I returned to this part of the build after I finished Chapter 24 because I noticed there was a mismatch between the canopy/turtle cut line. The mismatch is caused by the temperature change between summer and winter. To make matters worse, my mismatch is not even. I can see 1/16" to 1/8" mismatch at various locations. Since my canopy build is pretty much complete, I decided to stiffen it up to deter any more shape-shifting again!
Wayne Hicks has a nice write up on adding a hoop at the aft end of the canopy - I decided to follow. Here's what I did...
Before making the plywood template, I crawled into the back seat and closed the canopy. My dual actuator canopy lock and lift became handy again (Refer to Chapter 18 - Section 22A). I took a pencil and traced out the turtle back drip rail onto the underside of the canopy roof. The reason for this marking is to make sure the drip rail and hoop do not interfere with each other.
I followed Wayne's approach by making a plywood template and bondoed it on top of the canopy. Then I cut up a bunch of stir sticks (3 sections out of each) and used them as shims. I mounted the canopy back in place and started shimming the canopy edge to match up with the turtle back. I did not want to force the canopy too much for exact match, rather, I allowed a slight mismatch, but even throughout. The canopy edge stuck up above the turtle back ~1/32" throughout. I figure I can cover it up with the canopy lip in the subsequent step. Once shimmed, I bondoed them in place.
In preparation of the foam hoop, I removed the canopy (together with plywood template) and set it upside down on my saw horses. I taped a semi-transparent paper at the back side of the canopy and traced out its outline. BTW, my canopy is not crooked, its just the tracing paper not taped down in alignment with the rest of the canopy.
Anyway, once traced, I laid the paper on top of a piece of 5/16" foam and transferred the outline onto the foam with my Tracing Wheel. I did not have a large piece of foam at the time, so I cut up several strips and 5 minute glued them together. The width of my hoop is approximately 1" wide. Once the glue cured, I glassed the aft side with 1 ply BID. The reason for the layer of BID (and peel-ply) is to keep the foam flat when mounting it onto the canopy. Otherwise, the foam strip will be wobbling around like a wet noodle.
After the glass cured, I trimmed its edges and base such that it'll sit reasonably well against the canopy surface. To make sure that the hoop is straight, I took a ruler and marked 1" distance along the edge of the canopy edge and made sure it does not overlap the drip rail line. Then I 5-minute epoxied the hoop along the hoop line. Note that I used small blocks of wood and clamps to keep the hoop in place during cure.
Here's a picture of the hoop before flox and glass. I packed the aft edge of the hoop with flox as well as flox fillet. I wet out 3 ply of BID on my work bench and applied onto the hoop. With the original layer of BID, I have 4 ply BID altogether. I extended the BID all the way out to the aft edge of the canopy for added stiffness. I added peel-ply after the layup.
After the glass cured, I removed the peel-ply and trimmed the hoop. I placed the canopy back onto the fuselage and made a slight trim on the drip rail - just to accommodate the thickness of the BID over the drip rail edge.
After that I microed the forward edge of the hoop (because it has not been glassed yet). I added a micro fillet and glassed the forward side of the hoop with 3 layers of BID. Once cured, I removed all peel-ply and did a final trim and sand to make it look nice and smooth.
Recall that I still have a slight mismatch between the 2 edges. However, they are relatively small and I know I can blend it in with the canopy lip. I roughed up the canopy/turtle back surfaces and drew a 1/2" line behind the forward edge of the turtle back. I also drew a 1" line forward of the aft edge of the canopy. Then I applied packing tape over the turtle back edge for release.
I pre-wet 3 ply BID and applied along the canopy/turtle back cut line, using the previous marked lines as a guide. I added peel-ply over the glass. Once cured, I trimmed the lip to 5/8" wide throughout. Actually, I think 1/2" lip is adequate, but you can never cut it too short!
Here's a picture of the canopy lip - the mismatch has been hidden successfully. In addition, you can see the seams of my windows as well. I left the picture in its original size such that you can see the seam closely. Note that I have spray lat over my window panes.