jayseas

This is probably a dumb question.But what's the difference between a standard vent and a uniflow, and what would be the correct application for each?
Jim

PipeMajor

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This [link=http://beseenontheinternet.com/VAHS//pop_vent_diagram.htm]diagram[/link] will show the vent differences

The "pressure" or "flow" of fuel coming out of the uniflow remains a constant rate from full tank to empty. A conventional tank will have more "head pressure - the weight of the fuel pushing out the outlet" when full verses when empty. Therefore, a conventional tank will start to run a little bit leaner toward the end of the flight.

jayseas

Why would you use one over the other?

vauxhall

IMO there's no point in using a standard tank in any C/L model, whether you fly stunt or anything else. As PipeMajor has said, that type of tank will always start the flight rich, run O.K. mid flight, then go lean towards the end. All my C/L models use uniflow tanks, even scale and sport fun flyers. Uniflow tanks will deliver a consistent fuel flow right through the flight. I position the uniflow pipe near the front of the tank, makes the engine lean out just before the tank runs dry, so I get some warning.

Jim Thomerson

I agree with VAUXHALL. The only open vented tanks I have are small tanks on trainers. I've made uniflow tanks with the uniflow terminating anywhere from the front of the tank to 1/4 in from the end of the pickup with good succes.

Lou Crane

And, Jim (Jayseas, that is) the reason the uniflow venting does what it does is...

- The fuel's effective "gravity head"... Picture a town water tank, a big bubble supported fifty feet off the ground... The weight of the water provides a pressure into the pipes down at ground level, right?

When we fly CL, centrifugal force and gravity combine into what could be a single force pointed out and slightly down, with the effect of 3 or more g's of "weight." Like the town water tank, the effective gravity head is from the surface of the liquid open to the air around it, to the pipes which carry the liquid away to where it will be used.

The traditional "over and under venting tank" in the diagram has its fuel head at the surface of the fuel inside the tank, since the vent tubes are at the inside wall of the tank, and uncover to being both open shortly after takeoff. As the fuel burns off, the fuel surface moves nearer to the outer wall of the tank. The "height" determining 'gravity head' gets smaller. There's less pressure as the surface falls toward the pickup. That's why they run rich at first and lean out over the course of the flight.

The uniflow venting, on the other hand, sets the vent height that releases air into the tank interior very nearly at the same "height" (in the slanted direction of that equivalent force I mentioned) as the pickup tube. That "height" makes the fuel surface location irrelevant, since the tank is sealed, except for the uniflow vent end and the fuel pickup. NO outside air affects the fuel, except what enters the tank through the uniflow tube, and then, only at THAT height 'above' the pickup. (Along that slanted equivalent force direction.)

Uniflow tanks are not all sweetness and light, though. It may take some effort to place them where they work best. If they are too high, the act like an enema bag, pouring fuel downhill to the spraybar. If they are too low, the engine must draw fuel 'uphill' - like with a leaner needle setting. Once the uniflow tank is the right vertical height for the engine, not only is the same setting held through almost all of the flight, but it is the same upright and inverted, and in inside and outside maneuvers.

jayseas

Hello Lou,
Thanks for taking the time to respond to my question.I have one question now, i think i know the answer but i need to know for sure. The uniflow vent tube inside the tank, should end, almost at the same position, as the fuel pickup tube inside the tank? Yes or No?

downunder

I'll answer for Lou if he doesn't mind (and too late anyway ) but the end of the Uniflow line can be pretty much anywhere along the outside wall of the tank because in flight (recalling Lou's ~3G centrifugal force) the outside wall of the tank effectively is the "bottom" of the tank. This outside wall is of course where the fuel pickup will be even if it's a clunk tank. As Jim said earlier, he's used the Uniflow end anywhere from the front of the tank to nearly at the pickup. It's best to keep the end of the Uniflow line no closer than about 1/4" from the pickup to avoid the chance of air bubbles getting into the fuel pickup.

In my set ups I angle the clunk tank so the back end is further outboard than the front so the last dregs of fuel run to that outside edge of the tank to pick up every last drop. The Uniflow line is just brass tube bent to lay pretty much against the outside wall (to avoid interfering with the clunk line moving around) and back close to the clunk and halfway up the wall. That gives me a dead consistent run until maybe the last 10 seconds when it leans out for around 2 laps giving plenty of warning.

BobReeves

Just to keep the records straight, one situation where uniflow doesn't work all that well is if you are running a 4 stroke. 4 Strokes run much better with a conventionally plumbed RC clunk tank and muffler pressure to the vent.

jayseas

downunder, trying to fully understand, the uniflow vent line that faces into the airstream, is creating pressure inside the tank, correct?I'm trying to understand why the end of the tube inside the tank has to be against the outside wall. With the pressure ramming in thru the vent tube, it would seem that you could position the end of this tube just about anywhere inside the tank.I must be missing something here.
Thanks

Lou Crane

Jim, and much obliged Downunder, that fit right in...

A uniflow plumbed tank is much more sensitive to "relative tank height." I mentioned the advantages of getting it right, but not the reasoning.

But, first, yes - the forward facing vent tube end is exposed to some ram air pressure as the model flies. Not much... And, whether the vent air provided into the tank interior is below local atmospheric pressure, at that pressure, or slightly above, the reference point that the engine 'senses' is the inside tube end where the bubbles come out. For simpler handling of a hard (metal) tank, I usually take pains to get that uniflow vent tube at exactly half the height of the tank shell. That way I know where it is, if flight experience shows it isn't quite right.

This location is key, as several who use converted plastic bottle RC tanks 'trim' the run by twisting the uniflow vent tube to a different height inside the plastic bottle. Nothing else needs to be shifted.

Say the engine runs richer inverted and outsides than upright and in insides... The tank is too "low" when the model is upright. The engine has to draw fuel uphill, even if only a small distance. That same distance is doubled when the model flies the other way. If we can call the "perfect" tank height a sweet spot, in this example, the vent end in the tank is some distance below the sweet spot in upright/inside conditions. Inverted and outsides, it is that same distance above the sweet spot...

Since I have the uniflow outlet at a known height inside the tank, I can shim the whole tank, in this case up with the model upright, and see if it needs more or less to get it the same both ways...

And uniflow tank venting can be VERY sensitive to this, particularly for richer-running 4/2/4 engines. With muffler pressure, the difference is less than with the forward facing external tube open to the airstream. With more recently designed engines, almost invariably ABC/AAC or ABN's designed for RC then converted for CL use, their most natural running mode is somewhere in their quite wide range of 2-cycle RPM. ...And most often they run on muffler pressure, so the problem is quite a bit less. Even so, a careful ear, and a sensitive hand can feel the unavoidable variations in power when a uniflow tank is not just right.

I've developed a routine to check tank height before first flight on a ukie. Shortest description:

Hold the model with the fuselage centerline level at all times when checking...

Start the engine and rotate the wings to vertical, outboard tip down (re-read line above...)

Set engine slightly rich, so you can hear - or better, tach - RPM changes. (Fuselage level, right?)

Rotate the wings to about 45° from vertical, say cockpit-side up for the starter. Note RPM by ear, or better, by tach. Rotate wings to the same angle, cockpit-side down, and recheck RPM. (Keeping fuselage centerline level.) Repeat a few times to be sure any response is consistent, and that you can recall which way it went.

Now, you can relax that First Commandment about level fuselage centerline... Turn model nose-down, inboard-tip down to uncover the fuel pickup in the tank. If it doesn't shut off quick enough that way, you can cap an open uniflow external tube with a finger...

Your test wig-wagging crudely represents the angles of the "gravity vectors" the model feels in flight. NOT their intensity, just their direction. And, about 45° is both far enough to cause a response difference if things aren't just-so, and easy enough to approximate on both sides of vertical, outboard-down.

RPM drops from richening on the side where the tank is too high. Conversely, RPM rises when the tank is too low in that attitude. Adjust the height accordingly, and run thru the same sequence to see if that was enough or too much. After two or three times through this process, you should be close enough that, at least, you won't flame out or starve out on a first flight.

There is no hard and fast rule based on the layout of the engine type for the relative height of spraybar and inner uniflow vent end. Most individual engines of of a mass-produced type, e.g., Fox 35, or OS 40FP, or 40LA, share the same tank preference, but I wouldn't guarantee that the solution for an LA40 works with an FP40. I built a test bench which allows me to check unfamiliar engines for tank height preferences and sensitivities.The commercial engine mount (J'Tech) mounts on a board to hold the tank (adjustable as to height in the relevant direction, sidewinder is different from upright or inverted). The board can be rotated, roughly about the crankshaft centerline, while the engine is running. Testing is similar to the wig-wag process above. Inspiration for the wig-wag and the bench followed an experience with an engine that needed the reference height more than 3/8" different from usual practice. ...caused a few crashes... But when the engine was sorted out, it was excellent!

Jim Thomerson

Here is a reference on Edme Mariotte, the inventor of the uniflow tank. Go down the page to Mariotte bottle for an explaination of uniflow. Incidentally Mariotte was the first to test a model in a wind tunnel, not the Wright brothers.
http://en.wikipedia.org/wiki/Edme_Mariotte


When I run a uniflow without pressure, I put a one way valve on the uniflow so air can go in but fuel does not come out. I use the KK valves which have a tiny hole, maybe a #80 drill. I've also used a cap with a tiny hole. I don't like pressurizing the tank with outside airflow as that varies into the wind, with maneuvers, etc. Other folks use ram air and are happy with it. With muffler pressure I don't use a one way valve. But I do use one with crankcase pressure.