F21 Assembly order/tips?

[NWade]

Hi All,

So I have both aft fuselage side panels riveted per the plans and am ready to start putting together the cross-ties, bottom skin, tail web, etc.

The cross-ties don't seem to have precise lateral spacing called out at each station, so I'm guessing the aft shape & width should all be driven by the lower skin (in addition to the one width specified at the tail and the other width specified at the skin-bend)... is that correct?
(NOTE: My kit is pre-matched-hole parts)

Do you have a recommended order of assembly? Any tips for keeping the aft fuselage square throughout the process?

Thanks!

--Noel
Sonex #1339

[peter anson]

I built the aft fuselage assembly upside down so everything is square to the build table. Obviously the build table must be flat. I just propped the sides up and clamped in a couple of the cross members. Check and recheck, making sure everything is square before you drill. Once the end dimensions are correct, sight along the longerons to make sure they are straight before fitting the intermediate cross members; I recall that mine weren't quite straight so I had to take a halfway measurement. As a final check, temporarily fit the bottom skin in place. It should fit perfectly.

Peter
Sonex 894

[Bryan Cotton]

I have some pictures starting on page 13 of my thread:
viewtopic.php?f=39&t=578&start=120
I agree with Peter's comments. With the prepunched holes this is really easy - don't overthink it! Besides checking squareness verify the channels and cross members are all oriented correctly as per the plans.

[Rynoth]

Noel, look at Darick's pictures in the following thread: viewtopic.php?f=39&t=913&p=14957&hilit=cross+tie+box#p14957

Most of us have had to trim the upper forward cross tie box in order to get the 40" width dimension, otherwise the box ends up pushing the longerons outwards when you insert the upper splice plates.

[NWade]

With the prepunched holes this is really easy - don't overthink it!

Thanks, but just to reiterate: I don't have pre-punched holes. Some of us bought our kits many years ago, before that was offered. :P

Appreciate the tips and info, happy to hear any more that folks have!

--Noel

[Bryan Cotton]

Noel,
I misread your earlier post. I think using the bottom skin to set width is a good idea in your case.

You could use it on the top and bottom to help ensure squareness.

[aferddaberts]

Noel,

The way I made sure my fuselage was square I made 4 L shaped supports out of 2X4's, clamped them to my workbench, then clamped the fuselage sides to the uprights (upside down) drew a line down the center of the work bench and adjusted the clamped uprights thr proper distance from the center line reference point. When installing the cross ties, they fit in with no need to do any trimming. The belly skin fit perfectly.
I could email you photos of that procedure if you want.

AL Roberts (robieal2011@att.net)

[Direct C51]

I am coming to this point as well, and have a build strategy I would like to get opinions on. I'm thinking of putting the fueslage sides upside down on leveled saw horses, drilling and clecoing the lower skin, then clamping, drilling, and clecoing the cross ties from underneath. This method will ensure a perfect fit of the lower skin and correct width of the fueslage. Anyone see why this would be a bad idea?

[Bryan Cotton]

DC51,
That should work. Does your kit also not have matched holes? I did have my tailcone on sawhorses a lot working from underneath, primarily for the turtledeck.

An option would be to set it up on a flat table (more stable), clamp the formers in place, and check the alignment of the lower skin. If it was poor pull the skin off and tweak positions as required.

It will be hard to clamp the skin on except at the front and maybe rear.

[NWade]

I am coming to this point as well, and have a build strategy I would like to get opinions on. I'm thinking of putting the fueslage sides upside down on leveled saw horses, drilling and clecoing the lower skin, then clamping, drilling, and clecoing the cross ties from underneath. This method will ensure a perfect fit of the lower skin and correct width of the fueslage. Anyone see why this would be a bad idea?

Yes, I can see one (literally)! After spending 6 hours on this exact step yesterday, I have learned a thing or two. The biggest issue is that without the lower cross-ties you are going to see the skin sag a lot in the middle of the fuselage.

Instead I settled on a process similar to what Al wrote in this thread (which sadly I didn't see until it was all over; but thanks anyways Al)! I'm really happy with how it turned out, so let me explain it in-detail, below. The list of steps is long because I'm building from a kit without matched holes and with a few scratch-built parts, so I was extra-careful and wanted to be very precise with this procedure.

NOTE: If you have matched-hole parts and those include pre-drilled lower cross-ties, you can skip all the wood block shenanigans in the list below. Simply cleco the lower cross-ties to the vertical fuselage ribs and that will set your spacing and ensure square-ness.

If you don't have matched-hole cross-ties and/or don't have pre-drilled longerons, here's a complete step-by-step list:

1. Cut some 2x4 material up to make blocks for later. You need 8-10 short sections (roughly 4" long is big enough). Use a bandsaw so you get nice square ends on the blocks. Look at your 2x4 blocks and note the "factory" rounded edges. Your bandsaw-cut ends have nice square edges which is what we want to use against our parts.
2. Lay the bottom skin out on the work-table, fitting the fwd end of the skin to the edge of the table.
3. Clamp or weight the skin down temporarily, so it doesn't move.
4. Using a sharpie, draw several short (6-18" long) lines along the sides of the skin - do it at each end of your work-table, plus in the middle.
5. Remove the bottom skin from the work-table.
6. Since the aft fuselage is a box with square corners, you now have an outline on your table that works for positioning the longerons. The side fuselage skins should cover up the sharpie line (ideally the outside edges of each longeron would be just touching the sharpie line - but this is hard to see because the skins are already riveted on).
7. Remove the clecos and splice-plates from the fwd end of your fuselage sides. They will interfere with the fuselage sides sitting flush on your work-table, and they will interfere with your bottom skin sitting flush on the longerons. Put the splice plates in a safe place for now.
8. NOTE: A second pair of hands helps for the next few steps
9. Lift one the fuselage side up onto your work-table and rotate it so the upper longeron is on the work-table surface.
10. Place a block on the outside of the fuselage side, with the cut (square) end up against the fuselage skin. Pick a spot just fwd or aft of a vertical rib/channel (you don't want your block position messed up by rivet heads, but you do want to press against a relatively strong area of the longeron and skin). Mark the corners of the block on the table with a sharpie (you will only do this with your first block on each fuselage side).
11. Move the fuselage side a few inches clear and use 1-2 wood screws to fasten the block to the table at the marked location.
12. I find it helps to stack a second block on top of the first, and then screw the second block down to the first. Make sure your inside edge (the one that will press up against the fuselage skin) is square and flush with the first - so you get a nice vertical support that helps keep the fuselage square to the table!
13. Slide the fuselage up against the blocks and check to make sure its still in the proper location.
14. Repeat this process 6-12" from the fwd and aft ends of your work-table. You now have the outside of one fuselage side blocked up and square to the table (if you hold it up against these blocks).
15. To prevent the fuselage side from falling "inward" use a couple of blocks on the inside of the longeron. Check each time and make sure that the fuselage is pressed up against the outside block by the inside block, and ensure that you use the cut (square) end of your block. This is because the "factory" rounded corners on a wooden 2x4 can slip up and over the longeron as you screw it down. Your cut ends will sit flush to the table and push the longeron up against the outside blocks nicely.
16. Repeat this process with the other fuselage side. Check a few times to ensure that your sides are still following the sharpie lines on your work-table, and don't apply so much inward/outward pressure with your wood blocks that you bend them.
17. If you have the aft fuselage bulkhead formed and the tail end of the fuse sticks off the back of your table, slip the bulkhead in at the proper location and clamp it (or cleco it, if you have the matched-hole part) in-place. This will help set the spacing at the aft end of the fuselage. If you use clamps, attach the clamps down near the work-table. You want to leave the "top" clear to test-fit the fuselage bottom skin.
18. Check that your fuselage sides are vertical by using a square, pressed down to your work table and then slid up against the outside skin of each fuselage side, in several places. Adjust your blocks as-necessary.
19. Check the 40" outside width at the fwd end of the fuselage sides, both down near the table and up near the top - it should be really close.
20. You can make a measurement tool that also serves as a jig by clamping two large metal squares together, making a large "C" with the inside measurement locked at 40". Then you can lock the fuselage to that width by sliding the tool onto the fwd end of the fuselage and clamping it to each fuselage side. I recommend doing this
21. Examine the plans and notice that the lower cross-ties should be trimmed so that their corner cut-outs exactly line up with the corner cut-outs on the vertical ribs/channels that are already riveted to the fuselage sides. If you manually cut/trimmed your cross-ties or fuselage ribs and they don't exactly line up, then simply equalize the amount of overlap on both sides and mark where each end of the cross-tie should be. This gives you the proper left-right alignment of the cross-tie. To line it up in the vertical dimension, do the following process for each end of the cross-tie: take a sturdy metal ruler/square and place one end of it on the lower longeron, with the ruler running across the fuselage "above" where the cross-tie is. The ruler simulates the bottom skin resting flush on the lower longeron. Now slide the cross-tie up until its flange is touching the ruler, check your left-right alignment, and clamp that end of the cross-tie in-place. Repeat for the other end and you should have a cross-tie that is centered in the fuselage and with its flange at the proper location to support the bottom skin.
22. If you placed your clamps on the inside corners of the fuselage cross-ties, gently test-fit your bottom skin to ensure that you haven't somehow pulled the longerons in too close to each other (pay attention to the plans for proper bottom skin placement). If, on the other hand, you positioned your clamps on the outside ("above") the fuselage, then use a square on the inside corners to check that the cross-tie and the fuselage vertical ribs/channels are sitting at a 90-degree-angle. Or, if you're a freak like me, use two sets of clamps and add/remove them in pairs so you can perform both checks.
23. Now pilot-drill and cleco your cross-tie in-place!
24. Repeat for the other lower cross-ties.
25. I have steps for the upper cross-ties, but I've run out of time and have to get to work... I'll edit this post and add more later. I also have photos and may make a video of this - since a video would make it easier to see/understand.

--Noel
Sonex #1339
TD, Center-stick, Flush-pulled-rivets, Acro-ailerons, AeroVee Turbo