Fuel tanks part 1

Fuel lines and fittings have arrived.  Stainless braided Auto-Flex line and Swivel Seal fittings.  Expensive but the peace of mind this provides is worth it.  This is one area of the plane I refuse to compromise on.  Very impressed with the quality of Earl's Performance products.  With these in hand I can start laying out the fuel tank plumbing. 

I've been wanting to figure out a way to measure the fuel level in the tank that requires no internal sensor - less holes in the tank means less chance for leaks.  There are several stories on the builders forums about tank sensors mounted through the sides of the tank leaking or being terribly inaccurate, hard to service and generally failing.

I wondered if there was a better way, and I originally came up with the following.  A very sensitive presssure transducer (the chrome unit with yellow label below) that will measure the "head" pressure of fuel in the tank via a "T" in the fuel line next to the tank.  In theory this should work using an Arduino microcontroller to read the sensor and output a value to a gauge.  Here is the assembly mocked up on the bench.  From right to left - finger strainer (inside the fuel tank), tank fitting (through wall of tank), threaded adapter, threaded "T", coupler and pressure sensor), fuel line fitting elbow.

Testing of the system has proven difficult.  I've been unable to get consistent readings from the sensor in a static set-up.  The programming of the Arduino works fine, I'm just not happy with the accuracy of the output due to the very low spread between empty tank head pressure and full tank head pressure, which given the depth of the tank is about 1.5 PSI.  I'd hoped a programming algorithm within the Arduino to amplify the input values and stabilize the readings would work, but it's become too much of a tail chase to get it right.  From there I'd still need to figure out how to smooth the values to account for changes in atmospheric pressure (fuel changes "weight" with changes in altitude) and aircraft attitude - banking towards the sensor increases head pressure, banking away reduces head pressure.

In an effort to focus on the build getting moved forward, I've decided to abandon this concept in favour of a top mounted float sensor.  Yes, this means another hole in the tank, but if mounted from the top, the risks of leaks is minimized and the ability to service the sensor if needed is much easier.  More on this later.

So removing the pressure transducer from the fuel line leaves just the tank strainer, the tank fitting and the elbow.  I'll need to order the fitting that goes between them, but they are inexpensive and easy to obtain.

The tank fitting is a one direction NPT thread.  This picture shows the fitting as it looks from the outside of the tank.

These fittings were gifted to me by another builder.  They are actually the larger style, so I need to modify them slightly to fit the tank side rib so the strainer sits nearest to the bottom of the tank as possible.  Trimmed them using the bandsaw, then cleaned them on up the disc sander.

The smaller diameter of the fitting needs to fit through the tank rib from the inside.  I don't have a drill bit this size to make the hole, so I had to improvise.

I confirmed the fitting was trimmed enough by laying in on the outer face of the tank rib:

I drilled a pilot hole equivalent to where the centre of the fuel strainer will be on the tank:

Used a large step bit to create the general shape of the hole almost up to full size in the tank rib:

Laid the fitting centred over the hole on the tank side of the rib and traced the final size/shape:

Clamped the rib to the bench elevated on some blocks, then used the Dremel tool with spiral cutting bit to carefully bring the hole close to size using the trace lines as my guide:

Filed the hole to final size to ensure a tight fit:

Here is the fitting in it's final position looking from the inside of the tank rib.

​On the outside of the rib, the fit is really good:

​With the finger strainer in place

Bending the tank skins can be tricky.  It is paramount to ensure the bends are as tight as possible to the corners of the tank ribs to reduce any gaps that will need to be filled when the tanks are welded.

I wrapped and finger clamped one of the ribs with the seamstress measuring tape (very handy long ruler for any project) to get a general measurement of where the corners of the tank skin will be.  This picture is of the second tank taken later in the day (left tank had already been through this process and I forgot to get a picture for the blog):

The throat depth of the wide bender isn't deep enough to get to the middle tank skin bends, so I needed to use the heavy bender. To allow full access we moved it out into the open floor area, rather than roll up the paint booth curtain (Ron is painting currently).

First I laid out the 025 skin and marked the approximate location of the first bend.  Like the slat skins, order of bending is important here, working from middle out to ends. I've left the skin a bit long at each end to make sure it doesn't come up short once it is bent to shape:

The heavy bender allows for material to pass through, giving full access to the middle of the sheet:

First bend complete, next was measuring the second bend closely to make the corners tight

Finger clamped the ribs in position to get a good measurement where the next bend would be:

Second bend line laid out (note the little circled "2" I used to remind me of the bend order):

Back on the bench, I discovered the second bend was slightly overbent and the process on the bender had opened up the first bend as well....  I corrected both before moving forward, but swapped the bend order on the second tank which prevented this from happening again.

Left tank skin rear corners corrected and test fit shows good:

Marked out the location of the fuel filler neck:

Front side of the tank skin bent up to meet the top skin and close off the tank:

The front side of the tank skin needs to have a 5mm flange bent forwards to create the seam for welding.  I puposely waited to bend this flange as I didn't know where the bend would be until I mocked up the tank:

I tipped the tank forward on its nose and clamped the forward tank skin onto a piece of aluminum angle at the bench edge:

Using a deadblow hammer I caefully bent the 5mm flange forwards to match the top corners of the tank ribs:

Reassemble the tank and use Cleco vise clamps to hold it together.  Some final tweaking of the rib flanges squares it all up - very happy with the final shape!  The extra material overhang on the top side of the skin will be trimmed back to match the flange.

Marked out the fuel tank drain location which is directly below the fuel line fitting.  This is the lowest spot in the tank once it is mounted in the wing.  It is from here that fuel is drawn during pre-flight to check for contaminants such as debris or water:

The right wing tank went faster now that I have the benefit of a process from the first.  Fuel outlets are on the opposite side of the left tank (the face inwards towards the fuselage:

I drilled and sized the fuel line fitting for the right tank after creating the tank so we could move the bender back out of the way.  The process for the fitting was the exact same as the left wing, just the opposite side of the tank:

Picture of the top front of the left wing tank.  I've laid out the location of the top mount fuel tank sensor and the fuel filler neck.

Both tanks clamped together, resting on top of the wing and awaiting parts for the fuel filler neck and fuel level sensor.  These will be fitted before final weld-up.

Very pleased at how the tank assemblies turned out, hopefully welding and subsequent leak testing goes as well.  Getting ready to order the fuel filler necks, fuel caps and the parts needed for the fuel level float sensors.

This doesn't fit under either the engine or avionics category, I guess it is part of the wings?  Maybe under other......

Thanks for following along, stay tuned for more.