wing skins and a spot for the go juice

Continued working on the right wing.  Got the upper outboard skin mostly matched drilled to the ribs.  I've only pilot drilled the skins at the spar and rear channel as I will need to fit the nose and trailing edge skins first, the right size them as a group.

first thing was laying out the rivet spacing on the upper surface as per the plans:

On of the keys to good rivet spacing is knowing where the rib fluting is.  Marking it out ensures two rivets between each flute.  I marked the distance of each rivet back from the spar centre-line so I can easily transfer the same measurements to the other ribs which are the exact same:

A3 holes clecoed before right sizing to A4 across the top of the wing chord.  This really makes the curve of the wing apparent:

​With the ribs confirmed as right size, the rear channel is drilled out to A3, waiting for trailing edge skin:

Next up was the upper inboard skin.  This makes up the panel that covers the fuel tank.  It's installed much the same way as the outboard skin.  I'll wait to drill the rear channel here too:

finger clamps help to keep everything straight for pilot hole A3 clecos.  Again, I'll wait to drill these up to A4 when I'm ready to add the upper root skin as there is a root angle to attach at the junction of the two skins that help form the taper to the root:

The main upper skins are now complete:

After cutting the 2nd fuel tank skin, I roughly laid out the tank ends and some other parts I needed on 025 for the fuselage.  Minimal waste is the goal:

The next steps are joining up the two halves of the fuel tank form template and the two halves of the tank end aluminum templates:

Confirmed the templates match the measurements of the plans.  This is very similar to the templates and forms of the wing ribs:

The form template fits well on the outside of the inner wing rib and this confirms the extended tank will fit in the wing bay as I expect.  Kinda cool to see it work :)

With that confirmation, I laid out the aluminum templates on the pre sized 025 section.  Then I used the centre punch to mark the relief corners:

I also punched the inboard tank ends where the plans show the out-port of the fuel tank.  I've yet to completely decide on how this will look on my system with regards to the fittings, lines etc.  But the out-port will be here:

To remind me where I made punch marks, I circled them as I went.  Always drill and debure the holes before making relief cuts - so much easier

All four ends laid out for the fuel tanks - one left, one right:

Templates cut out, awaiting final relief holes and corner cuts:

Seeing as I only have four ends to make, I decided some pine boards would be just as easy to use and much cheaper than expensive 3/4 inch plywood pieces.

Stacked two boards and traced the form template on the top one.  Screwed the boards together.

Cut the template line out on the bandsaw.  Fresh pine getting cut smells real nice :)

Once I sanded the edges to the correct size, I marked the edges for rounding off on the router: 

Both sides of the form, edge rounded and beveled on the sander for springback allowance on the aluminum blanks:

The rest went the same as the wing ribs, except the forms needed to be clamped around the periphery as there are no tooling holes to use like the wing ribs.  Holes in the fuel tanks are not welcome here for obvious reasons!  Next step will be start laying out the bends for the wing tank skins.

Before I move forward on the tank construction, I need to finalize what needs to be built into the tanks, including fittings for the filler neck, the out-port where the fuel will travel to the engine via the fuel line and where the quick drains will be mounted (more on this later).

The other thing I need to decide is how to monitor fuel tank levels - this is the kind of stuff I love to figure out, but also keeps me awake at night.  The plans call for a float type fuel level sender similar to what you have in a car fuel tank.  Essentially it's a float on an arm connected to a sweep contact that changes electrical resistance or voltage in the fuel sensing circuit, which is fed to a gauge on the dash similar (simply) to this:

The drawing above is simple enough, however there are two flaws for this to work in my airplane.  There isn't room between the top of the tank and the upper wing skin for the the sliding contact/arm pivot.  Second flaw, related to the first is the plans call for the same float arm system, but mounted in the side of the tank.  All I can think of is why would anyone want to cut an unnecessary and large hole in the side of the tank?  That's just asking for trouble with leaks and the builders forums are chock full of stories regarding just that.

So, like the trim control and lighting, I'm going to create my own Arduino solution.  I've been doing some research on other methods to measure liquid quantities (the level) in a container (the tank) without being invasive (cutting holes).

My challenge is to find a method that can provide accuracy over 190mm of fuel tank thickness (top of tank to bottom) at it's thickest point and not require holes in the sides or bottom of the tank where it can leak fuel.

There is some limited information on the interwebs about using ultrasonic sound waves to measure the distance from the sensor to the fuel, but that requires a large range between full and empty to be accurate and again would require a sensor at the top of the tank, something I'm trying to avoid.  The math to make this work and the shape of the tank doesn't make this easy.

I briefly thought I could make something like this I found on Amazon.  It uses a float that slides up a column open/closing magnetic reed switches as it rises/falls, but it would still require a hole and mount on the top side of the tank and a bunch of circuitry to complicate things:

I've decided to try something like these.  A pressure transducer that measures the weight of the fuel in the tank inline with the fuel out port via pressure.  These transducers are fuel proof and output proportional voltage in a linear ratio to the pressure sensed - solid state, no moving parts and maintenance free They come in various pressure ratings and configurations, but most importantly are threaded the same size as my planned fuel fittings.

The outputs from the transducers can be read and interpreted by the Arduino microcontroller and with some simple programming the Arduino can output a signal for a readable gauge in the cockpit (one for each left/right tank).

What I want for gauges is really up to me as they can be displayed on a LED panel by simply programming whatever images I want to use as the display.

I could go with something simple such as the traditional automotive gauge on the right, but I kind of like the sweep/ribbon style on the left.  The numbers in this example represent percentage, but could be made to show litres/gallons as well - it's all customizable in the programming. 

I started to play with LibreCad to make my custom display.  I created the sweep and used Microsoft paint to colour each section of the arc.  Each arc represents a reading correlating to what the Arduino is reading, giving me a moving gauge as fuel is consumed.

The LED display uses low resolution bitmaps for display, but they can be in colour.  I plan on green for anything more than 1/3rd full, yellow between 1/4 and 1/3rd orange then flashing red for anything less than 1/4 to draw attention to it.  I might even have the programming sound an aural alarm as well.

A simple animated GIF shows what a declining tank would show (with an added funny at the end):

Progress is leading to more thinking and I love it.  It's the true core of what this adventure is about.

Next up, flipping the wing over and fitting the bottom skins and mocking up the fuel tanks for welding and fittings.

​Thanks for following along.