Inner cores and shear web layup

CNC technology has come a long way in the past few years - making it possible to produce CNC parts at much lower costs. So much so, that there is a company who produces and sells all the airfoil cores of most popular composite aircraft (www.eurekacnc.com) at affordable prices.

I ordered mine through Eureka and couldn't be happier with the results. It took all the hotwire guess work away, saving me time and headaches... and the accuracy of these cores are spot on - far way better than what I would have been able to do by hand. They even drilled the 10 dowel holes that are later used to realign the leading edges to the aft cores.

The first step was to bond the inner aft cores. The plans say to jig the inner cores using 2x4's, bondo and a bunch of nails. My OCD mind would not allow this technique to be used - I needed a more elegant solution to assure the cores would be perfectly straight.

So I clamped the left core down to my worktable... and clamped a perfectly straight aluminum 'L' angle extrusion along the trailing edge of this core. 

Now all I had to do was apply micro to the mating surfaces, slide the second core into place, and clamp the second core to the aluminum extrusion. Here is a 'before shot'...

Here I am test fitting the inner cores...

I positioned my laser on one side of the table to verify straightness...

I slid a mixing stick along the trailing edge to help me see the laser as I moved it along the trailing edge...

...straight as an arrow all the way to the other end!!

Happy with the straightness, I mixed up some slurry and added it to both mating surfaces...

Notice the wax paper underneath... this will make sure excess slurry doesn't bond to the table..

I then slid the second core into place and clamped everything together. Notice how I used plastic cards to help distribute the clamping force and not damage the foam.



I then rechecked for straightness...

That is pretty darn straight!

I then wiped off excess slurry from the top surface only.. I had no way of cleaning the bottom surface and will just have to dremel it off post-cure...

Letting it cure overnight...

It was now time to cut some cloth and prep for the shear web layup. The Roncz plans say to follow the original plans for this part... page 10-3 does a good job at explaining what to cut.. watch out for CP#26 LPC#38.. the plans say to "cut 12 pcs of UNI"... it should be 6 pieces.

After the inner cores cured, I removed the clamps, flipped the cores over and as expected, had some cleaning up to do...

No worries.. with the dremel, I cleaned up the joint in less than 5 minutes... any imperfections will be filled with dry micro prior to glassing the shear web.

And why not, lets check for straightness again... looking good! This time I used a string...

Since I didn't build the crummy 2x4 jig, I needed a way to hold the foam cores upright so I could glass the shear web.

To do this, I purchased a 10-foot 1.25" dia steel pipe (perfectly straight)... and clamped it to the trailing edge in 3 locations (the center and at both ends)... all propped onto 2x4's to keep the clamps from hitting the table.

Notice I used mixing sticks to help distribute the clamping force and not damage the foam...

Now that I had the core straight as an arrow jigged upright on my bench, it was time to remove foam material to recess the aluminum inserts where the lift tabs later get bolted onto. I measured the distance - 6.5" away from centerline...

Centered the aluminum insert and outlined it with a marker. I should note that I purchased my inserts and lift tabs from the 'cozy girls'...

I then used a dremel to remove ~1/8"...

I'm simply test fitting the tab to make sure I removed enough material (note that the insert is upside-down)...

Using the dremel, I then removed more material to allow the K1000-4's enough room...

Presto! Perfectly flush!

I then repeated for the other one...

I should note that I used the ol' cling wrap trick to keep epoxy from making its way down into the threads.. I scrunched up some saran wrap and fed it through...

I then used a knife to trim them flush...

Ok, with everything prepped, it was time to get started! I should note here that I skipped over the step of building a drill guide for the inserts. Instead, I created a template which I will later use to place over the centerline of the shear web and mark where I need to drill.

First, I poured micro into the pockets...

And placed the inserts inside..

I then cleaned up the excess micro and filled any voids in the foam with dry micro...

I then slurried the rest of the foam... notice the tape running span-wise to protect slurry runoff...

I then proceeded to apply the first ply of UNI at 45... with edges butted... and then the second ply of UNI at opposing 45...

After the two full-span UNI plies were wetted out, I proceeded with the two plies that span BL +/- 30

The 5th ply is BID at 45 bias from BL +/-20... followed by the 6th ply of BID from +/-10..

With the shear web complete, I now had to do the 9 plies of BID at 45 over the aluminum inserts. Since I was going to preg them, I drew a box under the saran wrap as a guide to make sure I wasn't deforming the small rectangles...

After wetting out the 9 plies, I positioned them over the inserts... and added dry micro around the edges as stated in the plans.

I then placed the BID 'cover' over the 9 plies. I should note here that I had to cut a larger rectangle (larger than the 4"x8") than the plans indicated in order to cover the 9 plies. Reason was because the 9 plies stretched out a bit.. making them longer than 'per plans'... so I had to cut a wider BID cover to account for this extra length.

I then repeated for the second aluminum insert...

After completing the layups, I trimmed the skirts using the ol' card and knife trick.

Here is an example of the card and knife trick... note that this picture was taken from the center section spar shear web layup....

With the layup complete... it was time to peel ply the whole thing...

I did a pretty poor job at peel plying... it was difficult to get a full sheet of peel ply onto the shear web. If I were re-doing this, I would use multiple strips of 3" wide peel ply tape.

But other than that, I was pretty happy with the way this layup turned out!