A really good couple of days in the shop this week.
With the first slat underway, I was time to begin the task of wrapping the top skin over the ribs. To make this easier, Ron has built a "slat box" as show below. Made from plywood, it is essentially a reverse or negative pattern of the top surface of the slats. Green painters tape is added to any contact point to prevent scratching the skins:
Before getting the slat mounted in the box, I had to tuck the underside into the folded over trailing edge. It's tight, and I used a thin piece of wood as a slide to get it tucked under. The resulting pinch of the trailing edge fold is enough to keep everything together for mounting in the box.
Now it fits in the box and can be strapped down to complete the final wrap over.
I used a piece of HSS tubing to act as my spreader across the rear of the slat.
A look inside the each end confirms things are close enough and I can begin drilling the top side rivet holes.
To access the top side (which faces down in the box) I tilted the box on it's side:
Strategically cut access holes in bottom of the slat box line up with the 3 internal ribs so I can get at least 3 rivet holes drilled through the skin into the ribs:
The fourth hole is to far down inside the box to drill it accurately, but that can wait until everything is out of the box.
Put the box back upright and added some more blocks. This allows the force of the straps to transfer down more vertically, tightening everything up and I can begin to drill and cleco the trailing edge down on the underside:
Take the assembly back out of the box again and finish the top side rivet holes I couldn't access before. The finger clamp holds down the rear curve of the skin to the double bend flange underneath:
Layout the rivet line across the top rear. These will be A4 rivets with 50mm spacing:
To ensure I had everything locked down where it needs to be, I flipped everything over and drilled out everything to A4 on the underside:
Back ipright again, drilling the topside rear rivet line, A3, then up to A4 on 50mm spacing. I also completed the front rivet holes in each of the nose ribs. Everything is tight and square.
And it all comes apart again for final debur and priming:
One of the challenges I'm facing is how to make the inside of the slat structure accessible for the inspector to see my workmanship. The fold-over design of the skin makes leaving it open like the flaps, elevator etc impossible. I discussed this with Ron and confirmed with Roger at Zenith that adding a lightening hole on the flap ribs was acceptable here. I'm not looking to save weight, just want an easy way to see inside. This viewing can be done with a scope.
I carefully added some small holes in the centre of the slat ribs using a step drill. This will allow a camera scope inside.
I flanged the hole slightly to add strength:
I scratched off some of the primer doing the holes, but they cleaned up nicely and I re-primed them.
The new access hole creates a new small problem. The skin support L now protrudes over the hole:
the quick solve for this is to trim the L a bit before riveting. I also trimmed it back a bit on the top of the bend flanges to ensure clearance for the top two rivets.
Primed the skin and once dry started the re-assembly which goes back together fairly quickly
With everything drilled out to A4 and clecoed, the slat skin is tight to the ribs and looks good for riveting. The access hole turned out really nice - there should be lots of room to look inside using a scope.
Very pleased how this turned out using the steps I came up with worked well, I'll be following the same order when I build the other three. Again, I'm rather surprised by the size these are, it gives a good impression about the wing dimensions.
One to the next one, it all starts with alying out the bends. Thankfully I wrote down the measurements and bend order from the first one - that will make the next three the same.
Very happy how the first slat turned out considering how complex and tight the bending that is expected of the skin. It's not a complicated structure, but "fun" to do.
Back in the shop soon to get the rest of the slats done. I'll be continuing work on the 3D scanning/printing project too, exciting things coming up. Thanks for reading.
Back in the shop for a few hours this week, continuing work on the first of four slats.
When we last visited my progress (ha!) I had just completed laying out the first rivet line on the skin where it attaches to the slat support angle (pseudo spar). With that complete, I flipped the skin over on the bench and marked out the second rivet line.
With the slat support angle in correct position, I drilled out the rivet holes to A3 on both edges. The plywood edge of the bench works well to anchor everything. I waited to do the holes where the skin/support/ribs meet to ensure fit. All the holes will be drilled out to A4 eventually.
Flipping the clecoed assembly back over, I began the process of fitting the three slat ribs. The ribs fit as expected so I drilled through from the skin, through the support angle an into the corner of the rib:
I continued the process for all three the same way and the fit well. The rib flanging die works well as a weight to hold the skin upright at the edge of the bench as I double check alignment.
Happy with the alignment, I removed the slat skeleton from the skin and began the process to layout the underside rivet lines of the ribs:
The inboard rib is 30 mm in from the edge of the skin and rivet holes in the skin are laid out, centre punched and drilled to A3. There are 5 rivet holes along the underside on each rib including the one at the spar.. I'll wait to drill the last two near the rear of the slat until the skin is wrapped around the trailing edge and I know where it will meet the skin on the other side.
Redlining the centre of the ribs makes line up easier when drilling through the pilot holes of the skin. This picture makes the rib appear twisted for some reason, it is not. Weird. To do this, I've removed the slat pick-up bracket - it will be added later once the slot in the skin is cut.
With the inboard rib close to the inboard skin edge, it was easy to start forming the curve of the skin. I used three finger clamps to secure the rib using the skin support L, using the redline visible through the pilot holes in the skin as the alignment reference. I further confirmed the alignment using a measuring square. The other two ribs were slightly more difficult to line up as I had no way to clamp them in place for drilling, but the skin was already starting to curve with the first rib so I managed to get them lined up well.
With all three ribs lined up, I drilled through the skin into the rib, starting at the 90 degree corner, gently pulling the skin across the rounded underside of the ribs. The other holes into the support L on the outer ribs will wait until the slat support brackets are installed because the slat support bracket thickness will change the location of any holes drilled now. It's hard to capture the curve of the skin in pictures, but it turned out well. I always pictured in my head that the slats were much narrower - seeing the initial assembly here makes me realize how wide/thick they actually are!
I want to make sure the slot for the slat in the skin is accurate and doing so took some head scratching. How do I mark out where on the skin the slot goes when I can't mount the slat attach bracket until the slot exists? It's compounded by the curvature of the skin and the tapering angle of the bracket. I also happens in two places, the inboard and outboard ribs on each slat section.
What I came up with turned out pretty good. I used a duplicate slat support bracket as an example.
Knowing that I really only need two reference points between any slat attach bracket and slat rib to determine position, I used the original slat brackets to drilled out two matching holes for each of the ribs. Shown marked below on the duplicate - two holes matched for the "G" rib and two holes matched to the "H" rib
The smallest diameter machine bolts I had on hand meant I had to upsize the holes in the example bracket a bit (hence the reason for a duplicate bracket - I didn't want to change the A4 size in my actual bracket).
The bolts are just long enough to make the bracket stand-off the rib to the skin edge so I can then scribe a line back from the edge to the rib. This gives me an accurate start and stop end for the slot I need to cut.
Here is the final picture with the slot cut, the attach bracket through the skin and mounted to the rib. I've also drilled for the rivets through the skin and into the support L. I'm happy how this worked. In hindsight however, this didn't work for the outboard rib as it was too far away from the skin edge to make this work. For the outboard slot, I just duplicated the same length and position as the inboard, slowly lengthening the slot until it fit well. the key concern is matching the rib holes and being equally positioned in reference to the opposite end of the slat section. I've accomplished that doing it this way, so I'll do the same again on the next slat. This picture also shows the curvature of the skin on the underside of the slat.
With the underside of the slat now secured, I began the process of laying out the rivet lines for the top side. This is a single row of rivets as there is no need for a support L on the top side of the slat.
I used a seamstress measuring tape for it's flexibility and made notes of where the centre of each rib flange was as well as where along the length of the spar the rib is. The inboard rib is easiest, as it is only 30 mm in from the edge of the skin. I measured all three and marked out rivet holes.
With the hole locations identified, I drilled the pilot holes from the inside which I will then back drill from the outside when the skin it rolled over the ribs. Again, I'll wait to drill any holes that double through the lower skin once I confirm everything is lined up.
At this point I also made the first bend in the trailing edge with the bender using a 020 shoe to maintain the required bend radius. In this picture you are looking at the top skin which curves over the top of the rib, forms the trailing edge and covers over the tail of the ribs (this will become more clear when I get the skin curved over the rib).
With the help of Ron and a long board, we bent the trailing edge over by hand as far as we could. The aluminum bar stock is taped in place to prevent us bending the trailing edge completely flat and the round rod inside the trailing edge helps form everything.
With the bend close, we needed to figure out a way to make a small crease along the trailing edge. This crease creates the return angle from the trailing edge to where the skin meets the tail of the ribs. We used a thinner board to clamp the skin down tight slightly inside the trailing edge while the rod remained inside the radius to give something to bend against.
I turned out great and I'll continue to take pictures as I move along. Next up will be moving the whole assembly to a specially created "slat box" that Ron built. we'll be able to tighten the top of the slat skin down across the ribs and drill through the pilot holes into the ribs. It should also be ready for the trailing edge rivet holes.
Thanks as always for reading! Stay tuned for more coming soon.
I wanted to get back to the 3D printer this week, but my time was better spent in the shop working on the slats. I did manage to get the 3D scanner to work, but more on those later.
Slats are aerodynamic lift assisting devices attached to the front edge of wings. There are many types of slats and methods to accomplish the same aerodynamic principles. Large commercial aircraft often have hydraulically activated slats that extend on command from the wing. Some aircraft have slats that automatically deploy when the right conditions exist for it to be beneficial. In both cases, these are overly complex to design and build and not very common in light aircraft. Slats benefit STOL aircraft because in normal cruise, the profile of the wing acts the same as a wing without slats. However, at higher angles of incidence, such as in climb or descent, the slat forces air from below to the top of the wing, increasing lift dramatically, allowing much slower stall speeds (and steeper climb/approaches typical of STOL aircraft).
On Zenith STOL aircraft, the slats look/work like this:
I've learned the importance of gathering all my parts/materials before starting to build a section, so I started by laying out all the parts I have made so far for the slats. There are four to build, 2 inboard and 2 outboard - just like the flaps. I just noticed in the picture below I'm missing one of the slat doublers.... hmmm. I'll have to double check my count.
With a good idea of what's needed and what I need to still make (skins), I had another look at the plans.
The slats are a fairly simple structure to make without too many parts - this keeps them very lightweight. Like the flaps, accuracy is important so that the pick-ups match the attach points on the wings. Also like the flaps, it took some sleuthing to deduce the "distance between slat supports" by flipping back and forth several times between the slats diagrams and the wing diagrams. Not sure why Zenith couldn't just place the measurement on both pages! Each of the drawings have different points of measure. If this was a match drilled hole kit, no issue but for a scratch builder it takes some figuring!
Next I started laying out the skins. The width of the skin for a completed slat is deceiving as it curves on both the top and the bottom around the ribs. As a result I was disappointed to find that I can't fit two slat skins on every sheet, it's about 20mm too wide.... argh! I'll use the remaining metal on other parts but it would have been much quicker and nice to get two skins from each sheet. While I had the rolls out, I got all four slat skins measured and cut to size.
Full size 4x12 foot sheets are cumbersom to work with, so I cleaned up the bench a bit in order to make room, which was long overdue anyhow.
Other parts I still needed to make were the slat support brackets for attaching to the wings. These are traced from my card stock template onto 040 aluminum and cut out roughly on the bandsaw. From there they get ground down to size and sanded smooth.
Here is a pic of the four skins rolled up and the slat support brackets all ready for their turn in assembly.
I put away the skins for now and started to formulate a plan to assembly the first inboard slat. The general construction of each slat consists of a 025 slat doubler angle, 3 slat ribs and 2 slat support brackets. The slat doubler is essentially the spar of the slat.
Each of the slat support brackets also has a doubler made from a bent piece of standard 025 "L" angle. I decided to make up all eight at thee same time, 4 left/4 right. It starts with a 120mm long piece of L angle.
Measure out 50 mm and 80 mm in the centre of the bend where the relief holes will be. Use a centre punch to make a dirll point:
This notched piece of hardwood makes an excellent backer for drilling the holes so the part doesn't wander:
Use snips to make relief cuts to the edges of the holes:
Next was trying to decide how to make all the support brackets consistent with the ribs. The plans call the back edge of the support bracket to be 140 mm from the nose of the rib and that the rear lower edge of the rib be 28 mm elevated. So I figured the best way was to layout a rough sketch of the dimension lines on a board and trace out where a rib sits in relation to the bracket. This should ensure consistency for each rib that needs a support bracket and doubler. At the top right you seen the curved doubler (more on this later).
I struggled for a couple of minutes to figure out how I was going to place rivet holes from the inside of the rib, through the support bracket and into the doubler on the opposite side. The I figured out if I used a right side doubler inside a left side rib (and vice versa) I could trace lines on the inside where the doubler would be approximately. Now I know the holes I'm drilling will mate up with the doubler.
I duplicated the same process for the opposite side slats. In order to keep parts together with their mates, I put a alphanumeric mark on each set of parts. Slat support "J" matches up with rib "J" and doubler "J"
here is a better look at a curved doubler as it would be oriented under the rib and slat support. The doubler provides extra skin support around the slot that will be cut for the slat bracket.
Each inboard and outboard slat has 3 ribs. The outers have attach brackets and support doublers, the middle rib is just a rib. I'm pondering adding a doubler to the middle ribs to further support the skin. They are real easy to make and attach and weigh nothing.
Here are the ribs for the inboard and outboard right side slats. The one in the foreground shows how the doubler creates the sandwich of the rib and support bracket go together. Again, I decided to complete all 8 at the same time as I had the layout and process readily available.
Next up I started figuring out how I was going to bend the skins. Slat skins from the factory come pre-bent and it's important to be accurate here. A few test strips of 016 measured out according to the plans, bent then adjusted and bent/tested again to correct errors had me in good shape. I documented where the bend lines are compared to the plans once the skin is actually bent.
With the lines laid out, I made sure to mark each line with an bend order number, where the bend setback would be and what radius.
Clamping the long straight edge down to the bench makes it easy to scribe the long bend lines.
The underside edge of the skin needs a corner relief cut at each end to allow the tip insert to fit correctly. Measured the required cut and corner drilled first to make a clean inside corner when cutting:
The first bend must be the middle one as the throat of the bender isn't deep enough to make the bend from the other end of the skin if I make the small (2nd/3rd) bends first. After measuring several times to confirm the first bend (everything counts on the first bend being correct) I placed it in the bender and used a long piece of 025 doubler as a forming shoe. It worked perfectly.
Placing the slat doubler inside the bend confirms the bend is correct and true. I pulled the slat doubler out past the skin to show the match in this picture.
The second bend adds the up angle on the lower part of the slat. Again the bend here needs to be exact - too narrow, the next flange will be too wide. Too wide and the next flange will be too narrow.
More double and triple checking and the 2nd bend turned out perfect.
Here is the skin back in the bender getting ready for bend number 3. This bender can bend aluminum sheet up to 025 easily, but is really designed for lighter/softer aluminum trim coil/flashing/soffit which is can be bent to sharp 90 degree corners. To adapt to bending smoother radii required of aircraft aluminum, we insert a pre-bent strip of aluminum called a "shoe" to help form the bending sheet around the shoe to create the correct radius. In this case, I use a "shoe" of 020 to bend the skin around, leaving a perfect 1/8" radius in the skin.
Careful measurment and planning leads to a perfect set of bends - very pleased how it turned out!
Next I needed to figure out how to lay out the rivet lines for the slat doubler that fits inside bend # 1 (the 90 degree corner). I placed the doubler on the edge of the bench and slid the skin over top. The goal here is to make the rivet lines line up with the centre of the slat doubler flanges. So measure the middle of the flange.....
..... then slide the mark on the doubler to the skin edge and mark the skin.
Do the same at both ends then connect the marks with the straight edge. Voila, a perfect rivet line.
The first rivet is placed 30 mm in from the edge of the skin which is also the centre line of the inboard rib. From there, the rivet spacing is 50 mm. Rivet locations marked with black marks along the rivet line. These continue across until meeting the location of the centre rib. Then the same process starts 30 mm in from the opposite end on 50 mm spacing towards the middle.
Next I'll flip the skin around the do the same layout for the other side of the flange of the skin/doubler.
Overall a productive couple of days in the shop. More coming soon!
As I mentioned in my previous blog post, I volunteered to assess, repair as needed and make functional again a 3D printer that belongs to our local library, in exchange for the use of it to experiment printing some parts for my airplane. The primary issue reported by the library staff was that it wouldn't print and they suspected it was a plugged extruder nozzle. They don't have the knowledge or salary dollars to assign a staff member to investigate further, so the printer sits idle and unused.
The brand of printer appears to be HICTOP which is one of many widely reproduced, made in Asia copies of a Prusa 3D printer. This is more common than one might think - there are literally hundreds of knock-off models on the market and this practice was commonplace at the time the unit was purchased by the library a few years ago. The market now has several different manufacturers competing for dollars.
No manual in the box, but I found one online to give me some hints for what to look for. Here is how I received it and what I found.
First up, an unlabled box of what appears to be left over/spare parts, typical of knock-off manufacturing, but this was likely where someone just dumped anything extra left over:
There appears to be a bunch of stuff in the box, leading me to wonder what's missing on the printer!
Some of the bags are labled, some aren't....
Initially I thought why would someone would keep (what appears to be) a broken cog-drive belt. Further looking at the HICTOP assembly manual I downloaded from the interwebs shows that the belt is left open ended for adjustment purposes (ties off on the chasis slides at each end). Learned something new.
Some extra split tube wiring conduit/loom that didn't get used.
The box has a tonne of filiment in it, however anything in a bag appears to be labled 3D "pen". I know the library was touting they had a 3D crafting pen, so I'll have to research if this also works in the 3D printer. It doesn't seem to be the same stuff - different diameter and texture, but I also know filaments comes in many different material types and diameters (you can switch nozzles on the printer), but for now I'll run on the assumption they are not compatible.
The printer itself looks well used (or well abused?) and typical of home built hobbyist kits. Everything seems to be in place and secure on first glance.
It looks like some attempts were amde to tidy up the wiring, either at the original build stage or later on - however it needs some attention. Probably what the spare wiring conduit loom was for:
Power cord seems intact. The other grey cable is an older style USB cable common in early versions of 3D printers, but nice and long.
Also typical are simple filament spool holders, in this case the model has a threaded rod with nuts on the end uprights:
The print bed is damaged but still flat. It's somewhat common practice to use glue sticks to spread a thin layer on the print bed for the printed item to stick to as the bed moves back and forth. I think the proper procedure is to clean the glue off between prints though. This glue is very thick and likely baked on over several heat/cool cycles of the bed.
Early users of 3D printers struggled sometimes to remove completed prints from the bed and I imagine the evolution to heated print beds (like this model) might have made hard adhesion of the extruded plastics even more prevalent. Looks like someone used a hard tool of some sort to remove prints and left some fairly deep gouge marks.
After cleaning up the glue residue a little, it looks like a previous user also dragged the print head nozzle across the bed when trying to print something, leaving a permanent hash shaped mark in the soft aluminum. In this picture you can also see the nozzle and heater is covered in old plastic debris. It also appears loose and freely turns on it's mounting pipe.
This printer has a small display screen and single control knob to both prepare and operate the printer.
With a bunch of initial tidy up, everything looked good for power up. There doesn't appear to be any master switch to control power on/off, so I plugged it in hoping to not see any blue smoke! The power supply groaned a bit then the LCD display lit up. The menu selection knob seems to be functioning correctly and I can navigate through the menus easily:
After a couple of minutes, the extruder began to spit out melted filament which I caught in a tissue. Seemed more of a drip than a push by the extruder, but maybe the clog managed to clear itself?
Using a socket on a drive handle, I removed the nozzle once it cooled down after power off:
The nozzle is most definitely blocked and this confirms the string that appeared was just melting debris. So the nozzle will need some serious attention.
A few more attempts to clean the bed while on the printer proved difficult, removing four anchor bolts and unplugging the electrical connector is easy. I'll take both to the shop where I have access to lacquer thinner which should make short work of the glue that appears to be baked on over a long time. Acetone will melt the blockage in the nozzle.
Looking down from the top, everything looks good externally, maybe a bit dusty from sitting. Looking left to right at the extruder assembly is the extruder cooling fan, heat sink fins, filament loading latch (hole) and the extruder driver motor). On the front of everything is another fan which is ducted towards the nozzle tip.
Remove a few screws and the drive parts of the extruder assembly comes off the carrier. This revealed a partly melted pieced of filament which looked like it had jammed in the feed tube. It came out easy with a small tug on the exposed end. I removed the set screw that holds the feed pipe and heating element (called the "hot end").
I had some other supplies to drop off at the shop, so I took the nozzle and print bed with me to clean them up.
Previously, I had purchased a ultrasonic jewelry cleaner at a thrift store. Turns out it's perfect for cleaning small parts like the printer nozzle. I placed it in the tray and added just enough pure acetone to submerge it.
The cleaner has a bright blue LED light that makes it hard to see anything due to the reflective stainless bin bottom, but I'm not complaining, the cleaner was less than $7.
A short video of the cleaner when on.
While the nozzle soaked and buzzed away in the cleaner, I used lacquer thinner and a paper towel to remove the mess of glue buildup. It worked well when let to soak a bit and "encouraged" with a plastic scraping tool.
Removal of the glue reveals even more deep scratches. I originally thought about sanding them out with some fine sand paper, but I'm concerned about damaging it further or worse taking out the flatness. I'll have to research options.
The long acetone soak in the ultrasonic cleaner worked very well. Little bits floating in the acetone were a good sign. A small piece of wire in the tip of the nozzle pushed the rest of the debris out along with some high pressure air from the compressor. All good again!
Next up, reassembly of the printer including improving/replacing the print bed. Stay tuned.
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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.