Chapter 14 - Section 3

Interior Layups

 

Fabricate LWA Aluminum Pieces

I cut up all the required LWA aluminum pieces per Page 14-7 with my band saw. Then I rounded off the edges with my bench belt sander. Since I was not able to cut the thicker aluminum pieces with high precision (with my band saw), I cut them to within .05" (a bit jagged), then took them down to exact dimension (along a straight line) with my bench belt sander. I then dulled the surfaces with 220 grit sand paper and wiped off the surfaces with acetone.

 

 

 

 

 

 

 

Applying Lay Up 2 - First Trial...

This step is to apply one layer of BID on the entire interior surface of the center section spar. It sounded pretty simple and the plan recommends to do the entire lay-up in one session. However, from the web sites of many builders, it took them 6 to 7 hours to lay-up ~half of the center spar. I figured I'd better plan it all out before I start. It took a few hours to cut up all the glass to the appropriate dimensions, roll them all up and set aside. I also cut out all the necessary 4 mil plastic to pre-wet the BID and UNI. I marked out the mid-point (a center line along the spar) and locations for all the 6 LWA1s for subsequent operations. 

 

It is important to note that the location of the LWA's are based on butt line (e.g. BL33) and not along the slope of the fixture - though the difference may be small. Here's how I established the LWA positions. I clamped a straight edge along the top edge of the fixture, then I placed a laser cross hair at the designated distance from center (i.e. BL0). The laser drops a horizontal line straight down onto CS1. I marked the positions with a pen for future reference. Just to make sure my laser is leveled, I placed a small bubble level for verification.  

 

 

Recognizing the following step is time consuming, Susann offered to give me a hand - twisting my arm here ... While Susann finished microing the interior surfaces of the spar foam with micro slurry, I formed all the fillets with drier micro. Since there were 2 of us, we decided to do the entire lay-up in one session per plan (that was our first mistake). I pre-wet all four 36"x18" BID strips and sandwiched them in 4 mil plastic (that was the second mistake). After removing the 4 mil plastic from one side of the BID strip and lowered it onto aft face (CS1) of the spar, the fillets start putting up a fight. We were having a tough time smoothing out the BID at the bottom surface, shaping the BID to the corners AND at the same time, keeping the vertical sides of BID making contact with CS2 & 3. Once the BID made contact with the sides, we were not able to 'pull' the BID down to hug the fillets. We removed the remaining 4 mil plastic and tried to smooth it out with a brush and epoxy with no success. We peeled the BID back away from the sides, as expected, the micro came along for the ride and the BID stretched out of shape - what a mess! To make a long story short, we eventually had to pull the BID out and discard the 36"x18" strip. 

 

Since we microed the entire inner surface of the spar, we needed to move on before the micro cured. We repeated the above process and ended up tossing out our second 36"x18" strip - evidently we didn't learn the lesson the first time. Since we were at it for 8 hours already, we knew the battle was lost. I decided to repair the micro on CS2 & 3 surfaces before they completely cured. We ended up tossing the remaining 36"x18" strips as well. We have to do Lay Up 2 the next day .

 

Applying Lay Up 2 - Second Trial...

After thinking over the above problem overnight, we took a slightly different approach. By now, the micro at the inside of the spar has cured. Therefore, the first thing I did was to rough up the micro throughout and vacuumed. Then I wet out the 36"x18" BID with 4 mil plastic on one side and Saran Wrap on the other side.

 

We also changed the lay up sequence a bit... Starting from one end, we first painted a very slight layer of epoxy on the bottom side of the spar (CS1), then I removed the 4 mil plastic (leaving the Saran Wrap layer with the BID) from one of the end BIDs. Then we lowered the center of our BID onto CS1 surface. Since the sides of the spar (i.e. CS2 & 3) were dry, the BID would not 'adhere' to the sides, making it much easier to pull and tug as necessary. We also used thumb tacks to hold the BID sides up (loosely) against CS2 & 3 foam to keep them out of the way. Then we smoothed out the bottom (CS1) all the way up the fillet - stretching the glass and the widened ends as necessary. Then we removed the thumb tacks on one side, painted a thin layer of epoxy onto the side wall (i.e. CS2) and then smoothed out the BID against that side. We repeated the same on CS3 side. This time we worked half of the spar, then we took a lunch break.

 

 

 

 

 

 

 

 

 

After lunch, we repeated the layups on the remaining spar. The entire spar took us about 9 hours, but we completed the whole task in one day. I used a few clamps, small flat boards and plastic sheets to keep the foam against part D during cure. Here's a picture of one of the in-board LWAs after assembly. As always, I used the flox smoothing method and it turned out well. I picked up this technique from someone's web site a couple years back and I have also discussed it in my Methods page.

 

 

 

 

 

 

 

Here's a picture of the the completed layup for the inside - after the glass was trimmed and center holes drilled. Though it is not in the plans, it was suggested in Marc Zietlin's (FAQ) and Rick Maddy's site, that a hole can be drilled to accommodate future wirings etc. Therefore, I drilled a center hole (1" diameter) through the 3 inside bulkheads.