Cowling Surgery HOBBS 1638.0 hrs

I'm going to walk you through my cowling surgery. If you're wondering why I am going through this process, read my previous posts on this topic. This is mating my Sam James cowling with the intake scoop that Rod Bower sells for his ram air unit. This may seem difficult, if you don't have experience with fiberglass, but its really not a big deal with some basic skills. Except for the sanding, I enjoyed this...it was pretty fun and I like the way it came out.

 

Since pictures are worth a thousand words...here are a whole bunch of them for your viewing pleasure.

 

From my previous post, you can see in this pic...I originally planned to do my own 'custom' scoop instead of buying one. That's why you see this pink foam. This strategy is old news at this point, but this gives you an idea where I started from. Of course, this is the Rod Bower scoop that I bought in its untrimmed state.

 

Initially placing the scoop to see how it looks/fits...

Remove the pink foam...you can see my initial cutout...it pained me to cut my cowling up like this, but you'll see it gets worse before it gets better.

Scoop cleco-ed in place for initial fitting...off the airplane.

Next, I move it to the airplane to see how it all lined up with the ram air unit. I had to do some trim work to get it the way I wanted it.

Removed from the airplane, getting it ready for bonding. You can choose to make this removable also by attaching it with screws. You can probably guess my thoughts on this...my entire cowling has no visible screws, so placing screws here was not an option for the look I wanted for my airplane.

You can see that the width of the exhaust outlet on the Sam James cowl is narrower than the Rod Bower version...and I would guess the Van's stock cowling as well. So, this was my first dilemma...how to make this work? Create it from scratch? Or....work smarter, not harder.

The surgery continues...more cutting to fit the scoop.

A view from the inside...

Here was my strategy for the wider exhaust outlet...use the existing one, cut in down the center and then reattach.

Scoop in place, trim work done on the cowling...

Reattaching my two halves...spliced together with temporary pieces of aluminum just to hold it nice and straight. Don't worry at all about drilling holes in the cowling for cleco's...we'll fill them easily later.

Voila...she's now almost ready to bond everything in place...just need to do some additional prep on the surfaces.

 

Here is the final cutout...gosh, that was a lot of fiberglass that I paid for and then discarded...painful, but I think it is not uncommon. Surgery of this kind is pretty common for most builders I believe. Notice the bonding surfaces have been sanded and cleaned with acetone.

 

I cleco-ed a scrap piece of alum to bridge the gap between the two halves. This will give me a nice smooth transition...use some kind of mold release on this piece.

 

After fiberglassing...notice how I laid the fiberglass layups a little long, we'll trim this to size later. The seams you see will be easily filled with micro later.

 

The start of the finish work...adding micro (the white material), forming nice transitions and fillets (I'm all out of my West Systems filler-the rust colored stuff-that I used everywhere else on the plane...standard micro works fine too and is cheaper). This is just the first application by the way...it will need more later to get it ready for paint, but it gives you an idea about the process.

 

Fitting my aluminum inserts. This one on the scoop will be permanently floxed/bonded in place. But first, you will need to do some trim work to get it to fit. Don't trim any more than you need to since you will just be adding it back.

 

This is about where I expect the insert to reside...I will use flox to make a nice transition from the insert to the fiberglass. Right now I am just getting an idea of where it will sit in its final state...which will come later.

 

I used a piece of manila envelope from Van's to fashion an extension on my ram air unit so I can check alignment. File folders work good too if you have one handy...paper/cardboard mockups are always helpful when prototyping.

Since I was feeling pretty good about my scoop at this point, I now wanted to work on the rest of the cowling...which I will post more pics later. Here I am temporarily fastening the two halves together for more fit checking...again using scraps of alum as fastening devices.

 

Removed from the plane and then attached together with the side hinges. I need to make fiberglass 'lips' to bridge the seam between the two halves. First task is get it all lined up nicely. Although not shown here, make sure you leave a small gap (1/16" min) for paint allowance.

A parting shot showing the inside of the cowling...just to give you an idea of how it looks. My next posts will continue this fiberglass work on the cowling...adding the aforementioned lips, and also the cowl pin covers. Stay tuned...