green river rc

My Feedback: (2)

I'm just wondering, exactly how does a uniflow tank work, and how would I convert a regular vent tank, or RC tank to uniflow? I have had this question answered for me before, but it didn't quite sink in. Maybe some of you RCU boys can make it stick!

Jim Thomerson

I do not understand the physics of uniflow tanks. The functional aspects are that the tank is sealed except for the fuel outlet and the uniflow air inlet. The uniflow is submerged in the fuel, and the engine thinks it is sucking fuel from the uniflow outlet in the tank. The engine will not know the level of fuel in the tank is changing until the uniflow outlet is no longer submerged in fuel.

Hope this helps.

Jim

Sport_Pilot

The engine does not think or know anything! On a normal R/C tank the vent is located at the top the fuel inlet is allowed to move around, top to bottom, side to side, and somwhat foward and back. When the tank is right side up the muffler pressure is unimpeded and the tank pressure is equivalent to the muffler pressure. But the fuel pressure is equal to the average head of fuel in the tank minus the differance in height of the fuel inlet in the tank and the carb spray bar or jet, plus the tank pressure. When the tank is upside down the vent is now on the bottom below the fuel, the tank pressure is now equal to the muffler pressure minus the head of fuel in the tank. Because of the differance in tank pressure right side up and upside down the ideal location of the tank is slightly below the carb spraybar. To make a uniflow tank from an R/C tank you add another clunk and line just short of the pickup and tie the two together, now that the vent is always at the bottom of the fuel the tank pressure will now be the same rightside up and upside down. With a uniflow tank the ideal tank location is always at the center of the spraybar.

William Robison

My Feedback: (3)

Uniflow.

What it does is make the fuel draw constant from full to empty.

Using conventional venting the engine can draw the fuel more easily from a full tank than from an empty tank, because of the lowered fuel level and greater effective distance. Remember, it doesn't matter how long or deep the tank, the engine sees itself as drawing fuel from the top surface.

With the air vent at the bottom of the tank the incoming air has to be forced against the weight of the fuel. As the fuel level goes down. less force is required to vent the tank.

The uniflow tank uses these two in balance, as the fuel goes down the easier venting makes the fuel draw restriction constant from full to empty.

Hope I made it clear.

Bill.

downunder

The easiest tank to convert is a normal RC clunk tank which is all I've used in my stunters for maybe the last 30 years.

But first, how do they work. In a normally vented tank air is allowed in above the level of the fuel so it's the same as having a tank with a hole in the top. Because the needle valve in the venturi is centred roughly on the middle of the tank then when the tank is full the fuel level is above the needle valve so fuel runs into it with the help of gravity (in a CL tank it's actually centrifugal force). When the tank is nearly empty the level is below the needle valve so fuel has to be drawn up against the centrifugal force. So the engine gradually runs leaner as fuel is used up because the head of fuel gets less and less.

In a uniflow tank the vent line goes to the bottom of the tank so it's always immersed in whatever fuel is left in the tank. Don't forget though that as far as a CL model is concerned, the "bottom" of the tank is actually the outboard wall of the tank. But this vent line now tries to get full of fuel to whatever level is in the tank so before any air can flow through it to make up for what the engine has used the engine has to suck a little harder (hey, I'm trying to keep this simple ). As fuel is used up there's less trying to fill the vent line so the engine (venturi) doesn't have to work as hard to get fuel flowing to the needle valve. The end result is that no matter what the head of fuel in the tank, the venturi doesn't feel any change so the mixture remains constant from start to finish.

The idea is very simple but it's hard to put into words. One thing that should be mentioned is that it still works even with muffler pressure because all the uniflow does is eliminate head pressure regardless of whatever extra pressure is fed into the tank.

To convert a normal metal wedge tank you have to solder in another pipe who's end is right in the wedge. It doesn't really matter how you do it, you can lay the pipe along the wedge or go straight through the tank down into the wedge but of course the outer end of the pipe must be to the inboard side of the tank.

Clunk tanks are the easiest because all that's needed is for the vent line to have a bend that puts the inside end close to the outer wall of the tank. I use a third line which bends to the top of the tank to let air out when I'm filling it through the uniflow line but this vent line must be sealed after filling because the uniflow relies on having a totally sealed tank, no leaks at all.

telmore

LOok here:

http://home.cogeco.ca/~controlline/tip1.shtml

Tony

green river rc

My Feedback: (2)

Well...I still can't say I understand how it works, but at least I know how to convert my plastic RC tank to uniflow. Telmore-that is a great link you posted, just the info I needed! I like the way you ran the hard lines through the fuslage also. I have a couple smaller metal uniflow tanks, but I didn't want to break the seal on the end cap just to have a peak inside. My questions have been answered, but anyone else that has something to add, feel free to chime in.

Thanks boys!

gcb

Another try:
Start an engine on a plane with non-uniflow and hold it nose high and adjust the needle. Now hold the plane horizontal and note the drop in RPM. How much it drops depends on length of distance from fuel feed in tank to venturi, how much fuel is in tank, and fuel draw of engine (venturi size and shape). Now remember how the engine speeds up toward the end of a flight as the fuel reserve becomes lower and lower. This is because the weight of the fuel is aiding fuel draw. Several things affect these but we are only talking fuel draw.
Now lets consider the same setup, only we have a uniflow vent 1/4" behind (shorter than) the fuel pickup. The engine now runs as if the tank only had 1/4" of fuel left for the whole run, until the fuel level goes below 1/4". The same differences between horizontal and vertical will exist EXCEPT variances caused by the weight of the fuel.
The 1/4" is arbitrary. Some make it much closer but you must avoid having air bubbles enter the fuel pickup.
Some use muffler pressure instead of uniflow, some use both, although using pressure removes some of the need for uniflow (IMHO).
More confusing?

George

green river rc

My Feedback: (2)

That reminds me, I'm also installing an after market muffler, and I was wondering about back pressure. I assumed I was not going to need pack pressure?

gcb

Now you have me confused. Back pressure is usually discussed as the pressure in the muffler that detracts from the exhaust scavenging of the engine (except pipes). Although modern engines are tuned for this, old engines (pre muffler) are sometimes greatly affected by back pressure. It can cause RPM drop and over-heating.

Or are you referring to the pressure you get from the tap on the muffler?

George

green river rc

My Feedback: (2)

I put a muffler on my engine (S.T. 35) just to quiet it down a little. It has no pressure tap in it, I was going to use a uniflow tank and a muffler with no tap...is this right? The muffler is a Tatone #2.

iskandar taib

My Feedback: (5)

If the vent is open to the atmosphere, then the pressure at the end of the uniflo vent inside the tank is atmospheric (plus whatever ram air effect you get when the vent faces into the wind). If you use muffler pressure, then the pressure at the end of the vent is higher than atmospheric. How high depends on the muffler and the engine and the RPMs you're running. Both setups work, a lot of people use either one. Atmospheric is easier, of course (less one piece of silicone tubing to worry about).

Incidentally, you don't need a second clunk for uniflo operation. What a lot of people do is simply bend the vent tube so that it is somewhere near the outboard wall of the tank. For small/simple airplanes, you don't even need a third vent for fueling (you have to remember to plug it before starting the engine). To fuel the plane, stand it on it's inboard tip and fill through the feed line. The uniflo will be in air, so air will escape this way.

downunder

The pressure that's tapped off the muffler into the tank is the back pressure in the muffler. Any back pressure will rob a small amount of power from the engine whether it's a modern design or not. Virtually all RC engines have to feed this back pressure into the tank because their carbies are made very large to get as much air into the engine as possible so they can develop the huge amounts of HP at very high revs. The tradeoff is very poor suction especially at the revs normally used so tank pressure is used to help get fuel into the carb. In this case, the extra air flow more than makes up for the slight power loss due to the back pressure.

CL venturis on the other hand are generally sized far better so have much higher fuel draw which can mean that muffler pressure isn't always necessary for a good engine run. It's just a matter of trying it with and without pressure to see what suits you best.

iskandar taib

My Feedback: (5)

Unless you're one of those running the Saito 4 strokes in Stunt.. To get them to produce power, they've been running fairly large venturis, resulting in poor fuel draw. There's been a lot of discussion on Stuka Stunt about this. Apparently uniflo tanks don't work too well, since they operate at low pressure throughout the run (as opposed to a normally vented tank, where the pressure is high at the beginning and low at the end).

Sport_Pilot

Downunder, the muffler would have back pressure with out the tap. The tap actually reduces back pressure since the exhaust goes into the tank, but is negligable.

Iskandar, I don't understand your comment, unless you are talking about using the tank with ram air pressure. Which begs the question: Why not use muffler pressure with the uniflow tank?

gcb

For this discussion, two factors are important...the pressure generated by air and the difference in pressure generated by the weight of the fuel when the tank is full vs empty. Uniflow lessens the difference between full and empty.
When you add muffler pressure you make the air pressure more significant and the difference in fuel weight less significant, to the point that a uniflow is not needed. Some, however, like to use both.
Some avoid muffler pressure because the plumbing is just another thing that can go wrong.

Of course both methods work well. Heck, some still run on non-uniflow (normal?) vented tanks.

George

Sport_Pilot

I always though non presurized tanks were used in control line because it is almost impossible to get a 4-2-4 break with pressurized tanks. The needle is too closed for the differance in tank height to lean the fuel enough for the engine to break.

Jim Thomerson

Why there is a 4-2-4 break is debated in CL circles. I am inclined to think it is a result of changing prop loads in flight. The theory is that when the load is increased on an engine which is in a 4-cycle, it will go to a 2-cycle. According to this theory, if you run an engine on the bench in a 4-cycle, then change to a larger prop without changing anything else, it will go into a 2-cycle. You can try this at home if your neighbors don't mind.

We have all seen an engine go from 4-cycle to 2-cycle when the nose is held up, but there is a noticeable delay. Because of this, I don't think changing fuel head would make the engine react fast enough to give the effects seen in flight maneuvers. Because I fly mostly rich 2-cycle this is largely speculation.

Jim

Sport_Pilot

Jim, this is science not an art. It is not theory but fact. It has nothing to do with prop load, you can make a good stunt engine break on a test stand by tilting the stand back, no change in prop load, just that the tank moves back and leans the engine out.

Jim Thomerson

If you read my post carefully, you will see I said that tipping up the nose of the airplane would make the engine go from a 4-stroke to a 2-stroke. No one doubts that that happens. I also suggested an experiment you could do to test the load change hypothesis. We'd be interested in your results.

Jim

green river rc

My Feedback: (2)

The whole reason I requested uniflow information to start with is because I think my plane (Challenger .40 with a ST .35 with a MA 11x4) is a bit over powered, and it seams a bit to fast to me. I have only seen 1 C/L stunt plane flying in 4-2-4 break and it just quietly and slowly purred in a 60' circle until it went vertical and you could hear it break to 2 cycle, and then back to 4. The plane was a Nobler (with a modified OS .40 fp with a muffler) and it was in the hands of a real pro. He was flying some seamingly perfect square loops and figure 8's, and doing it slower that my plane will fly. I was impressed to say the least. After talking to him about it until he was sick of looking at me, I decided I was going to try a uniflow tank and a muffler. I thought the slower moving plane might help smooth out my flying. I still don't have a suitable tank to modify, the closest hobby shop is 1 1/2 hours away. I'll probably order a real uniflow tank from Brodak tonight. Right or wrong, I figured the tank is a good place to start?

downunder

green river....not all engines will do a 4-2-4 break and it's not a necessity for a good stunt engine, it's more a personal preference (although it sounds cool ). The first thing you want is to have as consistent a run as possible from beginning to end and this is where the uniflow tank has the advantage because it eliminates a variable (the changing head of fuel). If you have a uniflow tank with a third vent line for filling then just by leaving it uncapped you've instantly converted the tank to a normal vented tank so you can do back to back flights and see what difference (if any) it makes. Or you can blank the uniflow line and run muffler pressure into the vent line and have a normal pressurised tank.

Any of these setups will require a retune because of the varying air pressures inside the tank and this is what Iskandar was referring to about uniflows running a low pressure. A non pressurised uniflow always has an air pressure lower than ambient because this is what balances the head of fuel so the needle has to be more open than for any other setup.

gcb

IMHO, the tank is only part of a good 4-2-4 break. Engine compression (head shims), fuel formula, venturi size, and engine blow-down timing (diff between exhaust and intake timing) also affect the run.
All engines are affected by fuel draw somewhat.
As stated previously by someone, there are many who run large flat props in a constant 2 cycle to accomplish the same affect (constant speed) as a 4-2-4 run.
The late George Aldrich discussed the 4-2-4 break many times online and on tape.


George

Sport_Pilot

Jim,
The reason the engine broke into a two stroke with a larger prop is that the engine slowed down with a larger prop. With less air going through the venturi the engine will run leaner. That is why an R/C carb has two needles, an additional fuel source must feed fuel through as the venturi vacume decreases.

Jim Thomerson

According to an article based on measuring combustion pressure changes, published some years ago in Stunt News, an engine running in 4-cycle develops about 75% power compared to the same engine running in 2-stroke. Based on this, I would think a 25% increase in load, causing the engine to 2-stroke, would have the engine running at the same RPM. Of course, a larger increase in load would slow it down. I have no idea how much increase in load is necessary (assuming this happens) to break an engine from 4-stroke to 2-stroke.

Jim