Lower Cowl

I need to either make a jig to simulate the prop placement or wait on the prop itself to arrive before I do the actual fitting of the cowl.  In the meantime, I decided to go ahead and cut the lower cowl to accommodate the nose gear.  Finding the center line was more of a guess than a definitive location since the cowl is still in its raw form and the edges haven't been trimmed (but you can't trim the rest without putting it in place on the firewall, which you can't do unless the slot for the gear is cut).  I clamped  a piece of angle to the bottom edge of the cowl, marked the exhaust exit on the angle, then found the middle point on the angle.  That should be the center line on the aft portion.  I then drew a line at 90 degrees to that point, but apparently the aft edge is not trimmed square either, because that line veered off course when compared to the air intake snout.  So instead, I eyeballed the middle of the oval of the front air intake and just connected that with the rear center line and pulled a string.  It's probably not exact, but the gear won't get anywhere near the cowl in anything but a crash landing, so the cutout is really just to make dropping the cowl to take it off easier - precision doesn't really matter.

I pulled out my angle grinder and cut the slot out according to the plans.  It seems huge, but 15.5" long and 2.25" wide is what the plans call for.  I think its length is to accommodate a three blade prop (you have to be able to drop the cowl much further down to clear two angled prop blades on the three blade, which means the gear leg needs a longer slot - with only 2 blades that can be placed in the horizontal plane, the cowl doesn't have to drop down as far to clear them).  I went ahead and cut for the 3 blade prop, just in case someday I go that route.  It doesn't really matter anyway, since I'll be making a removable fairing that covers the slot unless you take it off to remove the cowl.

I feel like I'm jumping all over the place, but that's kind of the name of the game anymore.  Until I make the prop jig, or get the prop (I just ordered it last week).  One of the other big things I have to do is finish up the canopy.  I grabbed it out of storage and put it back on the frame to remind myself how it all fits.  I will need to do some fine tuning before I glue it down.  I think that'll be my next big project.  It's tough to place accurately by myself though, so I'll get it all sorted out and then sweet talk Rose or Perry into helping for a few minutes.

Random Unfinished Stuff

I got a little further on the baffles over the last few days, but this is essentially where I have to stop until I get the cowl fit to the firewall.  The cowl has to be cut to the final fit so that the baffles can then be cut to fit the cowl.  And the cowl can't be sorted out until I have the propeller chosen (to ensure I have the spacing correct between the spinner backplate and the cowl nose bowl).  This has been the name of the game for all of the firewall forward stuff - you can't finish one task without having to start another one, which also can't be finished without starting yet another task.

Before I took the baffles off (they have to be off in order to do the initial cowl fit since the baffles are too big to fit inside the cowl as-is), I did some hand gesturing and head scratching and figured out my routing for the right side upper spark plug wires and both PMag blast tubes.  The upper spark plug wires have to pass through the rear baffles to get to the PMags.  The wire pass throughs have to be drilled big enough for the plug connectors, so you need a way to cover the big holes to limit air leaks.  The OEM plug wire seal is almost $50, and I need 2!  That seems like highway robbery for a piece of plastic.  I found an RV guy who sells a 3D printed version for a fraction of that cost.  I could have designed it myself, but I don't have the high temperature filament, so in the end it was just easier to farm it out. The passthrough is two pieces that slide together over the wires and then are attached with screws.

You can also see the PMag blast tube hole in the pic.  It's the 11/16" hole between the spark plugs and the fresh air hole.  The PMags need a blast tube to direct cool air at them (a relatively common approach for electronics that live under the cowl, where it can get extremely hot).  I will just use some of the black, corrugated tubing that I used for the wire runs in the wings.  This is the right side.  The left side is similar, but with the hole in a different location.  It took a lot of hemming and hawing before I decided on locations.  I had to figure out how the tubes would best route around other items and the engine mount, yet still have a way to attach them to keep the pointed at the right place.

I removed all of the baffles to be ready for the cowl fitting.  One of the other things I can return to now that the weather has warmed up and the humidity has dropped is gluing the canopy on.  I need to remind myself of how I'm going to do that.  I brought the canopy frame down from storage and put it back on the fuselage.  It is just as horrible of a part as I remember!  The fit is just not...ideal.  I won't make any plans for it until I have the canopy on for good and see what that does to the shape.  My guess is I'm going to have to cover the front of the frame with fiberglass to smooth it out, and I'm probably also going to need to make fiberglass extensions on the side skirts to cover up the weird gaps.  This whole setup feels like the project of an intern who didn't pass the class.

I can't really get started on the cowl until I figure out the propeller dimensions, but I reorganized the garage and brought my other workbench back to the work area so I had more room to work.  One of the first steps is cutting the nose gear leg opening in the lower cowl.  I didn't get finished, but I spent some time finding the center where the cut will go.  

I'm pretty sure I have picked out the prop I will use, but even if I order it now, it'll be a couple of months before it's here.  I know people have designed jigs to use for cowl fitting in place of having a prop.  I'll do some research and figure out what it takes to make one.  I'll probably go ahead and order the prop anyway - leaning towards the 2 bladed Sensenich ground adjustable.  I was considering the 3 blade just because it'll be quieter and smoother, but 3 blades make it notoriously difficult to get the lower cowl off.  I really don't want to have an excuse not to take the cowl off when it really should come off.

Baffles (2)

I'm back home after a week away for work, so back to the baffles.  Before I jump on the baffles, I wanted to double check the fuel fittings from the mechanical pump to the fuel pressure sensor.  I finally found documentation about the torque values for these fittings (straight pipe threads with an O ring and lock nut), so I decided to loosen them and redo them to the right value.  I had just snugged them up before, but I didn't have the actual correct torque settings to go by.  I torqued both the fitting and lock nut to 23 ft/lbs each.

The difference between the Lycoming cylinder fins and the Titan fins has been making me wring my hands trying to decide on a solution.  The Lycoming fins are all in line from the outer barrel to the inner barrel of the cylinder head.  As you can see below, the Titan's inner fins taper and get smaller as they get closer to the engine case.  The baffles Van's supplies for the Lycoming assume a straight line across the fins, so that leaves a big gap on the Titan.  When it comes to baffles, any gap around the fins leaves a place for lazy air to go from the top pressurized side of the engine to the bottom without actually cooling the cylinders.

The front and aft baffles have curved portions that extend down and wrap around portions of the bottom side of the cylinders (the front and aft matching curved sections are tied together with either safety wire or steel rods to keep tension on them).

The inside portion of the cylinders have the same wrap around baffle, but since the taper exists, the baffles don't lay against the fins like they should.  I did a bunch of research on these.  Most people cut off the curved portion for the tapered fins and rivet on new aluminum that is shaped to fit the taper better.  That's a pretty significant undertaking.  Templates exist for these new pieces, but only for the IO-370.  The IO-340 I have is essentially the same engine as the IO-370, but stroked, which means the distance of the cylinder heads is greater.  The standard templates won't work because of that.

I came across a thread on VAF where one of the old curmudgeon A&Ps said that he didn't want to cut up his baffles, so instead he just twisted the standard Van's baffles to fit the taper as best as they can, then he blocked off the big gap at the top with RTV.  He said he's never had high CHTs using this method.  By blocking off the gap at the top portion, it helps force the incoming air down through the fins. More research led me to the fact that the base of the cylinders really aren't nearly as hot as the outer heads where all of the combustion happens, hence the smaller fins to begin with.  So instead of cutting everything up, I decided to take the approach of twisting the baffles to fit and blocking the air path at the top.

For a couple of the tapers, the top gap really isn't very significant, so for those I'm just going to use RTV to create a bridge between the baffle and the fins when I do the final install of the baffles.  The two front cylinders have pretty sizeable gaps though, and I'm not sure how well RTV would work.  I decided to fabricate aluminum wedges to rivet to the baffles to fill the gaps.

It's a little hard to see, but here's an example of one of the wedges in place.  You can see it following the shape of the taper.  This isn't a perfect solution like redoing the entire baffle to fit the taper would be, but it sounds like it's probably just fine in the real world.

For cylinder 3 (aft right side), I wasn't able to get the inside baffle in place because the tab of the curved portion was catching on the screws of the  hose clamps of the oil return lines.  I took the clamps off and turned them around so the screws are on the bottom, which gave me enough room to get the baffles in place.

I've had the baffles on and off about 100x at this point.  I'm getting close to a point where I'll have to put the cowls on in order to trim the baffles to their final height and length, so I started riveting pieces together and screwing them to the engine.  I need everything to be solid to do the fit with the cowl, even though I'll still have to take it all off at least a few more times to do the final trimming (and painting - I decided I'll probably go ahead and paint the baffles black, because why not).

The aft right baffle is where I'll get fresh air for the cabin heat.  I cut a 2" hole in the baffle and riveted on a fitting that the scat tubing will connect to.  It also has a piece of metal screen sandwiched in it.  I'll seal it with high temp RTV once the baffles are painted.

Unfortunately, I can't finish drilling and riveting the aft left corner together yet.  The oil cooler mounts here on the aft baffle, but it can't be put in place until the tops of the baffles are trimmed to match the cowl shape (because the oil cooler should be placed as high as possible for the best cooling, and until the trimming is done for the top cowl, you don't know where the top lines of the baffles actually are).  I did beef up this corner pretty considerably using a 1x1 piece of angle.  It's a known issue that the weight of the oil cooler will crack the baffle after a few hundred hours without some extra structure.

Baffle Boredom

I've been slowly chipping away at the baffles the past few weeks, but my time has been spent in 10 minute chunks, so there really hasn't been much progress to show in terms of pictures (still isn't).  I'm at the point now where I've gotten all of the main baffle sections trimmed to fit the contours of the engine pretty well.  The last big problem to sort out is the inner cylinder baffles that wrap around the bottom of each cylinder (the shape of the Titan cylinders are different than the Lycoming's).  I'll either have to cut off the existing wrap-around tabs and fabricate new ones that match the tapered shape of the Titan, or just twist and bend the existing material and call it good.  I'll have to do a bit of research.