HS Spar Assembly

It’s assembly time! I began with the rear spar. I found that all of these rivets could be squeezed using my Main Squeeze. The larger -4 rivets do take some effort, but this squeezer really helps compared with the cheaper traditional design.

I found that some of the rivets called-out are a bit longer than desired (based on the 1.5 x diameter rule-of-thumb) by about 1/2 a rivet length. No such half sizes were provided with the kit though, so I made due. The risk is them bending over and not setting properly, which I experienced with several rivets between both the front and rear spars. On the rear spar I had just one decisively bad rivet that clinched (bent over) and cracked.

I replaced the bad rivet by first center-punching the manufactured head, then drilled using a #40 bit just a bit deeper than the spar surface. This is to ensure I’m concentric with the rivet and gives me some breathing room. I then used a #30 bit to drill just shy of the spar. I drilled only a little at a time, checking often to see if the hole was deep enough for my #40 punch to grip the inside edge of the hole to break off the rivet head. I didn’t want to go any deeper than necessary. Having drilled many practice ones out, I found that as long as I repeated this process, I remained clear of drilling into the part itself. Once the head was off, I drilled a bit more through the center of the rivet with a #40 bit to provide some relief to the rivet (so that it could slightly “collapse” on itself to allow removal) then hit it with the #40 punch and hammer, using a piece of wood with a hole in it as a support backing. Worked great!

The crack in itself would actually have been acceptable according to MIL-R-47196A, which discusses defects that would require rivet replacement, states:

k. Cracks in driven heads (1) which fall within a circle which is concentric with and 1.10 times the nominal shank diameter, or (2) intersecting cracks, or (3) five or more cracks (see Figure 1L).

MIL-R-47196A 4.2.1.k

However, since it’s clinched and you can see the original hole edge on one side of the rivet, it clearly isn’t set right. You can see in the photo with the shop head removed how asymmetrical it is. Yikes! I’m glad I replaced it.

The last step in the rear spar assembly was to bolt and torque the HS-411 bearing assembly to the spar. Section 5 of Van’s building plans gives proper torque settings for these AN3 nuts of 20-25 inch-pounds. HOWEVER, what it doesn’t explicitly give you is the friction drag torque that you must add-on to the given torque value to account for the resistance of the nylon insert in the nut. This additional torque must be determined by measuring it. Section 5 describes how to do that. I found it to be about 7 in-lbs. So, shooting for mid-torque of about 23 in-lbs, I added 7 in-lbs to get the torque value I’d set my calibrated torque wrench to of 30 in-lbs. This worked well. I will add some torque seal to that once it arrives (on order) as a visual indicator should the nuts ever loosen. Rear spar complete!

Front Spar

Like with the rear spar, many of the rivet sizes were on the long side, but doable. I utilized the rivet squeezer for all but a few rivets. Starting with the HS-702, HS-00001, HS-710 and HS-714 parts, I Clecoed and riveted these. Sadly, there were 3 rivets that didn’t set so happily. Can you spot the ones with the X’s in the second photo? The blue tape is just to keep me from riveting the holes that will attached the ribs.

Here’s a closer look at the misfit rivets. Zoomed-in, you can clearly see a portion of the original hole exposed. These all clinched over unfortunately. On the bright side, more rivet drilling practice! The bottom photo shows these three plus the one I drilled out in the rear spar. Amazing how just a little clinching makes these completely lopsided.

The HS-00005 and HS-00006 ribs were next to be riveted. It is advantageous to do the -00005’s first since they have two rivets that are independent of the smaller -00006 ribs, so it’s easier to do those first. It is tricky, but these can be set with the squeezer, at least with the Main Squeeze. In hind-sight, I should have just shot these with the gun, but I was being shy as I had yet needed to use it for the actual plane. Anyway, there is just barely enough room to get the squeezer in there, so little in fact I was doubting it could be done. Then, I found the sweet spot:

Just barely! On the opposite -00005 rib, it was an even tighter fit. I realized that depending on how inboard/outboard that rivet hole was drilled makes a difference.

Onto the -00006 ribs, I attempted to set the first rivet. However, it was starting to get ugly and I bailed half-squeeze. Time to break-out the power tools!

Why didn’t I do this sooner?! I was able to get the first one set to where I believe it’s a “safe” rivet, but not the prettiest. There may be a smiley in the rounded head. Maybe two (Van’s Section 5 has a picture of good/bad rivets. This is referred to as a “creased head…O.K. but looks bad”). The remaining 3 rivets came out beautifully. Front spar, done!

Just a note on quality control. All rivets were checked with both the standard rivet gauge as will as my digital calipers. Since many of the rivet lengths were longer than the 1.5D “ideal”, the shop heads were respectively a bit long as well. However, virtually all achieved the “ideal” diameter of 1.5D and all well above minimum diameter and height for a -4 rivet.

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