I've been meaning to publish this edition of my blog since Monday, which was Halloween, but work commitments have kept me busy and I've also been in the shop getting stuff done!
With the lower fuselage skin almost ready for deburring and priming of the joining surfaces, I needed to get the lower fuselage access cover (the Hell Hole cover) final fit up done. I didn't time this to happen on Halloween, but it seems appropriate :)
Before taking the lower skin/longeron/diagonals apart for debur, I fitted the flap/controls mixer cross support by back drilling from the inside of the skin just forward of the hell hole. This cross member provides the rear support structure for the flight controls as they pass through the centre of the cabin space back to the flapperon mixer tube (more on this once completed) .
Also drilled out and deburred the control tube bearing hole in the centre support channel. This mounts centred on cross member to support the control tubes, but I'll wait until I have more of the rear baggage area complete to determine the correct vertical mounting position for it. Nothing in the plans shows a vertical measurement and I want to have it exactly correct so the controls don't bind. There are also some clearance holes to be made once mounted.
With all the diagonals and longerons removed, I laid the lower skin down flat and upside down on the table. The hell hole cover was placed into final position and squared to the hole, diamond bends facing out from the hole.
The goal here is to make the cover lay as flat as possible to the lower skin and use the tension created by the diamond bends to tighten the cover skin as it rebounds once attached. It isn't much, maybe 5mm of depth in the cover from corners to the peak where the diamond bends cross, but it certainly is an effective design by Zenith.
With the cover in position, I placed a bucket of clecos on the centre, which weighted down the panel flat. Then I drilled the cover down where rivnuts and screws will hold it in place, starting at the corners:
With all the holes drilled and fastened (not all shown in this picture) to the lower fuselage skin the cover maintains tension and stiffness once the weight in the middle was removed - excellent!
With everything match drilled to correct size where I can at this point, it was time to strip off the protective plastic and start deburring all the holes. I generally try to leave the plastic on as long as I can to help protect the finish of the aluminum from getting scratched - it's never perfect, but reduces the amount of scratches I'll need to remove before eventual prime and paint.
Once every hole is deburred (both sides of the skin), every inside surface of the skin that contacts another part gets a scuff with ScotchBrite, then a light spray of primer. Like all the other components I've assembled so far, grey self-etching primer in visible/outside areas, zinc-chromate green on inside surfaces.
The inside of the access hole gets the grey:
The rear inside of the fuselage and lower rear skin gets the green:
Had a couple of minutes down time while the primer cured, so I fabricated some nylon bushing plates that will be required for the flapperon mixer assembly. 40mm square, 1/8" thick.
Used a punch to centre a hole in each which eventually will be drilled out to 3/8". More on these later.
For now the lower fuselage skin gets rolled up and put back into storage so I can complete the diagonals and upper longerons for the side skins.
Laying out the right side fuselage skin on the table I needed to complete the cabin window cut-outs. I've been anxious about doing these as there is always a risk of tearing the thin fuselage skin which could render the entire skin un-useable and needing to be replaced. But like anything in this build so far, no risk = no reward, right?
Clamping the skin down to the table with a sacrificial backing board behind it and I used a bit of cutting oil to assist with the round corners:
The pilot holes I drilled previously are exactly the same on both fuselage side skins, but they are being cut individually. I was going to use the fly-cutter tool to form the corners of the cabin window cutouts, but it is too big to cut 25mm radius holes, forcing me to use a bi-metal hole saw. Much more aggressive cutting, but taking it slow and using the cutting oil worked very well.
Corner holes for the cabin window cutouts complete:
Inside hole cuts can be difficult, particularly in small areas as there is no way for the cut-away aluminum to curl out of the way of the shears as it is cut.
Getting metal shears into the 25mm radius holes effectively was too difficult, so I used a Dremel cut off wheel to first cut a relief line wide inside the window cut out. This freed up the tension in the aluminum and I could trim the outer edges of the windows smooth between the corners:
Next all cuts are deburred and sanded smooth. Eventually I'll flange the edges inwards just a little bit so the edges are tight against the Lexan windows. I'm looking into adding a silicone or rubber gasket as well which will keep out the rain and prevent any rubbing on the Lexan.
It took a couple of hours to complete the cabin window cutouts, but with a bit of patience and careful trimming, they both turned out great!
Window cutouts complete, time to start adding the vertical and diagonal stiffeners:
I've done the same as the bottom fuselage skin, adding additional opposite diagonals:
Once the diagonals are in place, flip the right side fuselage skin over and start fitting the longerons. A yet to be drilled longeron for the top edge and the lower right longeron from the bottom skin in place on the side skin for fit up. The lower one is only clamped in place at this time, it will be match drilled during assembly of each of the finished side skins to the bottom skins.
I still need to trim the forward edges of the fuselage side skins at the front cabin join location. I'll wait until later once I have the uprights and spar carry thorough channels fitted in place (just in front of the windows, better view in another picture further down):
Drilled out the upper right side longeron to A3, then alternatively to A4. A forest of clecos!
With the upper longeron final match drilled, the entire right side fuselage assembly is flipped back over and I can start to fit up some of the upright channels.
The baggage area back panel support channel shown here in rough location. I originally thought I should pilot hole fit this in place for future assembly, but I've decided to wait until the fuselage sides are mated to the fuselage bottom to see how everything fits best. Kit builders don't have these issues, they just cleco things together and right size the holes for rivets.
Test fit of other upright channels seems to confirm things will go together nicely when the time comes. The two channels at the left show where the fuselage side skin will be trimmed to match. The channel in the middle behind the first cabin window is one half of the flapperon control tube cover (more on these later).
Next up, everything comes apart for debur and priming. The repeat everything for the left side fuselage skin. With the left side done, I can start assembly of the fuselage and installation of some of the controls like the flap mixer and bearings. I'll likely be fitting the windows up as well, but the final install of the windows happens much later on in the build.
Very pleased with my progress so far on the fuselage. I got discouraged for a bit figuring out order of operations, but managed to get a process together that seems to work well so the left side should go much faster. Another bite of elephant consumed :)
Thanks for following along, more to come soon!
I stated before that the fuselage should go together relatively quick and so far so good.
More work on the lateral and diagonal stiffeners, drilling them out to A4 (final size). Now that the stiffeners are solidly in place, it's better in my case to drill from the exterior inwards, rather than taking a chance on rounding out an exterior facing hole. True kit builders have match drilled parts, and quick build kits all the riveting is done for them!
Same for the "Z" channels around the hell-hole. First all to A3 for accuracy, then right sized to A4:
With the lower skin facing exterior up, it's time to add the corner longerons to the lower skin. I've been looking forward to this as it really defines the shape of the rear fuselage. First, I added the lower rear skin at the tail and confirmed the overall lower skin is the correct length as per the plan:
The 4 corner longerons are custom extrusions (read expensive) that you can't buy anywhere else but through Zenith. As expensive as they are, they a a beautiful extrusion (on the left in the picture) perfectly matched to the fuselage skin edges:
As stout as they are, they are flexible enough to match the taper of the fuselage. To start, I clamped the extrusion to the skins at each end:
This picture really captures the taper, looking back from the cabin end of the fuselage. The secret I found was to drill and cleco front to back, through the pre-drilled holes in the skin that I laid out previously. Bending the longeron extrusion to match the taper was actually quite easy and I used clamps as I went to get the skin tight up against the outer radius of the longeron:
First one done (right side, skin is currently inverted on the bench, second one clamped and awaiting it's turn:
The longerons come a bit longer than required for the skin - they get trimmed later in the process:
Left side longeron pilot drilled:
Longerons, lateral and diagonal bracing complete up to A4 rivet size where required. I purposely left the section of A5 holes adjacent to and forward of the hell hole, as there are gussets to be installed there that will require back drilling from the inside:
The combo of the longerons and the stiffeners really squares everything up!
The lower fuselage skin gussets were next. I cut the 060 alumium to shape and then pre-drilled them as a stack to ensure they match for both sides of the lower forward fuselage skin where they are installed:
The gussets are not equilateraly triangle shapped and they go on the inside of the fuselage. They distribute and balance the load from the landing gear and cabin/fuselage mating point.
Time for the hell-hole access cover.
The cover is 540mm wide by 640mm long which allows for overlap of the hell hole. Cut the 020 aluminum to correct size and initially thought this wasn't going to be stiff enough a material over the hole once airborne. Trust the plans they say, so I continued to fabricate the cover, starting my radiuses on the corners (both cosmetic and functional):
Centre of the crossed lines represents a 5mm radius corner:
Turns out a bolt size template also works good for tracing the radius - an AN6 bolt diameter is close enough to 10mm to be used to scribe a 5mm radius:
Snips to trim the corner to roughly 5mm, then file and sand to make it perfect:
The plans call for "diamond bends" diagonally across the cover, so I laid out bending lines on the underside as the very faint bend needs to point outwards from the hole it covers:
Flipped back over after the diamond bends. Very surprising how much that stiffens to the whole sheet. I think it looks like the back-splash on a fish-and-chips kitchen, but this will be less apparent with paint and flattening when I secure it over the hole using riv-nuts.
From the top side, I laid out attachment pilot holes, 10mm in from the edge. These holes will be replicated onto the fuselage skin once in place, but I'm going to wait until I have the lower fuselage apart for debur, that way I can press it flat to the table as I position the riv-nut holes in the lower skin around the periphery of the hell-hole.
A few more things to do from the inside of the fuselage, namely the inside lower skin gussets and the front/rear horizontal tail framing.
First the gussets. They are installed over the inside surface of the longeron and but-up against the front edge of the lower fuselage skin. Using the pilot holes in the gusset, I matched the holes through the longeron and fuselage skin. The longerons also need to be eventually trimmed at the front to match, but having them long allows for a cleco clamp to hold the gusset in correct position for match drilling:
Eventually they get drilled up from A3 to A4 then to A5 (final size). Here is a festive Halloween colour photograph half way through the process:
All done with the gussets for now. The forward facing edge of the triangle will be done once the passenger cabin is attached and the landing gear channel is installed.
Next, the horizontal tail box bulkheads. The rear one wasn't to difficult to line up, using clamps as it the mounting flange is an open angle and easy to drill and cleco, up to A5:
The flange on the front HT bulkhead is more difficult as it is a closed angle:
To facilitate the drilling, pulled the lower skin assembly over the edge of the bench so I could drill from below, eventually up to A5. The precise angle of these bulkheads relative to the lower skin will be determined later when the side skins go on:
I mentioned in a previous blog that the 12 foot sheet of 020 used for the side skins isn't long enough to reach all the way back to the tail. So like the lower skin, there is an extension piece. I made these very early on in the build as it was a fairly simple flat shape to make. It was satisfying to clamp them in place on the lower skin assembly and see that they match!
Looking at this picture makes me think I have the rear side skins turned 90 degrees the wrong way, but that could be camera distortion. I'll double check that.
So pleased to see the lower fuselage skin coming together and some other parts fitting up too. Too bad it has to come all apart for debur, prep and prime of the mating surfaces :( The both fuselage sides need their vertical and diagonal stiffeners matched up, then they can go through their own prep (I wish they could do it themselves!) before adding them to the lower fuselage skin... phew, that's a lot of holes!
Oh well that is just part of #dreambuildfly! Thanks for following along, stay tuned for more.
With the wings in storage and the fuselage skins measured and laid out, I can start working on assembling the fuselage! It's been a long time coming, but so looking forward to this step. Big updates as follows.
First up I finished fabricating the last component parts of the fuselage that I can ahead of time. These baggage back support channels will be needed once the skins are together to start building out the baggage area in the rear area of the cabin.
I've said before how handy the CriCut Maker is for taking CAD drawings and cutting them out for use in the project. Here is the template I used for the top ends of the baggage back channels.
I orginally planned to use the crosshairs in the middle of the circle to mark where to bore the hole in the blanks, but I realized I cut the blanks to length first. No way to expand the hole to the correct size by drilling (step drill only works on full area, not edges).
Cut the circle portion away, then traced the curve on the blank.
Carefully cutting them out with the bandsaw and gently finishing with a round file worked fine. The baggage back channels bent up nice.
To measure out the side skins, I used the same method as the lower and upper fuselage skins. They too have a gentle taper curve from front to tail. I adapted the plans into CAD and added 250mm sections, to be measured out on the skins for better accuracy of the taper.
Another template I cut out after CAD entry is are the cut outs for the rear cabin windows.
Unlike the bottom and top fuselage skins, both side skins need to be mirror images of each other. To accomplish this, I stacked two full sheets of 020 and clamped them to the bench.
Then I proceeded to lay out the balance of the measurements on the side skins. To ensure both side skin sheets stayed aligned, I drilled and clecoed indexing holes at the corners together. These index holes are on part of the aluminum that will later be trimmed off. From this point on any holes drilled for windows, supports or other items will be exactly the same on each sheet.
The plans are somewhat difficult to interpret here with regards to where the windows actually are on the side skins. Great measurements if you are using a CNC machine to cut out the holes - not so much for a scratch builder! It took some time and several cross-checks to be sure but they are correct.
Drilled A3 pilot holes along the measured edges of the skin, through both sheets and 10mm in from the skin edges where the fuselage longerons attach:
Balance of pilot holes drilled and clecoed. My camera really distorts proportion on long lengths like this, the taper from front to back of the side skins is much greater than what is seen here (see the CAD diagram above):
Another challenge when scratch building is being sure of the best order of drilling holes - i.e. what will I need to pilot drill, from which side and what attaches here. You can see some notes on the skin in red reminding me to wait on these holes until later assembly as there are doublers here that need to be back drilled on later assembly steps:
Extended the lines on the window templates to confirm they match and line up with what's drawn on the skins:
Taped the templates down in the right locations, then drilled pilot/index holes through both sheets, at the corners of the windows. These pilot/index holes will eventually be widened out to 25mm radius in the skin - but both skins will be exactly the same, just like everything else.
A trace out of the template edges onto the aluminum sheet confirms window orientation to other cabin area components - perfect!
Like the lower skin, the overall dimensions of the side skins are too big for a 4x12 sheet of 020 aluminum, so it requires a extension on the tail end. This doubler skin extension also bolsters the "box structure" of the fuselage tail that supports the horizontal tail and rudder. These side skin extensions are some of the first pieces I made in this project, several years ago - glad to see them being put to use after taking space on the shelf!
Took a few minutes to finished sizing and sanding the rear wing pickups - eventually these plates will attach at the top front corners of the rear fuselage to be mount points for the wings - will be needed very soon, so wanted them done and available.
Unstacked the two side skin aluminum sheets and put the drilled but unmarked lower sheet aside. With the marked sheet back on the bench, I trimmed the edges to the correct shape using shears:
Hard to see in this picture due to camera proportion distortion, but the top edge of the side skin definately has a gentle curve from front to back:
This angle shows the side skin taper well. This is after I trimmed the bottom edge:
Re-stacked the skins again, re-indexing them using the same pilot holes I drilled as before. Then I traced the outer edges of the first skin (now cut to correct size) onto the second skin, making a perfect copy:
Removing the first side skin and returning it (rolled up) to storage, I replicated the layout lines on the second skin - this was easy as the pilot holes already exist where the support angles will be, then this exact copy was cut out using the traced lines from the first skin, then rolled up and put away for now into storage.
Next I got the top skin back on the bench an cut it out from the sheet. Once done, it too was rolled and put into storage.
Bottom skin, back on the bench for trimming to size. Here the pilot side has been trimmed away:
Again, camera distortion at work. Here is the trimmed to size lower skin looking from the tail to where it will join the cabin. It does show however show the curved taper of the fuselage sides. Very happy how this turned out:
It looks really lopsided in this picture - but dimensions between the edges and the access hole are completely equal and square - weird.
Next up, fitting the internal bracing around the access panel (affectionately called "hell hole"). It is supported on 3 sides by Z shaped channels:
The first Z fits laterally across the fuselage skin just aft of the hole. Then one on each side:
Z channel is called that, but it's a bit lopsided to be a true "Z"
Two more overlapping Z's fit laterally on each side, really stiffening up the lower skin:
Next up, the lateral L stiffeners and diagonal L's in each lower bay:
There are literally tonnes of discussions on various forums and websites about "oil canning" of Zenith fuselages. Oil canning is where the skin surfaces between the lateral stiffeners tends to drum a bit as rough air passes over them during certain aerodynamic situations. Some say it's not a Zenith unless it does this but I don't think it needs to be that way. I remember going for a demo flight in a very early model and couldn't believe the noise in the cabin on slow approaches or steep turns (where the airflow over the fuselage is turbulent or "dirty" as they say). Almost too much to endure.
When the original Zenith 701 came out to build it was a plans only design, built in a garage and to be absolutely the the lightest structure possible. Zenith intended it to be be flown as an ultralight on 65HP Rotax two-strokes - so I understand that less weight was important and made it easiest and cheapest for the average person. I guess the drumming of the skins was considered an acceptable trade off.
As the design evolved into what is now the 750 STOL (like mine), the 750 Cruzer, the 750 Super Duty and the 4 seat 801 which all use larger and heavier engines, the drumming remains. In my opinion, Zenith needs to update their designs in this regard. Current larger engine horsepower choices allow for more overall aircraft weight and by extension the reinforcement of these areas - the weight penalty is extremely small for what it resolves. Less drumming is better on pilot fatigue and more importantly airframe metal fatigue.
So to improve my airplane I'm adding additional diagonals to all fuselage skin bays. None of this additional weight is significant nor does it impede any further components form being installed or functioning - all it does is stiffen up the skins to reduce (or hopefully eliminate) skin drumming. Here are the first two bays in the lower fuselage skin with the additional bracing installed:
Next up, I'll finish adding the extra diagonals where needed, then start to prep the lower skin for the addition of the lower longerons. In the meantime, I'm headed to the Zenair/Midland Huronia Airport open house soon and will pick up a couple of more parts from them for the fuselage I can't make in house and some more stuff from Aircraft Spruce - exciting progress ahead.
Thanks for reading along!
Some quality time in the shop this weekend.
The spar carry-through channel and doublers are critical components that define symmetrical mounting points for the wings and cabin frame. Kit builders truly have an advantage here. Their components come pre-drilled match up perfectly. I originally decided to buy the main channel from Zenith then fabricate the doublers myself once I figured out the order of operations to match drill the other two parts (the factory part is already drilled).
Ron and I had the plasma cutter out for some other fabrication work (engine stand for his O-200 engine) so we decided to give it a try cutting my doubler blanks from 063 aluminum sheet.
We'll need some more practice using the plasma cutter on aluminum (material feed rate, amps, air-pressure) to get nicer results, but with a bit of clean-up, the first doubler angle turned out acceptable. This doubler runs along the upper rear of the spar channel. Here is a picture of it lying flat on the bench in front of the channel and my smaller test piece to confirm bend angle with:
With the test piece confirmed correct, I used a piece of channel as a straight edge to mark the bending line:
Used the heavy bender to form the doubler angle to the correct 65 degrees closed angle. My shorter test piece was perfect, but it took some extra effort and convincing to bend the doubler as 063 is almost too stiff a material to bend at that length with the bender we have.
Once confirmed as correct when matched in position on the spar channel, I clamped the assembly together and elevated it on the bench to allow the clamps to be where I wanted them. Both arms of the doubler angle must lay flat against the spar channel in order to ensure the match drilling works properly.
For balanced drilling, I started in the middle and worked alternatively left and right to drill the rear facing rivet holes to A3 (eventually will be upsized to A5 here):
With the rear side of the doubler angle matched to the spar channel, I clecoed it in position on the upper skin (remember, this is upside down on the bench right now) to check for fit. The sub-assembly is held in position with the outermost A5 clecos that can be match drilled from above. All good so far:
Rear arm of doubler angle is perfectly flat against the channel and also sits flat against the skin under the doubler:
To finish the doubler match drilling on the underside, I flipped the top skin over, clecoed the top fuselage doublers to the skin. Then I elevated the skin onto square tubing (not shown in picture) and added the spar channel sub-assembly under the skin:
The outermost A5 clecos through the top skin doublers, the skin and into the sub-assembly holds things in position. The I used a deep c-clamp and some scrap wood blocks to hold the "sandwich" together for match drilling:
I also added a long thin strip of scrap 063 on the front facing edge as well - this temporary spacer represents where the front doubler channel would be once I have it in place, keeping the sub assembly square to the skin and the holes in lined up perfectly vertical for match drilling. I worked from centre out to the sides, drilling though the existing A5 skin holes in the skin, then the doubler and into the existing spar channel holes:
Rear facing spar channel doubler angle match drilled perfectly. This orientation of the fuselage skin on the table confirms the top doublers are positioned correctly to the drawn skin edges as well.
Doubler is in perfect position flat against the skin and the spar channel. Eventually this will be drilled out to A5 across the spar channel and A6 rivets at the shoulders of the top fuselage doublers at the wing mounting points.
Very happy with how this first doubler turned out. The front facing doubler which is actually another channel, will need to be purchased from Zenith however. We just don't have the ability to fabricate this in the shop accurately enough to ensure good match drilled holes. I believe we could try to do it, but the effort to do so could lead to unacceptable bend and match hole accuracy - not something I willing to save a few dollars on.
More to come, thanks for following along :)
With both the wings into going into storage, I added the fuel tank drain/test ports. I need to make sure that nothing can get into the tanks while they sit in storage, and the any openings on the wings, like the lightening holes at the root have been covered up with tape and plastic to prevent bird or mouse nests!
First time in a very long time the bench has been completely clear of wing stuff! Time to start laying out the fuselage skins.
Had a couple of minutes one afternoon after work to finish one of the horizontal tail fuselage bulkhead frames need for the fuselage:
The plans are a bit deceiving when looking at the them when scaled down from full size. Close attention shows that the four corners of the fuselage aren't actually directly straight - the fuselage tapers very gently from the back of the cabin to the tail along all four longeron corners.
I've been pondering this for a while and when I was at the Zenair factory in Midland buying my longerons last month, I asked Nicholas Heintz (Chris' son) what would be an acceptable way for a scratchbuilder like me to make the correct tapered curve of the skins (the longerons curve to match the skins and are pre-cut in the kit versions).
Nicholas said the taper is very subtle and I should just connect the measured points out from the centre-line as shown in the plans. As long as the taper is equal on both sides of each of the four fuselage skins, all would be acceptable, but the taper is important as it provides some longitudinal rigidity to the fuselage..
Apparently that's not good enough for me! I wasn't confident that each 500mm point would give me enough points close enough together to draw through to make the gentle curve. So I grabbed the plans and entered them into CAD, like a lot of the templates I've made.
CAD has a great "spline" tool that is like using a draughtsman's French curve ruler to average out the points in the plans to establish a smooth curve. Then I used tools within CAD to section each of the 500mm sections on the plans into 100mm smaller sections, then measured where the points along the curve so I could transfer this onto the aluminum. Enough points along this curve means I can connect them with a real French curve ruler and have my taper drawn correctly on the aluminum sheet.
Here is a snapshot of the fuselage bottom skin, half way through the sectioning process in CAD:
The fuselage skins are all 0.020 thick, with several doublers between the longerons laterally and diagonally to stiffen everything up. The bottom skin is laid out on the table and I used the long straight edge to draw the centreline from which the edge points will be measured:
I measured out each of the sections lines along the centre-line, then used a square aligned with the long straightedge to plot the section lines out the edge of where the skin will be. I soon realized that Nicholas was probably right, curves based on the 500mm sections would probably be enough, so I decided to divide the sections into 250mm sections instead. This is a good compromise and accurate enough for me to make the taper correct using the French curve ruler we have.
This picture shows the lateral lines drawn out from the centre - the circles are where the curve of the edge of the skin will be and the lines from circle to circle are the skin edges which will be trimmed later. From there, I laid out the lower hatch door opening (often called the hell-hole) and some of the other lateral lines for the stiffeners, diagonal "L's" and torque tube bearing channel (supports the control torque tube):
A 10mm line scribed inboard from the edge skin is the rivet line where the skin will be fastened to the longeron once the skin sides are trimmed. This picture is the tail end of the lower fuse skin.
The lower fuse skin is actually too long to fit on a full 12 foot sheet of aluminum, even if I "tilt" the lower skin outline on the sheet, so a lower rear skin is added - it becomes part of the horizontal tail "box" and further strengthens this part of the fuselage to support the tail structure. Aligning it on the centre-line at the proper location ensures the lower skin is the correct length from the cabin to the rudder supports. Here it is temporarily clecoed in place:
Z-channels are added around the sides and rear of the lower hatch. These still need to be trimmed, I just wanted a rough idea where/how they interact with each other and how the rivet spacing should lay out. The square of cardstock is a quick cut out for me to use to visuallize where the flap motor will mount - again for rivet spacing, etc.:
Cutting out the access hatch was fairly easy. I used the fly-cutter to shape out the corners:
Then connected the edges of the circles and cut out the hatch. Some filing and sanding to take care of some rough edges and the access hole is done. I'll make the hatch cover from some of the left over 020 of the top skin.
I finished (for now) the bottom skin. All the rivet lines are laid out and drilled. Before I cut the bottom skin out, I've rolled it back up as as full 12x4 full sheet. Much easier to store.
Returning to CAD, I sketched out the two side skins and the top skin - this time with 250mm sections. It worked so well with the bottom skin, this will be the path to obtain the tapered curves for the other 3 sides of the fuselage (these are snapshots from CAD, not scaled together):
The top skin is somewhat shorter in length than the bottom skin, so I can comfortably "tilt" it on the aluminum sheet to save wasting some of the sheet.
Same layout method as the bottom skin. This skin has a flanged hole near the tail - this is where the elevator control cables will pass through the fuselage into the vertical tail assembly. Cut it out using the fly-cutter. I'll flange it later once the skin out cut out form the sheet so I can clamp the flanging dies easier (can't reach the hole now):
A second hole is cut near the tail end. This will form the round end of the channel as laid out by the lines drawn rearward from the circle to the rear skin edge:
Now onto the part that has been keeping me up at night.
We don't have the ability to bend complex shapes such as the upper top channel shown below. It's a slightly leaning C shaped channel, tapered at the ends that forms part of the wing spar carry-through on the top rear of the cabin. It is made of understandably stout 063 aluminum and both the shape and dimensions are critical to ensuring the wings mate to the fuselage at the right angle and location. So I ordered this from Zenith and picked it up the same day as the longerons.
Problem is, I hadn't thought it through and asked them to provide the channel pre-drilled, as they would in a kit. What I didn't consider is how to transfer the holes to the cover channel and doubler that make up the other sides of the top channel to form the spar carry though box.
I could order those pieces too, but when I spoke with them, they couldn't guarantee they the holes in the other 2 parts would be an exact match to mine as they drill them together at the time of manufacture, and mine was a one-of ordered part.
What to do. Start looking at order of operations and see if I can match drill the holes somehow, while respecting the bends yet to be made in the other two parts.
I can access some of the holes at the end of the top channel where it is cut diagonally to match the cabin uprights, so I placed the top channel in the proper position of on the top skin, drilled/clecoed the accessible holes form above into the skin. With that done and the channel secured to the skin in the right location I then duplicated the remainng holes along the bottom of the channel (actually the top when the skin is in place) onto the top skin because I can use the strap duplicator before the other pieces are added and drill from below the table level:
Unfortunately, as well as that worked, several of the holes on the channel are the bigger A6 river size rather than the standard A5 in the rest of the channel. Here is the channel lying on it's back. The 5 holes grouped close together are the ones I'm taking about. I don't know if an A6 duplicator is available, but we don't have one. These A6 holes are not accessible from inside the channel either.
Time to get creative.
I bent a matching piece of 016 to fit inside the channel and long enough to cover both the A6 holes, two inboard A5 and the A5 hole outboard holes and clamped it in place:
Back drilling through the channel into the 016 (using clecos in the three A5 holes to hold it together straight) and A6 size in the other holes gave me an excellent template to transfer to the skin that will match the channel holes:
Template added to skin, secured with clecos in the existing A5 holes, then A6 holes drilled through the skin:
I added more holes in the template from the forward facing A6 holes so once I have the doubler and channel angle bent, it too can be duplicated without having the access from inside the channel:
Fabricated and drilled the upper baggage area rear panel support angle to the upper skin:
This angle won't change once needed, so I finished all the river holes, deburred it and put it back into storage.
Next up was fitting and predrilling the upper fuselage top doublers. These doublers are almost like the shoulder blades of the wing/fuselage junction which transfers the loads back over the fuselage longerons and upper skin.
First I lined them up on the top skin and used extended lines from the top channel holes and the longeron rivet lines. Once in place, I traced those same river lines onto the doubler and laid out the rivet locations as per the plan. The doubler actually fits onto the outside of the fuselage, but laying it out this way allows me to see the lines before drilling through the doubler and the skin.
Once I was satisfied with the layout, I drilled pilot holes in the doubler on the drill press in the require locations, but not where the top channel lays. Then I back drilled through the doubler in 5 locations to secure it to the skin. The rest I'll drill once the longerons are in place:
Both doublers in place, secured enough for the next steps:
Shifted the doublers to their proper position on the outside of the fuselage skin (remember, I'm working on the inside of the skin and it's actually upside down on the table) and clecoede them in place from above. Then I placed the top channel back in place and secured it from below using the holes duplicated earlier in the skin:
I drilled the forst A5 hole and the two A6 holes I can access at the end of the top channel, then removed the channel and marked the others through the skin and into the top skin doubler:
Removing the top doubler and finishing the holes on the drill press, proves the matching worked (for some reason the picture below seems to show the holes are out of round, but it must be a shadow from the flash as they are actually perfectly matched):
I've also been scrounging a bit online on various marketplaces, classified listings and forums. Scratch building makes you keen to grab deals when they come up and I've been scoring well lately.
Both Ron and I want to make epoxy resin castings for our navigation light lenses and strobes. One of the tools needed to cast clear lenses is a vacuum chamber which is used to de-gas both the silicone molding mixture and the epoxy itself.
I spotted this complete set on Facebook from a seller not to far away. She wanted $125 for everything but I managed to get it for $100 just by asking. It's really brand new, she told me she'd only used it a couple of times but couldn't handle the fumes, so she was looking to sell it to someone who could use it. I looked it up after I bought it and there is easily $400 worth of stiff here, so I was a bit surpised she was willing to sell it for so little.
I brought it to the shop and tried it. Ah. It's not working, that's why.
The vacuum pump appeared to be pulling lots of pressure at the end of the vacuum line, but nothing was jhappening in the pot. Originally I thought maybe the valves were bad or something and I would have to replace them.
When I looked closer however, it looks like someone let the some casting epoxy get into the vacuum port on the pot lid, sealing it completely over! A quick drill and clean out of the port, freed up everything and it is working like new again. Score! We can also use this to de-gas paint for the planes too.
On the Zenith website classifieds, I spotted a suitable airspeed and altitude indicator. Used, but in excellent used condition, clearly taken care of by the previous owner. $200 USD for the pair - Score!
Another Facebook marketplace find were these cable turnbuckles, cable swivels, and throttle cable. Less than $200USD for everything - Score!
Ron continues to look for some parts for his Continental O-200 conversion for the Aeronca Scout rebuild. He bought a Cessna 140 for the engine, but it needed a new intake spider which was cracked.
These are getting very VERY hard to find as they often get damaged during prop strikes and need replacing. New ones are available, but cost north of $800USD!! I spotted one on Facebook marketplace, contacted the seller and managed to get this good used one and a box of other intake parts for $100USD shipped! - Score! I've given it to Ron as thanks for all his help with my build and his kindness in letting me use his shop. That's the kind of karma that I believe we need more of in this world :)
I'm getting close enough now that I need to consider what I need for my Corvair engine install with regards to firewall forward stuff.
I ordered my Corvair/Zenith installation manual as well as the MOP (Maintenance and Operations Procedures) manual.
They arrived in my hands and I sat on the dock the following morning looking through them. This build is always on my mind, even in the quiet times :)
More to come, thanks for reading along!
Been over a month since I've updated the blog, but that doesn't mean I haven't been very busy in the shop making tons of progress!
Second fuel tank went together very quickly. Using the 2 part sealant enough times now, I've become used to best methods of applying it.
Outboard side of 2nd wing tank. Rivets on the lower seam are held in place with masking tape. This allows me to apply the sealant bead, insert the tank wall and then squeeze the rivets in one (almost seamless) process:
With the edges closed up and rivets squeezed, another layer of sealant is applied to all outside edges and seams. I learned from the 1st tank to make things cleaner by masking off the edges:
Waited the required 6 days after the last of the sealant was applied before leak testing the tank with water. Like the 1st tank, a couple of small weaping leaks required some re-work of the seams and some more sealant.
While that cured (for another 6 days!), I got some work done on some of the smaller parts for the controls and fuselage.
Some of these templates are ones I created a few years ago using CAD, a PDF conversion program and then printed out on cardstock using a laser printer.
Lots and lots of small parts from aluminum plate and aluminum sheet:
More recently, I've been using the CriCut Maker machine more as it has the ability to not only cut my templates from card stock direct from the CAD file, but I can also print on the templates as well - handy for part numbers and drawing centre marks for holes etc.
Some of the last parts I needed were from 063 aluminum sheet.
Some templates don't need to be the full length either. For example, these flapperon control rod covers are 890mm in length, but the same width at both ends. A short template serves dual purpose for both end cut measurements:
I laid out several of the pieces I have made for the cabin and fuselage - including seat pans upper and lower fuselage doublers and more. Like I said, I've been very busy! Here they are in an animation using the online gif maker Kapwing. My animations skills won't have me working for Pixar!
With the tank sealant cured and tested again (this time no leaks!), I got it installed in the wing. Fuel line and fuel tank wiring is done:
Wing is sealed up again:
Fuel cap and filler neck fit perfectly! Wing is ready for storage!
Next I started final bending/fabrication on some of the parts I've been cutting out from sheet.
Seat pans require lightening holes, so I drilled those out:
Then I used the flanging dies and some large C-clamps to form the lightening holes:
Same for the seat pan supports:
Major milestone completed with both wings now complete and ready for storage. Next, we'll get this wing into the storage barn and start the process of laying out the rear fuselage.
Thanks for following along! More to come soon!
The second fuel tank is being prepped for install in the first wing. Confusing I know, but happy to get a few hours in the shop this weekend and tonight.
Most of the work on the second tank is done - I've fitted the centre support rib (added to the inside of both tanks) and the fuel outlet and rain outlets are already fitted. This tank install will go much quicker because most if not all of the planning, thinking and decisions are done.
First up, I added the internal rib rivet line support strip to the bottom side of the tank. This time I have the advantage that I can access the inside of the tank without having to reach through the access hole to maneuver the rib into place.
With the access available, I decided to mount, rivet and seal the rib to the bottom of the tank skin now - no point in waiting for the rest of the tank sealing. More sealant will be added as I mix more up later on.
Fuel drain installed and sealed (inside of tank view):
Fuel drain fitting, all sealed (outide of tank):
Repeated the fuel access cover install using the same template as the other wing:
Hole drilled in access cover (under the cardstocl template):
Transfered the mounting holes from the access plate to the wing skin.
Used a hole saw and Dremmel tool to cu the access hole then mount the access cover via Tinnerman clips (just like the other wing):
Drilled out the drain plug hole in the lower wing skin and the fuel outlet pass-through hole in the tank bay rib.
Fairly short update and not a lot of pictures to share. Like I stated above, most all the thinking and planning from the first wing tank has been done, just got to apply the same process. So far, so good :)
More to come.
Finally it is Saturday and time to get into the shop and work on the left wing. Time to get the fuel tank final install complete, plumbed and wired for operation. Thanks to Brenda and Caitlyn for coming to the shop to help flip the wing back upright - it really made today a productive day.
The left wing tank has been tested for leaks and is ready to be permanently put in the wing tank bay.
Placed the tank back in the wing bay for final fit up. All the extra sealing didn't change anything from prior fit-up.
1/4 inch truss head screws fill in the centers of the rivnuts securing the fuel sensor plate to the top of the tank. Ez-Turn threat sealant on the threads of the screws - it has the consistency of vaseline.
16 screws for 16 rivnuts.
The fuel tank is rquired to be grounded to the airframe to prevent any static discharge from igniting the fuel or fuel vapours in or around the tank. Grounding is via the inboard tank rib - I used an A4 rivet to secure 18AWG aircraft wire with a matching ring terminal. This wire got twisted together as a bundle with a pair (orange/green) wires for the fuel sensor.
The twisted bundle is secured near the top of the rib using a wire clip and A5 rivet as the head on an A4 waa too small.
Used a trick I read about online to make nice wire bundles - put the loose ends in a cordless drill and slowly twist them together.
Green ground wire from the rib connected to the tank via a ring terminal under one of the rivnut screws. Confirmed with an ohm-meter that the tank is electrically connected to the rest of the wing structure.
The orange and other green wires are crimped connected to the white and black wires (respectfully) of the fuel sensor. The sensor has no polarity, but I decided to use the black wire as "ground" feed and whire wire as "positive" in case I need to troubleshoot later.
Next up, I installed the finger screen into the fuel tank outlet. Ez-Turn thread sealant is used here too.
Next the AN6/3/8NPT adapter - it only requires sealant on the NPT threads, AN6 fittings are self sealing.
AN6 90 elbow connects the braided fuel line to the tank via the adapater and finger screen at the outlet. The looks like a very clean and professional installation, but more importantly it is the "right stuff" for th job.
This close up show the assembly and also the back side of the Tinnerman clips that hold the inspection cover on the lower side of the wing that I insalled last week.
Next up, I used contact cement to secure cork strips to the underside of the upper fuel bay skin. I figured if the bottom cork strips are attached to the lower wing skin, these might as well be attached to the underside of the top skin. Two layers of cork strips and a square with a hole to support the threaded tank filler neck.
Some heavy containers on top of some wood scraps hold the cork in place while the contact cement sets up:
Once set up enough, I installed the upper skin in place. It quickly became apparent that the strips were enough to support the skin over the tank, but I needed to add another layer of for the fuel filler neck.
Third layer in place, closes the gap nicely (sorry about the out of focus picture!)
Fuel neck threaded in with Buna-N gasket to seal it. It tightened up perfectly and the fuel cap faces in the correct direction that I want - perfect!
Coiled up in the wing root awaiting connection to the fuselage when the wings are mounted now are the blue static line, the yellow pitot line, the braided fuel line, the orange/green fuel sensor twisted pair conductors and run of orange string which i can use to pull wires out to the wingtips when the time comes to add the navigation lights.
The left wing, almost final and ready for storage. Need to finish some minor items on the wingtip fit up, but otherwise it's ready to go.
Once the wing goes into storage, I'll bring down the right wing back to the shop and get the right wing tank installed. It should go much quicker as it will be the same process as the first bolstered by the lessons learned on the first.
It has been forever since I updated the "completed features" picture so here it is. Both left and right wings are now complete (with the exception of the right wing tank as mentioned).
As always, thanks for following along. Soon the fun of the fuselage begins! Can't wait - it will be nice to look at a all new part of the build.
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.