Tag Archives: countersinking

The spar is finally done….

The work on the spar really dragged on.  Section 13 is another one of those nasty little sections that just drag on and on…  It is only 3 pages of instructions, but oh, there are so many holes!

I had a long break in the work as I was off in Houston helping my brother out.  He was starting a new job (actually returning to an old one after a few years of trying a startup). He needed some help watching his family while he went to Chicago (in the midst of the Polar Vortex) and followed that with a week in frosty Delaware.  The long and the short of it is that I lost 3 weeks in the aircraft factory (but earned the love of my brother).

It was pretty cold in the shop, a pretty hash 18 degrees.  My torpedo heater brought the temperature up to almost 60, but with the high roof it just kept sliding back to freezing.

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I finally got to the point where I could install the 120 some odd nut plates.  This went pretty smoothly.

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I did mess one up when I didn’t have the squeezer set when I hit the trigger.  I managed to drill it out (with a small bit of my finger).   This is a plane building right of passage, apparently, glad to get that out of the way.

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While I was doing that, I put my son to work deburring and smoothing the lightening holes in the many, many ribs.  I had a little Scotch-brite flap wheel installed on the drill press.  It made pretty quick work of the task.

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He did some nice work!

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I was still in nut plate hell, so I let him head home to warm up. I kept plugging away at the nut plates.  Eventually, you do get to the end of it.  Yay!  All the flange nut plates are in place.  The blue tape is there to (1) keep the aluminum chips out of the gap between the flange and spar web and (2) mask it off when I spot prime all the countersinks.

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With the nut plates in place, it is time to countersink the fuel tank screw holes and the inspection plate screw holes.   There’s a couple hundred of those to do as well.  Van’s provides specs for the max hole size.  The bottom of the spar went really great.  My holes were all smooth and consistent.

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A quick hit of primer, and I wrapped up for the day.

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The top countersinks did not go as well.  I chipped my countersink bit which made very rough holes.  I ordered new ones Sunday night from Brown Aviation.  They were delayed a day (FedEx wouldn’t deliver in the blizzard), I still got them Friday.  Saturday, I finished the top screw countersinks (much easier with a good, sharp bit) and spot primed the spar as well.  With that, I finally finished the first two pages (it took a full month of build sessions, sigh).  The last page covered another dozen nut plates on the spar face, 10 final AN470 4-6 rivets at the end of the spar, and the aileron bellcrank bracket.  The hardest part of the bellcrank bracket was finding all the parts.  The W823-1 brackets were hard to dig up (they are in one of the small parts bags).

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I had to mentally put myself back into part prep mode — I really hadn’t done any deburring and priming since before Christmas. It was nice to install some green parts on the spar.

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At last, Section 13 is done!

I got started on Section 14.  It begins with some part prep.  I had to replace my bandsaw blade since some of the teeth had gotten bent and were messing with the cut.  This meant a trip to the aviation department of my local Home Depot (I was breaking for lunch and ice skating lessons anyway).  I deburred and primed the parts for the whole section.  You have to be careful because some of the left and right parts are nearly indistinguishable.  There are many cautionary notes in the instructions and the blogs to mark them before separating them.  I added the markings to drop paper so that I could keep them all straight (important since the acetone wipes off all the marks).    I put them far apart on the paper (left parts on the left side, right parts on the right side naturally!).

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The parts were rough from the bandsaw cuts, so they needed some filing before moving to finish on the Scotch-brite wheel.  The aileron brackets are very hefty chunks of aluminum.  They appear to be cut with a water jet and are very rough.  They needed some serious work in the vice with the bastard file.

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I made a lot of magic pixie dust!  They polished up very nicely though.

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That gave me a nice pile of parts to end a solid day at the shop.  I’ve got all the parts for Section 14 ready to go (including the two bearings).

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Sunday was a lot warmer than Saturday —  The mid-40’s seemed almost tropical.  It was the first day in months that I could work in the shop without running the heater.  I even left my jacket in the car!  I did have to shovel a foot and a half of snow to make a path to the shop door though 😦

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The first real step in Section 14 is to square the rib flanges.  I made a little tool from the end of a paint stir stick to check them.  Most were pretty square and didn’t need any adjustment.  My seaming pliers fixed the rest. Not hard, but there are 28 ribs to check!

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The ribs also needed fluting.  They look pretty straight, but if you lay them along a straight edge you’ll see a pronounced bow.  Three or four flutes on each side of each rib took care of that.  But with 28 ribs, that’s 150 flutes!

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Lastly, I reamed the #40 skin holes, deburred them with some Scotch-brite, and dimpled them.  I now have two piles of ribs.  After I dress the flange edges and prime, they’ll be ready for assembly.

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So many holes…


I didn’t fully appreciate how long the countersinking on the spars would take!  There are SOOOO many holes.  116 nut plate holes for mounting the fuel tanks, nut plate holes for mounting access plates, and 300 skin attach points — for EACH WING.

I first countersank the nut plate holes as called out in the plans.  You are trying to remove just enough material to get a AN426-3 rivet to sit flush.  I checked each countersink as I went along.  It felt like it was going fast, but then I turned around and saw how many holes were left to go!

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Luckily my son came out to help.

He did a great job on the skin counter sinks for the very visible left upper side.

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I made a test dimple in a scrap piece of .032 aluminum to make sure that the skins would eventually sit flush.  You can see it in the close up below.

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After finishing the top left, my son took off (can’t spend too much time with the old man in a cold garage after all!).  So I was on my own to finish the countersinking for the left bottom.  That was it for the day.  No build time on Sunday because I actually went flying instead!

With the left spar fully drilled, I hoped to move on to riveting the nut plates so I could finish Section 13.  I decided that it would be better to finish drilling and countersinking the right spar to keep the wing builds roughly in sync.

I was building on Friday (normally my “honey-do” day since I’m only working 4 days a week as a consultant) because I’m headed off to Houston to visit my brother.  I woke up early with dreams of hitting the shop at 8 and working all day.

Didn’t happen!

My wife woke up and I ended up doing a couple of small honey-do items anyway 🙂

Then I had to mail off a package and fill the minivan with gas before the trip.  The first station was closed, so I went directly to the post office instead.  A huge line for mid-morning Friday!  Then off to an alternate station to fill up (but no left turns allowed to exit).  I finally dragged myself in to the shop only to discover that I left my heat gun at home.

My shop gets really cold.  And it is “power” challenged (one shaky 15 amp circuit shared with the rental house).  And if the power blows, it takes an hour to get it back on again (breaker panel in locked basement only accessible by landlord).  So, I have to baby my compressor to get it started by preheating the block.  So, another 50 minutes was spent heading home to find it and bring it back.

Then my son called to see if I wanted to grab lunch with him between classes.  So we had a quick lunch, but at least I gained another set of hands.  I finally hit the shop around noon.

The j-channel was already set up on the right spar.  It went much faster with a second pair of hands to put the clecos in.  With that done, my son headed back to class and left me alone with my countersinking.  At least that’s all done now.  3 build sessions to take two expensive spars and to transform them into something that looks almost identical to what I started with.  Sigh.  At least next time, I can actually do some riveting (before finishing the final countersinks into the nut plate holes).


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Dis-assemble, deburr, dimple, and prime (rinse lather repeat)


Rivet count: 1819 + 0 new = 1819

I had a half day to work on the plane, so I jumped back into the elevator.  After finishing the initial assembly and match drilling, it was time to take it all apart to prep for final assembly.  I finished getting the blue plastic off the left top skin.  I finished some of the holes and polished the edges.  I also “broke” the trailing edge and the rolled leading edge so that they would sit flatter after riveting.

Then it was on to dimpling.  The DRDT-2 is really nice for this.  The table is big enough to hold most of the skin and then I can pull the lever one handed to set the dimple.2014-08-03 13.45.09

This worked fine until I got the the trailing edge.  The instructions warn that normal dimple dies will compress the slight bend.  So they ask you to grind down an old/cheap dimple to fit.  As luck would have it, I had a dimple die that was sticking, so I bought a nice new one from Cleaveland Tools.  I took the old one, cleaned up the pilot, and used a belt sander to grind down one edge.  This went into the DRDT-2.  Then I just had to watch the die orientation so that the ground down part was facing forward.  Hard to see in the photo, but the ground down part stays inside the crease.

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I also had to countersink (part of) the rear spar.  The trim tab hinge needs to lay flat under the spar flange.  So, you cleco it in place and countersink the top.  I think you cleco the hinge in so that you countersink into the hinge piece if you go too deep.  I found that even with a deep countersink (enough to hold a rivet just below the surface), the top of the hinge was unmarred.  Clecoing the hinge piece does insure that you don’t countersink more of the spar though!  The other holes are just dimpled.  The instructions indicate that you should grind out/modify one of your squeezer yokes just to get it to fit inside the small, slanted spar.  I think that was total overkill.  You could use a pop-rivet dimple die or use a different squeezer.  I used the vice-grip dimpler I used to dimple rib flanges.  It fit fine and made short work of the holes.2014-08-03 16.38.04

There are many more small parts to get here.  I finished the top skin, the root and tip rib assemblies, all of the ribs, the mounting plate for the trim motor, the sheer clips, the gussets and the rear spar.  I still have to countersink the trailing edge piece and the larger front spar.  I also haven’t even touched the bottom skin!  Sigh.

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So I probably have another day’s work to finish the left elevator and do the right one.  The right one is a little easier (one fewer sheer clip, no trim tab, no trim tab mounting bracket), so hopefully I’ll get all those done soon.  Then it’s off to the rivet races again for the 1154 rivets in the elevators and trim tab.

Horizontal stabilizer assembly (and many countersinks)

Rivet count:  779 + 32 = 811 rivets!

The family visitation is over (nephew and 3 friends plus brother-in-law and his family), so it is back to work on the plane!

With only a half day to work, I wasn’t sure I would get much done, but it was a pretty productive day.  I started by priming the horizontal stabilizer skins.  I ran a full coat over each rivet line and then a lighter coat of filler between.  The hardest part was getting the blasted blue plastic off the skins.

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The next step was countersinking the spars and the stringers.  There were a lot of countersinks to do.  426 of them!  I used a rivet to check the depth as I went.  Even with the countersink depth set, you still have to watch it.  The drill needs to be kept perpendicular and it can still be overdriven.

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With all of the countersinks done, I added a light coat of primer on the flanges.  Hopefully it will help prevent corrosion in the countersinks

So finally, assembly can begin.  The first part to go together is the stringer assembly.  This is the V shaped part that fits into the very end of the tail cone.  The instructions showed assembling it separately from the spar, but I wanted to assemble it on the spar to make sure I got the orientation correct.  The first part is a brace that goes between the modified intra spar ribs.  I put the shop heads on the outboard side… probably would have been better to keep them on the inboard side.  Next I had to cleco and rivet the stringers.  A little bit of primer got in the holes, so I had to run my 1/8″ reamer through the holes to clear them.  I was able to squeeze all these rivets.

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Next up… I start assembling the rest of the skeleton and put the whole thing into the riveting cradle to get the skin riveting.  I’ll have to shoot most of those, but I’ll finally crack the 1,000 rivet barrier 🙂