While doing the scarf joints, I’ve also been dimpling the wing structure. Got the left wing done today. Once again, boring stuff…so no pictures.

Over the last two weeks I worked on scarf joints between the overlapping portions of the inboard and outboard skins on both wings. The idea behind these scarf joints is to taper the thickness of the skins where they overlap. The top surface of the bottom skin gets tapered so that the edge is about half the thickness of the rest of the skin, and the bottom surface of the top skin gets a similar taper. When the two surfaces come together on the wing, the overall thickness of the joint is reduced. The whole joint isn’t scarfed, just the portion within 3″ or so of the spar.

Here’s one scarf area marked out on a skin, with some duct tape in place to protect the places I didn’t want to taper.

Preparing to scarf a wing skin

Some buiders use sandpaper or files to taper the skins. As you can tell from the picture above, I used scotchbrite discs in my die grinder. By varying the grade of disc and grinder speed, I was able to control the amount of material removed. I worked outward from the duct tape, increasing the grinder speed, pressure and dwell time as I approached the corner.

Here’s one finished taper. After the sanding was done, I cleaned and alodined the area using a Touch-n-Prep pen. Worked great.

Preparing to scarf a wing skin

Here’s one finished scarf joint – if you didn’t already ‘get the flick’ on how this works, you will now. This is the joint where the inboard and outboard skins overlap on the left wing’s upper side. Remember, just the forward 3″ or so of the joint is ‘scarfed’.

Preparing to scarf a wing skin

This was a time-intensive process…tapering eight skins for upper and lower scarf joints on both wings. But in the end, I think the results were worth the extra effort.

Over the last week I deburred the primary structure (front/rear spars and ribs) on both wings. Important stuff, but boring…so no pictures.

The only thing worthy of note is the process I used to re-torque the fiber-lock nuts that hold both tiedown plates to the spars – one of the steps in getting the wings ready for leading edge and skin riveting. Until recently, I think most builders just used a torque wrench to snug those nuts in place without regard for the drag exerted by the self-locking feature of the nut. Overcoming that extra drag “uses up” some of the force applied by the wrench, so it’s important to add that extra ‘drag torque’ to the torque specified for the nut. That ensures that the nut itself gets enough torque.

So how do you figure out what that drag torque is? It’s easy…just set your torque wrench to the lowest possible value (1-2 in-lbs on mine) and see if the wrench will turn the nut. If it doesn’t, add torque in 1 in-lb increments until the nut moves…that’s the drag torque. For the batch of AN365 fiber-lock nuts I was using, that worked out to be 4-5 in-lbs – but your settings may vary. Adding that 4-5 in-lbs to the normal 20-25 in-lbs for the nut gives the proper torque setting. Easy!

With the leading edge prep work done, I clecoed them together. Here’s the left leading edge.

Left LE clecoed together

The next day, Captain John and I riveted them.

LEs riveted

John and I have a pretty good riveting rhythm going…I shoot and he bucks. Using my Sioux 2x gun with a straight mushroom set (yep, we’re going bareback here – no swivel set or rubber guard) and the tungsten bucking bar I bought from Bucking Bars by Webster, we got really good results.

After cutting landing light holes in both leading edges and fitting the AoA probes in the left leading edge, it’s time to debur, dimple prep and prime the leading edge skins, landing light parts, joint plates, and a lot of other miscellaneous parts like the inboard leading edge ribs. So…after deburring and dimpling – not very visually exciting, so no picture – that means the obligatory wash/degrease/scuff/etch process.

Here are the landing light brackets and the inboard LE ribs drying after being alumiprepped.

LE parts prep

And here’s everything after priming, and after back-riveting nutplates to one of the W-423 joint plates that fasten the leading edges to the tanks.

AoA port holes

My goal is to get both leading edges ready for riveting in the next week.

I’ve been tying to finish as much of the leading edge work as possible before they go on the wings for good. That includes the Angle of Attack (AoA) system I bought from Advanced Flight Systems. The “A” kit for this system includes the pressure probes that provide static pressure above and below the wing to a processor that computes AoA. If you’ve got a hankerin’ to know how this system works, click here (MS PowerPoint required).

These pressure probes go in the outer portion of one leading edge; the locations are specific to each airplane design. Captain John had already installed an identical system in his RV, and he lent me a drill template he fabricated for the port. Here it is taped in location on the left upper leading edge.

AoA port template

Here are the holes…two for the #4 screws that hold the probe in place, and a #60 hole that actually provides static pressure to the probe.

AoA port holes

Here’s the upper probe temporarily screwed into position. The Duckworks instructions hint that the screw holes can be dimpled with a #30 die, but I couldn’t get the screws to set flush in a test piece of scrap – so I called Cleaveland and ordered a set of #4 dimple dies – worked great.

AoA port

Here’s the upper probe itself, the lower probe looks very similar. The quick drain on the bottom of this probe allows moisture to be drained from the probe. The kit comes with a ‘poker’ that’s inserted through a hole in the underside of the leading edge to push in the quick drain.

AoA probe

Small-diameter tubing connects that barb on the probe to the processor in the cockpit. I won’t install them until the leading edges have been riveted on to the wings.

Welcome back to the wings! I’m wrapping up the tanks, but in between tank sealing and riveting sessions I started to do some work on the rest of the wings. Over the last five days I worked on the leading edges, getting the W-423 joint plates deburred and dimpled, and starting work on the landing light installation. No pictures of the joint plates – pretty boring stuff.

The landing lights are more interesting, I’m putting 100w Duckworkslights in both leading edges. The Duckworks kits come with decent installation instructions, and following them I had no problems with the installation. The first step in this process is using the provided template to mark holes on the outboard ribs for the screws that will hold the landing light brackets…

Template for screw holes

…and for the hole that will be cut to let the light out.

Template for light opening

Hole marked for light opening

The next step is cutting the leading edge hole. As you might guess, there’s a slight bit of a pucker factor here because of the potential to screw up the leading egde skin. To make the hole, I pilot-drilled holes at each corner of the hole and opened them up with a Unibit. Then I used a side-cutting bit and a cutoff wheel in the Dremel to rough-cut the whole opening. The side-cutting bits don’t cut agressively so they’re a little easier to control.

Rough-cut landing light hole

I followed that up with a sanding drum on the die grinder, followed by a small scotchbrite wheel on the same tool. The end result was a nice hole. After that, I match-drilled the lens mounting brackets to the skin – sorry, no pictures.

Rough-cut landing light hole