Cowling - Day 1 & Day 2

TLDR

• Test fitted cowling on the firewall
• Made jig to hold cowling in place at the front
• Cut & fitted the top cowling

Details

This entry is a summary of about 2 days’ work. I’ve lost track the exact number of hours I spent on this task, either 9 or 10 hours maybe more. I’ll call it 9 hours.

So in the last few days I started to work on fitting and cutting the cowling to fit on the firewall and around the engine. This is a necessary step to finalize the relative position between the cowing and engine, so I know where the air intake and cooler etc will live.

As usual, cutting composite material is very tricky and unpleasant due to the dust. The huge size of the cowling doesn’t make it any easier. Knowing this is gong to be a long task, I watched and re-watched the video posted by Sling on their official channel and tried to memorize the pro-tips, then started my own process.

Fitting

First step is to get both the top and bottom cowling onto the firewall, and cleco them in place. It’s probably a two people job though I managed to get it done just by myself. It’s actually not too difficult. I placed the top half first, then used a sawhorse under the engine as leverage to move the bottom half gradually in place. Then I clecoed as many holes as I could. It’s not necessary to cleco all the holes between the two cowling halfs, as I was really just trying to get a rough fit.

Spacing jig

The next critical step is to get a even gap between the front of the cowing and propeller hub flange. The Sling instruction suggested 5mm, and the Airmaster instruction suggested 6mm - 12mm. In my case I chose 6.5mm.

To get the right amount of gap, I bought a bunch of angle brackets from amazon. The thickness of the brackets are 3.5mm.

I used a bench grinder to remove the coating on all of them, then riveted them back to back. This gave me 6.5mm - 6.8mm total thickness on the angle bracket.

Then I screwed the brackets (4 sets of them) onto the propeller hub using the existing rivnut holes on the flange.

The thinking is that when I fit the cowling, I would push it all the way forward so the cowling touches the brackets. Then I will have roughly 6.5mm gap in front of the cowling.

And for the 2 jigs on top of the cowling, I replaced the super thick metal with a thinner (but still pretty stiff) aluminum, and bent the aluminum piece so that it just touches the top cowling.

Then I drilled a hole through the aluminum and the cowling, and clecoed them together.

This gave me a steady gap all around, and also ensured the cowling doesn’t change position as I put it on/off when making adjustments.

After all of this work, I took a caliper and measured the gap one more time. I got 6.1mm - 7mm all around. The minor variation could be fine tuned once the aft edge is cut and the cowling sits nicely in position.

Marking cut lines

After the cowling’s spacing was set, I started to mark where to cut at the aft end.

Evan showed two ways of marking in the official build video. I chose to use the tape method: first put tape just next to the edge of airframe, then place the cowing, then a second layer of tape on top of the cowling and exactly overlap the first tape layer. Then cut off the second tape layer.

I also drew a line at the tape’s edge to be clear where to cut. The line would stay during the cut asdf

Cutting

Honestly, cutting was easy compared to work making jigs and doing the measuring.

I used a diamond disc on my die grinder to make the first cut. Then switched to a 80 grit sand block to smooth everything out.

During this process I fitted the top cowling maybe 5 or 6 times onto the firewall to ensure I sanded every point on the cowling to the exact position, and the gap in the front did not change.

Fluting

After cutting and sanding, some part of the cowling did not sit flush to the firewall. I did not expect the cowling to sit perfectly flush, but some area was standing up pretty high. This was because the stiffening strip on the firewall was sticking outwards.

Evan’s video talked about this issue, and he suggested to adjust the stiffener with a flute plier and that’s what I did.

I marked where the cowling was sticking up a lot, took the cowling off, and used a flute plier to bend the stiffener back down. This procssed lasted 30 minutes or so and I test fitted the cowling a few more times.

After this, the top hakf cowling sat almost fully flush with the airframe skin.

To be continued

I got pretty exhausted after getting the top half ready. I will continue with the bottom half later this week.