Spar foam panels

The spar box is made up of 12 foam panels (plus two interior rectangular bulkheads and two end bulkheads). Page 14-7 in the plans gives us the dimensions of each panel.

I should note here that I substituted all 1" urethane foam with 1" H45 Divinycell foam. As I mentioned in the overview, I don't like to use urethane foam - the stuff just falls apart in my hand!





Taking a look at Page 2-3 (the foam layouts page), I noticed that I could fit all CS2 and CS3 panels into a single sheet of H45 as follows (Aircraft Spruce only carries 1" H45 sheets at 32" x 48"). This meant I had to split my CS3 (right side only) into two halves - which is not a big deal.

 I wasn't worried about cutting the above panels to exactly 6" as the plans indicate - these panels later get trimmed down flush to the jig's top edge - so I divided 32" (the height of the sheet) by 5 (number of panels) and got 6.4"... which gave me a larger buffer. However, I later did cut the panels to exact length as indicated in the plans.

I drew all the dimensions onto the H45 - 1" foam sheet as per my drawing above and cut everything with my bench saw.

The H100 foam sheets come in 42.1" x 42.5"... as much as I wanted to fit all H100 foam panels into 1 sheet - I couldn't. Unfortunately, I had to purchase two of these sheets and had a lot left over.

The first H100 sheet I cut as follows. Notice that rather than splitting the CS1 center panel at the edge (as indicated on Page 2-3), I made it so they would join at the center line. Recall that CS1 center panel is 46" long... the sheet is only 42.5" long... so either way, I was going to have to join two sections. Joining them in the center made it a lot easier.



Since this sheet is only 0.25" thick, I used a blade knife to make all cuts. Look close and notice I labeled each 90-degree square corner accordingly. This made it very easy to identify its orientation when bonding them in the jig later on. (Not shown here, I then used the second H100 sheet to cut my CS4 center A/B panels and my interior bulkheads: CS6 and CS7.

Before I could bond the foam panels together, I had to fine tune a few things. First, I test fitted CS1 left panel with the center panel. Since this is where the spar sweeps (BL23), it makes a subtle angle change.

In order to have the faces flush, I had to bevel the CS1 left panel face. Using a metal file, I held the panel in place and filed away.

There we go... much cleaner mating surface! (I then repeated this for the right side.)

The filing of the foam removed very little material and made no noticeable difference to the end. I still had a 0.25" lip for the end bulkhead to attach later on.

With the 0.25" placement of the end in place, I drew the line of where to trim CS3.

Taaaa-daaaa! Perfectly flush with the 23" butt-line from center line. I then repeated this for the right side. And the whole process over again for CS2.

To keep the panels from sliding, I centered CS1 into position, clamped it down, and installed 3 nails to hold it in place.

At this point, I'm only test fitting everything...

With the center panels nailed down, I could now push the side panels up towards the center...

And butt up against it like this...

Same for the other side...

0.25" lip... perfect!

Yup.. looking good!

I then screwed on 2x1's onto the jig to give the 90 degree angle to line up the CS3 panels..

Oooooooooo.. the spar is starting to take shape!

Remember I said not to worry about cutting the CS2 and CS3 panels to exactly 6"... That's because they get flush trimmed to the jig's "D" panel backboard. Here I am showing the excess foam..

Normally I would use my router to flush trim things... but I couldn't here since the router is too bulky and wouldn't fit - having to go on the inside of the spar box. So I had to switch to my dremel. Notice the tip of the bit is solid...



The solid tip of the bit rides along the edge of panel "D".. and the rest of the bit trims the foam...

Slow and steady, I was amazed how well this worked!

I clamped the foam panels onto position and trimmed along the entire length. I then moved the clamps to a new position and trimmed those portions. Notice I used squeegee cards to protect the foam from the clamps.

I then removed CS2 from the jig.. replaced it with CS3 panels and trimmed those flush with panel D as well.

Now to make the 0.7" (for CS2) and 0.4" (for CS3) thick ends. Using my digital calipers, I set the correct depth on my router...

A LOT of work went into trimming all the panels to correct size and shape. So before routing the ends, I used a scrap piece of foam to test my router setting... 0.70" - per-fect-o!

Happy with my test trial, I brought a CS2 end panel over to my temporary side workbench and clamped it down. I measured 3.5" inboard (per plans) and drew my reference line.

Using my router as a mill... I milled away...

Not bad!

Notice the use of shims so the clamps would not damage the foam panel...

Now that I milled the 0.7" and 0.4" ends... I had to give them a nice transition... so I doodled a nice guideline to follow...

So I brought the panel back to my bench.. clamped it.. and brought out the big guns.. (a bit over kill, but hey.. clock is ticking!)

I started out by running the belt sander perpendicular to the panel to make a 45-degree bevel...

Then switched it parallel... using the belt sander's front lip to round the ramp...

Finished it off by hand...

Daaaaaaamn!! Not bad.. not bad at all!

Then the end bulkheads... CS8 and CS5... per plans.. 6.83" x 6.30"...

With all foam panel pieces trimmed and ready... I lined all corners of the jig with tape so the epoxy wouldn't stick to the jig...

I placed the left CS1 center panel on the jig... re-inserted the nails to hold it in place. I then mixed up slurry... applied it to the edge of the right CS1 center piece... and mated the two panels together. I then inserted the nails to hold CS1 right panel in place.

Showing the CS1 center panels mated... right on center line of the jig.

I then applied slurry to the CS1 left panel...

Aligned it along panel "D" back panel...

And slid it up to mate with the center CS1 panel.

I then inserted the 3 nails to hold it in position and wiped the joint clean.

I then repeated that for CS1 right panel.

Now to bond CS2. I applied slurry to the center panel...

And bonded CS2 center panel into position. I held off on inserting nails to it for now so that I could fine tune its position after the left and right panels were installed.

I then slurried CS2 left panel...

Placed it into position and fine tuned the center panel so that their mating edges were perfectly flush. Notice the tape holding the panel back up against the jig's backboard.

I then clamped the joint so that it would cure perfectly flush. Notice I used cling wrap on the squeegee cards to keep the cards and clamps from bonding to the foam.

I then repeated that for the right side. So now I've got the CS1 and CS2 panels bonded in place.

I then removed the 2x1 pucks and replaced them with these shims - believe it or not, these were a lot more sturdy.

Slurried CS3 center piece...

Then CS3 left...

Clamped the mating joint...

I then scooted CS3 center panel so that it was flush with the jig's lip (lip of panel "A").. and inserted nailed to hold it in place...

CS1, CS2 and CS3 complete.

Here's a close up of CS3 center panel.. showing it is exactly on center line with the jig and flush with panel "A".

Now to bond the end bulkheads. Simply slurried the edges.. and placed it into position. I then inserted 6 nails (2 per edge) to hold it in place.

Repeated it for the other end bulkhead...

Since the ends of CS3 kink inwards toward CS2, it causes the center piece of CS3 to want to lean forward a bit. So I used a scrap piece of foam to hold CS3 away from CS2 - and up against the shims.

While I was at it, I decided to bond my CS4 center pieces together. Nothing new here.. mixed up some slurry and butted the two halves together. Used a vice to hold one piece in place and clamped the other piece to the table.

Used a straight edge to make sure the back edge was properly aligned..

 

And then placed a scrap piece of melamine board with a weight on top to assure both pieces were flush with each other.
 


After everything cured, I marked the locations of the interior bulkheads. To do this, I drew a line 4" outboard of where the CS1 center panel mates with CS1 left panel. Since the CS1 center / CS1 left panel joint is located at BL 23, this means that BL 27 is located 4" outboard of this joint - OR you could just measure 27" from center line.

Any who, I then drew a perpendicular line up against CS2 and CS3  to show the interior bulkhead plane. I also drew out LWA1's puck position. Plans say the interior attach points are centered about BL 25... so that is 2" outboard of the BL23 joint.

Applying LPC #28 (from CP #25), I measured 0.75" from the inside face of the end bulkhead and drew out where the outboard LWA1's needed to go.

I then cut out the interior bulkheads and trimmed them down to final size.

The spar box is now ready to receive its inside layup - step #4 of chapter 14!