This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

Wednesday, December 14, 2022

JT’s miscellaneous updates #6 - Reversed Wing Bolts

This is gonna require a little explaining…

The Center Section spar (CS spar) is a hollow forward facing C-shaped structural member with lighter fiberglass coverings enclosing the front of the C, and both ends.


CS spar/wing junction

The wings are attached to the fuselage CS spar with three bolts, one close to the engine, and two further outboard. Each bolt is driven forward through a hole in the wing, then into a corresponding hole into the CS spar.

CS spar bolt holes

The nuts that engage with the two outer bolts must be inserted into the CS spar from a small hole below the spar. It takes at least a team of two to tighten them.

My finished CS spar with various holes

The inner nuts require someone to sit in the passenger seat and stick an arm into the CS spar up to his shoulder with a wrench, then blindly feel backward for the bolt, use the one hand to thread the nut onto the bolt, and use the wrench to hang on to the nut while someone else from outside cranks on the bolt. This is a pain in the rear, but not nearly as bad as the two outer nuts.

Tightening the CS spar inner bolt

Tightening nuts on the outer bolts is a very difficult process in my experience, and the smaller the hole the builder cut into the bottom of the CS spar, the harder it is to deal with them. The top bolt can barely be seen inside the CS spar, but it is not easily reached with a tool, while the bottom bolt can be more easily reached, but not seen at all.

Let it be said that mounting the wing even once is enough to make you never want to take it off again.

Enter the “Reverse Bolts” mod.

The main idea is to have the bolts backward from inside the spar, and mount the nuts on the outside.

How is this any better?

Well, the key to this modification is to mount a U channel inside the spar that captures the head of the bolts and prevents them from
backing out when mounting the wing, and from rotating when tightening the nuts outside. To this last point, the fit of the U channel over the bolt head must be as tight as possible.

Thick aluminum U channel fitting tight over the bolt's head


These bolts are loaded in shear (perpendicular to the bolt's shaft) so the torque on them is not very high (200 in/lb or 22N/M)

Spoiler alert… this mod is awesome!

“Alright! This is going to require surgery, so let’s get started!”

Clearly the wings need to be removed first, so Chris and I got to work on that. We have done this multiple times due JT’s various upgrades, so there’s nobody else I trust more than Chris to help me with this task on JT.

Chris helping me remove the wings

Right wing separated from the plane

After that, I took JT home because of a number of other upgrades I had in mind.

JT's going home for more comfortable working conditions

Once at home, I did get a lot more time to plan my approach, where to cut and how much, and how and whether to close the holes back up.

To reach the inner bolt holes to mount the U channels it was simply a matter of reaching into the spar from the back seat access hole, but the outer bolts are in an enclosed section, so the easiest access would be through holes on the far end caps. These caps are non structural, thin foam, with one ply on either side, and have holes for Nav lights wiring, and antenna coax cables. Some holes are bigger than others, and I know of a few builders that cut big holes there and never closed them up. Still, when I asked around, the consensus was that I should close them back up.

I picked a 4” (51mm) diameter hole saw, because I could fit my hand through it, and the rest went fast.


Right side CS-spar end-cap hole planning

Right side CS-spar end-cap hole

Wing bolts installed backward, and U channel going in for fitting.

Using a scrap aluminum piece as a drill guide. This hole goes all the way through the U channel.

A look with the U channel removed
Added a nut plate to the U channel and reinstalled

Screw with tapered washer to prevent fiberglass damage

Left side CS-spar end-cap cut, bolts inserted, and U channel being fitted.

U channel installed over the outer wing bolts

Outer wing bolts in their permanent location

Working on the right side CS-spar inner hole

Bottom hole drilled, bolt installed. Top of U channel is fitted over the original wing bolt's head.

Drilling the middle hole using the drill guide

Bolt holes must be exactly on the U channel's centerline, or they won't line up when the U channel is installed facing back inside the CS-spar.

Installing nut plates on the U channel

I used dome-head rivets since I have a ton of them, and they don't require countersinking.

U channel ready for action

Wing bolt reinstalled backward, and screws holding on to the U channel.

U channel in its final resting place

A view of the U channel inside the hollow CS-spar

Going to work on the left side CS-spar inner-hole

A gauge block on top of the U channel (with clear tape) allows the channel to hang from the wing bolt

Holes drilled

Here you can see how the U channel was hanging from the wing bolt

Nut plates installed

Left side of the mod completed. The three bolts are permanently installed.

A closeup of the finished mod

Left U channel holding on to the inner bolt (at the top)

Another look at the U channel from the passenger seat access hole

I had saved the material I had cut from the end caps so I could put it back later to close the holes, but I obviously couldn’t fiberglass from the inside of the spar, so I decided to make a flox corner style tie-in with the foam/fiberglass biscuit. I’m sure it was probably not necessary, but made me feel better, and I figured that if I ever had to cut it again, the hole saw would cut through the flox, then I could flox it back easily.

The small issue I faced now was figuring out how to hold the biscuit flush with the end caps while the flox hardened, and, at least on the right side where the hole saw cut the wiring hole in two, how to clock it at the same time.

3D printing to the rescue once again! 
This should allow me to clock the biscuit, and hold it flush.

The linear piece on the inside of the CS-spar is designed to be removed via the hole cut into the bottom of the CS_spar

Testing the concept fresh off the 3D printer
Because a flood corner requires removal of the foam and the micro slurry between the foam and the fiberglass, sanding was definitely going to be involved, and a big mess to be made inside the spar and on the shop floor.

As luck would have it, my vacuum hose fit exactly into the bottom CS spar hole, and sucked up everything that came off JT.

This worked really well



Removing the foam and sanding off the micro on the fiberglass inner surface was tedious as usual, but not difficult.


Removing foam from the right side biscuit

Same-same with the left one

Using the Dremel to remove foam

Dremel in action

Sanding the micro slurry off of the fiberglass

Then JT got prepped, and the floxing began.


One really needs to mask well, because epoxy gets everywhere.

"Alright, this is working!"

Might take a couple of applications of flox

Left biscuit getting floxed

I let the flox cure overnight, and prepped for glassing over the top.


Next day, got a few spots to rough up and fill before glassing.

Dremeled the shiny spots

Masking is mandatory

Dremeled the shiny spots on the right side as well

Masking as usual

Prepping the plastic sheet for pre-preg glassing

Painted a little pure epoxy before filling

All holes filled with flox

Painting epoxy on the left biscuit intersection

Depressions floxed


I then prepped two plies of BID for each end, and glassed them on.


BID with epoxy on top between plastic

Epoxy soaked BID

One side of plastic removed, and BID attached to the CS-spar like a Band-Aid.

Plastic removed from on top of the BID

Peel-ply added over the BID

Left side gets the same pre-preg treatment

Then the plastic cover gets peeled away revealing the BID

Peel-ply gets added over the top for the overnight curing cycle.

Next day, I removed the peel ply, sanded down the patches of BID, reopened the wiring holes, and ran the wires back through.

Peel-ply removed from the left side

Left end-cap gets sanded, and the original wiring holes reopened.

Transponder/ADSB antenna in the middle, COM, NAV, and Strobes out of the bottom hole.

Finally, I designed and 3D printed new T shaped grommets in a rubber-like material, and mounted the transponder/ADSB antenna back up.

Four custom rubber grommets

Grommets removed from the base

Grommets in place in the holes

Transponder/ADSB antenna mounted

Left side of CS-spar completed

Right end-cap without peel-ply, and sanded.
COM coax, NAV lights, and Strobe wires coming out.

Once finished with this mod, JT went back to the hangar, and Chris and I got to put the wings back on in this new configuration.

Mounting the wings with the new mod in place.

Went on pretty smoothly

"This was great!"

Mounting the wings is so much easier and quicker now, and JT has been flying in this configuration a few years already without troubles. The wings have also had to come off at least once more recently (more on that in another post), and they have been a pleasure to work with, so much so, in fact, that with proper preparation one could now even consider mounting the wings solo.

I like this mod so much that I will eventually do it on my original project as well (after the wing bolt holes are drilled). I wouldn’t remove the wings just to do this modification, but should your wing ever need to come off, I would seriously consider doing it. It’s not difficult, and takes just a few days.

Removing the wings is a pretty big deal, but it doesn’t have to be a pain. This mod goes a long way toward simplifying the process, and earns Marco’s two thumbs up of approval.