As explained in a previous blog post, I've decided to use magnetic reed float style fuel senders to measure the fuel levels in the wing tanks. Much simpler to manage and accurately interpret the signals for display in the cockpit.
Figuring out the best position for the senders is a compromise between tank design and expected fuel position in different phases of flight. I also want them mounted in a way that the top of the sending unit doesn't require a blister or bump in the wing skin to cover it.
It makes the most sense to measure at the tallest part of the tank, but also consider the orientation of the tank. Wing dihedral means the fuel will accumulate towards the centre of the airplane, that's why the outlet is on the inboard side of the tank, so it seems to make sense to keep the sender in the inboard front corner of the tank. But that only makes sense if the tank remains in the same position in all phases of flight. What about when the tank tilts outboard in a turn? Or inboard?
So I believe the right thing to do is to have the sender furthest forware I can at the thickest part of the tank, and centred laterally in the tank. This should give the most stable level measurement regardless of bank angle as the mid point will stay the same or the average. More thinking on this required to confirm.
In the meantime, I do know I have to fabricate a way to mount the sender.
The "empty" level of the tank should be considered to be the distance above the bottom of the tank to the top of the fuel outlet - if the outlet of the tank is anywhere below the fuel level, the fuel line will draw air. I want my fuel indicator in the cockpit to show "empty" before this happens. I measured this distance and it is about 25mm, so for an extra margin of safety we'll design it at 30mm.
To simulate the top of the wing skin, I laid a piece of aluminum angle across the ribs approximately where the sender will be mounted. The goal here is to get an idea of how deep the recess will be in top of the tank for the sender.
The senders I've chosen at KUS-USA 6.5:" marine fuel senders, available on Amazon. There are many cheaper versions of these from various retailers online, but I wanted something I can easily replace if the need ever arises and KUS-USA has been in business for years, is the industry standard and I know the quality is good.
I was very pleased on the build quality of the units. The float is fuel/chemical proof, the sender assembly and mounting screws appear to be high quality stainless steel and it comes with a fuel proof neoprene gasket, not cheap cork or silicone. Even the sender wires are properly tinned from the manufacturer.
A reed style float sender works by having a series of reed switches and resistors inside the sealed stainless tube, which are activated by a magnet inside the float that travels up and down the outside of the tube. The position of the float in the fuel tank activates a certain combination of reed switch and resistor than can be interpreted by the fuel guage.
I clamped the unit into position on the aluminum angle with a wooden shim in between to simulate the space between the upper skin and the top of the wing tank skin:
A wooden shim placed on the bottom of the wing tank bay approximates where the bottom of the tank will be once installed. The fuel float in this position represents empty and although there will still be fuel in the tank at this position, the fuel will still be above the fuel outlet.
Next, I needed to fabricate the recess tube that the sender will mount in as shown above.
I started by creating the lower plate of the tube. I chose 0.125 thick scraps I had left over as I wanted something thick enough to tap threads in.
I'm sometimes amazed about what I remember from math class and more specifically geometry. One of the easiest ways to find the centre of any circle is to remember that 90 degree perpendicular lines drawn from the the mid point of any two bisecting lines will cross at the centre of the circle:
It works no matter how long the bisecting lines are:
I drilled out a pilot hole in the centre, then used a scrap of wood as a support for making the cut on the bandsaw. I can rotate the piece as I cut. It didn't create a perfect circle, but the bench grinder took care of any rough edges:
I used a large pilot drill on the first plate to create the sender passthrough and that worked, but for the second one I used a bi-metal hole saw on the drill press. Much easier and cleaner.
From left to right: Sender, lower plate and tube stock which will be used for the sender mount:
The sender mounting plate is just a bit wide to fit inside the tube:
There is enough material on the outer edge of the senders to allow a bit of grinding to fit inside the tube:
A look at the bottom of the fabrication where the sender protrudes through the bottom plate, prior to cutting the tube to size:
Next to fabricate was the top mounting plate. I've decided to make these extra wide for a couple of reasons. First and foremost, spreading the weight of the sender assembly across a wider area of the upper tank skin and secondly, giving me wide opening in the top skin should I ever need to look or reach inside the tank for maintenance/repair reasons.
Centred on the plate, two pilot holes to guide the fly-cutter (didn't have a hole saw big enough):
Cut the hole slightly undersize then used a Dremmel bit to slowly expand it until the tube fits tightly in the hole:
A rough estimate where the sender will sit in the tube, prior to cutting the tube to length. This is what the top of the assembly will look like on the tank:
Laid the sender beside the tube and roughly marked out where the float will bottom out. The float buoyancy means it sits about half submerged in the fuel. Also marked where the tank bottom is, top of the fuel outlet and where guage empty is.
Once confirmed, I can mark the tube where the bottom of the recess will be - this is where I need to cut the tube, the lower plate will attach here:
Lower plate, sender gasket and recess tube:
The mount tube assembly stack:
The mount stack upside down on the bench:
With the sender in place:
Next up was the mounting screw holes. To ensure the 0.125 bottom mounting plate was thick enough to be threaded, I grabbed some scrap and did a test. Drilled a 5/32" pilot hole and tapped it to 10-24 thread that matches the sender mounting screws:
The screw holds nicely and will do fine on there own, but I'll add some thread locking compound when I install these just to be sure. much nicer than trying to capture a nut on the back side.
With that determined, I laid out the mounting holes in the lower plate and tapped them the same:
Both sender assemblies as seen from the tank side ready to be fitted to the wing tanks once they are welded up.
Very happy how these turned out. I'll do a bit more research on tank posistion just to confirm I am on the right track, including simulating a tank using a shallow(ish) Rubbermaid tote and some water.
More to come, thanks for reading.
Good day in the shop today. Time to start plumbing the wing for pitot, static and fuel lines. 50 feet each of blue and yellow nylon line and 20 feet of Earls's Auto-Flex fuel line:
I'm not ready to get the fuel tanks welded yet, but they are taking up a bunch of room on the bench, so I "dismantled" them for storage. Before putting the skins away, I decided to drill out the drain hole.
Step drill brings the hole up to 18mm diameter:
The tank drain fitting will be welded here and eventually will have the drain valve installed.
Before flipping the wing upright, I cleaned up the bench completely. It was getting quite untidy with all the bits, pieces and other tools. I laid down some green tape runners on the lower skins to help protect the skins where they will ride on the steel bench tubes.
Ron helped me flip the wing over and I was pleased to see the tape runners I put on the upper skin helped prevent most if not any scratches:
The nose skins are just wider than the bench when they lay flat, so I tagged the corners with a bit of yellow tape . It won't protect the corner, but it catches the eye when walking by.
Here is a better look at the pitot/static tube inside the nose skin now that the wing is upright. As mentioned before, I'll need to trim the length a bit to leave room for the the union connectors:
Here the connector unions in place on the end of the tubes. They point right at the spar web, bending the tubes into the lightening hole isn't practical.
A couple of holes in the spar web are necessary for routing the lines, but I like this better as it is another way to support the lines as they pass towards the rear channel:
Grommets protect the lines in the holes. I won't be connecting the probes until later as I work on wrapping the nose skin. Blue line is pitot (airspeed) and yellow line is static pressure (altimeter, VSI etc.):
The lines pass through the wing bay and out through the rear wing channel, again through grommets to protect the lines:
Not all leght of this line will end up in the airplane, but I'm not cutting anything I don't have to (less cuts means less potential leaks/failure points). For now it stays coiled up. Eventually it gets routed through the cabin, under the seats/floor and up to the instrument panel. I'll likely need more union/elbow connectors, but that's a ways off yet.
Next I worked on laying out where the fuel line leaves the left wing tank and travels though the inboard wing bay rib and eventually through the rear channel and out inside of the trailing edge to the fuselage.
The outlet fitting isn't welded yet, so for test fit I taped it in place:
The inboard tank rib (side of tank) sits inside the fuel bay of the wing once part of a completely assembled tank:
I lightly clamped the rib in place. The goal here is to approximate the location of the pass through hole in the wing rib foward/rear, not the distance laterally (left/right):
I marked the location of the pass through using a Sharpie through the centre of the fitting then drilled a pilot hole in the wing rib:
Using a the step drill, I enlarged the passthrough hole a little at a time until the finger strainer fitting from the tank could fit through. The aluminum plate is a piece of scrap I placed on the lower wing skin to protect it from scratches from the drill chuck:
I traced the size of the fuel outlet fitting centred on the passthrough hole, then enlarged it a bit more. This will give me a bit more room when I install the tank. The fittings and line just need to pass through here, they don't actually fasten to the rib.
Looking at the wing tank rib, facing inboard with the finger strainer in place:
Looking at the finger strainer, outboard towards the wing tank:
I can't do much more tank fit up until I get the fuel caps and fuel level sending units I ordered.
Looking back inside the wing, I needed to come up with a way to support and protect the pitot/static lines from rubbing on anything from vibration. The lightening hole flanges of the nearby wing rib face toward the lines, so clipping them to the surface of the rib won't work.
Using some scrap 025 aluminum that was already bent 90 degrees, I created some stand-off brackets
While the standoffs dried after priming, I remembered I needed to final torque the forward wing strut attachment bolts - so much easier with the wing upright!
Some 1/4 inch cable clips facing back to back get rivetted to the standoff then the standoff rivetted to the wing rib:
Nice and tidy and secure from vibration damage:
The standoffs are perfect, very pleased at how neat this arrangement is. Once the final fit of the lines are done at the pitot/static probes, I add a wire tie on the lines to prevent them line from sliding back and forth in the clips if needed:
Gentle bends supported well prevent the lines from kinking. There is plenty of room inside the trailing edge for a large radius bend in the lines as they exit the rear channel:
Cable clips avery 11 inches spaces out nicely along the face of the rear channel as the lines head inboard towards the fuselage:
Tied the nose skin up a bit with some twine which allows anyone walking around the bench when I'm not working on the wing to do so:
Good productive day. Waiting now on some plumbing fittings, fuel caps and fuel senders. I've got to drill and grommet the fuel line hole in the rear channel, then I can fir the trailing edge. While I wait for my orders to come in, I'll get the nose skin slotted for the slat pick-ups and wrap the nose skin.
More to come, thanks for following along.
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.
Happy New Year to my dedicated and loyal followers!
A bit of a slow start to 2022, but back in the shop for a few hours over the past few days. I'm still waiting on the fuel lines and fittings to come from the supplier, hopefully soon as I'd like to get them test fitted before I flip the wing over in the next couple of weeks to build up and fit the fuel tank in this wing.
While I wait for those to arrive I got the backing strips epoxied to the inside of the wing tip. EcoBond epoxy is spread thinly on both sides and left to set up for about 5 mins:
The 016 aluminum backing strip is clamped in place and left to set up. I leave these overnight and have no problem with them bonding well.
A test fit to the outboard wing reveals little trimming to be done. I'll need to confirm the measurements of the other wing to see where the other wing tip positioned for final fit and make sure this one is in the same place making the wings symetrical.
My push-connect unions came in from the supplier. If you are familiar with SharkBite plumbing fittings these work the same way. I'll be using these to connect up the pitot-static lines once the wing is flipped over.
Long time readers will recall the genesis of this entire project was largely credited to the mentorship of my friend Captain Barry Morris. When he passed away suddenly in Sepember 2015, I made it my mission to get working on this project and not wait for it to just come to me (see my earliest blog posts for more).
I've been in regular contact with Barry's widow, Linda Morris since his passing and we've shared many memories, laughs and tears. She gave me the honor of gifting me a damaged wooden propeller that belonged to Barry that he "acquired" and had cut down to a "trophy" of some sort. I wish he was still here and could share how this prop came to be, but somewhere in his adventures it came into his possession and I will treasure it forever.
For Christmas from Brenda, I was surprised to receive a hand made clock insert for the prop. Here is a picture of it installed. The words scribed on the face mean a lot to me, it is in a prominent position in our kitchen where I see it every morning:
Next up in the shop are fuel tanks and today I'm working on the design of strobe/position/nav lights for the wings among other things. Now that I have finalized the controller design, I want to test some higher power LEDs that I found on Amazon and see if I can really make the lights punch it out by boosting the voltage up. Also pondering lens design using clear casting resin - all in with experimental right? I'm NOT paying over $500 for what is currently offered on the market for lighting solutions.
Stay tuned, more to come.
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Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.