Firewall Forward

Fitting the engine mount

Well, I’m still paused on the canopy front until I get in the replacement slide block from last week’s shenanigans, but in then meantime I get to kick off a whole new category with the first bit of firewall forward work. Most of the weekend has been spent working on other stuff around the house, but I wanted to keep making progress on something and so I decided to see about fitting the engine mount to the airplane.

The firewall and the structural bits behind it came with an existing pilot hole at each corner, and I drilled those holes out to 3/16” quite some time ago to allow for the fuselage rotisserie mount. The final bolt holes are 3/8”, but final-drilling those requires some attention, since individual engine mounts can vary even though they’re welded up in a jig.

So the first step is to do a rough evaluation of fit out of the box. To do this, I installed a couple of long AN3 bolts in the upper two holes and used those to loosely hang the mount in place. Doing this allowed me to establish a good starting point, as I could verify that the width of the two upper mount points were at (or very nearly) the same width as the two pilot holes. That meant I didn’t need to do anything dramatic to get those first two holes drilled…well, almost. The construction manual calls for drilling the holes using the engine mount as a guide, which I imagine helps ensure that the holes through the fuselage structure are in line with the actual mount sleeves. So the most interesting part of all of this was figuring out how to “clamp” the mount against the firewall. I probably should have gotten a photo of the final setup, which involved some ratchet straps going from a piece of lumber across the mount to the setback support in the cabin…

In any case, the upper two holes are now drilled and bolted in place. This gives me a nice base to work off of for the other two holes at the bottom: 

However, things down here are a bit less simple; the engine mount holes are a good 3/8” or so above the pilot holes in the firewall. I figured this out by putting flashlights inside the fuselage, pointed at the pilot holes; with that light sources inside, I can easily spot the pilot holes from outside, even if I’m looking through the engine mount itself. Note the pilot hole at the bottom of the engine mount hole, mostly occluded by the mount itself:

So in order to get these drilled, I’m going to need to rig something up to precisely stretch the engine mount. I made a first rough attempt at this tonight, which didn’t work so well. I figured since both lower holes are high, I’d see about trying to push both down, so I flipped one of my large hand clamps to make it into a spreader, and put it between the upper and lower center of the mount. I was able to move the lower holes by maybe 1/4” or so, but still nowhere near into alignment.

I’m not entirely sure yet how I’m going to work this problem, but so far I’ve had some vague ideas involving threaded rods, nut, and bearing blocks, but the hard part would be getting any of that stuff braced in the right place on the mount. Normally I’d prefer pulling for something like this, but I don’t really have anything to pull against…unless I were to put concrete anchors in the floor just for this one operation, and I don’t really want to do that.

So anyway, something to mull over. I’m sure it’s a solvable problem, I’ve just got to work it out.

Posted in Firewall Forward | Hours Logged: 1.5

Finishing with the engine mount, doing some wire routing

As I headed out into the hangar to get to work this evening, it occurred to me that I hadn’t had a single super bright idea about how to stretch out this engine mount to get the second pair of holes drilled. All that random thinking about making a jig with threaded rod and so forth hadn’t gone anywhere concrete. So I ended up just trying to stare at things and work out a solution, and eventually I got to something.

Basically, there are two ways to approach this: either I need something inside the mount exerting outward force to spread things, or I need something pulling down on the bottom of the mount. The latter seemed impossible since I had nothing to pull against, but finally I realized that if I stopped thinking in terms of using something like a ratchet strap, there were other ways to approach this. What if I could just hang weights off the bottom of the mount and try to work with the positioning that way?

And that’s how I ended up with this amusing Rube Goldberg setup:

We’ve got a 30-pound kettlebell hanging on the center of the mount, using an S-hook normally reserved for hanging up a caliper while working on the car brakes, plus another 30 pounds of dumbbells lashed together with some spare paracord and hung from the corner with more spare paracord. The only real issue with the hanging-weight idea is that you don’t have a lot of fine control over how much the mount stretches, and that’s where the big clamp comes in. Hanging the weights as shown here got the hole just a tiny bit above where it needed to go, so with the clamp in place, I could carefully apply that last bit of force to get it all lined up nicely.

The only remaining issue was how to pull the mount snug against the firewall while doing this; as it sat, the mount face wanted to sit about 1/4” off the firewall. I initially tried using a ratchet strap secured to a long piece of lumber going through the center section, but the strap would have been bearing on the edges of the forward side skins and the firewall flange, and surely would have bent something if I’d started cranking down. Then I had another epiphany – the other lower hole could be used for this. I just had to run a 3/16” bolt through the hole and the mount, add a large washer so it’d bear against the mount hole, and tighten the whole thing down.

That pulled both lower holes up nice and snug, and once I had the lower right hole drilled and bolted, the other was already lined up nicely and didn’t even need any drama to get it set up. The only concern here is how annoying it’ll be to attach this mount unless I permanently stretch it, so I’ll probably have a go at that at some point, but not tonight.

Instead I decided to continue thinking about FWF type stuff. One item I’ve been putting off for a while is drilling the 3/4” hole down the right side of the aircraft for all the engine sensor wires and such to pass through. That seemed like a nice digestible project for tonight, so I got to it. I’d been thinking that laying the hole out properly would be a challenge, but once I got down to it it wasn’t tough at all. The only hiccup I had was that the unibit drifted a bit as I was opening the hole, and got into the edge of one of the firewall angles, so I had to then go in with the dremel and sanding drum to smooth things out.

Once that was done, I installed the snap bushing and had the fun of feeding the wire bundle through from inside, and now I’ve reached the milestone of having random wires sticking out through the firewall:

Assuming I don’t get that replacement canopy block in sometime soon, I think I might start marking up the firewall for component locations, just to sort of spitball ideas. The FWF plans include some suggested layouts that I’ll likely start with, but I’ll have to consider some other items not shown there that I’m adding – like the battery-bus fuse block and E-bus relay. Those are pretty small but they still need homes, and routing of wiring is also a concern.

The other item I could work on now would be feeding the CPI2 FWF harnesses back through the firewall and finishing up the lacing of the relevant harnesses in the cabin. Those had to wait until now, as they’re already terminated no the engine side and so can’t be fed through in the other direction. It’d be kinda nice to finish up those harnesses inside, I suppose. We’ll see.

Posted in Electrical, Firewall Forward | Hours Logged: 2

Firewall planning

So not a ton to talk about tonight really. Initially I’d figured on threading those CPI2 harnesses back through the firewall and getting the pertinent harness bundles laced for hopefully the final time…but then I got to thinking about whether that was a good idea. The whole reason I had to wait until I pulled the rest of the harness through the firewall is that the engine side of the CPI2 harnesses are already terminated, meaning they have to be threaded through bulkheads from the engine compartment. Thing is, that firewall passthrough may not be the only place I need to thread them. For example, one of the ignition coils will be mounted on top of the engine case, which means that harness will presumably have to pass through the baffles.

Now, that might not be a huge deal down the line – the coil connector plugs can probably be depinned if necessary, I’d just have to identify the connectors and get the right tool. But the crank sensor is a different matter – the wire connections aren’t detachable there. I’m not 100% sure whether or not those wires will need to pass through baffles, but in the end it seemed that the better choice here was to wait on these bundles. I might well end up with a chicken/egg type setup here, though…if I wait until I’m doing the engine baffles, that’ll mean I’ve fitted the cowl, which almost surely would require that the forward upper skin be riveted in…and once that’s done, finishing up the harness bundles will be a lot less fun. But still, I have little to lose by waiting; there’s nothing at stake right now other than me wanting this task done.

So instead I turned to the firewall layout. Some low-hanging fruit here was to go ahead and mount the brake fluid reservoir and cabin heat control box, both of which already have prepunched holes in the firewall. Getting the brake reservoir in place also required riveting a couple nutplates, which I guess were skipped way back when I first did the firewall. The heater box I just clecoed in place for now; it’ll need to be sealed when I final-mount it, and even though that’s still just using a couple bolts, I figure I’ll wait on that.

From that point forward, I was kind of on my own. The one thing that I figured I’d definitely lay out was the manifold block that’ll be used for the fuel and oil pressure sensors. The FWF drawings provided specify a location for that block, and while I don’t necessarily have to abide by that, I see no reason not to. Rather than actually mount it – I’m definitely not ready to commit to anything like that – I just drew an outline of the block and marked the two hose attach locations.

It’s not much, but it’s a start:

The next thing I tried to tinker with was the location of the battery box; since this is a relatively large item and accessibility is a concern, it seems like something to work out early. The FWF plans call for putting it on the right side of the firewall, about halfway up; I’m sure I’ll end up on that side of the firewall, but I’m less enthusiastic about the vertical location. The big question is how to tie the box into the firewall angles; they’re not exactly conveniently located for this. I suspect I’ll probably end up needing to add another angle on the back of the firewall to accommodate this, but the main takeaway is that this requires some thought. Something else to mull over a bit, I suppose.

Of course, somewhere near the end of the night I remember that I could also have been working on stretching the engine mount to better fit the firewall mount holes. Maybe I’ll work on that next; I’m sure I’ll want to install and remove it a few times while working out some of this firewall layout stuff.

Posted in Firewall Forward | Hours Logged: 1.5

Motor mount massaging

So here I am, back after another little break. I have yet to figure out my headset noise issue from last time, but fro some asking around I did dine one other person who’d had a similar problem with a Lightspeed headset. I’ve been meaning to shoot them an email asking about potential issues, but things have been kind of bananas with work and other stuff.

Continuing on the electrical front, I looked a bit at handling my control stick disconnects a few days ago, and was pleasantly surprised to find that I already have all the connector hardware I need on hand. Still need to actually do those disconnects, though…and even better, my engine has been assembled at Titan and should be shipping pretty soon…well, once I send them a check for the remaining balance.

The engine news has me thinking about FWF stuff again, which got me to revisit the engine mount fit. I’d been wanting to see about stretching the mount vertically a bit so that inserting the bolts was less of an ordeal, so I decided to look into that. The good news here is that I only need to stretch things by about 1/8”, but the bad news is that this structure is far beefier than the canopy frame I bent before.

To make a long story short, I tried a couple different setups before settling on the one that finally worked, as seen in the following photo:

Here I have the bottom portion of the mount secured to my work table with a 2×4 and a few clamps. This gives me a good angle to attach a ratchet strap from the mount ring to a large eye bolt under the table. Once again considering the forces at play here, instead of a small ratchet strap I grabbed one of the big ones I have for securing a car on a trailer. I’ve also placed some hard plastic tubing, split down the middle, on the table corner to avoid a potential stress concentration.

I’ll just say up front that exerting this kind of force was more than a little scary, especially since with me standing by the table cranking on the strap, I’d be directly in the line of fire if my clamping setup were to fail. With that in mind, I added a backup safety strap between the mount and the clamp side of the table, hoping that if a clamp did fail, that’d keep the mount from becoming a deadly projectile.

In the end it took about three iterations – each one just going one tooth further on the ratchet – to get the stretch I wanted. The end result still isn’t perfect, but the force needed to align the mount while inserting the bolts is easy to exert with one hand, a huge improvement. Overall I’m very pleased; going into this I figured there was at least a 50/50 chance that I’d chicken out on cranking the ratchet strap before anything moved at all.

So at this point I’m working on a punch list of inside work I want to get done before the gear legs go on. I kind of want to go ahead and build the cover plate for the rear stick base, but I can’t be sure the full range of motion of that stick until the wings are installed and rigged, so I may end up waiting on that…or maybe I’ll fab up the plate but skip cutting the actual hole for the stick, I dunno.

Other things on the punch list are rigging most of the brake lines (but probably not leaving the pedals in place afterwards) and securing a few more electrical components under the seat floors, but…I think that’s about it.

Posted in Firewall Forward | Hours Logged: 1.5