Chapter 21 - Section 5

Top Skin Cores


Pour Foam

The area between baffle TTE and the center section spar must be filled with pour foam. Since the pour foam mixture ratio is 1:1, Susann poured out a 3 oz cup of each part and then poured them into a 12 oz cup. I stirred the mixture like cream in coffee. Once it started to expand, I poured the entire cup of pour foam into the cavity. Meanwhile, Susann readied the next cup for me to mix. The cavity got filled quickly, I only needed 1/2 of the second batch. Here's how the pour foam grows out of control...








I allowed the foam to settle down for ~45  minutes before I started cutting it down and sanding it flush to the ribs and center section spar. I have a few small gaps and voids but decided not to add any more of this crazy foam in them.


The entire process was quite easy and fun!








Preparing The Inner Surface of Top Skin

There has been much discussion regarding the most effective epoxy coating to protect our fuel tanks from degradation - caused by auto fuel, in particular. Though I have no plans for an auto engine, it was recommended (per Cozy Forum) that E-Z Poxy is a good protective coating. Following the good advice of many senior builders and fliers, I proceeded to coat my fuel tank with E-Z Poxy (using 87 hardener). I cleaned off the inner surfaces of the top skin, laid it down on a flat surface and brushed on 3 layers of E-Z Poxy, except the T-hat contact areas. Recall I already had some experience with the this E-Z Poxy on fuel tank in Section 3. 


I experienced an unexpected result with this 3 layer E-Z Poxy. Craters and pin holes started forming on the surface even with thick layers of the epoxy. Here's a closer look at the pin holes / craters formed. They were random and were difficult to predict. I posted this unusual phenomenon on the Cozy site and received many responses. I was, in a way, relieved to learn that many builders before me had similar experiences. (You can probably find many of these correspondence in the Cozy forum archives).


From all the responses, I concluded that this may be a surface tension issue though it looks like surface contamination to me. I decided to change my procedure a bit and re-do the surface. Here's what I did:

- I lay down a piece of plastic on the bottom surface of the top skin;

- Traced and cut out the plastic to the exact size and shape to the area of intended coverage (i.e. avoid the t-hat area);

- Sanded the previously coated E-Z Poxy off the surface;

- Vacuumed the surface completely and carefully;

- Warmed up the resin and hardener to about 100 degrees F;

- Spread a thick layer of E-Z Poxy onto the surface with a brush;

- Laid down the plastic over each area;

- Squeegeed out all air bubbles and smoothed out the epoxy under the plastic;

- Wiped off the excess epoxy that spilled out to the plastic edges;

- Let cure;

- Once cured, I removed the plastic and re-prep the surface one more time;

- Then I brushed on 2 more layers of E-Z Poxy and let cure.


Note the smooth and shinny surface without craters and pin holes...You can see the reflection of the ruler on the top of the table. Advice from the Cozy forum saved the day again! 










Access Holes

I  have pre-marked the profile of both the top and bottom skins along the fuselage. As a matter of fact, I traced the profile from the right side onto the left to make sure they ended up the same. With the profile marked, I decided to cut the access holes BEFORE I put on either the top or bottom skin.


When I installed the Feather Lite leading edge in the earlier section, I already started the forward access hole. I decided to deviate from the plans a bit and changed the shape of the access holes to give them a better flow. Following the shape of the leading edge (at the front) gives a bit more elbow room, though not as much as the 'extended strake'.









Then I cut the rear access hole to follow the seat line. It always gives me a bit of uneasiness when taking a large hole drill to any part of the airplane.









Eventually, it turned out reasonably well...











Flox Top Skin in Place

I vacuumed the fuel tank twice and wipe down the inner surfaces (including top skin) with a wet towel 3 times before I started. Since I lined my fuel tanks with E-Z Poxy, it would be prudent to use the same epoxy system for floxing the top skin to the strakes. One of characteristics with E-Z Poxy & 87 hardener is that it cures much quicker (~2 hours) than the MGS 287 with slow hardener (~4 hours) that I am used to. Making up a bunch of wet flox and piling up 1/2" of it along all the matching surfaces will take some time. In addition, I need to add new epoxy onto the inner surface of the top skin prior to assembly. I figure I need to come up with a simple procedure to get a uniform bead of flox - quick! Here's what I did...     


I wrapped two straight edge strips with packing tape. In my case, I have a metal strip and a wood strip. Susann helped me to draw a 1/2" straight line on the wood strip - that's a guide for the thickness of the flox per plan. I sat the 2 strips perpendicular to each other - with a sheet of plastic underneath them (to keep the flox from getting all over the place).


First, I measured the length of the T-hat strip I plan to apply the flox to. Then I made a small batch of flox enough for that T-hat strip. I used a stir stick and slopped the wet flox along the corner of the two straight edges and smoothed it out uniformly. I was able to do that really quick after a couple of trials. You cannot see it on this picture - but I have a 1" grid drawn on my table top since I started this project. That guides me as to the length of the flox bead I need to prepare at that time.








While I was preparing the flox bead, Susann painted a layer of pure epoxy onto the top of the T-hat. Once the flox bead is ready, I picked up the metal strip while scraping up all the flox along the back vertical face (picture left).














I transferred the wet flox bead to the T-hat location. With a narrow stir stick (thank you, Starbucks!) and using a quick 'jig-saw' motion, I dropped the flox bead onto the desired location. 










 I repeated the same procedure until all contact surfaces were covered with 1/2" of flox. This method allows me to put a uniform bead of flox without much fussing and shaping the flox bead at the T-hat locations. 










After the painted top skin is carefully laid in place, I applied pressure all over the top skin to squeeze down the wet flox within. Actually, I felt the 1/2" flox (as called out per plan) is a bit too much, especially since I had a nice tight fit between the top skin and T-hats to begin with. Anyhow, picture left shows the top skin being weighed down for cure... 









Here's a picture of the top skin after the flox cured.