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Post  ZACATTACK Fri Jun 22, 2012 7:42 pm

Ive been looking at other fuel systems for my Combat/Stunt Planes. Does the metal Uniflow Tanks have clunk lines set up in them? ( They Should)...Lots of guys are using bladder systems in place of uniflow...Why?? The uniflow set-ups in my helicopters are very reliable...why are guys not using them in here??
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Post  Ken Cook Fri Jun 22, 2012 8:12 pm

Hello Lew, a metal tank that you purchase commercially doesn't have flexible lines inside. A Uniflow tank has a addtional pipe soldered directly to the pickup which goes to the rear of the tank. The "Uniflow pipe is approx 5/16"-3/8" from the rear of the tank and like I mentioned soldered to the pickup which is in the extreme right hand corner. Many flyers take Sullivan plastic tanks and use a clunk setup that again has one line to the extreme corner with the other right next to it. Both pipes go up and down together.

When you use a Uniflow tank, the Uniflow pipe is open to the atmosphere and the vent or vents are tightly capped. Generally the needle setting is set slightly rich because as soon as the plane is released, the engine goes lean. Uniflow is used because it somewhat tricks the engine into believing it has a steady head pressure of fuel. A standard tank leans out through the run while the Uniflow remains virtually steady throughout usually resulting in a clean shut off at the end and not a screaming lean run to the end. This is desirable for stunters and also racing planes.

A uniflow tank must be filled through the uniflow pipe and no where else. If it's filled through the vent it will be short tanked due to it coming out of the engine pickup line prior to the tank being completely filled. Getting uniflow to work properly is a science in itself. If there are any vibrations foaming the fuel, all bets are off and the tank won't work correctly. Many times I plumb a tank for uniflow due to it being universal. If you can get it to work, great. If not the tank can still be used like a standard vent tank just by capping the uniflow pipe. You can also run muffler pressure to the uniflow pipe and I found this to work quite well for me at times.

A bladder is used on a fast turning maneuvering plane. Certain tanks aren't going to take the g-loads your placing on the fuel. This means the fuel is going to get slammed and held into a corner that usually results away from the pickup line. The plane in turn goes overlean, burps, quits, etc. This is what happens when your flying a wing. The speed of the plane and the rate it turns keeps adequate fuel flow from getting to the needle. This is why bladder is used. The fuel is under pressure at all times and therefore the engine never quits or hesitates. The venturi can be larger resulting in more power from the engine as well. This can't be done using a tank as you would have little fuel draw. Bladder keeps constant fuel in the engine case resulting in cooler operating temps as well. A bladder is also lighter and extremely simple. In combat today, I can say the majority are all using bladders. When most were still flying slow combat using Fox.35's, they used a Chicken Hopper tank which worked well. The Chicken Hopper look like someone grafted a small tank onto a large tank. Internally there was a baffle in between the main tank and the small tank grafted onto it. The pickup was in the small tank. This always kept the pickup submerged in fuel even during maneuvers keeping a steady head pressure on the engine. Here is a diagram of a plastic clunk tank with the uniflow line attached to it. Ken
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