Cowl Cutting & Baffle Tensioners

The Sika has cured on the rear window.  There's a bit of a mess for me to clean up, but for the most part everything lined up pretty well and the window is setting where it needs to. I still need to clean up the rollbar area and put a final bead of Sika on it to make the transition to the rear window look a little nicer.  I'm waiting to do that until I see if I need Sika out for anything else though - easier to do it all at once vs a little here and there.

I'm still chipping away at the lower cowl fit.  What a pain!  The right vertical side is where I needed to extend the edge.  The flox worked really well here, other than being a nightmare to sand.  This picture is before I took it down to the final edge - a lot of work for 1/32-1/16" addition.  It looks nice and consistent now though.

I had originally built up the bottom (inside) of the cowl with flox to try to get it to lay on the fuselage better.  About 5 hours of sanding later, it turns out that didn't work.  Oh well, there's a lesson in everything I guess.  So I pretty much sanded the entire flox addition off the bottom and am now almost back to where it was from the factory.  That factory shape still doesn't work though, so I need another solution.  The only thing I can think of at this point is to cut a relief in each corner to give the bottom the flex it needs to bend up toward the fuselage edge.

The cut lets the bottom snug up against the bottom fuselage flange since it's no longer constrained by the strong cowl corner.  I will push the cowl into position, then fiberglass over the cut to hold the shape. Once it's dry I will pull it back off and put more fiberglass (probably flox again - yippy) on the outside to fill the crack and smooth the outside shape.

The corners and bottom of the cowl need to be pushed in while the epoxy cures.  A few well placed boards did the trick.  I made a small square of fiberglass (3 layers about 2x2) and put it in position on the inside over the crack.  I'll know tomorrow if it was a success or not.

I can't do much additional fitting while the lower cowl dries.  The top cowl gets inlet ramps to help smooth the air out as it enters.  They don't fit very well (shocking, I know).  I put them in position as best as I could guess and drilled some holes to hold them.  They will get epoxied on once the top and bottom cowl are in place and attached to each other.  The instructions warn that epoxying them with the top off will cause fitment problems because they will make the top cowl more rigid.  So everything has to be in position while the epoxy cures.

This is a little out of order, but the other day before the lower cowl went back on I worked on the baffle tensioners.  Each cylinder has a couple of baffle sections that wrap down and around the cylinder fins.  These have to get pulled snug to the cylinders and held there (otherwise the air pressure will just push them away from the cylinder and allow air to leak past without actually cooling the fins).  Each cylinder on a side has a baffle on the outside and inside fins, then they get tied to the matching baffle extension on the other cylinder.  There are a lot of ways to do that, but the most common are with stainless rod that you thread each end of and use nuts on, or safety wire.  Van's supplies stainless rod.  All of the experienced builders say it's a real pain though, and safety wire is the way to go, so that's what I went with.

Each baffle has a flange with a hole in it that the safety wire can go through.  My first approach was to cut a small piece of stainless rod for the wire to wrap around.

I put the L on the rod to insert it into a second hole.  I'm using large safety wire, and it doesn't hold the rod very tight.  By having the second hole in the flange, the rod can't go anywhere.  I slid some old brake line over the safety wire to protect it from rubbing against stuff.

The rods worked fine, but then I saw that an even easier method that most people use is to simply wrap the wire around a washer.  There is very little tension on these baffle pieces, so the wire really just holds them in position vs pulling them much.  The washer is plenty of surface area.  When I pull out the high temp RTV to seal all of the baffle gaps, I'll put a dab on each washer to keep them from vibrating.

Rear Window (2)

Yesterday I marked up the window where I needed to tape it off for the Sikaflex.  I also sorted out what spacers I needed under it in order to have it match up relatively evenly with the main canopy.  Since I'm going to put a fiberglass targa strip over the whole thing, it doesn't have to be perfect, but the closer it is the easier it'll be to fiberglass.  I just used various pieces of wire - I think that'll be easy to pull out once the Sika is cured (then I'll just fill the holes in).

 I pulled the window back off and scuffed up the plexi plus the rollbar and inside of the fuselage skin.

I cleaned the plexi with the Sika activator, then put the primer on and let it flash for 30 minutes.

The rollbar and skin got the same treatment.

Now for the messy part.  The window has to be slid underneath the skin and set on top of the rollbar.  The center of the aft part fits in a very tight space between the skin and the center brace that goes to the rollbar.  I think I sat and stared at it for 10 minutes before I got up the nerve to go for it.  In the end I got it slid around and in the right spot with all of the spaces correct, but in doing so I made a total mess!  Even though I had most everything taped off, I still managed to get Sika all over things that shouldn't have Sika on them.  Luckily, the adhesive doesn't stick very well to anything that isn't primed, so once it cures I will be able to clean it off.  I pull a strap over the top to pull the window down onto the spacers, and I used some blocks cut to the shape of the fuselage to pull in the skins at the side.

The window naturally springs out against the skin, but the 10 and 2 locations still sag a little.  I used some old scrap flooring (a little flexible) and some foam to push the window out.  Now I just have to wait for a day and cross my fingers.  I pulled most of the electrical tape off while everything was still wet, but I didn't crawl around too much to see how things look.  At this stage it's too late already anyway, so I might as well let it dry before I poke around and just make it messier.  I'm sure there's going to be a decent amount of clean up to do, as well as another layer of Sika in places.  I don't think I actually put as much adhesive on as I could have (I didn't see it squeeze out in most places), but in terms of strength, a little Sika goes a long ways, so it'll be fine.

Rear Window

Hours of sanding and shaping fiberglass doesn't look like much!  Lesson learned with flox - it is HARD to sand.  At least everything after what I've done up to this point can be done with micro, which will be easier to work with. I needed a break from dealing with flox though, so switched over to the rear window.  Because of how the canopy and rear window set, the original cut that split the canopy into two pieces has to be modified.  It took a dozen or so iterations to get it where it needs to be.

The window slides under the skin.  It's fairly tight, but I think I'll have to figure out a way to put pressure on it from the inside while the Sika cures.

I ended up making the gap between the canopy and window about 1/8".  It's not perfectly straight and even, but I decided not to obsess over it given that I'm going to be putting a fiberglass strip over it anyway.

Fiberglass the Cowl

The fuel hose from the fuel flow sensor outlet still needed to be clamped to the engine mount.  I did that and also connected the wires for the sensor.

I made the rough cutouts for the louvers yesterday, but they still needed to be finished up.  I sanded all of the cutouts smooth and countersunk the rivet holes.  I'll probably wait to rivet in the aluminum pieces until I'm done with everything else on the cowl and it gets a coat of epoxy primer and paint to seal the inside.

I can't drag my feet any longer - it's time to tackle the awful fit of the cowl.  I wrote a bunch of notes on the cowl in an attempt to remember what needs to be done.  Here are all of the reference pictures for me.  Nothing interesting to see other than how ugly the cowl fits out of the box.

I mixed up a full cup of epoxy with flox and a little cabosil.  The flox is for structural strength, and the cabosil is simply a super light filler whose purpose is to help keep the epoxy from slumping with gravity.  Flox doesn't spreads very smoothly, so I know I'm going to have a lot of sanding in my future.  I tackled the right side vertical edge by sanding a scarf and adding flox so I can ultimately extend the edge about 1/16" to fit the firewall.  The lower edges that skybolt to the fuselage pucker out (have to be sucked up against the fuselage) and need to have more material to allow me to sand them even with the fuselage, so I added 1/8" of flox to the inside.  That will allow the inside to set against the ledge of the fuselage and give me enough meat so I can essentially taper off the original material on the outside.  I also added meat to the inside of the bottom corners (the round corners of the fuselage) so I have more material available to sand the outside to match the fuselage curves.  The front portion of the cowl is going to take the most work.  Matching the top and bottom cowl will be a lot of iteration I think.  I built up a lot of the areas that currently have gaps, then I'm just going to have to slowly chip away at sanding them to fit.  I don't expect to get perfect with the flox.  Once I'm close, I'll be able to fill in the small gaps and smooth it all out using micro.  Flox is very hard to sand and shape, but it's strong.  Micro (small glass balloons) is not nearly as strong, but much easier to sand and is what is usually used for the vast majority of fiberglass finishing.  Once the main shape is better, I'm also going to have to use glass cloth to fix the nosebowl flanges.

The cowl is going to take a day to dry, so in the meantime I started working on the rear window.  It's still too big and needs to be cut down to size.  The first thing I did was prop the front canopy up a little and scribe a line so I can make the first cut to get it parallel to the front.  I think Van's usually recommends something like 1/16" gap, but I'll probably do about 3/32 or 1/8".  I've heard of guys getting stuck canopies if they make the gap too small and the plexi swells because of heat.  1/8" sounds like a lot, but I'm 99% sure I'm going to do a fiberglass targa strip over the back edge of the canopy, which will cover the gap.  The targa strip is just a fiberglass/carbon fiber strip that goes from the side canopy rails, over the top to the the other side rail.  It's not commonly done, but from what I've read it provides a tremendous amount of stiffness to the canopy.  As is, the canopy flexes/twists pretty bad when lifted open and closed, making it super easy to catch the right side plexi corner on the roll bar when lowering it closed.  Do that too quickly or accidentally drop it and you've got a broken canopy corner.  The targa strip will also help me smooth out the uneven gaps on my side rails.

Fuel Hoses, Cowl Louvers

It's been a while.  Between being away from home for a bit and starting a new project for work, I haven't had much energy to work on the plane.  Hopefully I can get back to it now though.  I've done a number of random small items over the last few weeks, but nothing too exciting.  I finished up most of the FWF wiring, so I sealed the ends of the pass throughs with fire barrier.  then I wrapped them in silicone tape and put the hose clamp on (no picture of the final product).

I received the last few fuel hoses I needed.  The recommendation by the fuel flow sensor manufacturer is to mount the sensor in a way that just suspends in with the fuel hoses (with the hoses adel clamped within about 6 inches of the sensor), as opposed to rigidly mounting it on the engine anywhere.  I have quite a bit of room on the lower left firewall, so that's where I decided to locate it.  The one remaining task I have is to adel clamp the output hose that goes to the throttle body.

The last fuel hose is the one that goes from the firewall fitting to the intake of the mechanical fuel pump.  I ordered a 45 degree fitting to go on the firewall side to help the hose clear the engine mount and allow more room for the scat tubing to the cabin heat box (removed in this picture).  The one gotcha I discovered was that the fitting on the fuel pump was also a 45 degree fitting, which kind of torqued the hose in a very tight radius and made it rub the engine mount.  I ordered a new straight fitting from Aircraft Spruce, only to discover the next morning that I already had a spare straight mount.  Oh well, I guess now I'll have another spare that I can forget about.  With the straight fitting in place, the hose fit much better, although it's going to make for a very tight route for the cabin heat scat tubing.

Switching gears - It doesn't look like much, but I spent an afternoon prepping the cowl hinges.  I drilled large holes in between the rivet holes.  It's not absolutely necessary, but it's recommended that in addition to rivets attaching the hinges, that epoxy or proseal is used as well.  The addition of an adhesive just gives the rivets some additional support and minimizes the chance that they'll work loose over time.  The holes provide space for the adhesive to get some extra grip.

I figured I needed to cut my teeth on something fairly simple to learn a little about fiberglass.  I scuffed up the area around the oil door and epoxied/riveted the doublers in place.  I already learned some lessons with it -  I didn't add quite enough flox to the epoxy, so it was a little runny after clamping/riveting and made more of a mess than it should have.  I've heard that the oil door can pucker out a little bit during flight due to the high pressure in the top of the cowl, so I decided to bond on the chunk I had cut out from the cowl to double up the strength of it.  With that in place, it's very stiff.

After spending hours and hours looking at the cowl fit and marking it up everywhere to help me remember where I need to thicken it up, extend edges, etc, I needed a more mindless task.  Nowadays Van's recommends adding an optional louver on the lower cowl to help with cooling.  Apparently the change from the simple front landing gear to the new elastomer type impacts the exit air enough to raise engine temperatures.  The louver provides a little more exit air.

I laid out where each louver would go and match drilled them to the lower cowl.

Once the marker holes were drilled, I expanded the holes that will form the sides of each cutout.

Lastly, I cut out the material between each hole.  The aluminum piece will be riveted to the inside of the cowl and the aluminum tabs will be bent inward to catch the air.