Matt's RV-7 Project

After a seemingly never-ending process of measuring, marking, trimming, and test-fitting, I finally have the forward baffles trimmed to fit the cowl inlet ducts.

I ended up totally re-making the center crankcase baffles from new parts. It was really tough to figure out the proper shape here, so I'll post plenty of photos.

The key thing here is to try for a nice smooth transition up the side of the inlet ducts to the inside surface of the top cowl. This is highly dependent on how you shape your inlet ducts so there is a feedback effect as you work on the shape of the various parts.

Close-up of how the parts join together just behind the spinner:

All the various baffle parts are cut about 3/8" behind the aft edge of the cowl opening:

A view from the other side:

I bent the forward edges of the crankcase baffles inward to help the rubber baffle seal material bridge the gap:

Trimming the outboard baffle on the #2 cylinder was fairly easy. I already made the corresponding modifications to the outboard #1 cylinder baffle in a previous work session.

I had to grind away more baffle material in front of the #1 cylinder to make room for the back of the flywheel:

A view with the cowl on… the gap between the crankcase baffle and inlet duct looks huge due to the camera angle. In real life it's between 3/8" and 1/2".

Same area on the left side. This photo shows where I still need to do some fiberglass work to square up the face of the cowl behind the spinner… that can wait till next summer, though.

Looking in through the spinner cutout. This is the best photo I could get of how the crankcase baffle runs up the side of the inlet duct towards the apex of the cowl. It's almost impossible to see in real life – you mostly have to figure it out by sticking your hands in through the inlets and feeling around. Naturally this makes trimming the baffles a long and tedious process.

On to the next step…

The oil cooler is attached to the baffles behind the #4 cylinder, at the left rear corner of the engine. (I did briefly consider mounting it elsewhere, but I couldn't find a place that it would fit) Unfortunately, the plans are especially vague about exactly where to position the oil cooler on the baffle, leaving you up to your own devices to make it fit. Also, the thin sheet metal baffles are really not strong enough to be hanging an oil cooler from, which is widely known to eventually lead to cracks. Many builders add reinforcement to the baffles to better support the weight of the oil cooler, which is yet another thing you have to design and build on your own. This will be a long post showing how I fit the oil cooler and built up the baffles to support it.

The CB-705B brace is a prepunched part that forms a box at the top of the #4 baffle, and adds some stiffness to it. Its rear flange is supposed to share some rivets in common with the oil cooler doubler ring. They don't give you any guidance on where to position it vertically, but I think this is how they want you to install it: (rear baffle removed for illustration purposes)

The problem with this is that positions the oil cooler so low that almost half of it is blocked by the cylinder fins. For an RV-8 with its narrow cowl this is your only option, but in a side-by-side RV there is room to mount it higher for better cooling airflow.

My solution was to cleco the aft flange on CB-705B to the next row of prepunched holes on the oil cooler doubler. This allows the doubler, and thus the oil cooler opening in the baffle, to move upwards a substantial amount.

Obviously the aft flange on CB-705B now blocks an equivalent amount of the oil cooler opening, but there's a solution for that.

Test-fitting the oil cooler to make sure everything is going to fit. You want to mount it as high as you can to get the most airflow through it, but you don't want it to get so high that it hits the inside of the cowl. There is also a constraint related to the clearance between one of the mounting bolts and the diagonal engine mount tubes. And, you don't want to put the rectangular opening so close to the edge of the baffle that you run into edge distance problems.

It's typical to have to grind away a portion of the flange where my finger is, to get the proper clearance between the oil cooler and the engine mount.

Drilling the doubler to the rear baffle locks in the oil cooler position. The sharp-eyed will noticed that I drilled a hole for a third bolt in the middle of the outboard corner. Since the oil cooler has a hole for a bolt there, I figured why not use it…

The three clecoed holes in the CB-705B flange will be used – the rest of the flange will be cut off.

If you are working on this part of the baffles right now, this photo contains enough information for you to position your CB-705B:

I riveted a piece of 0.040" angle to the top of the horizontal part of CB-705B. It picks up the row of rivet holes along the top of the oil cooler doubler. It's all one big puzzle that you have to figure out how to fit together, while you're also designing the pieces.

Lots of work between the last photo and this one. I built a number of reinforcement pieces to be attached to the baffles, which I'll detail below. Here's the master shot of all the parts that will be riveted together to form the #4 cylinder baffle and oil cooler mount:

From left to right, this is the aft cylinder baffle, 0.063" spacer, and oil cooler doubler ring. These are all kit parts, no bespoke items yet. Note that the outboard top corner of the doubler ring is substantially ground away to fit the inside of the cowl.

I used a unibit and air nibbler to make the rectangular opening in the aft baffle, using the doubler ring as a template. A complex pattern of dimpling is required to accommodate the different rivets that are flush on one side or the other. The small piece of angle picks up the two inboard bolts on the oil cooler, and will become the attachment point for a diagonal brace later on (a future project).

Another view of the aft baffle and doubler ring. Flush rivet holes everywhere – be careful to pay attention to which way each one faces!

I used a piece of 1/8" angle to stiffen the corner where the aft baffle and outboard side baffles meet. I had to file a radius on the apex of the angle (not visible in this photo) to get it to nest into the bent flange in the corner. The holes where the oil cooler bolts come through are just barely far enough from the angle web to install the nutplates – careful measuring is required here. Note the countersunk holes on the aft side of the 0.063" spacer where it faces the dimpled holes in the aft baffle.

The leftmost and rightmost pieces in this photo are designed to stiffen the side and rear baffles by providing a load path from the oil cooler to the forward mounting screw. This shown more clearly in one of the photos below, keep reading.

This is what 705B looks like after it's trimmed to fit the cowl, and after the bottom flange has been truncated and a new flange riveted to the top. I also added an extra little tab on the left side to tie it into the cylinder baffle.

Here's a final fit test to make sure everything is going to work. It all seems to line up, thank goodness.

I used a hacksaw, rotary file, and scotchbrite drums to notch the oil cooler flange where it comes close to the engine mount. No clearance problems now, but probably no more warranty either!

It took hours to rivet the whole thing together. Many of the rivets are hard to get to, and you have to pay careful attention to the order you install things so you don't block access to any of the holes.

Another view from the side:

Here's how the stiffener pieces on the outboard side work:

And a closeup of the triangular, joggled attachment at the aft end. The two blind rivets here are the only ones in the entire #4 cylinder baffle, believe it or not – I managed to squeeze or buck all the rest.

Top view:

A view from the inside. Note how the horizontal part of CB-705B is even with the top of the oil cooler opening. That turned out pretty well – not sure why Van's didn't just design the part that way.

Nutplates on the corner stiffener. Some of these rivets were an absolute pain to set properly. It wasn't my best or prettiest riveting work, but it's adequate.

Back side, showing the doubler ring. Rivets are flush where the oil cooler flanges go.

Test-fitting the oil cooler to the completed baffles. I cut and ground a bunch of AN970 large-area washers to spread the load on the cooler flanges, which are made of some very soft type of aluminum and don't seem very strong. Later on I'll prime and paint the steel washers to keep them from rusting where I cut through the cad plating.

I turned some spacers on my lathe to fit around the bolts and prevent the oil cooler flanges from bending.

The inboard side has one long bolt (with spacer) and one short bolt where the rear flange was ground away. The lower hole doesn't get a bolt since the cylinder fin would be in the way on the forward side.

Believe it or not, it's possible to install and remove this whole huge thing with the oil cooler still attached. With the baffle screwed into the cylinder head, the oil cooler mounting is very solid.

Plenty of clearance between the inboard flange and the engine mount – the plans call for a minimum of 1/2", and I have between 5/8" and 3/4".

Mounting the oil cooler up high allows more of the face to be exposed to the local airflow, which should help with oil temperatures in the summer.

The only part of the oil cooler mounting arrangement that's not as stiff as I'd like is the inboard side, which flops around a bit when shaken. This is only natural, since there's no good way to attach anything to the cylinder between the case and the head. Later on I'll rectify this by running a brace from the flange on the inboard side of the oil cooler to a convenient screw on the cylinder head.

I dabbed a bit of zinc chromate on an area of the oil cooler where I lightly gouged it while trying to fit the short bolt on the inboard side. Got to protect this expensive and vital piece of machinery.

That's it for the oil cooler mounting, for now anyway. A friend once told me, "Keep adding stiffness to the baffles until you can attach a chain and tow the airplane around by the oil cooler." While it may not be quite that strong, it's definitely a lot better than if I'd stuck with the flimsy factory design. Hopefully by putting in the extra effort now, I'll have saved myself some heartburn down the road.

Once the baffles are roughly trimmed far enough to generally fit under the top cowl, you then have to trim them further so you get a nice even gap all the way around the engine. I used the "old paper clip trick" to figure out what to trim and what to keep. I didn't invent this method, but here's how it works…

You start by putting a whole bunch of paper clips on top of the baffles. The jumbo size seems to work best.

Then, very carefully put the top cowl on, and push it down onto the paper clips. Here's a view of what's happening inside, looking in from the oil filler door:

This is another view looking aft from the spinner opening:

Then you carefully remove the cowl, and if you're lucky you're left with an impression of the inside face:

Mark a line the desired distance down from the top of the paper clips – without bumping them out of position! – and proceed to trim, file, and deburr.

Then, repeat a dozen more times! Seriously, I did this for hours, tweaking the fit a little more each time. The baffles came off and went back on many many times, which I didn't bother to take pictures of. But the finished product looks something like this:

Not a very interesting picture, I know. So here's a shot of the whole airplane with the cowl on, which is more fun to look at:

It's finally time to start fitting the baffles to the top cowl. The upright baffle parts all start life being extra-tall, and then they get trimmed down so they end just short of the top cowl. You don't want too small of a gap between the fiberglass cowl and sheet metal, which could cause the shaking engine to damage the cowl; you also don't want too large a gap because it prevents the rubber air seal material from doing its job.

But, since the cowl isn't transparent, how do you know where to trim without a lot of tedious trial and error? I used a method suggested by someone else, which begins by using wood strips to elevate the top cowl some known distance above its usual position:

After making sure the gap is even at each corner, you then reach through the gap to trace the contour of the top cowl onto the baffles.

It just happened to work out that a popsicle stick is just the right length to give me a 3/8" offset from the inside cowl contour. I notched the end to locate the tip of a sharpie, then used this contraption to draw a line on the baffles inside the cowl.

This is a picture looking in one of the cowl inlets, showing the line I traced. This worked pretty well and wasn't too difficult, other than requiring some painful contortions to get the pen onto every corner of the baffles while working in a limited space.

After removing the top cowl, I retraced the wobbly line so I could see it while cutting:

Then I removed all the baffle parts (quite a chore).

This is the first time in quite a while that the engine has been baffle-less:

The bandsaw made quick work of the initial trim cuts. I stayed outside the line, since there is some error inherent in this process, and I didn't want to cut off too much too quickly.

Everything goes back on for fitting, for the nth time:

After only one cut, it almost fits! Still a little trimming left to do in the aft corners, where I had a hard time making sharpie marks.

Here's a view into the right inlet, with the top cowl in place. I'll have to trim the cylinder baffles down a lot further once I glue the upper inlet ducts in place, but that will come later. For now, I'm glad I haven't attached them yet, since they would just be in the way.

Next: Yet more trimming…

It's relatively easy to remove and replace the engine baffles when they're still individual pieces clecoed together; it's tougher to do it when the various pieces are assembled into big unwieldy shapes. I am trying to design for future maintenance as much as possible, so I am trying to ensure that the baffles can all be removed without drilling out any rivets. In certain places this might require using screws where rivets are specified – we shall see. I know that the finished baffles are all going to have come off at least once before the airplane flies, since there is already at least one airworthiness directive on the cylinders that will need to be addressed.

With that in mind, I tested my ability to remove the baffles attached to the #1 cylinder as a complete unit – cylinder head baffle, inlet ramp, and crankcase baffle all clecoed together. With the flywheel removed, it's just possible to twist and turn it enough to get it off the engine and put it back on. So, all the parts you see here will eventually be riveted together as a single assembly.

I riveted the stiffener angle and the various brackets to the inlet ramp. Some rivets could be squeezed, and some had to be driven. Surprisingly, considering I haven't used the rivet gun in ages, none of them look too bad. Note the use of flush rivets between the three screws that will eventually attach an air dam in front of the #1 cylinder.

Then I riveted the cylinder baffle and corner gusset to the inlet ramp With careful planning, these rivets can all be squeezed if you do it in the right order. You can see where I used some red RTV between the mating parts to fill some gaps that would otherwise be air leaks.

I'll wait to rivet the crankcase baffle until I finish fitting the baffles for good, since not having it permanently attached makes it a lot easier to remove and reinstall the baffles… and unfortunately there's still a lot more of that to come.