If you’ve ever built a plane powered by a .10 engine, you probably know that the fuel tank can be kind of a pain. For one thing, most planes in this size don’t allow a lot of room for a tank, so you have to use a really small one. Then you find out that a 1 or 2 ounce tank doesn’t really allow the clunk weight to clunk, so you get to fly inverted only for about the first quarter of the tank of fuel. After that the engine sucks air when the plane goes inverted.
One way to avoid this kind of trouble is to make a fuel tank from a plastic bag. A plastic bag will allow the engine to suck all of the fuel out without ever getting an air bubble. Also, the tank will be able to hold a greater amount of fuel than a rigid tank in the same fuel tank compartment.
The tank is made from a sandwich bag. You could try other types of bags. Here’s mine:
Cut a notch into the end of a piece of fuel line to prevent the line from becoming blocked if the bag gets sucked onto the line.
This is the pickup line. Attach the other end of the pickup line to a piece of brass tube. Attach the engine line to the other side of the brass tube.
Put the notched end into the bag. It should reach almost to the bottom of the bag. The brass tube should be inside the top of the bag, positioned in the middle of the open side.
Fold the open end of the bag over the fuel line.
Roll the bag around the line and secure it with a cable tie.
Here’s what it looks like after being tested for leaks. (There weren’t any.)
I used a small piece cut from a manila folder to protect the bag from sharp edges.
The muffler pressure line is not used. There is only one line coming through the firewall.
Turn the crank on your pump until all the air is out of the line, then connect it to the line on your fuel tank. While observing the bag from inside the airplane, pump some fuel in. Then reverse the pump to remove the air from the bag. Empty the bag completely, so you can start from zero and determine how much it should hold. While watching the bag from inside the plane, count how many cranks it takes to fill it to a comfortable size. If you land your plane with fuel remaining, empty the bag first, then start from zero again. If you pay attention to the sound and feel of your pump, you should be able to tell when it is empty. Then pump the amount of fuel you have already designated as a full tank.
Purging air should be necessary only once at the beginning of the day.