Fuselage top skin fit up, starting on rudder fittings and engine re-work quote (aka what is the price of your dreams?)
Another productive day in the shop yesterday. Continuing my work on the fuselage, soon to be taken apart again for debur, prime, reassembly then final rivets. I have some plumbing of wire to do from the front cabin are to the tail section and I've got fit up the rear cabin windows before I close everything up.
Like the bottom fuselage skin, the top skin tapers towards the rear. The large hole in this picture still needs to be flanged, but it is the passthrough for the upper elevator control cable. The hole is covered by the horizontal tail assembly and fairings once the are mounted.
Finished trimming the front upper edge of the top skin at the longeron corners and trimmed the skin across the blue line. With this properly fit, the forward end of the longeron can now be drilled and clecoed. Eventually the rear edge of the cabin skylight window will attach to this front skin edge.
Reaching the top of the cabin was tricky, but there is actually just enough room to stand on the workbench to drill, trim and debur holes and edges.
I've bought a 90 degree aircraft air drill which I will use to back drill the wing mounting plates at the top corners of the rear cabin, just waiting for it to arrive this week. I'll post pictures when I get it, but think of the shape of a large dentist drill and you'll be close. For now I'm holding the wing plates in place with C clamps to check fit as I square up the cabin.
The fuselage currently sits on the bench in a tipped back position, so I needed a way to extend the centreline of the top of the fuselage in order to plumb bob that centre relative to the bottom of the fuselage. I cleco clamped a strip of 093 aluminum extended forwards along the top centreline and strung a plumb bob weight downwards:
I know the fuselage cabin is slightly out of square (leaning very slightly to the pilot side without the other cabin structure in place yet). To adjust the square of the cabin, I borrowed one of Ron's turnbuckles and strung it passenger bottom corner to pilot top corner of the cabin. Tightening the turnbuckle pulls the cabin square. It didn't take much to bring it into line, maybe two or three turns.
For the plumb bob, I used the pipe reaming tool head. It is heavy enough to quickly dampen any swaying of the plumb bob line, but not so heavy as to pull down on the fuselage cabin roof. As added bonuses the reamer can be tied and centred on the plumb line easily AND it has a pointy end for really accurate line up with the centre line of the lower cabin skin.
Here is the original square of the cabin - surprisingly close considering nothing else of the cabin structure is connected yet:
After a couple of turns of the turnbuckle:
Obviously string and turnbuckles won't be what holds it all officially square, I'll bolster it more externally when the time comes to tighten things up. But this gets it in position to measure the cockpit side walls and floor and where it attaches.
Also with the cabin squared up, I finished drilling the top skin to the upper longerons from cabin back to tail. I found it helpful to use a ratchet strap lightly around the fuselage to pull the longerons inwards to line up with the skin edges as I work towards the tail. It's this tension that gives the fuselage it's rigidity once riveted.
Every 3rd rivet hole on the top longerons drilled to A3. Next will all the holes aft of the baggage area up to A4. The picture below really captures how tapered the fuselage actually is. The tail is clamped for now to ensure the longerons maintain their taper curve until I figure out order of operations for the balance of the rivet holes and the bolts that hold the fuselage side rudder attachment plates. The longerons need to be trimmed to match the side skins. The small hole just in front of the clamp is the front edge of a passage slot for the lower elevator cable that runs between the hole and the tail end of the top skin hold for the lower elevator cable. It will be slotted soon:
Super stoked to see the fuselage coming together and that it is almost perfectly square as it sits!
Rough placement of the upper rudder hinge plate - this was one of my earliest pieces I cut from 0.125 aluminum plate using the early cardstock templates. I know I'm on the right track as it fits exactly where it should on the tail:
All four of these longeron ends get trimmed to match the side skins:
A look inside the tail looking forward through the tail bulkheads. This large area gets covered by an aluminum fairing that is attached to the rudder and pivots inside this spot:
I marked where the top, side and bottom skins ends meet the longerons, marked them for trimming then peeled back the skins a bit to facilitate trimming with the Dremel (just like the front ones):
Looks like the top skin is a bit short? It matches at the front edge correctly, so I know that is right, so what's going on here.
I remeasured the entire length of side and bottom skins and they are in correct relative position too. But once I remeasured the top skin, I seems I cut the tail end 10mm too short! Everything else on the top skin including the cross members, the upper doublers, the passage holes - all correct. Thankfully this 10mm shortage has no impact on the function of the tail or the rudder attachment structure. It would have much worse at the front end where the main spar channels are or if I'd cut the longerons too short. Still not sure where I made the mistake, but glad it didn't require replacing the entire top skin, that would have been a major set-back (and expensive!)
As more of this enormous project comes together, more intersections of items to complete start to cross on my list of things to do.
Back in September 2018, I travelled to Mexico Missouri for the Zenith factory Open House Weekend (full 6 part blog story about my trip)
One of the prime reasons to travel all that way was to meet with other builders, get some airframe parts that I couldn't fabricate/bend myself direct from the factory and most importantly have some of my Corvair motor parts assessed by the Corvair aircraft engine authority William Wynne who was also in attendance that weekend (he hasn't missed one since they've been doing them)
William had a look at my cores and offered to take them to his shop in Florida for re-work and at the time my plan was to start that process in parallel with my build. Of course that didn't happen for a number of reasons, cost being one of the them.
Fast forward to last week when I contacted them to inquire of my cores were still available in their warehouse as now I was ready to proceed with the work - four + years of them holding them for me was a lot to ask. Thankfully despite my tardiness in getting them moving through their processes, my cores were in fact still available and sitting in storage awaiting my decision.
I received a quote last week for the required work and in discussion via phone with William I now have a solid plan to get the engine work rolling. It's expensive work, but the value in having the recognized expert and his associates do the required work is great value in having a solid engine to fly behind.
Work includes deep cleaning the cylinder heads and piston chambers, new valves, new valve seats, exhaust value rotators and welding on intake pipes. The engine case I sent unfortunately doesn't make the cut and requires too much work to make it viable, so I'll need to replace that with another core (they are easy to obtain). The core crankshaft I had was too rusty to be ground, so they will supply the core which will heat treated (nitride process), stress relieved, the journals be ground/radiused, bearing surfaces polished, new crank gear welded on and the crank will threaded for the propeller hub safety shaft. When that is done, a 5th bearing is added to the crank, ground concentric and shipped back to me with a 5th bearing housing.
All this work is being done by experts and gets me past the half way point of having a ready to bolt on engine. Again well worth it.
William is a great writer. He often reminds people that none of this is possible if you aren't in the arena. I am in the arena and I continue to participate. It can be an expensive undertaking, but ask yourself honestly if you can put a price on your dreams what would it be?
Holiday season and IT change freeze is generally over and I'm back into the swing of ongoing projects at the paying job and was gone on the road all week. It was great to be back in the shop this weekend working my other (non paying?) job.
The more this aircraft comes together, the more complex the order of operations seems to become. I probably spend way more time figuring out not what to do next, but what should come next. Every drilled hole connects something to something else, and I have to always be cognizant that I don't drill something before all parts of that particular joint/juncture/assembly are ready to be positioned. So far so good, but I've got many, many things in the future to consider as I work.
As the greater fuselage assembly comes together I need to decide how to fit and add the tinted Lexan rear cabin windows. These windows are is one thing that doesn't actually join with something else. Just a row of rivets around the perimeter of the window opening holding the Lexan in place (remember, these windows are not pressurized or designed to open and close, essentially just portholes to let the light into the rear cabin area). As the appearance of the rivet lines on the outside edges of the windows is what shows, I think it best to layout and drill the holes in the fuselage cabin skin, then drill through the Lexan window. The windows will be added after painting of the plane is complete, but I want to fit them soon before the cabin is closed up and access will be more difficult.
The rivet line needs to be 10mm outside the window opening, the window Lexan is 20mm wider:
I want the rivet line to exactly match the window curvature, 10 mm outside. I could cut yet another cardstock template and trace it, but I thought why not make a simple tool that can mark the edge with a fine tip Sharpie, so this is what I came up with.
I cut a simple triangle from a scrap piece of 025 aluminum. Two A4 solid rivets are the edge guides and the hole for the Sharpie exactly 10mm offset below that. I used longer rivets and the rivet squeezer to just pinch them enough that they hold in their holes tightly:
Here is the backside. I rounded the tips of the triangle and bent them forward slightly so the tool rides smoothly across the skin.
Here's a short video of how it works. Careful with the volume on the video - Ron is working with the grinder in the background and I didn't get a chance to edit out the audio before posting!
I might remake the tool and use a smaller Sharpie hole as I don't like the thickness of the rivet layout line this one leaves:
Finished adding the rear horizontal bulkheads. The required "L" angles are added to the top of them where the top fuselage skin will connect. This is also where the fore and aft horizontal tail supports get mounted once the tail is ready for fitting, so it's a "no rivet zone" until later:
This middle cross "L" doesn't have a bulkhead to connect to, but is an important support for the entire tail structure. It's going to be fun to add later once the top skin is on! - more on this later in the build
Rear cabin uprights now in place between the rear cabin windows with a couple of clecos to hold them. This helps square up the sides more and eventually will make up the rear half of the vertical flapperon push rod tube covers. Also in the picture is the centre bearing support channel. I have it propped up close to where it belongs, but will wait for the control rod placement before drilling the rivet lines on it.
Speaking of control rods, I got more hardware and 4130 tubing of various sized from Aircraft Spruce this week, all part of the control system. I'm planning on dual control sticks so I had to add some more to my original order plans:
Also picked up the 3mm tinted Lexan sheet for the rear cabin windows. I realized when I got it home that it is not only tinted, but it also has UV coatings. Not required, but nice to know it will be better able to withstand sitting in the bright sun when I fly to Tahiti (just kidding) :)
Another weird camera angle? The top right longeron looks like it is huge and touching the ceiling - it is not. I clamped the main wing spar channel in place to get a feel of where it will sit once the top skin is in place and how the cabin side channels meet up to it. Not pretty, but this fit up confirms the side skins are equally matched on their respective lower longerons. This is only a temporary fit to check things out and won't allow the top skin to be rolled out, so I took it down immediately after the picture was taken - couldn't take a chance of it falling and punching a hole through the bottom skin either!
As I rolled the top skin on top of the upright fuselage sides I realized that there isn't much room to brace the top skin on and any slight outward swing of the upper longerons would allow the top skin to fall inside. To prevent the longerons splaying outwards, I tied them together (see the orange twine below) in a narrower width than the upper skin. I also added the the upper skin stiffeners to help as I rolled it out:
It's starting to look like a fuselage!
I'll need to fire out the best way to add the upper fuselage skin diagonals. Like the sides, they overlap the longerons on the inside of their curvature, but at least with the bottom and side skins, I had access by reaching over or around to fit them:
With a little of wiggling and sliding back and forth, I got the top skin aligned in the proper position relative to the front edges of the side skins. The forward edge still needs to be trimmed - I left it long when I was laying it out to ensure fit before trimming if needed (see order of operations comment above). I temporarily clecoed the forward upper fuselage doublers in place to check fit:
Blue line on the left is approximately where the front edge will eventually be trimmed back to - 30mm forward of the rivet line for the skin/main spar joining point. Blue line on the right is a marked to show the trimming of the skin where it meets to the upper longeron edge. This will be trimmed later as well:
With the forward upper fuselage doublers in place, I now know where I can trim the outside edge of the skin - it has a notch that matches the doubler, but I can't make that cut until I trim back the longeron and the skin will lay completely flat on the longeron - yet another C before B but only after A order of things:
Picture of the right cabin side which shows the eventual notch to be cut out - this too will be more clear later:
Now with a confirmed location of everything, I can safely cut the longeron to match the cabin sides. Trace a line with a fine Sharpie:
To avoid damaging either of the fuselage skins, I tied back the the side skin with orange twine and propped up the upper skin on top of cleco I inserted in the hole - perfect!
Used the Dremel tool with a cutoff wheel to trim the longeron back close to the line:
Untied the side skin, laid it flat against the longeron and clecoed it in place - still some minor trimming to do:
Slow work with a sanding drum on the Dremmel and some careful filing by hand and he longeron is now matched to the side skin:
Now I can fit the rear wing attachment plates to the front side of the cabin upright channel at the front edge of the side skin:
With the wing attach plate in place and confirmed flush with the cabin side skin I finished pilot drilling several of the cabin upright channel holes to finalize its position on the skin. The attach plate will be drilled to the channel later, after back drilling the existing holes through the main spar channel when it is in place - then, with it secure in place, I can square up the cabin before finish the drilling on the uprights - man, that's a lot of steps in order to get this in place, but probably one of the most important connections in the airplane, so it's worth the extra effort to think it all out. The cabin upright channels still need the inside corners bent, but that will happen when I fit the rear upright covers, another thing to keep my mind occupied I suppose.
Lots more to come soon, including more bending, drilling, deburring, priming, head scratching, followed by more bending, drilling deburring, priming and riveting. Always learning, but also always getting closer!! :)
Thanks for following along!
Happy New Year everyone!
Back in the shop over the holiday weekend and moving forwards with the build-up of the rear fuselage.
With the passenger side fuselage skin now in place (see above picture), I can duplicate drill the forward side gussets. A LOT of the parts on the fuselage are left/right clones of each other, so once I have the right side skin in place, I can match drill the right gusset, then backdrill through it so the left gusset is a perfect match. Both of these will be added later, once the cabin floor and sides are fitted.
Disassembled the passenger side skin, and prepped it for priming paint at the join locations:
Prime completed on all sections aft of the baggage support channels.
While the passenger side priming paint dried, I rolled out the pilot (left) side skin, clecoed the diagonals into place and mounted it in place on the left lower longeron, same method as the right side. The only difference on this side was matching the exact fore/aft position of the right side. A bit of measuring from several locations confirms all is square:
With the primer now dry, I added the diagonal supports back to the right side skin and prepared to lift it back into place on the lower right longeron:
Both fuselage sides in place, clecoed to the bottom skin. Here I've temporarily added the torque tube bearing support angle in-front of the hell-hole, and started fit-up of the pilot side baggage support channel:
Wood supports screwed to the table helps support the skins and keeps the fuselage somewhat close to square:
Top longerons in place (these were fitted to the side skins previously when I was completing the diagonals):
More looks rearwards inside the rear fuselage. Even without the top skin or forward uprights installed yet, the fuselage is fairly straight and stout. The passenger side baggage angle is now fitted:
Camera doesn't capture the full length of the boxed up fuselage, but it takes up most of the entire workbench:
Next up, finish upsizing the pilot side rivet holes at the lower longeron, and complete the horizontal tail support structures/bulkheads. Also need to run the wiring conduit along the inside edge of the fuselage that will carry the wiring for the tail navigation lights and the control wires for the elevator trim system.
With those done, I can start adding the top skin. That will require standing up on the table I think!
Thanks for following along, lots more to come.
A couple of weeks has passed since my last blog post. Christmas is a very busy time for everyone and I'm no different. Two back to back work road trips right before the holidays, but any chance to be in the shop that came up I went.
With the left (pilot) cabin side fit up, time to replicate the right (passenger) side to match.
Used the same method for the matching the cabin longerons, placing them back to back, prior to back drilling them to the skins.
Before heading home one evening, I decided to see what the cabin width would look like. Very rough estimate, but liking what I see - lots of room for two people sitting side by side!
Next time in the shop, I decided to compare the cabin sides and they are perfectly matched - everything will be straight and equal once I create the cabin floor and attach it to the rear fuselage.
Both cabin sides, initial fit-up complete:
A somewhat more accurate placement on the bench using the landing gear channel as a guide. This picture shows the rear cabin longerons as they bend inwards to match the tapering fuselage in the baggage area:
Upsizing the cabin side longerons, doublers and uprights to A4 in preparation for debur, prime and assembly.
Further investigation was required regarding the forward edge of the lower cabin doubler. Shown at bottom right in this photo, it needed to be trimmed back.... but....
The plans show two different things. Circled in red below the plans show the doubler extending forward to the front edge of the cabin skin. However the view from the bottom on the same plan drawing (circled in green) shows that the corner has a relief cut.
The cabin skin plan drawing shows the relief notch at the corner. That seems to match the green circle above. Time to email Zenith.
While waiting for Zenith to respond to my query, I continued to drill out the cabin parts to the correct size (A4)
I heard back fairly quickly from Zenith and it turns out the drawing in the plans is wrong. The doubler only extends forward as far as the front most upright angles. That leaves the notch free to allow the boot cowl skin to form around that corner later. So with that questioned answered, time to pull everything apart for deburring and prime:
With all holes deburred I put the door sill longerons/angles back in place briefly in order to final sand the cabin skins to match them. I also stripped the protective plastic of the the outside of the skin along the rivet lines so I can prime the rivet lines ahead of placing rivets:
Added the last two cabin side rivet holes, but only up to A3. These will be the pilot holes for joining the cabin sides to the rear fuselage. Still need to bend the rear edge inwards to match too:
Debur, scuff with Scotchbright and clean prior to priming:
Decided to change track a bit while I had room to work on the bench and got started on laying out the cabin floor. The camera distortion doesn't show it well, but the cabin floor is a tapering rectangle, narrow end (furthest from camera) being the front end towards the firewall:
Laid out the location of the rudder pedal bearing channel and started to secure it with A3 clecoes:
Next headscratcher.... I see in my plans that there is a gap above the floor skin between the heel support channel (on the left in pic) and the pedal bearing channel (on the right):
Is there any reason this gap needs to be there? I don't see anything in the plans that says that the gap needs to be there for other stuff and to my way of thinking it will just be a place to gather dirt, dropped pens, travel snacks debris on long trips etc.
What I was thinking of doing is widening the heel support channel to overlap the rear flange of the pedal support bearing with his eliminate the gap, then riveting both flanges together (green stars):
I asked in the forums and in an email to Zenith and there is no reason not to widen the heel support. Turns out the gap is a carry over from the 701 design where every chance to save weight was made. The 750 Super Duty model has already covered this gap in the same way I am suggesting, so no issue to proceed, so I will.
Next up is the lower cabin floor corner longerons. Cut them to match (one for each side) and mounted them. This support the cabin sides, so I'll wait to drill them together when fitting the floor into the cabin:
Also on the bottom of the cabin floor is the centre stiffener whih runs from the firewall back to the gear channel. Additional lateral stiffeners will be added later:
Seat support channel is added on the top side of the cabin skin:
Lots left to do on the cabin floor, but off to a good start, next will be the heel support once I fabricate the new wider one:
Back to the cabin sides. Primed and ready for assembly.
Priming of outside of cabin skins:
Final assembly in progress after priming. I'm waiting to add the long forward and long rear longerons until I confirm where they meet the firewall and cabin:
All of the cabin side A4 rivets are driven with the air hammer, similar to the wing spar caps. Fortunately, some of these A4 hard rivets are also close enough to the edges of the cabin skin that I can use the rivet squeezer to complete the assembly:
Forward upright now fully installed:
The others I could reach witht e squeezer are the tops of the rear uprights, the doubler L's and the door sill angle.
I set aside the cabin sides for now to work on the fuselage bottom assembly. First thing was to cleco forward corner doublers to the bottom skin and the fuselage longerons. Next I clecoed all the diagonals and lateral stiffeners to the fuselage skin:
Installed the riv-nuts that will be the mounting points for the stainless screws to hold the hell-hole access cover in place:
All riv-nuts secure and tested with the screws that will eventually be in place - very happy with how these turned out.
Worked the air rivet gun back and forth equally across the diagonals and lateral stiffeners, leaving the ends free to tuck in the side fuselage skins:
Incredible how rigid the lower fuselage skin is now with the rivets lines across the middle of each bay:
I started to work on the end plate gussets and cross angle, but soon realized the plans don't define at what measured location this gets installed. Spoke with another builder and he noticed the same thing with his kit and we agreed this needs to wait until the controls they support in the hell hole are installed ensuring correct alignment.
Next up, the front and rear horizontal tail frames. I'd previously fitted these to the bottom fuselage, now it is time to start building up the box that becomes the tail support area.
Kit builders have this step much easier. The rear side skins and longerons are drilled already, whereas a scratchbuilder like me needs to think a bit how to place them in the right location relative to the rest of the fuselage.
To that end, it is time to start fitting up the side fuselage skin to the bottom skin.
I decided on starting with the right side, mostly because was closest to the edge of the bench!
I added a couple of upright boards which are screwed to the bench, then lightly clamped the skin one of them through the cabin window. The it was just a matter of sliding the skin fore and aft a bit until the front lower corner matched the front right corner of the bottom skin. With the rear side skin clecoed together, I know the distances are correct (and I confirmed this as well).
I drilled the side skin to the lower longeron along several places. As mentioned before, the fuselage has a gentle curve towards to the tail on all four corners. By matching the side skin taper to the longeron and securing it as I went, the fit up was relatively easy to accomplish. I still need to remove each of the diagonals and uprights on the side skin for priming, I just clecoed them in place for now give the side skin some rigidity for this task:
Here is the left side with the rear side skin temporarily clamped in place. I also clecoed on the the top coner longeron to further stiffen up the side skin for back drilling the rear tail frames:
Front and rear tail frames fit up and drilled to A3, eventually these wil be brought up to A5:
A bit hard to see among all the clecoes, but I added the rear frame diagonal as well, these too will be brought up to A5. I've got a clamp holding the ear corner of the skin to the lower longeron and will drill them later once the left side fuselage skin is in place and I confirm everything is square.
Very pleased so far. All the work to get the fuselage skins laid out, cut and replicated side to side is working very well. I'm also pleased that all the fore-thought on order of operations (what to drill now, what to wait for later) is paying off now in the assembly stage.
Thanks for following along, more to come soon!
Another productive weekend at the shop. Work continues on the cabin sides which are being built ahead of time. Once complete, they will be temporarily stored until the rear fuselage is put together.
With the radii complete in the cabin side skin blanks, I realized I needed to duplicate the holes where the longerons and other cabin wall supports would be installed. Once trimmed to final size the pilot holes around the perimeter would no longer be available.
With new duplicate rivet lines established, I started to cut away the door sill on the master cabin side (left/pilot side). I started with rough cuts between the radii circles:
This allowed me access to completely trim away the door sills. I used a scoring tool to ensure perfectly straight lines between the radii, then snapped off the remaining trim away scrap. Same with the bottom edge where it will fold inwards to form up part of the cabin floor.
Also trimmed away the last part of the forward cabin edge where it will join the boot cowl (area between instrument panel and engine cowl:
In order to preserve my ability to stack left and right cabin skins until absolutely ready to bend them, I left a pilot hole dog ear at each top corner. These will be trimmed off later.
Fiipped the left cabin side over and stacked it on the right side cabin skin. Radius holes match the door sill radii which confirms both will be equal once cut out:
Trace the left onto the right:
Follwed the same method to remove the trimmed scrap from the right skin, restacked to confirm and now I have a cabin skin blank for both sides:
Now to start laying out the longerons and supports on the left cabin skin, looking towards duplicating them on the right skin. Until I install the 040 doubler along the folded lower section, I left the longerons and supports a bit long. I can trim them later to fit.
The forward longeron which connects the cabin to the boot cowl and firewall is the only piece of this assembly that crosses completely from corner to corner, so I started with it. Plans call for 3/4x3/4 inch 093 thick angle, but I'm upsizing this to 0.125 thick. The extra strength will be worth the very small weight penalty, giving the cabin that much more rigidity when the aircraft gets put on floats later on:
The lateral leg of the longeron extrusion forms part of the door sill, so it abuts to cutout in the cabin skin:
Next I started fitting the uprights and once positioned correctly, laid out the rivet lines on the extrusions and drilled them on the drill press to A3 hole size. This is kind of like a jigsaw puzzle figuring out what piece to do next to best suit the trimming order required. Again, I intentionally left the uprights and diagonals long until I bend the lower section inward ( on the line just above the red material stamping) and fit the 040 doubler angle (more on this later):
Working at the edge of the bench allowed me to clamp the upright to the edge of the bench and back drill two cleco holes through the extrusion into the skin.
Next up was the forward lower extrusion which makes up part of the cross bracing. An adjustable angle tool was handy here for determining the trimmed edge where it meets the forward longeron:
This piece was left long for later trimming:
Temporary placement of rear upper cabin longeron so I can determine trim angle of the top end of the rear cabin upright extrusion:
Any part that will interface at the inside corner of the side skin and cabin floor is left long for later trimming to match:
The rear cabin longeron runs from the centre of the cabin skin at the forward longeron back through the rear cabin and into the baggage area. In order to fit, the longeron needs to be bent inwards to match the taper of the baggage area at the rear of the cabin side skin. A taper of 11 degrees inwards. Another thing done by the factory for kit builders to overcome. I also had to make sure the radius of the bend was in proper location at the back edge of the cabin skin.
I figured someone else MUST have done this before in scratchbuilding, and a quick search revealed others have used several methods including hydraulic press bending, hammer forming on a jig, etc. The method described by one builder seemed simplest - build a bendging block on the bench edge and use a length of pipe over the extended edge to bend the extrusion being carefull to avoid any twisting.
To that end here is what I came up with and it worked very well. Start with a stout block (left) and a small support block adjacent to it:
The support block is rounded on the end to spread the bend over a radius and the upper inside edge is sanded round to match the inside curvature of the extrusion:
The extrusion is held tightly against the the support block by another board screwed to the table and the whole thing is clamped down tight via a bridge of blocking screwed to the outer blocks. To approximate the 11 degree bend, I stuck a piece of card stock underneath. Not perfect, but close enough for this purpose. I also chose a bend location that left me enough room to trim the angle at the X and long enough to meet the rear of the baggage area as per the plans (total length 1440mm):
Bent it past 11 degrees to account for springback and nailed it pretty much the first go. This worked amazingly well. I would have liked the radius tighter, but this is acceptable and I didn't want to risk cracking the aluminum. There was a bit of twist that I expected, but I was able to flatten the twist out with some gentle persuasion using the vice and deadblow hammer:
Placed the bend at the rear edge, then trimmed away the "X" edge. Transferred the rivet line to the extrusion, drilled the A3 holes out on the drill press and fitted the extrusion to the skin:
With the location of both forward and rear longerons now confirmed, fabricated the extrusion for the bottom of the door sill.
Careful trips to the bench grinder to taper each end until the match the longerons and the door sill:
Used a couple of blocks to hold the bottom door sill extrusion in place then back drilled through the skin. Then I added the vertical L doublers that are at the edge of the cabin skin doubler:
Remove everything except the rear and forward uprights.
Slide the cabin skin doubler blank under the cabin skin, secure using clecos in back drilled holes through the skin into the blank once the radius holes in the blank match the radii on the skin:
The doubler width is from the rear doubler L to the front doubler L. Trace the edge of the cabin skin onto the doubler blank, then cut the same as the edges on the skin.
So everything is ready on the left cabin skin assembly that I can bend the lower portion inward at this point. However, in order to duplicate the extrusions to the right side, I need to mirror them for drilling. I started with the rear upright, clamping the now drilled left one to a blank and undrilled one for the right, after matching the angled end on the new one:
Back drill though the left one, making a perfectly matched hole on the right one:
Did the same for all the other extrusions, with the exception of the long rear longeron, which I sill need to bend and fir correctly to the right side skin:
Now I have perfectly hole matched extrusions for both cabin sides. The lengths are approximate, still waiting on final fit-up once the lower cabin skin corner is bent:
Repeated the process for matching up the doubler blank to the right cabin skin:
For curiosity, I stacked both the now trimmed left side doubler onto the right side doubler and then onto the right side skin. The door sills match perfectly!
I've been very consciously aware that the bottom bend of the side cabin skin is important do do accurately in order to ensure both left and right cabin sides are symmetrical. A test piece of 032 cut to replicate what the bend will look like when complete serves the purpose.
With that in mind, I proceeded to bend the left cabin skin:
I couldn't be more happy how this bend finished out. Dimensionally perfect, now I just have to replicate the same process on the right side skin.
On the inside of the cabin skin bend is a 040 doubler angle. 30 pitch rivet line laid out:
The green tape helped protect the cabin skin while I drilled in the tight corner. The doubler will be trimmed back to match the front and rear edges once final hole sizes are complete.
Prior to drilling the corner inside rivet line in the doubler, I added the cabin skin doubler and clecoed it in place:
Then I trimmed the uprights, the L's and the longerons to properly match the new corner doubler:
Pilot side cabin skin fabrication complete, awaiting final hole size drilling and solid rivets. This picture really shows the rear longeron taper:
Outside facing cabin on the pilot side. It doesn't look like much, but this is where the door will be and the seats of the cockpit! I'm slowly getting closer to sitting in my creation and making airplane noises!!
Next up, the rest of the right (co-pilot) side skin fabrication. Thanks for following along!
It's that time of year. Shorts and Tees one week, then freezing cold the next. October into November has weather been beautiful and I have no right to complain , but November into December has been less than stable. At least I can light a fire in the shop woodstove and keep working on the plane.
Flipped the right side fuselage skin over and finished right sizing the holes I could in preparation for debur and riveting.
Top longeron on the right in progress, going just as well as the left side:
Moved the right side fuselage skin aside on the bench and started fitting the rear fuselage side skins. Left side skin is the "master" so I used it to back drill the seam holes (camera distortion makes this look shorter in length than it actually is):
Stacked the other side rear fuselage side skin to match drill it from the "master":
Rolled out the right side skin and stacked the left side on top to match drill them:
I realized very quickly that by adding additional diagonals on the other skins, I needed to make more "L" angles for this second side skin and for the top skin to come. The method I like best for making "L" in the least amount of time is first scoring the 025 sheet in 38mm wide strips, but then waiting to separate them until I have several in line:
Once I have the number of strips or blanks scored, I debur the edge that still sticks out proud of the bender (left side in picture below):
Then, I pull the scored sheet outwards and snap-bend the first blank - this leaves the first blank in the bender for deburr and sanding:
One by one, the blanks are snap-bent off the sheet, deburred and cleaned, ready for bending to 90 degrees:
From blanks to "L", ready for placement and fit up:
Laying out the diagonals and fuselage skin stiffeners:
Flip the skin over and fitting up the upper longeron:
I sometimes think there must be literally pounds of drill shavings to clean up and dispose of as I work through this project!
Still amazed how distorted the camera image is. Here is the rear most side skin. It almost looks wider here than the front of the fuselage!
Layout the skin flat again for window cut-out, using the same method as the right side skin:
Slow and steady, lots of cutting oil:
Deburr and cleaning up the edges with sandpaper and ScotchBrite pad:
Now that both fuselage side skins are matched to the same stage, I trimmed the forward edge of the left fuselage skin where the cabin and door will meet the fuselage:
Stack the trimmed left fuselage skin on top of the right:
Use the match drilled rivet holes to line both skins up perfectly (windows are perfectly matched too!):
Trace the edge onto the other skin:
Trim away at the blue line using shears:
Both fuselage side skins now match each other.
Rolled the fuselage sides up and put the fuselage top skin back on the bench to add the stiffeners:
Match drilled the upper wing spar carry-through channels and angle together. These were the pieces I decided I couldn't bend accurately enough so I bought what I needed from Zenith. The purchased pieces matched up perfectly with each other and with my scratch-made rear doubler angle:
All the effort and thinking about the drill/assembly order to match drill the spar carry-though to the upper fuselage skin and corner doublers paid off. It fits perfectly (although I still need to do some trimming of the forward edge once the fuselage is assembled):
What appears to be waviness in the assembly is just a rough edge on the top skin. This will be trimmed back later as the fuselage gets assembled. The picture shows where the wing pick-up plates will attach as well (holes on left side):
Took a break from the fuselage for a bit to contemplate some the control assemblies I'll be making in the coming month or so as the fuselage comes together First up are the mounting plates for the flapperon mixer control tube. These are made from 080 thick 4130 chromoly steel, common to aircraft structures:
The flapperon control tube is 1-1/4inch diameter seamless chromoly tubing:
Eventually these plates will be welded to a stub end of a piece off the end of this tube and a AN6 bolt will protrude through the hole. The bolt will be the mounting point for the flapperon bellcrank, sandwiched between two nylon bushings. More on these later.
The push rod tubes for the flapperon controls (and others) are smaller diameter tubes. In one end, a nut is inserted that will allow threaded rod to be inserted which connects to a rod end. This allows almost infinite adjustment of the push rod length when it comes time to adjust the rigging of the control surfaces.
The plans state to weld the nut inside the tube, but the required nut doesn't fit inside the tube?
Rounding off the point corners of the nut on the bench grinder, rounds it out enough to fit the inside diameter of the push rod tube:
I used a long bolt here to test fit the rounded nut in the tube. These push rod tubes will be welded up later along with the threaded rod inserted.
Before assembling the fuselage and taking up the entire workbench, I decided to start assembling the cabin sides. These normally come pre-assembled in the factory kit, but as I am scratch-building, it's another assembly I need to make myself from the beginning.
Laid out some 032 sheet and started laying out the cabin side skins as per the plans:
Took a fair amount of time to lay everything out correctly - lots of tapering angles and considerations for cutting door sill radii, etc.
Much like the fuselage side skins, these cabin skins are mirror images of each other. So I used the same method as the fuselage. Make one as a master, use pilot holes in the master to create the twin. Here the side skins are stacked, with pilot holes in the perimeter for holding them square and pilot holes for the radii where the cabin door sills will be:
Each cabin side also has an 032 doubler sheet that is fastened between the outward facing cabin side skin and the internal extrusions that make up the cabin skeleton. Using the two inner door sill radii, I made up rough cabin side doublers which will be matched drilled later with the extrusions - so many things to think about and decide on order of fit-up/assembly as I go!
Next up was cutting the radii that make up the lower rear cabin skin. The step drill is good for this 16mm diameter hole:
The cabin door opening is defined by three 47mm diameter radii connected together from the rear of the cabin skin forward, eventually curving upwards at a 19mm radii to meet the upper cabin frame. It's hard to describe verbally, but the following pictures may show it fairly clearly.
Used the fly cutter to cut the 4 radii in each of the skins and the matching doublers:
Here is the rear most radius, before trimming the door sill. The angled reactangle represents the eventual rear edge of the cabin skin where it matches the rear cabin of the fuselage. The small hole in the top left circled in red is one of the perimeter pilot holes that I use to keep the skins aligned for match drilling later. Eventually once everything is complete and ready for solid riveting, these pilot holes will be trimmed away as scrap metal, leaving just the completed cabin skin:
A quick stack of the rough cabin side skins confirms the second skin pilot holes match where the new radius holes were cut (measure thrice, cut once!)
Second cabin side skin radii cuts in progress:
Cabin skin doublers also have the door sill radii cut to match the parent cabin side skin, using the previously drilled pilot holes:
More aluminum debris from the fly-cutter to clean up!
So far, so good on the cabin sides. Have some challenges coming up, including how to bend the rear cabin side longeron extrusions. 0125 angle extrusions by default do not like to be bent , but the plans call for just that. Time for an email to Zenith to find out how they do it for the factory assembled units.
Thanks for reading along. Still more to come soon.
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 :)
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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.