Productive day at the shop today.
First order of business, sorting the parts we ordered from Aircraft Spruce. Some hardware machine screws, lock nuts and castle nuts for Ron. I also ordered 4 drain valves for the wing tanks (2 for me, 2 for Ron). I wish this pandemic was over. I hate paying shipping when I could drive to Brantford to get stuff - probably cost more in gas than shipping, but who doesn't like going to an aircraft parts store?!?!
One of the appealing features of the 750STOL is the visibility from the cockpit. Most high wing airplanes have a wing spar/structure that carries through the top of the cabin or the cabin roof itself is solid. Look at the roof between the wings on this stock photo of a Cessna 172:
Compare that to the 750 which has a clear panel roof - the visibility from the 750 cockpit is amazing:
How does the 750 design accomplish this? The wings taper sharply at the root, but the curvature of the upper wing surface creates a very complex curve. To make this work, the root skins need to a way to mount at the varying taper angle from the tank rib to the root laterally and from the main spar to the rear channel as well. I needed to make a root skin support angle out of 025. I started with a piece of 025 strip and bent it to a 30 degree angle along the length of it.
I marked the angle 400mm up from the end and used the dead blow hammer to flatten from the end to the mark. The goal here is to go from completely flat near the rear channel to a full 30 degree angle at the 400mm mark. It went easier than I thought.
Next I marked the location for fluting bends between rivet locations on the tank skin. It takes a bit of tweaking to get the flutes just right, but eventually the angle starts to get close to the curve required and I could tuck it under the tank skin to get the first rivet in near the reach channel:
The strap duplicator is used to match drill the holes in the tank rib:
Continue to massage the flutes and work the angle until it matches the top of the nose rib:
It took a lot of fussing with flutes, adjusting here, tweaking there to get the angle to lay straight and flat. I soon realized the cleco I used on the taper side at the spar doubler was preventing me for properly adjusting the flutes. Once I removed that one cleco, it was much easier. Unfortunately that misaligned the hole slightly, but I can recover from that when I upsize the holes in final drilling.
Rivets across the angle and into the nose rib. That's better:
Root angle, looking forward:
Sometimes it's easier to take the plans out of the binder and put them beside me on the floor cutting mat instead of walking back and forth to the bench.
With the root skins laid out on 016 sheet, I used the bender as a straight edge and cut the templates out:
First fit up of the root skin looks promising. The instructions aren't very clear on how this interacts with the nose skin, but my measurements look good. The root angle also seems correct:
Laid the skin back on the bench and used the template cut out on the CriCut maker to lay out the curve that sits inboard and will match the curvature of the roof line and the offset of the root rib:
Finger clamps at the rear channel and spar, along with some wide masking tape to get the first cleco in the rear channel:
With all the holes drilled and clecoed along the rear channel, the skin appears to be fitting well:
Happy with how the root skin is fitting up, but I got a bit or work yet to do on it, including rivets along the root rib and the root angle.
Progress is progress. I've got some more hardware for the fuel tanks, and I expect the pressure transducer I ordered online to arrive this week so I can begin testing it's suitability for fuel quantity measurement. Soon I'll be able to flip the wing over on it's back and work on the bottom skins.
Thanks for following along - more to come soon!
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.
No update for a couple of weeks, but that doesn't mean I've been idle on the project. Picked up six sheets of 020 aluminum last week. They are 12 feet long and fit (barely) in the back of the family truck. The shipping materials actually weigh much more than the aluminum itself. Lots of straps to hold it in and a cheap fabric red flag on the end to keep it (closer to) legal.
I griped a bit about this on Facebook (who doesn't). It's very hard to understand why raw aluminum made here in Ontario is shipped out of country to be processed into sheets, then sold back at a premium here in Ontario. I'm all for North American jobs, but this is a good example how much manufacturing base our country has given away since I was born.
Off to the shop last Saturday. Nothing better than a hot tea in my Canuckmug.
The wing skeleton takes up much if not all the workbench and the other workbench is being used for Ron as he works on the Aeronca Scout finishing touches before it gets painted. A roll of plush carpet on the floor makes a good cutting mat:
First wing skin cut to size and rolled out on the skeleton. This one is the upper outboard skin:
Lined up, the pilot A3 holes at the spar clecoed in place. With those secure, the skin naturally curves down across the wing ribs. It actually starts looking like an aerofoil! Sweet!
Marked up the rib lines and the cross L lines that define the rivet lines for both. The cross L's help stiffen the skin over the wing bays. The whole skin will come off again once I finish the pilot holes at the spar, to drill pilot holes for the ribs and L's.
Spent the rest of the day making the L's and helping Ron install the new windshield in the Scout. Learned a bunch about drilling windshields so they don't crack - this will be handy when I install mine.
I also started working on the fuel tanks. I've got the first tank skin cut to size which is just a large folded rectangle (think four sided rectangular box). I'll need to make the tank sides and the wooden form to bend the edge flanges for welding it all up.
I managed to secure a copy of the drawings for the extended fuel tank. I put these into LibreCAD. This is the template for the wooden form directly from the dimensions in the plans. The tank will fit in the first wing bay between the first and second wing ribs, main spar and rear channel:
The template for the aluminum sides was derived form this using the drawing tools available in LibreCAD - I learned a few new tricks how to make parallel lines, offset by 6 mm which leaves room for the flanges. It's very close in shape to the wing ribs, just a bit smaller. I also created crosshairs on the drawing where the relief holes at the corners are drilled and also where the fuel outlet of the tank is, including allowances for the fold of the flanges:
You may recall from a previous post, my wife Brenda bought a CriCut Maker craft printer/plotter/cutter. I have a fair amount of time to think while driving to/from work sites and it donned on me to ask if the CriCut could accept native .DXF files that LibreCad produces and if it does, could it cut these templates from Bristol board?
Turns out, it CAN! I wish I knew this a long time ago, it would have saved me a ton of time cutting templates from converted PDF files, but the CriCut is fairly recent in the crafters market. Oh well, still have lots of templates still to make!
The CriCut can cut and print up to (almost) 12 inches wide by 24 inches tall with a little margin included in that. My wing skin templates had to be sliced down a bit in order to fit, but I was able to get the most important shapes (the complex curves) plotted and cut.
It starts with a standard piece Bristol board, cut down to 12 x 24 inches:
The CriCut uses an adhesive cutting mat - something similar to 3M Post-It notes on a larger scale. I laid the mat on top of the marked Bristol board to make sure it fit correctly. (The whimsical black cartoon cat and logo on the inside of the machine lid is something Brenda added herself, using vinyl and made on the CriCut):
With the Bristol board cut to size, it gets stuck down on the mat to wait loading into the printer:
The Design Space software for the CriCut takes a bit of getting used to, but isn't too difficult to figure out. As I stated above, to make the upper wing skin tip template fit, I had to slice it in half in LibreCad and import both halves into Design Space.
Follow the directions on Design Space to choose and load your material, and press go. The CriCut cutter automatically checks depth of the cutter and goes about making the cuts defined by the DXF file:
Almost impossible to see in the picture below but the cutter follows the lines of the uploaded DXF file. Once the cutting is complete, unload the mat:
Bristol board isn't one of the default materials in Design Space, so I just chose the heaviest card stock listed and asked the machine to cut with more pressure. Almost completely through and enough to easily remove the cut outs:
Emboldened by my success, I make the template for the outboard wing nose skin. It too turned out perfectly:
Here is what I got done over the course of about 45 minutes. Like I said, the machine and software make it easy to create perfect templates. On the bottom are the two template pieces that make the tank sides. You can even have the CriCut print and cut on the same piece just by choosing what you want done with each line segment. You can see the crosshairs I spoke of above, drawn right on the template.
Back in the shop for the day tomorrow. Goal is to get the pilot holes drilled in this upper outboard skin, make the upper inboard skin and drill it (it covers the fuel tank) and maybe get the blanks made for the tank sides.
I'll need to obtain the hardware for the tanks soon. These include the fuel outlet fittings, the quick drains and the fuel filler necks. Lots to think about :)
Thanks for reading, stay tuned for more.
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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.