I said "vacuum at the spraybar" I was referring to the vacuum on the inside, which the vacuum on the outside, due to the Venturi effect, has to overcome. If you had a pressurized system (muffler, crankcase, or bladder), then the venturi wouldn't have to work as hard (or at all with crankcase or bladder pressure) to pull the fuel out of the spraybar.
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Appreciate all the explanations, theory and tweaks to convert an RC clunk tank to UniflowTM. My first iteration attempts on my S1 Ringmaster and McCoy 35 RH and 3 oz clunk tank didn't pan out, couldn't get consistent run. I resorted to the standard vented and works fine, but as stated will lean out toward end of run, which for now I can live with.
Also, appreciate the input on using muffler pressure with the UniflowTM vent. Will keep the 35 unmuffled, but will keep in mind for the future.
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Two out of three correct isn't bad Tim, I must have learned something so far. :)
Is there a simple explanation for the source of the partial vacuum in the tank and spraybar?
Is there a down-side to muffler pressure?
Terry
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Is there a simple explanation for the source of the partial vacuum in the tank and spraybar?
Yes. The opening of the uniflow vent is below most of the fluid in the tank, and all the weight of that fluid pulls down. So in order for the fluid to not just run out of the uniflow vent the pressure at the "top" needs to be less than atmospheric.
Did you ever take a straw, stick it into your drink, cap the end with your finger and pull out a straw-full of drink? Then you take your finger off the end and it all comes out in a whoosh (possibly accompanied by some dismayed comments from your nearest female relative)? The reason that the liquid stayed in the straw when your finger capped it was because there was a partial vacuum at the top of the liquid (to the tune of 1/2PSI per inch of liquid in the straw). The reason that the liquid came out when you uncapped the top was because that partial vacuum went away. The reason that your nearest & dearest was dismayed is because females have an instinctive distrust of anything that involves men aiming streams of liquid accurately.
Is there a down-side to muffler pressure?
I know that some people don't like it but many do. I believe that it has a certain self-regulating effect (more engine speed = more pressure = more fuel = less engine speed), which is either good or bad depending on whether you're seeking a strong 2/4 break or not.
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Is there a down-side to muffler pressure?
Terry
downside,, some exhaust residue will get back into the tank,, ( I never gave a rip about this personally,, and never found any more than color)
on the upside,, one gain is that in MY experience,, I had observed the engine leaning and richening as I went around the circle into and out of the wind,, due to the pressure on the uniflow vent in the airstream,, the muffler pressure eliminated that annoyance
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Tank vacuum is clear now Tim, spraybar vacuum not so much.
Or at all, actually.
What causes it?
And I like muffler pressure until I find I don't. ;D
Terry
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And while I'd considered asking in a separate thread, since you've conveniently brought it up ( ;D), why does the engine's fuel demand vary with flight position?
EDIT: Upon reflection, I think I should be asking why, since constant pressure is the point of uni-flo, does the engine's fuel supply pressure vary?
Gravitational/centrifugal forces upon the remaining fuel?
Terry
Good question.
I see the engine as (obviously) a pump, an asymmetrical pump that works more efficiently in certain orientations than others. It simply breathes better one way than the other.
The question more correctly be posed as "why wouldn't it vary with flight position?"
Could be down to condensation or fuel pooling in the transfers and it clears its sinuses better in one orientation, venturi direction, spraybar height, angle to the wind, exhaust port rotation etc.
The engine can only induct what it exhausts and if it can't clear the same percentage of residual charge due to varying G forces than it can't induct the same percentage either.
But as to exactly why that happens - good luck tracking it down mate!
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Good enough Chris - there are certainly some depths of knowledge which I simply will not attempt to plumb. ;D
But I'm still curious about internal spraybar vacuum.
Terry
Edit:
After thinking hard about it as Tim suggested, I found this: "The air starts out at atmospheric pressure, goes into the venturi, speeds up, the pressure goes down and the fuel is drawn in by that lower pressure. This is called the Bernoulli effect and that's what a VENTURI does."
I sorta knew that, so is the low pressure in the venturi extending into the spraybar, as this (non-model engine) diagram indicates, the partial vacuum source?
Terry
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Really don't like that word "vacuum" as it implies a void, nothingness.
Far better to think of this as areas of varying pressure, high pressure is the driving energy and low pressure wants to be equalised by it.
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No prob (or probs ;D) Chris - I say "nineteen", you say "nineteen". LL~
Terry
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Hi.
I couldn't read the whole thread but one function of fuel head pressure variation in engine adjustment seems to be completely neglected here. I mean the tank's sideways position. I've seen many cases where the engine is blamed for instance for lack of power in overhead or not ideal power supply, when the origin of problems is in fuel system.
Especially engines like DR and similar are sensitive for it. With Retro for example, the ideal position of fuel pick-up inside fuel tank is 5..10mm outside the point where fuel sprays out of spraybar. Obviously this calls for a side mounted engine or fat fuselage. But of course, using no nitro also helps as it's easier to play with tank position when tank is smaller.
The simple logic is that inboard tank richens the setting in low maneuvres when the model likes to sped up in windy weather, and it leans and gives more power, ot just better running stability in overhead maneuvres.
L
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Hi.
I couldn't read the whole thread but one function of fuel head pressure variation in engine adjustment seems to be completely neglected here. I mean the tank's sideways position. I've seen many cases where the engine is blamed for instance for lack of power in overhead or not ideal power supply, when the origin of problems is in fuel system.
Especially engines like DR and similar are sensitive for it. With Retro for example, the ideal position of fuel pick-up inside fuel tank is 5..10mm outside the point where fuel sprays out of spraybar. Obviously this calls for a side mounted engine or fat fuselage. But of course, using no nitro also helps as it's easier to play with tank position when tank is smaller.
The simple logic is that inboard tank richens the setting in low maneuvers when the model likes to sped up in windy weather, and it leans and gives more power, ot just better running stability in overhead maneuvers.
L
Thanks for the post, Lauri! I was wondering, as I have a very wide wedge tank on one profile plane, how much additional pressure differential was needed to keep fuel flowing sufficiently to overcome the centrifugal force on the fuel, and what affect it had during maneuvers. Probably a good reason to ditch the old wedge type tanks and go to the newer versions that are more box-like and don't extend outboard so much, or just make a couple uniflow tanks out of the clunk tanks sitting around. H^^