Closing the Hell hole
The geometry of the NACA duct on JT’s belly is critical. The air it channels into the cowling serves several vital functions: it feeds the pressure carburetor, cools the engine oil through an oil cooler, and most importantly, cools the engine cylinder heads.
In part #3, I explained why I needed to cut a hole into the delicate NACA duct. Any irregularities on its surface could easily disrupt the airflow, causing it to exit the duct prematurely. I was determined to ensure that didn’t happen.
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| That's one big hole to have to erase! |
I chose to design the hole digitally instead of freehanding it. This approach allowed me to create a cover while working at the bench rather than being upside down under JT. This plan would allow me to machine it, 3D print it, plasma cut it, or use other precision techniques if needed.
The goal was to fabricate a door that fit flush with the NACA surface, without any external hardware. I envisioned a two-layer design: a bottom layer that fits snugly and flush, topped by a larger layer to secure the door in place. For the initial attempt, I decided to try fiberglassing.
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Using the CAD drawing to lay fiberglass on
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That's just about wide enough to cover the hole
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Peel-plying one side as more layers might have to be added to reach flush
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Outline of the hole on glass drawn by my CNC plasma cutter using a marker
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The door was trimmed along the line and fits nicely right off the bat
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Getting ready to glass the larger top layer
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Top layer glassed
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Bottom layer pressed over the to layer and left to cure overnight
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Testing for fit the next morning
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Trimming the top layer
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To secure the door, I crafted eight click bonds from stainless steel using a lathe, then floxed them to the inside of the NACA.
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Drilled eight holes for closing hardware
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Clickbonds floxed to carbon fiber NACA inner surface
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With the door secured, I increased the thickness of the bottom layer to ensure it remained coplanar with the NACA surface.
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Hard to see here, but the door is still a bit short of flush.
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Adding a couple more layers of glass to bring the door to flush with the NACA
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Glass cured the next day
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Perfect flush fit
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Looking good
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Though the door is flush, there are a few spaces to fill aroung the perimeter.
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Next, I filled the small gaps between the door and the NACA with flox for a seamless finish.
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Protecting the paint for floxing
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Flox added to the gaps around the perimeter
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I replaced the nuts on the click bonds with hand operated nuts to prevent over-torquing and risk damaging the flox. These handnuts can be easily removed from the firewall hole, or with a bit more effort, from the backseat.
I did make the mistake of using blue Loctite on them once, and let’s just say that’s a lesson learned the hard way!
Initially, I considered leaving the door flat black but ultimately decided to add a splash of color.
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Door painted black with white tape added
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Door finished
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“Another problem solved.”
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