Firewall Forward

Once again, I’m absurdly behind on writing these blog posts. Not that I’ve been hilariously productive, but I have been doing stuff. The fun part will be remembering half of this.

Basically, with the oil door done, it seemed like a good time to switch gears to the engine baffles. Some of this stuff I’d fitted before, but that was a while ago. The first order of business was starting back with the baffle pieces along the back of the engine. These were a royal pain when I was fitting them, mainly due to maneuvering the pieces that wrap around the cylinders into place. But here’s the thing – since I have a Titan engine with tapered cylinder barrels, I need to modify some of this stuff anyway. The factory baffles are design for non-tapered cylinders, and won’t sit pull tightly in place, which in turn will compromise cooling, After all, the entire objective here is to force as much air as possible through the fins on the cylinders.

The fit for this is to cut off these parts, and fabricate some new custom bits designed for the tapered cylinders. Fortunately, that last part won’t be so bad; other builders have gone before, and one person was nice enough to upload templates for making the new pieces. That portion of the program is still in the future. though; for the time being I just had to do the cutting:

Once I’d gotten all that done, I reinstalled the rear and side baffles onto the engine. This, again, was stuff I’d done before, so at least a lot of the head-scratching was behind me. The one thing I hadn’t yet figured out was how to brace the center of the rear baffle. Normally this is supported by a brace that attaches to a pair of the case through bolts, but those are the bolts that are not being used to mount the upper coil pack.

I did a ton of thinking about this, and even spent some time with cardboard trying to mock up a complex bent piece that would tie into the coil pack mount, but none of it really seemed right to me. It’s hard to design a piece that provides decent bracing, but still stays clear of the actual plug wires. Plus it’d probably have to attach via the bolts between the coil pack and the mount, which would mean modifying the spacers, and probably some other annoyances.

Bottom line, I still haven’t solved this. The idea I landed on after that night of experimentation was to just get a big block of aluminum and sandwich it between the back of the coil pack and the baffle. The would let the coil pack do the bracing work, while also spreading the load over a fairly large area. I order a big chunk of aluminum, and…it’s heavy. Maybe heavier than I want for this task.

It’s kind of tough to see in the photo below, but it’s a split between the left and right sides of the rear baffle, right behind the coil pack:

Eventually I kind of left that issue alone for a while, and instead got to looking at the baffle pieces that attach to the front of the cylinders. These are long flat pieces that lead up to the cowl inlets, and do the first part of directing the air to the cylinders. Fitting these things was quite a bit of fun – they interfered with the complex curved part of the case just behind the crank snout, and there was nothing to do except endless rounds of trial-and-error – fitting, marking interference points, filing off some more material, fitting again, and so on.

The real fun came with the right-side ramp; over in this spot is the crank sensor mount for the SDS electronic ignition, which I mounted in place ages ago. That mount, along with the guard I fabricated for the wiring harness (to protect against damage in the case of an alternator belt failure) interfered heavily with the baffle. But before I could even deal with that, I had the same fitment issues with the crankcase itself mentioned above, so I started by just removing the mount.

After handling the initial fitment issues, I came to the realization that having the prop on the plane was a real annoyance. Not just working around the blades, but having the flywheel in place would make it a royal pain to work with the sensor mount while handling those fitment issues. So I decided it was time to pull the prop off. I always knew I’d have to do this, since I neglected to install the alternator belt before hanging the prop.

The main difference is that instead of briefly pulling it off to install the belt, now it’s going to stay off for a while, probably until I’m feeling somewhat close to first engine start. That means I need to figure out storage, since I (perhaps unwisely) disposed of the crate it came in a while back. For now it’s just sitting on top of a big trash can – which actually works pretty well as a place to lay the prop down, but definitely is not OK for long-term storage. I think I’ll probably end up modifying my rolling engine cart for this purpose – I’ve got some ideas there.

In the meantime, I get to look at a prop-free airplane for the first time in quite a while:

Removing the prop was today’s first task, and after that I was freed up to get to work on that pesky sensor mount. At first I’d been intending to trim the baffle around the mount, but I was worried about air leakage, and that thought had been bothering me for a while. At some point today, it occurred to me that it’d be way smarter to modify the baffle as little as possible, and instead modify the cable guard (which was the most egregious source of interference).

So I only had to take a small notch out of the baffle for the mount, and that finally let me sort-of hold the mount in place and get an idea about how to start trimming the cable guard. Basically, I needed to shorten it from the rear, and then cut off some of the lower material on an angle to follow the inlet ramp:

That, too, was another long run of trial-and-error fitting, but eventually I had it where it fit nicely. This photo is a little more cluttered, as I have another baffle piece in place – the vertical portion behind the mount:

That vertical portion is the last major part of the baffling, which basically encapsulates the area behind the prop and none of that inlet air goes tumbling around that area instead of actually cooling the cylinders. Those pieces are temporarily held in place for the the being:

The last bit of work I did for the day was to make a first attempt at putting the lower cowl in place to see how the ramps fit to the cowl inlets. This, too, became another gradual trim party – both the ramps and the side baffles are too long as-is to allow the cowl in place. I took a couple iterations at trimming them, and got pretty close, but I never did actually get the cowl in place before it was time to quit for the day.

Maybe I can get that done tomorrow. I’m off work until January 2, so hypothetically I should be able to get some good work done. Today was a very productive day, and I feel like I’m back in that productive mindset – let’s see if I can keep that momentum going.

Not much in the way of shiny content today. Mostly I’ve been working on getting those tapered cylinder extensions fitted in place and drilled for rivets. Part of what made this fun is that for the most part, there’s just no good way to clamp the extensions to the adjacent baffles, especially with everything installed on the engine – which is what I wanted, so I could ensure the extensions laid across the cylinders properly.

The one exception to this was the #1 baffle, where I was able to get a cleco clamp through the large hole for the prop governor oil line. I wasn’t so lucky with #2, but that’s still at the front of the engine, where there’s decent access, so it wasn’t so bad. #3 and #4 were quite a bit more challenging, since I had to work around the engine mount. I ended up using some creative methods involving scrap wood and popsicle sticks to sort of hold things in place while I marked one hole at a time, then drilled it on the bench, then went back to mark the second, and so on.

Here are a couple photos of the #1 and #2 extensions in place; the extensions are the little wedge-shaped pieces just barely visible near the cylinder bases I didn’t even bother trying to get similar photos of the other two, because 1) it would have been a lot tougher to frame and 2) I was more in a “just get it done and move on” mode.

With those in place, I went back to the instructions and started…following them (a rare luxury at this stage of the build). The ensuing work was just getting the two inlet ramps baffle assemblies deburred and match-drilled as needed. There were a couple pieces to get riveted together, but for the most part everything is staying clecoed for the time being.

So those ramps are back in place, and I have the lower cowl hung again right now. The next step will be tweaking and trimming the baffle pieces in this area so they’re in the right spot relative to the cowl inlets. There’s also some fun to be had up here to get things fitting nicely to the rounded corners of those inlets…stand by for fabricating some fun conical aluminum pieces…

The baffle journey continues! The overall theme here was to refine the interface between the inlet ramps, the forward side baffles, and the cowl inlets. As seen below, as provided, the ramps sit well below the lower portion of the inlet:

This isn’t what we want. We need those ramps to fit closely to the inlets, so they can eventually seal together. So I got to get even more practice taking the cowl on and off while tweaking the bends of both ramps. At least I was able to bend the ramps in place, instead of having to remove and reinstall them repeatedly; that at least reduced the tedium a bit. After a few iterations, I had both ramps resting gently against the bottoms of the inlets.

This led to the next tweak: since the inlets aren’t perfectly flat, we need more bending. The idea is to mark the spot where the ramps contact the inlets near the spinner, and draw a diagonal line outboard. Then the ramp gets bent along this angle, until the outboard portion of the ramp sits up nicely against the inlet as well. Unfortunately this did require taking the ramps on and off a few times (along with the cowl, of course).

The result is a nice close fit across most of the width of the inlet:

Next up, the ramps and side baffles need to be trimmed. They can’t actually overlap the inlets as seen above; if they did, then installing the cowl would require starting an inch or so forward of the final position and sliding it backwards. That can’t work once the prop is in place.

Instead, the ramps are trimmed until they stop just short of the aft edge of the inlets. This will allow the cowl to be lifted vertically into place, so it can work within the space behind the prop spinner. Eventually there will be flexible material added to the cowl, which will seal against the ramps…but that’s later:

With that trial-and-error work done, the next step was to fabricate a support bracket for the left ramp. This will prevent the forward board edge of the ramp from being pushed down by high-pressure air entering the cowl. The bracket consists of a flat plate that screws into the engine block and a thick angle to tie that plate into the ramp. Both pieces are precut, but the plate needs to be trimmed to fit. I ended up cutting off about half an inch or so to get it to clear the ramp.

Then the plate gets screwed into place, the angle is clamped to the ramp and against the plate, and the four holes get match-drilling. Here are the two pieces prior to drilling:

Then, after demurring and such, they get riveted together. The angle also needs to be drilled to the ramp. Curiously, the plan images show the angle having three prepunched holes in this spot, but my angle had none. The instructions say to only rivet the two outboard holes, and not the middle one – I think it ends up being in common with another attach angle that will be added later. So I just laid out and drilled the two outboard holes for now – I’ll figure out the middle one later.

Here’s the support angle after drilling the ramp attach holes:

The next step would be to rivet the ramp assemblies together, but I decided it was a bit late for riveting (these will require use of the rivet gun). Instead I jumped ahead to adding some bends to the forward side baffles. Previously, after finishing all that tweaking, I traced the edge of each ramp on the intersecting side baffle. These lines then are used to bend the lower flanges inboard; this bend provides some more support for the forward outboard corner of each ramp.

The instructions sort of gloss over the details here, but I had to do some trimming of both pieces to accommodate the bends. One of them I was able to do on my little bending brake, which was nice, but the other one I had to do the old manual way – clamping it with a piece of wood and beating the crap out of it with a hammer.

Once the bends were done, I remounted the side baffles on the engine; they’ll be ready to receive the ramps again once I get those assemblies riveted together:

Next session will involve some fun scratch metal working, as I address those square corners between the ramps and the side baffles.

So…as I start typing this, it looks like I missed documenting some work a couple weeks ago. There was some time spent fabricating a conical gusset to close out the square corner I mentioned at the end of my previous post, but I guess I didn’t take any pictures. I’ll have to try and sneak them into a later post, I guess.

After doing that work, I almost let myself get distracted by the air intake snorkel, which has to meet up with the left inlet ramp…but then I remembered the main reason I’m in here, which is to get the baffles trimmed so I can work on FWF wiring. And so I moved on to working on that trimming work.

The basic deal is that the upper portion of the baffles need to be trimmed to provide 3/8-1/2” of clearance with the upper cowl. Later, rubberized seal material will be riveted in place, which will, well, seal against the cowl. This is a somewhat annoying task in any case, and more annoying in my case since I don’t have an RV-8-specific baffle kit. My baffles are intended for a side-by-side aircraft, which means there’s even more trimming to do than in a normal case.

Anyway, there’s a pretty universal approach to this trimming task, which involves putting a bazillion paper clips across the top of the baffles, then laying the cowl in place, then using the disturbed paper clips to see the contour of the cowl and work on trimming the baffles to fit. The pictures I’ve seen have people doing the entirety of the baffles at once, but this seemed overcomplicated to me, especially for the initial trim.

So I started with just the forward side baffles. These need the most trimming of anything; they need to match the curve of the upper inlet ramps, which means 1) removing a lot of material and 2) matching a surface that’s way more curved than the rest of the cowl.

Here’s a pic looking in the front of the cowl from my first fitting attempt. You can see that the top of the baffle is straight, unlike the curved ramp, and as a result only a small area of the paper clips have been pushed down. You might also notice the huge amount of open space to the right, between the cowl halves, The upper cowl is nowhere close to sitting in place:

After a couple iterations of trimming those forward side baffles, I decided that having hacked out most of that curved ramp area would make it OK to try fitting the whole thing. So I installed all the baffles and then put paper clips all the way around. In case you’re wondering, there are 190 of them. Yes, I counted:

The really fun part is that 15 minutes after taking that picture, I removed all those clips again, after marking the first conservative cut line all around… Also, because the cowl was back to not being anywhere near sitting in the right place, and especially because that made it hard to get the orientation right, I knew these cuts wouldn’t get me far.

So after this iteration, I went back to doing a sort of piecemeal fitting, starting with getting those forward side baffles trimmed to the proper length (as opposed to “kinda close” like before). Below you can (hopefully) see the little hash marks I made at each paper clip location; this is the beginning of laying out the cut line:

A nice little game of “connect the dots” results in a trim line:

I think it was after cutting to that line – or maybe there was one more iteration, I can hardly keep track – I was finally able to get the upper cowl in place. Peeking in the inlet, we can see that there’s still more trimming to do; the gap at the very front of the inlet is good, but where it curves up, it’s still too close:

I did decide that this was the right time to add the rear side baffles into the mix. The left rear side baffle shows the contrast between the trimmed and untrimmed state:

I did go ahead and get the initial trim done on those rear side baffles, but that was when I called it a night. I’d done enough repeated installation and removal of paper clips for one day.

Next up I’ll get those rear side baffles trimmed to where the cowl will sit in place, then I’ll move on to the rear baffles…and after that I guess I’ll do paper clips all around again so I can get the final trim done.