Lower Engine Cowl Reinforcement

[TimHicks]

I have about 5 patches that reinforce cracks from the trailing edge of the lower engine cowl of my '46 BC-12D.
And I have 2 new cracks that are unrepaired.
I'm planning to remove all of the existing individual patches and make one long reinforcing strip out of 0.040" 6061 aluminum.
You can see from the template pictures below that the exhaust hole will be included.
Any pitfalls to doing this?
Before I drill any holes in the patch, I plan to fixture the cowl in its installed position (more curved than shown).
I'll replace all the Dzus fasteners.
Also, my exhaust muff was rubbing up against the edge of the hole in the cowling. So the dashed line shows where I plan to make the new opening to allow a little clearance.

[TimHicks]

Re: Lower Engine Cowl Reinforcement

Here's my next iteration of template. I used thicker paper to make sure that it was laying right. And made it a touch wider in spots.

[Mark Julicher]

Re: Lower Engine Cowl Reinforcement

Tim,
Is this going to be a flush patch?

Consider making the steel reinforcement doublers cover about twice the area that the originals did. This technique helped minimize cracking on my cowling. Consider making the right hand steel doubler one piece with the exhaust outlet reinforcing ring and the corner steel doubler as one piece. If you do increase the size of the steel doublers, then you may need to use double countersunk rivets so the rivet heads do not gouge the boot cowl.

[TimHicks]

Re: Lower Engine Cowl Reinforcement

Thanks Mark,
That's good stuff to consider. I think that the picture below roughly describes one of your comments. This picture is not mine. I saw it somewhere else on this site.
I'm not sure exactly what you mean by a flush patch, but my intention is to make basically an aluminum doubler in the shape of the template shown above.
My intention was also to use flush rivets and dimple both pieces of aluminum (kind of like the crude sketch below). The outer piece is the existing lower engine cowl and the inner piece is the new patch/doubler.

But I see your point about doing this near the trailing edge where it overlaps the boot cowl (the rivet bucktails might gouge the boot cowl).

Let me go back and look at my existing patches and my boot cowl and see if I can see a way through this. If I use double-countersunk rivets (which I had never heard of previously), I would countersink instead of dimpling and the rivet would be flush on both the inside and the outside, right?.

[TimHicks]

Re: Lower Engine Cowl Reinforcement

Below are some picture of my aluminum cowl doubler. I checked the fit of the lower engine cowl to the boot cowl. Having rivet bucktails did not seem to be a big deal at all. It looks like there is a little room. And where the rivet would touch is actually against the steel flange of the firewall. Since I've had rivets in this area for decades, I checked the firewall flange to see if there were any signs of rubbing or wear. But I couldn't find any.
So I don't plan to double-countersink the rivets.

[Mark Julicher]

Re: Lower Engine Cowl Reinforcement

Tim,
That looks nice!

[TimHicks]

Re: Lower Engine Cowl Reinforcement

Thanks Mark,
I really appreciate your and everyone's feedback.
Below are some pics of the stainless-steel corner reinforcements.
I made these out of 0.018" thick 304.
The rivet holes are irregular because I'm mostly using whatever rivet holes were already in the cowl.
The right one got trimmed a bit more after this photo was taken.
Last night I was practicing my riveting. I need a little more practice. But I'm getting close to riveting everything together.
Tim