Spar interior layup

The inside layup of the spar is not as easy as it sounds.

First, you have to cut out all the glass, which includes the following:

-1 ply BID (@45) for the entire interior of the box, including the end bulkheads.
-1 ply BID (@45) for the interior bulkheads (both faces)
-3 plies of UNI per wing attach hard point (4 sections)
-1 ply BID (@45) to go over aluminum hard points

And ALL these layups must be done together without curing in between.

I had to study the plans very carefully to understand the proper layup schedule. One thing I didn't find very clear in the plans was how to apply the BID over the bulkhead faces. The plans say to apply the BID onto the bulkhead and then "lapped onto the top, bottom and aft faces of the spar box".

Ok, great... but how do you lap onto three 90-degree corners without disrupting the fibers.. or ending up with a scrunched up corner?

The only hint I had was the comment to "overlap where needed".

So I came up with the following origami scheme... this particular one is for the interior bulkhead, where it must lap 5" onto CS 1 and CS 2... and 1" onto CS 3.

So here are my two 5" panels that will lap onto CS 1 and CS 2... and 1" that will lap onto CS3.

I cut along the red lines...

And ended up with something like this: (please excuse my photoshop paint work; I forgot to take pictures of the intermediate steps so I am recreating the process here).

So now when I go to glass the cloth in place, I start off by placing the center onto the interior bulkhead face. I then fold along the blue lines such that panel "A" rests on CS1... panel "B" laps onto CS 2.. panel "C" lays on top of panel "A" which is on the same plane as CS 1... panel "D" laps onto CS3.. and finally panel "E" lays on top of panel "A" just like panel "C".

Simple, right? The same process is repeated for the other interior bulkhead - just remember to mirror it since the 5" strip will be on the other side. For the backside of the interior bulkheads, all overhangs were cut to 1" - the face that is on the opposite compartment of where the aluminum hard points go.

Once I had all my bulkhead glass cut, I moved on to cutting the UNI strips. Whatever UNI I had cut previously had a 45-degree cut... so I simply staggered the strips and cut per plan.
All 4" wide...

These are for the interior attach points...
10" (x2)
8" (x2)
6" (x2)

Since the outboard attach points have two hard points, they share a common UNI strip:
15" (x2)
13" (x2)
11" (x2)

 As for the first BID ply that goes on the interior of the box, I cut three 30" wide strips of BID at 45 degree.

Here are all the glass pieces...

Before I could glass, I had to make the LWA1 aluminum pucks. Page 14-7 in the plans gives us the dimensions. Note LPC# 119 (from CP #43) which says to extend the height of LWA4 and LWA5 by 0.25" (corrections shown in red below) - I only had to make the LWA1's for this layup but decided to go ahead and make all of them now. I also went ahead and sanded them dull with 100-grit sandpaper as stated in the plans.

Here is something interesting I found when ordering aluminum from Aircraft Spruce - I went to their "2024T3 square aluminum bar" page ( at the very bottom, there is a "Q&A" tab which states the following: "This material is T4. If T3 is required, must be requested at time of order." Ummmm the title says "T3"... why are you giving me T4?!

So I called them up... they checked their stock and told me "Yup, all stock is T3..." I placed the order online with a special note saying "All aluminum must be T3". About an hour later, I get a phone call saying "we only carry the 1/8" x 2" aluminum bar in T4". I told them to go ahead with the order since I didn't want to hold up everything else.

After doing a little research, the difference between 2024-T3 and 2024-T4 is that T4 is solution treated and naturally aged. I also came across this:

Funny how the same issue I'm questioning now was talked about back in 1997! Case and point, the LWA1's act like a "washer" to distribute the bearing load from the wing attach bolts - so substituting T3 for T4 is not really a big deal (for the LWA1's only). If this piece was being pulled in tension, I might reconsider. Any who - moving on...

So with all metal and cloth cut to size, it was time to do the interior layup. Remember, once you start, you've committed to doing the WHOLE interior layup in one shot!

I prepped the interior (sanded everything smooth, including all butt joints) and vacuumed it.

Do not under estimate the amount of epoxy this layup consumes! I must have used more than half a quart in all!

I first slurried all interior surfaces. Then mixed up flox to make a nice fillet on all corners. (The plans say to make the fillets out of micro - but I find flox to work so much better. I rarely ever get air bubbles with flox.) I used the "baker's icing" bag trick to get a nice fillet all around.

I made sure to fillet the end bulkheads as well. Here you can see where the outboard LWA1's go. The plans say 1" from the end bulkhead but is not clear on whether it is referring to the inside face or outside face. LPC#28 (on CP#25) clears this up for us which states to measure 0.75" from the inside face of the end bulkhead (1" from the outside face).

And in goes the first ply of BID at 45 degree bias.

I used a brush to 'tuck' the fiberglass into the corners and then used a squeegee card to wet out the glass. Again, do not under estimate the amount of work this takes! I must have spent a good 3 hours to finish the 1st ply! The real pain were the end bulkheads - since you have to trim the corners so the flaps do not scrunch up.

After the entire interior received its ply of BID, I installed the interior bulkheads. Notice the nails on the top corners holding in position. I had made the markings of where to place these bulkheads in the previous blog post.

I had plenty of slurry left over from when I prepped the foam earlier - so I added more micro to turn into dry-micro and made a fillet around the bulkhead (looks pretty terrible here.. but I cleaned it up before installing the cloth).

Since I was trying to hurry so that the layup would not get too tacky, I didn't take many pictures in between the next few steps.

Here is a shot of the interior bulkhead (inboard face). You can see I installed the 1 ply BID at 45 degree bias which then folds and laps onto CS2 (right side of the picture) and onto CS1 (the bottom) - 5" each. You can also see the 1" flap lapping onto CS3 (left side of the picture). Again, I used a brush to tuck the corners into position and used a squeegee to wet everything out. I then prepregged the 3 strips of UNI and bonded those plies into position.

Here's a picture showing the outboard face of the interior bulkhead. Notice all flaps lapping onto CS1, CS2 and CS3 are at least 1" wide.

I don't have a picture of it, but I also added 1-ply BID to the end bulkheads. These plies had 5" flaps all around.

Now that all bulkheads were covered with their respective BID and with the UNI plies installed in their positions, I applied wet flox to an LWA1 puck and installed it in place - making sure the radius edge was facing the corner between CS1 and CS2. I then applied a bead of flox all around the puck and installed the 1-ply BID cover over it. (Again, I apologize for the lack of pictures - everything was happening so quick!)

The next day after everything had cured, I used a knife blade to trim all the overhang.

Here is a good shot showing the exterior attach points. Notice the 5" ply BID at the very bottom (lapping from the end bulkhead onto CS1, CS2 and CS3). You can also see the 3 plies of UNI that go over that, followed by the aluminum pucks with floxed fillets all around... and lastly, the 1 ply BID that goes over everything.

Here you can see the outboard face of the interior bulkhead. Notice the 1" flaps all around and how the bottom of the side flaps lap onto CS1...

Inboard face of the interior bulkhead.

Next step will be to glass the interior faces of CS4 and bond them to the spar to complete the box!


Capt Meatballs said...

Wow, that was a ton of work my friend. Nicely done! I'm sure glad you went first now.

Ciao, Marco.

Unknown said...

So are you ready to make my spar too?

Ary said...

Oh yes, of course.. let me get right on that :p

Alex Sabbatini said...

Amazing work on the center spar Ary!

Alex Sabbatini said...

Amazing work on the center spar Ary!