Fuel System Design
Designing a good, reliable and pilot error proof fuel system is quite an undertaking. Contrary to a car, aircraft fuel systems need to take into account balance problems of having fuel in their wings or even in the horizontal stabilizer (B747) and a lack of certain standardization among aircraft manufacturers doesn't help either.
You will find the tanks usually in both wings and sometimes there is a header or auxiliary tank in the fuselage. To get the fuel to the engine(s) under all circumstances and flight conditions the system is equipped with pumps, filters and valves.
But safety dictates that the system must be simple, safe, easy to operate and not confuse the pilot/ crew. Designing such a system can be a challenge and fun and frustrating at the same time.
Most homebuilt aircraft have two tanks, one in each wing. But if it works for you, use one only gravity feed tank and install a fuel shut off valve near the bottom of the tank. For a simple fuel level indication use a cork float with a steel wire running through the fuel cap. This is as basic as you can get and will always work, even without electricity.
Adding a second tank means more complexity and a possible pilot error with selecting the wrong tank. Unless the tanks are connected through a Y or T fitting. Connect the ON-OFF selector after the Y/T fitting and before any filter. Changing that filter is then possible without any spillage.
With a two tank system you will need a cross-over vent line connecting the tanks, even more so when only one tank has an air vent. This will keep a tank from collapsing when fuel is pumped out of it and the cross-over line keeps air pressure above the fuel at the same level so fuel should be drawn equally from the tanks.
Tank vents are either part of the fuel cap or installed next to the tank facing into the wind creating a positive airflow and increased fuel pressure. Be careful when using vented caps: if the opening faces backwards fuel can be draw out of the tank leading to starvation. Forward facing vent should also have a screen to keep insects and other bugs from clogging the vent.
Each tank must have a drain in the lowest part, so that any water or dirt can be drained during pre-flight. Make sure it closes after draining as you will loose fuel fast should this fail.
Some aircraft use a cork float type, but nowadays the resistive or capacitive types are used in modern aircraft. I prefer the capacitive types as these are more accurate (even with tanks that are oddly shaped) and show a much more stable indication when flying in turbulence. Calibration is also easier: set empty, set full and fill tanks with known quantities and note that in a table with indications from the gauge. Simple!
Ideally each tank should have its own shut off valve, perfect when working on the system and should leaks develop somehow you can close the tank without losing fuel.
Helpful with more than one tank but install the valve in a logical way, i.e.: moving to the right should select the right tank and to the left the left tank. Forward should mean both tanks and to the back fuel is shut off. The valve should be simple to operate and in easy reach with a clear indication too!
Fuel injected engines (diesels too) usually have a return fuel line routing fuel to the tank in use. This complicates the fuel valve installation, especially so with aluminum tubing which needs proper bending without cracking or collapsing the line.
Keep away from the automotive type flaring tools: these are at a 45° angle and aviation tubing and connections are at 37° angle flare. It won't fit properly and will leak with eventually caused by vibrations.
There are several types of strainers in the fuel system: finger strainer in the tank exit, gascolator and sometimes a screen in the carburettor. These serve to stop any dirt from reaching the engine preventing an engine failure. Engines with fuel injection require a proper filter to keep the fuel pump from failing, this filter will be installed after the fuel selector valve.
Make sure that you can reach each strainer with ease after the aircraft is finished, downtime due to maintenance is then kept to a minimum and cost too.
Fuel flow test
Make sure the system delivers the fuel flow required by the engine on take off including a healthy safety margin of at least 50%. To do this set the aircraft in a tail low position (climb attitude) and measure the flow from the tanks at the gascolator with the valves open. This will make sure that you always have enough flow and pressure in this attitude.