[Gary Maker]

Gary Maker said: " from what I understand this setup is suppose to help keep the engine from leaning out as the fuel is depleated from the tank."

[nosedive]

Originally Posted by Ed Smith

The theory, basically, is this. With a two line open vent, be the vent supplied by pressure or atmosphere, air can enter the tank uncontrolled. This means that fuel can leave the tank in the same uncontrolled fashion, resulting in a rich/lean inconsistant fuel supply.

With the vent submerged in fuel, air cannot enter the tank unless the same volume of fuel leaves the tank.

Ed S

Well I still don't get it. Even with the traditional open vent, air will not enter the tank unless a quantity of fuel leaves the tank. This is true regardless whether open to the atmosphere or pressurized. With the vent attached to the clunk the vent air will simply bubble through the fuel until the pressure is equalized to the atmosphere (non pressurized set up) or until the pressure is equalized to the pressure in the muffler. This is the same thing that happens with a traditional vent set up, less the bubbling through the fuel. One benefit that I can see is that with a pressurized set up and a submerged vent, tank pressure might remain more constant for a brief period when the engine is suddenly throttled back. Other than that-???

One more thought- on non-pressurized systems wouldn't this set up harm fuel draw? The carb would have to overcome the resistance of the vent air being sucked through the fuel. The typical model engine carb has minimal fuel draw on it's own. Obviously the majority of us use pressure or pumps but this just an added thought.

[Imhoff43]

I guess when you fly inverted you are flying a uniflow?

Even with the traditional open vent, air will not enter the tank unless a quantity of fuel leaves the tank.

Not strictly correct. Air under pressure could enter the tank without fuel leaving it. This uneven air pressure can result in an uneven fuel supply and an uneven engine run. We are talking about small amounts here. But if you have ever cooked an engine because of 1/8 turn too lean on the needle you will know what I mean.

With the uniflow tank, as fuel leaves the tank a vacuum is created in the tank. No more fuel will be allowed to leave as long as this vacuum exists. The air bubbling up through the fuel releases this vacuum. It can be seen that the needle valve controls the balance of fuel leaving/air entering the tank. It is kind of self regulating. The pulses if you like happen very quickly

I would like to sound super knowledgeable and say this system is based on the "Marriotte Bottle" principle. However nobody has ever described this principle to me. They just say "It is the same as the Uniflow tank"

Ed S

[Colin]

I like the normal setup, some heli guys run this, but mine works perfectly using the stock tank setup

Colin

[dondow]

Afternoon All

I am with Collin on using the standard setup. Some people swear by them about eliminating air bubbles or as a reserve tank. I spend a great deal of time ensuring the clunk in my tanks can find the fuel. Now having said that, my turbine choppers use header tanks but that is a different setup.

Cheers
Don Dow
Ottawa

While on the subject of fuel tanks, there is a tank that is superior to all others. The bubbleless tank made by Tettra or Jett engineering is the tank to use.

The tank looks like a regular hard plastic tank. The difference being it has a bladder inside it. The bladder is the same size as the plastic shell and does not exert any pressure on the fuel by being stretched, it cannot be stretched. The bladder is sealed at the neck and has one feed line into it. The pressure line is attached to the outer tank and applies pressure to the outside of the bladder.

The tank is filled by sucking all of the air out of the bladder. The bladder is than refilled with fuel. There is now a solid blob of fuel at the feed line. No air can get in so the engine run is constant from start to finish.

No clunks to come off, No fuel lines to bend or kink, no header tanks, no hopper tanks, shove the tank in any way you like. what could be simpler?

Ed S

[Frenchman]

I see two problems with the Jett or Tettra tanks. 1st is their cost, about $20.00 US, and second is their size. I believe the biggest tank available is about 6 ounces

[Vulcan1]

Frenchman,
Actually Tettra makes a bubbless tank up to 32 oz. and is around $30 US. I did use the Jett 6oz. for Q40 racing and they are very good. You get a good run right to the end of the tank with no burps. You get what you are paying for.
John

I see two problems with the Jett or Tettra tanks. 1st is their cost, about $20.00 US, and second is their size. I believe the biggest tank available is about 6 ounces

Cost?
How many airplanes have been damaged because of an unplanned landing due to engine failure? How many plugs have been burned out due to an engine leaning out at the end of the run? How many engines have been cooked for the same reason? The tank is cheap insurance.

As John says they do make bigger tanks and you get what you pay for.

Ed S