How to Assemble a Model Aircraft Fuel Tank

The fuel system in your aircraft is an extremely important factor in how your engine runs.  Not only must it be located properly, but it must also be clean, as close to the engine as possible and 100% free of leaks.

Take your time assembling it and do not hesitate to replace damaged parts.  For example, if a tube collapses or cracks when bending it, replace the tube.

1/8" tube (aluminum, brass, copper or stainless steel) is used for most tanks and it does not hurt to have some on hand.  Buy a piece or two when you purchase the tank so you have it when needed.

Important! Fuel line, tank stoppers and some tanks are made of materials specifically for glow fuel or gasoline.  Use the correct fuel line and stopper or the fuel system will decompose.

Assembling the Fuel Tank

Note:  Information contained in the instructions supersedes information presented here.  Before assembling the fuel tank, be sure to review the instructions that came with it. 

It is not difficult to assemble a fuel tank but if it is not assembled properly you can expect a great deal of frustration at the field.

All R/C fuel tanks assemble in a similar manner and these basic philosophies apply to all tanks:

• Always deburr metal fittings to prevent cuts in the silicone fuel line.

• Always set up the feed line inside the tank so that it is approximately 1/4" from the rear of the tank.  Hold the tank vertically so that it is back-lit to help you see inside of it.

  The feed line should hang straight down.  If it clears the back of the tank by 1/4" then it will be slightly farther away when it is resting on the top or bottom of the tank.  This prevents the fuel from surging into the line and flooding the engine when throttle is applied (so I'm told).

• Always disassemble a tank after initial assembly and wash the tank and fittings out thoroughly to remove metal shavings and other crud.  Be sure to run water through the brass tubing and pick-up line.  If you have an air compressor then blowing some pressurized air into the tank (without the stopper assembly in place) and through the feed lines will help remove any particles.

• There is normally a third hole in the stopper that is plugged.  Don't open this hole unless you plan to use a three-line fuel system.

Fuel tanks come with brass or aluminum tubes.  The feed line should only extend in to the tank about 1/2".  That gives plenty of tube for the fuel line to grip.  If the metal tube is inserted too far into the tank then it will inhibit free movement of the feed line.

The vent line should be slowly bent to 90°.  I generally make the bend far enough from the end of the tube so that it is too long.  Then I cut off the excess so that the vent clears the top of the tank by 1/16" to 1/8".  You can either buy a tubing bender or use a variety of methods to keep the tubing from collapsing when you bend it.  Some people fill the tube with sand or feed a weed-eater line into it while bending.

I purchased a tubing bender in Germany that works well.  The tube must be pulled while bending to prevent it from collapsing.  Soft tubing can be bent around your thumb if you bend the tubing slowly.  The tube does collapse slightly, but it still passes fuel easily.

Do Not use a razor saw to cut metal tubing (unless you don't mind ruining the saw).

The two easiest ways to cut the metal tubes is with a small tubing cutter (K & S) or a moto-tool with a cut-off wheel.  To deburr the tubes, turn a counter-sink by hand in the end.  The idea here is to remove the burr inside the tube - not to sharpen the tube.  Use very fine sandpaper around the outside.

What I normally do is put a cut-off wheel in my moto tool and then spin the tube at about a 45° angle against the wheel.  Check your work carefully because it only takes a small cut in the fuel line to make your engine run erratically and make you pull your hair out.

The feed lines should extend about 1/2" from the front of the tank as well.  If the lines do not route in a fairly straight line from the tank through the firewall, then you may need to bend the metal tubes in the general direction of the exits on the firewall.  This will help prevent the fuel lines from kinking in the tank compartment.

The fuel tank is sealed through the use of a rubber stopper.  On the front and back of the stopper are hard plastic or metal plates.  The metal tubes pass through the plates and the stopper.

A screw goes through the center of the front plate and stopper and threads into the back plate.  When the screw is tightened, the stopper compresses and expands sealing the tubes and the front of the tank.

You will notice that there is a third hole in most stoppers that is plugged.  Do not use this hole or open it unless you plan to make a three-line tank.  It is easiest to slide the tubes through the stopper if you spit into the stopper holes first.

A common mistake you can make when assembling the tank is to over-tighten the stopper bolt.  The stopper is sealed when it bulges and it will not come loose.  Just because you can turn the brass tubes in the stopper holes does not mean they are not sealed.  If you over-tighten the stopper it is very likely that the tank will split and spill fuel all over the inside of your aircraft.

Testing the Tank for Leaks

Put a fuel line on the vent and another line on the feed.  Plug one line, put the tank under water and blow in the open tube.  If you see air bubbles coming from anywhere in the tank it is not sealed properly.

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Copyright © 2003 Paul K. Johnson