It’s time to dimple the skins. Dimpling a relatively stiff part like a leading edge is easy to do with minimal help, but the wing skins are large and floppy. Even with Ellen’s help, some additional support was needed. So I dusted off the support table plans that came with the DRDT-2 and made a runto Home Depot for some particle board and carpet remnants. Throw in some wood glue, wood screws and a little elbow grease, and voila – a dimpling stand.

Spar-to-skin countersinks

With an hour’s help from my awesome, wonderful wife, the skins were dimpled. Following that, I whipped out the squeezer and dimpled the wing inspection plates and corresponding screw holes in the wing skins.

I’ve been avoiding countersinking the skin attach holes on the spars, but I couldn’t put it off any longer – I hate drilling/cutting those things. I reviewed Vans’ instructions on countersinking and found out – once again – that their instructions were right. They specify that countersinks which accept dimpled skins should be deep enough so that a test AN426 rivet should sit flush to 0.005″ below the countersunk surface. So, I tweaked up my countersink and did some checking with the depth measurement pin on my calipers just to make sure that everying was set correctly – then fired away. Here’s the beautiful finished product…

Spar-to-skin countersinks

…and here they are primed. I really don’t like to prime in the shop, but in this case I had no choice. I cranked down the air pressure and flow volume on my HVLP detail gun and had no problems.

Spar-to-skin countersinks primed

Got the wing skin edges and holes deburred this weekend. Four skins per wing times two wings = a lot of deburring. The edges were easy; the holes took a lot of time. I used Ellen’s cordless screwdriver and threaded deburring bit adapter that I fabricated last year.

Not much going on with the wing structure in the last three weeks – my time has been spent working on the tanks. All I accomplished was re-checking the jigs to make sure there’s no twist orsag as I get ready to put the skins on for good. I also started deburring wing skins. Boy, that’s a lot of fun.

Yet again, more stuff that I won’t waste your bandwidth with boring pictures.

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!