Big Hole, Small Wires

After riveting the main wing skins next up was to install several oddly-shaped skins and ribs that make up the wing tip.  The top ones went on and then it was time to turn the wing over (again, with assistance from neighbor Ken) and close it out with the bottom wing tip skin.

I'm also installing nav lights for night flying, so I needed to cut a hole in the bottom wing tip skin.  Since the wing-tip lights and landing light installation instructions were originally intended to be retrofitted onto a completed wing, Vans instructs builders to cut a fist-sized hole in the bottom skin to order to get a hand in there and fish through wires needed for the lights.  But cutting odd shapes like the one on the provided template into sheet metal ain't that easy.  Furthermore, the big hole is only needed if you are retrofitting the lights and need to get your arm in there to pull wires through.  If you're not doing a retrofit, and you haven't yet closed out (riveted on) the bottom tip skin, then a small drill hole, say 3/8", to pass lighting wires should be sufficient.  In my opinion Vans should have provided a small hole option for builders who are installing lights as they build and finish out the wing.  Alas they did not, and are not likely going to. Since deviation from the plans is not allowed under the ELSA airplane category, I'm stuck cutting a fairly large odd-shaped hole in the wingtip bottom skin whose purpose will be to simply pass three small wires to the wingtip strobe/nav light.

On the Vans Air Force RV-12 forum (thread link here), I posed the question on whether a small hole could be used instead of the big hole.  The answer I got from the unofficial factory rep who monitors forum postings was this:

The answer is really pretty simple... would the hole match what the plans say to do?  If not, then it is not per plans.  So it is obvious, the answer is no.

Would an inspector be able to see it wasn't?  Probably not.  Does that make it right?  You will have to decide that one for your self....

And that's the final word on that.  Scott (aka rvbuilder2002 on the forum) is right of course, so big hole it is.  Sigh.  But really, IMHO Vans should have provided optional/alternative plan instructions for a small hole.  Feel free to comment below if you disagree.  I'll get the flame suit on.  Who knows, maybe it'll help out the next builder who reads this.  Or help me feel better about cutting a 14 square inch hole just to pass three small wires.

    Now on to the pictures.  This is a pic with the top wing tip skins and tip rib riveted on.

     Here is the wing turned upside down (inverted) again.

    Here's the wing tip bottom skin with the hole that I wanted to use.

And here is the bottom wing tip skin with the Vans-supplied template sized hole.  I used a unibit, metal snips, drill with 2-inch sanding drum attachment, and of course the deburring tool.  Helps that the radii of the cutout corners are about 1 inch (same as the sanding drum).  I ended up spending way too much time sanding down to the marked line so for the left wing I'm definitely using the sheet metal nibbler that worked so well on the landing light hole.

The tabs at the top need to be bent down at specific angles in order to match the other tip skins.  A bending tool fabricated from plywood is used to make the bends.  Most of the bends are 30 degrees, some are more and some are less. 30 of the tabs are bent at 30°(nice symmetry eh?), 9 are 31°, 4 are 17°, 3 are 32°, 2 are 28°, and one bend each at 16°, 19°, 20°, 21°, 22°, 24°, 25°, 35°, 41°, 51°, 82°, and 116°, as we shall see a little further on.

Click to enlarge

Personally I think Van's draftsman was having too much fun with this plans page. One degree of bend angle on a half-inch tab is less than 1/100th of an inch -- barely measurable. If the bend is within +/- 5° it'll work. Some builders have some pretty fancy angle-measuring tools but I used my daughter's $2 protractor to mark and cut a bunch of business cards for checking the bend angles.

Bending, checking, and sometimes re-bending all those skin tabs was tedious and I kept putting it off but eventually I got 'em all done.  The result is that the bottom wing tip skin fit like a glove.  I wasn't sure about the location for the bend line for the last tab on the trailing end (and also remember reading something about it on the VAF forum), so I trial-fit the skin onto the wing to visualize how it should go before actually bending it with the tool.  I'm glad I did because the desired bend line is not where I initially thought it'd be.

Above right is a picture of the final aft-most tab without the bend, and the picture to the left shows the bend in the tab.

Finally it was time to cleco on the bottom tip skin for an hour-long session with the pop-rivet squeezer.  Can you tell which tab got the 116° bend?

Below is a pic with the bottom tip skin fully riveted on:

Three rivet holes are left open where a wing tip fairing for the nav-strobe light will be fastened.  The fiberglass fairing will also cover up the access hole.   The open rivet holes are marked No, No, & No, which is what I say to people who ask if the plane is almost done yet.

I saved the leftover rivet mandrels beginning from where I started riveting the first wing skin and then counted 'em up.  It takes 2,128 pop-rivets to skin a wing on the RV-12.