Spent some time in the shop on a rainy day.
Managed to make the rear elevator trim channel. It's critical to ensure this is made straight and to the correct dimensions as the covering skin leaves little room for error. A millimetre too big or too small translates to many problems, so I want to make sure I get this right, and it will be good practice for bigger parts later on in the build.
Unfortunately I've learned the plans sometimes leave out the "developed length" dimensions on some of the parts. In other words, what the dimension is prior to bending (which changes the length or width of a part as it bends). Not a huge deal if you are buying a kit as these parts come pre-formed for you, but for a scratch builder like me it's a critical piece of info. The math to figure out this isn't overly difficult, but making it repeatable using different material thicknesses and bend radii could be tricky.
Looking at the Zenair builders website, I found a great spreadsheet put together by another builder that does the math for you. It asks the basics (thickness, flange and web lengths and bend radius required). Input these or at least the ones known and the spreadsheet returns the "flat" or "developed length" dimensions. It's these that you use for cutting the raw aluminum. Here is a look at how a bend is calculated:
The spreadsheet makes it easy to apply the math, in this case for a piece of stock being bent into a channel:
So in the example above, entering 18mm for both flanges, with a channel web length of 80mm, requiring 90 degree bends with 1/8 inch (0.125) radius bends in the material 0.016 inches thick results in a developed length (dl) of 112.01mm. Just adding 18+18+80 mm together doesn't account for the bend radii or the thickness. The other result is handy too. Measuring 16.41mm in from the edge of the flat material provides you the sight line for the bending brake. I'm obviously only going to get as close as a half millimetre to these dimensions, but that's well within acceptable limits. Perfect, right?
One thing I learned early on in the process of making parts for the 701 wing repair is to use the never ending supply of scrap corners to try the theory before actually bending the full size parts (cheaper to experiment on scrap!)
So, I measured out the required dimensions on scrap, used the markings to align the piece in the bender and voila! Perfect example of what the full size piece should look like - this channel has two flanges, on at 90 degrees and one at 105 degrees.
Repeat the dimensions on the full size piece and bend the same way. Perfect and straight!
Finished off the relief angles on the rear wing ribs and root rib forms:
With this done, I made some headway on the 701 wing extension. Next step was figuring out the inside web doubler.
It's a bit of a puzzle as it needs to be fitted to span the spar caps top and bottom, the spar web and the closest wing rib. Consideration also has to be given to where the spar cap rivets will come through this doubler.
After careful measurement, I drew out the initial shape of the doubler. The dotted lines running left to right are the distance between each spar cap. The vertical dotted line represents the flange for attachment to the win rib:
To make the doubler fit correctly by sitting flat on the spar web and over the spar cap flanges requires a technique called "joggling". A joggle is a rise in the surface of the metal to overlap the adjacent panel or other piece. It's also used in reverse to create a depression in a surface to allow another piece sit flush, such as an inspection cover.
Joggles can be made in numerous ways, but I found a real neat tool that can be made out of scrap aluminum plate.
First, I measured out a rectangle of 0.125 aluminum plate:
Measure out the piece for a flex hole and bend slot:
Drill a hole on the press with a step drill bit. Makes a nice clean hole:
Cut the bend slot to approximately the same thickness as the plate, in this case 0.125:
Grind and sand everything smooth.
To use the tool, insert the piece to be joggled in the slot:
From here you have a couple of options. Thin gauge metal could be bent by hand by pressing down on the top of the "X" formed by the piece and tool.
Because this piece was made of 0.025, I decided to use the better option of pinching it all flat in the vise. This provides a much cleaner joggle and both sides of the bend lay flat. It's hard to capture this in a picture and I forgot to show it in the vise, but here is the result:
I purposely left both the flanges on either of the joggles wider than needed as the important dimension was the distance between the spar caps against the spar web. The flanges can be trimmed after test fitting it in place.
What a great tool. I made it big enough to do anything that needs joggling on my plane as well.
Next up, trail fitting the spar tip extension inner doubler and fabricating the rear rib channel. I want to get my 750 wing and root ribs final sized so that I can finish sand them prior to bending.
Until next time!
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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.