Looking to set up a uniflow tank with a McCoy 35 for control line. Should it bee 2 or 3 lines? And where should each go once it leaves the tank.  This situation is a built up fuselage.

     3. If you use two, you will have to hold the airplane in the extreme nose-down position for fueling. Since the vent and fuel lines are right next to each other inside the tank you will have quite a bit of trouble fueling it without forcing fuel out the other line.

      The third tube should go to the front/upper/inside corner of the tank, and be used as an overflow. Fuel through the uniflow vent, it will fill up and then flow out the overflow line when full. Then block the overflow for flight, and it needs to be airtight. A screw shoved into a piece of fuel tubing will work to block it, but you have to seal up the threads with silicone sealant or air will leak around the threads. This will cause the uniflow effect to be either partially or completely lost.

    The reason you put the overflow as far to the inside as possible is so you can use it as a vent if you want to defeat the uniflow. Fuel it the same way, but block the uniflow vent rather than the overflow, and that acts as a vent, but with conventional suction setup so the engine leans out as the fuel runs out.

     You an also use muffler/pipe pressure into the vent, but sometimes that has unexpected effects when you attach it to the uniflow line. Usually, the funny thing is that it works fine for some fraction of the tank, then goes lean for some reason. I have no idea why, but it happened repeatedly on several different arrangements and engines.

      Brett