Chapter 18: Canopy Hoop
It is simply amazing how much our plastic airplanes change shape with varying temperatures! This is especially true with my canopy. It is most notable where the forward turtleback and the aft turtleback meet at the cut line. I built the canopy in the summer months. So in the summertime, my forward turtleback and the aft turtleback match up perfectly at the cut line. But when it's cold outside (like it will be at 10,000 feet), the sides of the forward canopy want to expand outward, especially at the areas aft of the side windows.
To put it in perspective, let me tell you this real-world story. I pour-foamed and shaped the upper cosmetic strake fairings on a bright and beautifully sunny day. It was 60 degrees that day. I came out the very next day after a cold front. It was 30 degrees. The canopy had expanded outward about a quarter inch! The fairings were very noticeably mis-matched. They weren't even close. So, I decided to physically constrain that type of movement.
Taking a tip from Dennis Oelmann told to me second hand by Norm Muzzy, I installed a canopy hoop to hold the aft edge of the canopy in shape. I also installed a canopy constraint tab to help stop the outward expansion. Neither is addressed in the plans, although Nat has mentioned the canopy hoop in the newsletter. Several builders have used them successfully. The canopy hoop is made from 1/4th- or 3/8th-inch foam cut 1.5 inches wide into the circular shape of the aft edge. It is attached an inch or so forward of the cut line and is glassed with 3-BID on both sides overlapping onto the turtleback. The canopy constraint tab is just 10-BID tapes glassed onto the inside starboard surface of the canopy, extending over the inside of the top longeron. Details for each are provided below.
Canopy Hoop
Note: The following construction is not addressed in the plans.
Here's the process I used:
1. I removed the canopy from the plane and placed it onto my workbench. I tilted the canopy upward until the aft edge was flush onto the table top. I took a pen and marked the inside shape of the aft edge onto the table top.
2. I cut and 5-minute glued some 1/4th-inch blue sheet foam remnants into the shape of the outline on the table top. I slurried the foam and since I had some carbon BID lying around doing nothing, I glassed each side with 1-BID carbon fiber. 45 degrees orientation to the perimeter.
3. I took some left-over 3/4-inch plywood and cut out a form to bondo over the canopy's aft edge. I placed the canopy back onto the plane and re-inserted the canopy hinge pins. While it was cold outside (and inside too!), I placed the form onto the turtleback and bondoed it in selected spots. I then use shims to tweak the canopy back into shape until the forward turtleback once again matched the aft turtleback. I bondoed the shims and more of the form so nothing moved. In the picture below, you can see the shims are placed where the aft edge was expanded the most.
4. I repeated Step 1, but this time tilting the canopy onto the hoop from Step 2, and marking the inside shape of the aft edge directly onto the hoop. I cut out the hoop to be 1.5 inches wide and sanded the outer edge on a bevel to match its angled installation into the inside of the turtleback.
5. Turning the canopy upside down with the form still bondoed in place, I placed the hoop inside the turtleback and sanded here and there until the hoop fit perfectly. I prep-sanded the turtleback aft edge as well as the hoop surfaces, and temporarily held the hoop in place with small dabs of 5-minute glue. I filled all gaps with flox and I made a flox filets on each side. I painted both sides of the hoop and turtleback surfaces with epoxy, then BID-taped each side with 2-BID oriented at 45 degrees, overlapping at least 1 inch onto the turtleback inside surface. These pictures show the hoop just prior to glassing in the 2-BID tapes.
6. After cure, I trimmed the layups, removed a little bit of the foam from the interior edge of the hoop, and filled it with micro. I sanded the micro edge to make everything nice and smooth.
Canopy Constraint Tab
This is not addressed in the plans either.
I added a 10-BID tab onto the inside starboard surface of the canopy to ensure that the starboard side of the canopy stays aligned with the aft turtleback. I took a small piece of very thin aluminum sheet and bent a 90-degree corner into it. I grabbed my 5-minute glue, got into the rear seats, and closed the canopy. I put some 5-minute glue onto the tab and slid it into place against the top longeron. One side of the tab lay flush on the top of the longeron, the other side lay flush against the inside surface of the longeron. I put the glue on the side touching/facing the flat edge of the canopy deck. Once glued into place, the tab would serve as the guide for the laying up the tapes to make the constraint tab. Once the glue was cured, I removed the canopy and tuned it upside down onto two saw horse. A workbench works fine, too. I made up five 2-BID tapes about 2 inches wide by 3 inches long. I prep-sanded the area on the starboard inside area of the canopy and glassed the tapes into place. After cure, I knocked off the aluminum guide and trimmed up the tab. That thing really works! Just as the canopy is fully closed, the tab presses flush against the longeron, thus keeping the canopy from ever expanding outward. The left edge of the canopy is physically constrained by the aft canopy latch hardware tab. Guess what? The canopy is constrained from ever contracting inward (which I know from experience it never does) by the canopy drip rail. So now the canopy is physically constrained everywhere and it matches up with the aft turtleback each and every time. I am NOW a HAPPY CAMPER.