Started baffles

October 31st, 2010

Time to start on the engine baffles, which form a box on top of the engine to direct cooling air down through the cylinder fins. Everything I've ever read about the baffle kit has mentioned weak instructions and poorly fitting parts. However, Van's must have re-engineered the baffle kit sometime recently, as my baffle parts are CNC-punched and the instructions feature computer drawings. They are still confusing and contradictory, but at least the pictures are pretty!

Lots of sheet metal parts:

You start by chopping some partially pre-cut angle stock into six or seven separate pieces, which will become brackets and stiffeners:

Various pieces have stiffeners and doublers attached. I riveted on the bolt-hole doublers as instructed, but I am holding off on riveting anything else until I have a better handle on what's going to fit and what will need modification.

It didn't take long to get crosswise with the plans… on the second page, they want you to start cutting and drilling the port-side aft baffle for mounting the oil cooler. However, at this point you don't know how tall the baffles are going to be, which means you also don't know where the oil cooler should be positioned vertically. Not to mention, I'm not sure I'm even going to be able to put the oil cooler on the baffles anyway (see below).

I decided to leave the oil cooler stuff along and skip ahead in the baffle plans. In this area at the right-side aft corner, the preformed parts don't quite match up:

Rather than force the parts together – a recipe for cracking – I made a little 0.032" spacer to fill the gap:

Many hours later, I now have most of the bolt-hole doublers riveted to the baffles, and about half the baffle pieces deburred. Smoothing the edges of all those funny-shaped parts is highly tedious.

This is my oil cooler… it's a Stewart Warner 8432R, which is a bigger version of the 8406R, which is itself a higher-quality replacement for the no-name cooler that Van's will sell you. I let the guy from Pacific Oil Cooler talk me into buying this one – the argument being that it's easier to fix an installation with too much cooling capacity than too little. That does make sense, but if I'd known how little room I'd have to mount it, I might have gone with the next size down!

The oil cooler is beautifully made – look at those welds! – but it's also heavy and expensive and kind of fragile. I didn't want to risk dropping or damaging it while experimenting with mounting locations, so I built a dimensionally accurate stand-in out of scrap wood. If the wooden oil cooler fits, the real one will too.

The plans want you to put the oil cooler here, on the baffle behind the #4 cylinder. I might be able to make that work, although the flimsy sheet metal baffles will have to be strengthened considerably to properly support this brick.

Another possible idea is to attach the oil cooler to the engine mount instead. That would certainly be easier from a vibration point of view, but then I'd also have to come up with a way to duct a sufficient volume of air to it, while still fitting within the tight confines of the cowl. Hmm…

Posted in Baffles

Center cowl joint

October 3rd, 2010

On the inner faces of the air inlets, the cowl halves are joined together by three screws on either side of the spinner. Here I'm marking and drilling pilot holes using the spacing provided in the plans:

This is an area where you typically see a lot of wear on RV's that have been flying for a while – no surprise, since the screws only have a thin layer of fiberglass to bear on. I decided I wanted some reinforcement here, so I fitted a piece of 0.063" alclad to each side of the upper cowl. This will give the screws something else to wear against, and actually helps in another way by filling some gaps in the area where the top cowl overlaps the bottom cowl flange.

Then I glued each one to the inside of the cowl using an epoxy/flox mixture. After curing, it's now pretty much permanently attached.

After the doubler plates were affixed, I put the cowl onto the airplane and match-drilled the pilot holes into the lower cowl.

Oops! The dimensions in the plans will cause the aft-most screws to interfere with the engine flywheel and ring gear. No problem, I just filled up the holes with flox and redrilled new holes farther forward.

I figured it couldn't hurt to bond some more doublers inside the lower cowl too, so I did that before riveting on the nutplates.

Nutplates and doublers permanently attached to the bottom cowl flanges:

Test-fitting the screws – everything lines up, which is a relief. These screws are only barely threaded into the nutplates, due to all the stack-up of extra material. I'll probably have to use 5/8" screws instead of 1/2", but that's okay.

Posted in Cowl

Attached metal stuff to cowl

September 26th, 2010

Well, I couldn't think of a better title for this post. I mixed up some epoxy and flox and glued the horizontal splice plates and rear hinges to the lower cowl:

What I should have done next was wait half an hour, then come back and remove the half-dried epoxy mess from the inside surfaces of the metal with a solvent rag. But I was tired so I went to bed instead. That meant I had to come back the next day with a grinder and tediously grind off all the dried epoxy. This also took some of the alclad off the aluminum, but that's okay since it will all be painted in the end.

Countersunk the outside and squeezed all the rivets… pretty standard stuff:

Here you can see how the epoxy grabs on to the holes drilled in the aluminum. Be careful to keep epoxy out of the hinge material. I used a squared-off piece of popsicle stick to dig the glue out from between the hinge segments, and I used a piece of steel rod coated in Boelube to keep it out of the hinge eyes.

Back on the airplane. Now the only clecoes in the fiberglass cowl are in places where there will eventually be some kind of removable fastener – either a camloc or a screw.

From the side, you can see that I have some work to do to even up the face of the cowl behind the spinner. I'll probably save that until later, since it won't affect what I'm planning to work on next.

Posted in Baffles

Cowl shaping

September 12th, 2010

Here's what the corner cowl flanges looked like after being trimmed to shape. (They aren't really that white, it's just the flash)

The top cowl overhangs the bottom cowl by about 3/32" at the forward outboard corners, giving it a sort of "overbite" appearance. To build up the lower cowl, I first put packing tape along the mating edge of the upper cowl, then clecoed the two halves together on the workbench:

Then I mixed up a batch of epoxy and microballoon filler:

Filler is applied with a popsicle stick over a light epoxy wipe, then roughly contoured with a rubber squeegee:

After curing, most of it gets sanded away as the new shape of the cowl is formed:

Then comes another round of filling and sanding, and then another. After three iterations, it's starting to get where I want it:

You can see where I filled in some low spots on the bottom of the left air inlet. I think the vacuum bag got partially wrinkled here when the piece was being molded, but it's all smoothed out now.

The fit is pretty good now – no more overbite. I will wait to do the last bit of sanding until I have the cowl back on the airplane and locked in to its final position.

To sand the filler to shape during the above steps, I used a variety of different tools. Each one has its own strengths and weaknesses, kind of like a team of superheroes. Let's run down the dossiers:

Team Member: Two-Face

Background: Was an ordinary piece of aluminum angle until unfortunate particle accelerator accident. Then had sandpaper glued to it.
Zodiac Sign: Gemini
Strength: Squaring corners
Weaknesses: Bad for large areas, sometimes sands where it shouldn't

Team Member: The Strip

Composition: Stainless steel, adhesive sandpaper
Thickness: 0.020 inches
Strength: Good for sanding gap between cowl halves
Weaknesses: Limited applications, sharp edges
Ouch My Hand: What did I just say

Team Member: The Rod

Origin Story: Wooden rod with sandpaper taped to it. Also, bitten by radioactive spider.
Cover Identity: Nuclear-plant employee
Strengths: Inside corners, curved surfaces
Weakness: Flat areas

Team Member: The Block

Alias: The Preppin' Weapon
What Seriously: Yes, that's the actual product name
Yikes: Tell me about it
Strength: Most ergonomic sanding block ever
Weaknesses: Bad for inside corners, goofy name

Team Member: The Saw

Manufacturer: Xacto
Application: Adjusting kerf between cowl halves
There Is No Such Word: Yes there is. Kerf is a word.
Philosophy: Every problem in life can be solved by cutting
Hobbies: Cutting, slicing, windsurfing
Weakness: Not every problem in life can be solved by cutting

Team Member: The Hand

Technique: Holding a piece of sandpaper with your hand
Main Power Source: Engineer's hand (L or R)
Strengths: Contours to any surface; can also open cat food containers
Weaknesses: Stamina, precision, sports

Sanding team, assemble!

Posted in Cowl

Cowl corner flanges

September 6th, 2010

While waiting to see if my epoxy will still cure properly, I drilled a bunch of #30 holes in between the existing rivet holes in the metal side flanges. When I later bond and rivet these parts to the lower cowl, the holes will help the epoxy get a good grip.

My epoxy turned out to still be good, so it's time for what will probably turn into several weeks of glass work. The first thing I want to do is build up some flanges on the inside of the lower cowl, at the outboard corners. Eventually these will be used for a single screw and nutplate per side, similar to this example. First I applied clear packing tape to the areas where I didn't want the epoxy to stick, and clecoed the two halves of the cowl together on the bench:

I laid up five plies of cloth from my box of scraps and stuck it to the inside of the cowl, after first cleaning the area of interest and wiping it down with epoxy.

This is how it looked after it cured, immediately after removing the top cowl:

I should be able to work with this. It needs to be cleaned up and trimmed/sanded to shape, of course. Then a bit of flox on the front to ensure a good fit, a few more plies on the back if it needs stiffening, and I'll have my flanges.

Posted in Cowl

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