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!
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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.