12.4.2 Exhaust Fairing

This entry is part 39 of 50 in the series 12 - Engine / Propeller

A while back I installed the custom 3 to 1 exhaust and made the cowling cutouts.

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The manual says a 1/4″ clearance but Malcolm says that close and the glass starts showing signs of charring. He recommends 1/2″.

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It’s a minor thing, but I think it will look and function better with some fairings forward of the exhaust. Not only will it be more aerodynamic, but it should be another path for air to exit the lower cowling.

My initial plan was to make the fairing out of blue foam and simply cover it with some BID. But Malcolm advised against that. He said that method would result in unneeded weight (doesn’t sound like much, but I guess every little bit counts), be a pain to cover and wouldn’t be as effective in being an efficient exit for air. He suggested in making a form, covering it on the outside, cutting away the cowling from the inside, removing the form and then adding fiberglass layups on the inside.

My first attempt at the forms was to use a mailing tube of the same diameter at the exhaust pipe.

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But that didn’t work very well so I used some blue foam, formed it to the correct shape and used 5-minute epoxy to attach it to the bottom of the cowling.

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Then to aid in making a consistent radius, I whipped up some sacrificial micro. I call it “sacrificial” because it will be removed from the inside once I cut away the cowling on the inside.

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Then I covered the forms and radius with duct tape.

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I applied 2xBID over the forms. That’s really not enough for a rigid structure but it’s only to get the shape defined.

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Once the layups cured, I removed the cowling and then began working from the inside to carefully cut away the cowling under (above?) the foam. This took a while because I didn’t want to cut through the fairings. So what I did was to make very shallow cuts well inside of the fairings and removed the cowling which exposed the blue foam.

Then it was time for another Hangar 18 trick… Acetone. Pour a small amount on the blue foam and it dissolves. Then just work and the remaining cowling with carbide burrs and some 24 grit on the grinder.

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Now it’s time for the reinforcement. One layer of Triax and two more BID gets applied to the inside. The result is a fairing that’s roughly the same thickness and strength as the cowling itself.

Here’s the cowling back on with some rough cuts for the opening.

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I fussed about what the final cut should look like. If I made it to have a consistent 1/2″ clearance to the pipe, it would have a funny shape. If I cut it to have a nice, symmetrical, pleasing shape, it would be too close in some places and too far in others. I spent about two days worrying that issue.

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In the end, I split the difference. If the places that it’s too close start to show charring, I’ll trim it back.

Once I made the cuts, I started filling process.

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All filled, sanded and ready for primer.

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A comparison (Before & After)

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