Drag Wires

What gives the wings their strength and rigidity are three things: the spars, the compression tubes, and the drag wires that pull that structure tight. A similar structure is built up on a different level between the upper and lower wings: the wings are like the spars, the interplane struts and the fuselage are like the compression tubes, and, like each wing, you also have wires going between the upper and lower wings to pull everything together.

The drag wires are made from 3/16" 2024-T3 aluminum rod. The wing metals kit calls for 6-foot-long pieces, but I would order them a couple inches longer. I think you'd have less waste that way, but maybe there's a way to cut them to reduce the waste. I then measured out each piece and chopped them off on the band saw. Although the throat of my little band saw isn't big enough to fit these wire lengths in (each drag wire is around 24"), I simply made the cut at an angle, then used the grinder to square off the ends, with a little chamfer for ease of starting the threading (the oscillating belt sander would probably be better, so you don't clog up the grinder wheels with aluminum). You could easily cut these to length with a hack saw, too, if you don't have a band saw.

EDIT: I just discovered that Aircraft Spruce sent me 6061-T6 aluminum instead of 2024-T3 aluminum for the drag wires, so I will have to make all of them again. Ugh! 2024-T3 has a tensile strength of about 60,000 PSI while 6061-T6 has a tensile strength of only about 45,000 PSI. Of course the load on the drag wires is tensile. Glad I made this discovery now instead of after the wings were assembled and covered!!!

Next, you simply use a 10-32 die to create threads on each end, one inch on one end and one-and-a-half inches on the other. Frankly, I don't know why the instructions say to put one inch worth of thread on one end, because they then tell you to put two nuts on that end (locked against each other) with about three threads showing at the end. You probably really need only a half inch of threads on that end. Use padded jaws in your vise so as not to mar the wire, or just throw a couple blocks of wood in there, as I did.

I used AlumTap when cutting the threads. It's quite popular and, in my limited threading experience, seems to work well. I really enjoyed doing the threading. Of course I primed the drag wires with zinc phosphate primer. I think I might cover up the threads if I had to do it over, as it kind of gunked things up when I screwed the nuts on later. I'd be interested to hear what an expert has to say on the matter.

A completed thread.

Several completed drag wires. Each upper wing has six and each lower wing four.

Some completed drag wires in the foreground with a partially assembled upper wing in the background.

Starting Wing Assembly

Starting wing assembly. First thing you do is slide all the ribs onto the spars. You've got to make sure they go on smoothly and can be angled at 6° for the sweep of the upper wing.

Trim the spar opening uprights on the ribs wherever necessary so that the ribs can be angled at 6° without binding. This is why you use shims while assembling the ribs on the jig, but some fine tuning may be required during wing assembly.

I love seeing it come together!

ONLY AFTER the ribs are all adjusted properly so they don't bind when angled at 6° should you epoxy the scab plates to the spars. After these are in place, the ribs can't come off! Every place a bracket touches the spars, you have to have a scab plate.

Scab plates clamped in place while the epoxy cures.

The biggest scab plate: outer end of the main spars on the upper wings.

Make sure to watch my aviation YouTube channel, Aviation Fanatic, for lots of videos about this build.

Finishing Up the Wing Brackets

Each upper wing has three compression tubes which, along with the drag wires (and spars, of course), give the wing its strength and rigidity. On the ends of each compression tube where it butts into the spars, there is a bracket with a little aluminum "button," or "compression tube locators." Each of these is held to its bracket with a small countersunk screw. The smooth part of each screw should fit nicely into the reamed hole, although by the time you're done countersinking the bracket, there's not much of a hole left. I was about to countersink all the way through, when I realized that something was wrong. The head of the screw wasn't fitting the countersink correctly, so I took some photos and measured some angles in Photoshop. Upon perusing the plans later, I discovered that they actually tell you to use a 100° countersink, while the one I had been using was 85°. There's a lot to keep straight, so I'm not surprised I missed it. Thankfully, I figured it out before I ruined any (or all!) of the brackets.

This is a countersink I already had on hand. I had used it in woodworking before. It's included angle is about 85°.

No wonder the screw didn't fit the hole properly! Their angles are totally different! So, I ordered the correct countersink (from MSC, I think) and the screws fit the countersunk holes perfectly, albeit with very little contact area left between the screw grip (the smooth part of the shank) and the hole. It was a great fit, though.

Getting ready to paint some brackets with zinc phosphate primer. That's the stuff that all interior metal gets coated with to help prevent corrosion.

Primed brackets. This primer doesn't seem very durable to me, so be careful with your primed brackets.

These are the compression tube locators, or "alignment buttons" as I like to call them. I made them on my new lathe, a Grizzly G0752. Check out my YouTube videos on the process.

This countersunk hole is to make some clearance for the grip (smooth part of the shank) in the screw so the button can get screwed down tight to the fitting. The countersunk hole I had some difficulty with is on the underside of that fitting.

This is an example of one of the brackets onto which the compression tube locator will be screwed. The bracket will be facing the other direction, though, of course. The screw head will be against the spar and the locator, or "button," will be facing outward toward the other spar to hold the compression tube in place between the spars.

Make sure to check out my aviation channel, Aviation Fanatic, to see some videos of me making the compression tube locators.

Wing Metals Package and Wing Brackets

With the ribs done, it's time to move on to wing assembly, but before that begins, I have to build the brackets. There are dozens to make, and to get started, I had to order the wing metals package. I also ordered the two hardware packages, one for each wing, or rather, set of wings on each side of the plane. Those just contain a bunch of nuts, bolts, and washers.

The Wing Metals Package

The Hardware

1/8" 2024 Aluminum for the wing brackets

Templates made from poster board. I'll trace these, then cut them out with the band saw. I tried the jig saw first, but discovered the band saw worked great. That is an indispensable tool for building and airplane. On occasion, if the original piece of metal wouldn't fit the throat of my band saw, I would use the jig saw to make one or two manageable pieces.

Some brackets, roughly cut out.

Almost all the brackets, cut close to final size, ready to be sanded to final size on the oscillating belt sander, another indispensable tool.

Laying out the holes. I ended up just laying the piece over the plans, then laying the ruler along the lines on the plans, and using a scribe to mark the intersections of the lines indicating the holes. Then, I would center punch those intersections. After that, it was easy to drill them. Make sure to drill 1/64 undersized, then eventually ream them to size. First, though, I just drilled 1/8" holes to use roll pins to gang several fittings together for fewer pieces to  sand.

Ganging several brackets together with roll pins for sanding. Why sand four times when you can sand once?

One of the bigger wing brackets: outer bracket, main spar, upper wing.

The scribe marks where a hole will go.

Tools of the trade for punching holes precisely (hammer not shown).

Getting ready to drill and ream.

As always, check my YouTube channel, Aviation Fanatic, to watch many videos of the construction of my Super Baby Great Lakes.