September 2006
Chapter 7 Starts with construction of the NACA Scoop. The date above starts this process in September 2006 but it actually goes on for a full year. This is mostly* due to the fact that I have built the NACA Scoop not once, but three times! (There is a cautionary tale in here somewhere!)
*I've also had an incredibly busy year at work...too much to go into on a private website, so let's just say that things are getting back to normal and I am finally spending some more time on the plane.
The "Dip"
The main problem was that I found there was a dip at the front of the scoop where the foam transitions from the Blue PVC Foam to the Urethane "Scoop" Foam. I saw no mention in the plans for this and somehow this just did not seem right to my "Engineering Brain".
"Surely it's not supposed to be this way; who would want a dip right before a NACA Scoop?"
I tried searching the Cozy List Archives (at the time I found nothing and I wasn't aware of other builders who had discovered the same thing). I also emailed a few builders, but without the picture posted above it seemed I couldn't get my question framed right (the stock reply I got was "I don't have a 'dip' in mine...")
So, either I hadn't communicated the question properly, or the builders I asked were not aware of this feature in their planes, or the builders I asked did not have this problem (and I had done something wrong).
It became a little perplexing and I started wondering about the "Bow" in the bottom that required all the extra weight to get it to seat properly (See the end of Chapter 6).
Okay, now what?
Well, I didn't want the "Dip" and I decided that even if it was a feature on "standard" Cozy's**; I would "fix it" on mine.
**I later had a discussion with Marc Z. about this over a hamburger at the Oshkosh Cozy BBQ. He then told me that "everyone has that 'Dip'" and that it was normal. I later found some references to it on other builder's websites as well. However, I had by this point mostly remedied the "Dip"...read on.
Just note: If you want to avoid "The Dip", extend the forward Urethane Foam block to the point on the fuselage bottom that avoids this (this required approximately 3 extra inches on my plane; your mileage may vary).
The "Bubble"
So, I started to sand down the foam further; my plan was to remove 1/4" of the current foam and micro in a 1/2" sheet of Urethane foam that would take care of "The Dip". When I sanded through the Pilot-Side of the scoop, I got a shock. I found a 7" "bubble" of micro that the Urethane Foam had NOT bonded to.
I concluded that the cause of the "bubble" was the failure of the Urethane Foam to conform to the curvature of the Fuselage Bottom at that point. (Either not enough weight and/or too little micro to fill the gap).
But what if there were other "bubbles" under the scoop?
I started looking at the side-views of the NACA Scoop (see Chapter 7, Fig. 2 in the plans) and came to the conclusion that I was safe. The rear-most block was only 9" (longitudinally) and the section of PVC foam it was bonded to did not have much curvature, while the forward block was 15" long and had the most curvature for that section. Since I had sanded through most of the 15" long block on both sides (that was how I discovered the "bubble" in the first place), I think the likelihood of having a "bubble" under the remaining NACA Scoop is remote.
Evidence of the "Bubble" stared showing up around 1/8" thick as an area that "moved" as I sanded it and as I continued sanding the surface, it would break off in large quarter sized chips. Since no other area of the forward foam block exhibited this behavior under sanding, I think the bonding of the rest of the foam is okay.
The "Moon Bounce" of Foam Construction
This is where I made a critical mistake. I decided to try gluing the two pieces of Urethane Foam (the original foam of the scoop and the new, extended 1/2" thick foam that was going on top) together with "Great Stuff" expanding foam. I had heard other builders on the list talk about doing this; the advantage is that it is supposed to make sanding easier (i.e. no ridges) as the "glue" itself is also Urethane Foam.
The problem I had was the amount of weight necessary to "seal" 4-square feet of foam (while the expanding foam is expanding) is enormous. I did both sides at once (another mistake), and added all the weights I had in the shop; it wasn't enough. The foam lifted up the sheets by a full inch and several gaps formed. Uggh.
I think the builders who recommend bonding Urethane to Urethane with "Great Stuff" never tried it on a piece this large.
So, I cut and sanded out all of the 1/2" Foam and all of the Urethane Expanding Foam; my second attempt at finishing my Scoop failed.
I finally went back to the "tried and true" micro method and micro'd both sheets down and weighted them prodigiously.
I
now have the NACA Scoop Completed and I'm finishing up rounding the Fuselage
Sides to get ready for glassing the bottom.
I'll post more pictures soon.