Long time no update, but work still getting done. I apologize in advance if this blog entry jumps around a bit.
Trimmed the top side inboard trailing edge to match the other wing:
Then trimmed the outboard tops side trailing edge as well, in prep for fitting the wingtip:
The fuselage longerons that I ordered from Zenith back in December finally came back into stock. I was down to visit Dad and we made a day of it heading to the Zenair factory in Midland to pick them up. Four 13 feet long custom aluminum extrusions which will form the corners of my fuselage.
With the sealant cured for a week, it was time to test the fuel tank for leaks. Nylon threaded inserts and teflon tape tightened into the drain and outlet fittings works well for testing. They are soft enough they won't damage the threads on the tank:
Propped the tank up on some sawhorses and elevated the rear to simulate the "in wing" position.
Threaded in the fuel filler neck to act as a funnel and wrapped it in terrycloth to capture any spills. I didn't want any stray water to make me question if the seams were leaking or not.
Based on the dimensions, the tank should hold very close to 15 gallons. In order to measure accurately, I used a 500mm graduated flask. Unfortunately although it is accurate, it took a very long time to pour the water from the Jerrycan to the flask and into the tank - 15 USG = 56.78 litres. Ouch, this is going to take a while!
I hooked up my retro Radio Shack analog multi-meter to capture the changes in resistance of the fuel sensor as I filled the tank in order to map out what various fill levels mean on the sensor.
That wasn't easy to read accurately, so I switched to a digital unit - much easier to read.
Filled the tank right to the maximum in order to test all seams. Disappointingly I found a weeping leak on the top outboard seam about 2/3 of the way to the rear of the tank. Argh, this was going so well.
Water pooled on the lower seam and was also evident on the newsprint I laid on the floor below. Better to know now than later with fuel I guess, but this further delays getting this wing complete.
Drained the tank to fix the leak. In order to make sure everything was completely dry before adding more sealant, I used a small pancake computer fan over the sensor hole and left it run overnight. That worked extremely well.
Flipped the wing over and finished trimming the inboard lower trailing edge to match the other wing. Had to be careful here as the fuel and pitot/static lines already run inside the trailing edge.
Drilled the rivet holes where the inboard and outboard trailing edges meet.
Measured the other wing and matched the template for the outboard lower wing skin on this wing. lots of notes to myself on the template!
Trimmed the outboard lower trailing edge then installed the wing tip support angles on the rear wing channel and on the wing tip spar extension:
Test fit the wing tip and made some small adjustments for better fit.
Main wing strut pick-up riveted
Laid out the plan location for the fuel access cover on the underside of the wing on the inboard lower wing skin. The original plans require this access near the main spar where the side mounted fuel sensors are installed and serviced from (oval on the left).
I'm not using side mounted sensors, so I decided to move the access cover closer to where my outlets are, which are further back due to the expanded tank size. This gives me good future access to the fuel tank outlet should I need to ever change out the finger screen.
Original location in red on the left, new location in blue on the right (fuel drain hole above that):
I've also been making up some of the smaller parts needed for the fuselage as I find the time waiting for sealant to cure, etc. I had a perfect piece of 025 to use for the two longeron gussets. Most times when there are two, you can mirror them and save material. This is a good example of that:
One thing I observed when the tank was full of water - how much the tank flexes as water (fuel) moves side to side and back and forth. Over time, that flexing will likely lead to stresses on the seams and fittings that will leak. I decided to add a centre inside rib made of 032 for stiffness. It means some more rivets in the tank skin, but with tight fit rivets and sealant they won't be an issue. I added some passage holes in the new centre rib to slow flow of fuel from one side of the tank to the other. The rib will not only stiffen the tank to prevent flexing, but also slow the sloshing of fuel - an internal baffle.
To further bolster the seal I buried a tab of 025 in the sealant as I riveted the rib in place, then covered it all in sealant again (the rear section done, front to be done in the picture):
I've decided to go a different route with the sealing of the fuel sensor plate. As much as I thought the well-nuts and buna gasket would be enough to properly seal the plate to the tank, the well-nuts can not provide enough tension in this application to hold the plate against the gasket. Disappointing as this would have been excellent it it had worked. Back to the thinking board.
Switching tracks again, drilled out the rear rivet holes on the wing tip:
Unfortunately, the pilot holes on the forward portion of the wing tip did not line up well with the forward attach angle. Had to drill this one out and replace with a wider one which worked perfectly.
Back on the fuel access cover, I confirmed the new location wasn't interfering with anything where I planned to put it.
To secure the panel, I picked up some Tinnerman clips from Aircraft Spruce. These are very handy!
I needed to make sure the inside edge of the fuel access hole left enough space for the clip to be covered by the edge of the cover plate. As shown below, the original hole oval left no room at the outer edge:
I adjusted the inner oval, making it 5mm narrower than the original - this left enough space to cover the clips and be a viable access hole. I laid them all out, focusing on the front and rear edges and corners:
Clecoed the template on top of the cover plate and drilled the holes for the clips out to A3:
Used the cover plate as a template to match drill the hole locations for the clips:
Enlarged the holes in the lower skin at the access hole location to the size needed for the clips and the #8 stainless screws that will secure the cover plate. This picture is deceiving as it shows the original access hole size (the inner oval):
Hole saw to cut the access hole, centered on the pilot holes from the template:
Forward end of the access hole the same way. The mark connection between the holes via tangential lines:
I find a line of masking tape is a good guide for cutting straight lines with the Dremel cutting wheel:
Access hole cut and ready for deburring:
Cleaned, scuffed and primed:
Clips in place and underside of the access cover primed and ready for attachment:
Once the prime was set, I used the edge roller to slightly curve the access cover plate edge downwards. This makes the cover sit tight against the skin when screwed down:
Marked the forward end on the underside of the cover plate. You can see the curled edges well in this picture. I also filled the pilot holes with A4 rivets to clean those up.
Really pleased how this went, looks really good!
For the fuel tank sender plate, I've decided I to use rivnuts to secure it and ProSeal sealant as the gasket. I kinda overthought the whole access to the tank thing, so this will be more permanent.
Rivnuts are hole inserts that are pulled just like a rivet but allow a machine screw to be used in the hole. With the holes I drilled for the well-nuts too big to fill with traditional rivets, these rivnuts will work fine and really will secure the sensor plate properly to the tank skin. Ron already has the tool to pull these, so I picked some up at Princess Auto to use.
I had to drill out the mounting holes a bit more to accommodate the 1/4 inch rivnuts, but the edge distance to where the sensor mounts in the tank is still acceptable.
Mixed up some ProSeal and applied a thin bead around the mounting holes and perimeter of where the sensor mounting plate will be using a small syringe:
Didn't get any pictures as I went as the ProSeal is messy and I didn't need it getting all over my cell phone. Here is the final result with all the holes squeezed tight with rivnuts. I'll have to get some 1/4-20 machine screws to fill in the centres, but very happy this is going to work.
Had some ProSeal left over from this batch, so sealed around the rivnuts for good measure and smoothed out the edges of the sensor plate. This is messy stuff, but no question about leaks!
Maggy the shop dog decided she wanted a selfie as we were wrapping up for the day (got to love the 0's safety glasses)!
Lots of stuff done, more to do as always. Wing is almost ready for flip back upright, then the wing tank will be fitted, along with the rest of the wing tip.
Thanks for reading, stay tuned for more!
Busy couple of weeks at the shop since my last blog update.
After finding the fuel sensor assembly gasket dimensions weren't scaled correctly I redid the CAD drawings and cut new ones that have wider surface areas while maintaining the center hole diameter and the outside dimension of the sensor plate:
Original test gasket laid on top of new gasket. This should improve hole edge tolerances. It will shrink the access hole a bit in the wing tank skin, but I believe it is worth the trade off:
With the fuel sender mounting holes already drilled to full size for the well nuts, I needed to come up with a way to secure the new gasket and drill mounting holes at full size instead of upsizing them as I went. I took some scrap plywood big enough to fully sandwich a sensor gasket and drilled an index hole in the plywood at opposite corners of the original gasket:
Opened the sandwich, put the new gasket in place using the index holes as a guide, then screwed the sandwhich down tight to hold the new gasket for drilling:
Placed the fuel sensor assembly on top and indexed it over the gasket using the same index holes:
Used the sensor plate as a guide, pilot drilled then right sized the holes in the gasket sandwich:
Much cleaner result and the hole edge distances are now improved.
Tsped the gasket in place to confirm the holes do indeed match up on the skins and traced out the new access hole dimensions on the tank skin:
Removed any non-pertinent lines after corning drilling the new access cut-out:
Dremel tool with cut-off wheel to final shape the access hole, then filed the corners and final deburred the edges:
Took a quick peek at the truss head screws inside the tank that hold the drain bung and gasket. This is where the fuel will drain from the tank for pre-flight testing and any future required fuel tank maintenance if necessary. Very pleased at how the gasket I made fits, but I'll add some sealant on the screw heads as well:
With all the cutting and fitting of access holes, filler necks and other outlets complete, I can finally move onto the dreaded sealing of the tank seams. This process has been keeping me awake at night figuring out order of operations and how to do many things at once.
First, I fully cleaned the tank inside surfaces to remove any contaminants or markings and used Scotchbrite pads to scuff the joining surfaces on all seams inside and out where sealant will be applied. This included a full wipe down with lacquer thinner to remove any other oils or residue that could impede the correct adhesion of the sealant or curing of the sealant once applied.
Next I hand placed all the A3 squeeze rivets in the bottom seam of the tank on the outboard side and used tape to hold them in place so I could flip the tank upright.
Tank on the bench with rivets placed in the lower seam, the back edge and front side of the tank.
Next up was preparing the ProSeal sealant. It gets mixed 10 parts sealant (big can) to 1 part activator (small jar) - this is done by weight not volume so I used a gram electronic scale to measure out exact ratio of each and once in the same mixing container, stirred them together to start the sealing process. Working time of this sealant is about 2 hours after mixing depending on ambient temperature with early cure time of 7 hours once applied. Full cure time is suggested as 1 week before applying any pressure or liquids to the tank.
I was so wrapped up in applying the sealant to the outboard tank side lower seam I didn't take any pictures of the process. Needless to say, it's messy and stinky stuff to work with and anyone using it can tell stories of it getting on everything so I wore old clothes I didn't mind throwing out if needed.
A peek inside the tank after it sat overnight shows that the sealant I applied in-between the joints did squueze out exactly as I planned into the inside seam of the tank - perfect!
Same with the sealant between the front upper edge and the folded angle I had to add:
I used the remainder of the sealant I had mixed up to cover the rivet tails on the tank fill neck flange and it cured up very nicely. It doesn't look pretty, but it's inside the tank where no one sees it and is fuel tight which is the real goal.
With the outboard tank side started, I started on sealing the inboard tank side which included copious amounts of sealant on the fuel line outlet bung. I figured to to do it first where I had 360 degree access to it before the tank side gets placed in the tank skin:
Here is a progress shot for the inboard tank side. Same as the outboard side, hand placed rivets, secure them with tape on the outside of the tank. To apply the sealant easily, I used some large bore disposable syringes filled with sealant - kind of like a miniture caulking gun and ran a bead of sealant on the inner side of the rivets as I want the majority of the sealant to move inwards as I squeeze the rivets:
As you can see in this picture, sealant squeezes out between the seams on the outside as well as coming up through the rivet holes. A good complete seal on the seam and the rivets. More sealant applied arounf the fuel outlet bung as well.
Extra sealant doesn't go to waste (it's way too expensive to throw away!) so I added some around the fuel drain bung screws as well. Anything to improve the fuel tightness of the tank. You can also see how the sealant squeezed properly into the inner seams of the tank:
While I let the inboard lower seams cure, I placed the right tank into the left wing and finished fitting the fuel sensor assembly and gasket for the tank in anticipation of bringing the other wing out of storage.
The new version of the gasket fits perfectly and I could drill the sensor plate and gasket out to full size mounting holes right on the tank this time instead of the plywood sandwich shown earlier:
I took the Dremel tool and a grinding wheel to both fuel sensor assemblies and carefully cleaned up the sharp weld edges where the fuel sensor wiring may come into contact to prevent chafing with vibration. No issues any more.
Next I repeated the process for the top seams of the tank. As there is little room to waggle the rib into place onto the rivets, this time I laid down the bead of sealant on the rib, folded down the top tank skin then hand placed the rivets into the holes. Messy, but effective as I needed to do both inboard and outboard sides and the front edge as the top skin laid down. This sealant gets on everything!
Once mixed it can't be stored, so to use up each batch I made (40 grams at a time) I started sealing the outside of the tank edges - rivet heads and rivet tails:
I purposely went sealant heavy on the inside corners to ensure the corner plates I added formed a fully tight corner - again, none of this gets seen inside the wing so it doesn't need to be pretty.
First section of the top seam, working forward from the back corner of the tank. The sealant clearly is doing it's job at the rivet holes.
Forward edges done, sealant on top of squeezed rivets:
Extra sealant on the front corners
All edges sealed and left to cure. An inspection mirror inserted inside the access hole shows excellent sealant fillets on all the inside seams.
Picked up a couple of nylon NPT plugs for each of the outlet bungs. These will tightened in place when I pressure/leak the tank. Nylon seals well but won't damage the aluminum threads of the bungs. Permanent thread sealant will be added once the final fittings are in place.
Got the final truss head screws I wanted from Fastenal and used them to seal the fuel sender plate with the gasket. All part of sealing the tank for testing.
Fuel sensor in place - the relief grinding I did makes a huge difference in helping the sensor wire to lay flat across the top of the tank and to prevent chafing.
Fuel tank test fit in the wing, first time completely riveted and sealed - nothing changed, tank shape still the same and sitting in proper position for fittings and filler neck - YES!
Next step is to add buffer strips to the sides of the tank to prevent tank movement in the wing especially when close to empty or less than full of fuel. I have some high density foam that I secured to the tank sides with high strength contact cement. Once dried I can literally lift the tank by the foam blocks.
Next close up the bungs with the nylon plugs and test the tank for leaks. If it passes the leak test, back into the wing it goes where I'll add the top cork strips and connect the fuel lines.
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.