Anyone use these balloon tanks? Dan Banjok has been trouble shooting a Tettra 6 ounce on an LA 46 powered Brodak ARF P40. The run is consistent and predictable until the last 20 laps when the engine breaks screaming lean. Setup has used muffler pressure and crankcase pressure. Engine did not like crankcase pressure, but worked as described when using muffler pressure. Pickup line has been run to middle of the tank as well as to the front of the tank with the same result. When approximately one ounce of fuel is left, the middle of the balloon collapses while both ends retain fuel. This appears to create a negative pressure in the tank, causing the over lean setting.

We are interested in the Tettra tanks because they eliminate air bubbles from the tank/fuel/engine system. It's hoped that this can tame some of the weirdness that occurs in profiles with vibration issues. The first 2/3rds of the run is good. The tank performing like a standard vent hard tank that works well. Gradually leaning out during the course of the run. Usable (even desirable?) for doing the pattern. It's hoped that if the over lean condition of the last 20 laps can be solved, the Tettra tanks would prove to be a useful option. Especially on profiles.

We know these tanks are commonly used in RC Pylon Racing and RC Pattern. Are the engines shut off before the tanks create a negative pressure? Or do the RC boys have a TRICK we can use in CL Stunt.

I prefer the Tettra Tanks above all others. Amazingly consistent runs. My version of "the proper" setup is:

Feed/fill line inserted about 1/3 of the tank length back from front, bent over to the outboard side (no clunk), with a ½" length of silicone tubing to prevent wear on the bladder. Muffler pressure to the other fitting. I also install the tank with the longer axis of the rectangular cross-section vertical, although this probably isn't critical.

Filling requires 2 - 3 cycles of sucking air and adding fuel. I use a Jett "Tanker", which is about 13 oz. max. Put the needed amount of fuel into the Tanker, hook up to the fill line, suck air out, invert the Tanker and fill the bladder. Turn the Tanker back over and suck as much air out as possible, re-fill. Repeat. It is necessary to turn the wing vertical with the inboard tip down to completely fill the bladder. Then you can remove fuel as required to adjust runtime. Clamp the feed line with a hemostat until ready to start the engine.

I have not experienced the problem of the bladder collapsing in the middle, but I can offer two possible solutions. The simplest would be to disassemble the tank and rotate the bladder 180⁰. Then reassemble the tank and try again. The other solution is to use a longer piece of fuel line with several "v" notches cut into one "side". Then, if the bladder collapses, it will seal off one or more notches, but not all of them.

One other possibility occurs to me. There may may be a problem with the location of the muffler pressure tap. Randy Smith informs me that it's possible for some positions in the muffler to generate a negative pressure. If high enough, this could certainly collapse the bladder.

If none of these ideas work, the bladder may be defective. I would certainly try another example before giving up on this style of tank!