Fastback 50


After my last session of priming the canopy skirt, it finally became time to look towards cutting and fitting the canopy. This post encompasses the work of several sessions in the shop over the course of the past week. The canopy skirt looks great – I am really proud of how its turned out.

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Canopy fillet

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Canopy skirt

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Left canopy fillet where the latch mechanism will reside.

The first step in cutting the canopy was to gather my tools, plan the cut, and make measurements. After some research I decided to use a 506CU cutting wheel on a dremel. Any cutting disc WITHOUT teeth should work fairly well.

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Dremel CU506 cutting disc.

Keys to a good cut: 1) Make sure its a warm day. Anything over 70 should be fine, but the warmer the better. 2) Use the proper tool – don’t use a saw or any blade with teeth. 3) Take your time. 4) Measure at least 4 times.

There are essentially two initial cuts that need to be made. The cut along the red line at the base of the canopy, and the rear cut for intersection with the fastback turtledeck. The first cut, in terms of difficulty is rather trivial. This is the cut along the base of the canopy. The second cut, at the rear of the canopy, requires careful measurement.

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Basic outline of cuts to be made.

Measurement. Showplanes instructs you to measure a line 77 inches from the front center of the canopy (on the pre-marked red line) and 72 inches along the side. The canopy itself isn’t necessarily perfectly true, and the extra material is certainly anything from square or equally spaced, so determining the exact center of the canopy can be a challenge. I measured in both directions around the side and used my eyeball to mark the center. This ultimately resulted in about 1/2 inch disparity between the left and right marks at the base. My friend Jack and I debated what to do about this. Certainly you could move the center mark 1/4 inch and level it out, but again, with things not perfectly true you’re still not guaranteed to be right on. In the end we decided to cut an inch less (moving the cut rearward). We could then put the canopy on the skirt and see the true fit.

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Trimmed to pre-marked red line

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Marking the rear cut location with tape

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Canopy after initial trim

Ultimately this was unnecessary. The 77 and 72 inch measurements were conservative. Had we made the cut at that point, we would still ultimately have had to trim at least another inch if not two. After we trimmed the both the base and the rear of the canopy we placed it on the canopy skirt and both on the plane. After reviewing the fit, I was delighted to find that I had not made the un-repairable error of trimming too much. Quite the opposite in fact – we had plexi to spare in every dimension. The question was then, which area to trim and fit first? I settled on the front of the canopy. The front will require shaping to match the curve and to be far enough aft to intersect the existing fairings.

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Initial rear cut – 2 to 3 inches aft of actual cut location

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Initial lower left cut. 1/2 inch to 1 inch extra.

My thought was that the front of the canopy, by virtue of its angle coming to a point was relatively set in stone. It must touch the skirt at the intersection points which thus dictates the forward/aft positioning of the canopy. Without shaping this gives you the distance, but further shaping could slightly affect the rear angle. So, in short – shape and fit the front first. This will provide the proper distance to trim the rear of the canopy. After you trim the rear, the canopy should be seated in the proper up and down position allowing trimming the sides. Trimming and shaping the front was a slow and iterative process. I took my time and got it right.

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Front intersection after trimming and shaping to match curve

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Second photo of front intersection post shaping

Trimming the sides is, perhaps, the most iterative portion of the trimming. The canopy is a circle that cannot lay flat while it isn’t fully trimmed. As you trim the canopy, the radius slightly reduces and thus your trimming dimensions reduce.

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Left lower cut line marked in tape

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Close up of left cut line

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Post cut fit

My technique here was to position the front and rear of the canopy in their final locations and centered, and then tape them in place. I then taped the canopy as tightly in place as possible and used tape to mark the trim of both the left and right. I only trimmed the left side. After trimming the left side, I again put the canopy back on the skirt and this time I was able to tape the canopy in position on the front, left, and right. As predicted, this reduced the radius and resulted in the trim line on the right moving about 1/2 to 1 inch! Had I trimmed both, I would have still had to trim the right again.

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So, now I trimmed the right. And, again, placed it back on the skirt. You’d think it’d be a perfect fit, but again, a small change meant I still needed to trim a little more.

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Right side ready for trimming

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Right side after trimming

Unfortunately, I’m still off by about 1/8 to 1/4 inch when the canopy is squeezed into its final position. I will evaluate options in my next trip to the shop.

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