Steve
I have a number of 'square wedge' tanks (Perfect?) with uniflow plumbing that are drop in replacements for the homemade Barnstormer tank in the photo. I'll try those first for comparasons sake when the flying season returns. Al Rabe described an angled tank he designed for a shut off on command with a loop at the end of the pattern. I wasn't quite sure where the angle was and thought maybe the angled rear wall of my tank was what he described. Was it instead the angle he created by the outside of the tank being wider at the rear than the front?
Ara
It's the angle of the outside wall of the tank. With a wall that's parallel to the nose of the plane the fuel tends to collect in the front of the tank, because the centrifugal force in front of the center of gravity tends to point a bit forward as well as out. In full-blown form, when you run low on fuel the pickup uncovers, then the engine briefly goes lean. The resulting acceleration on the airframe flings the fuel to the back of the tank, and (usually) gets to the venturi before the engine cuts out. Then the process repeats. The result is that classic "brrrrt ... brrrrt ... brrrrt" run at the end of a flight.
With a pre-made tank that doesn't have the sloped back wall, kick the whole tank out so that the back is about 1/4" more out than the front. If you're building a tank, taper the whole tank. It makes a nice puzzle for cutting out the sheet metal before you bend it. If you look at any of my profile planes you'll see a plastic clunk tank with the rear of he tank kicked outward.
To get a few "warning laps" of lean running before the engine cuts, locate the uniflow vent a bit inward of the pickup tube. When it uncovers, the engine will start going lean. I can't tell you exactly
how much to do this, because it's not an effect that I seek.
I have never managed to get the cutoff loop thing to work, so I'm not even going to try to tell you how to do that.