Matt's RV-7 Project

When last we saw the air filter, its brackets were fitted but there wasn't anything to hold it in yet. So earlier this week, after doodling various methods for securing the filter, I drove up to Airparts on my lunch break to buy a sheet of 0.040" aluminum. Then I cut out a nine-inch square and bent a lip and a joggle into it. Since I only have a small hand brake and not a real joggling tool or any kind of press, this was harder than I thought.

I put a diagonal bend into the new piece to match the inlet ramp:

Then I bent the excess material at the front end into a 90-degree flange. This was kind of a chore, as I wanted the two flanges at the front to match up exactly, as well as for the joggled lip at the back to fit over the transverse stiffener angle. It took some doing to get it all fitting properly. Once I was finally satisfied with the fit, I match drilled the holes through the filter brackets and inlet ramp, plus two more holes at the front to tie the front flanges together.

To attach the side baffle to the inlet ramp and filter retainer, I used some 0.040" angle. This will be riveted to the side baffle, and then the whole stack of angle / retainer / inlet ramp / filter bracket will be held together with screws and nutplates.

I marked the outlines of the cutout, and rough cut it with the air nibbler:

To cut the hole to the final size, I used the drill press like a mill again, followed by files and various deburring tools.

Still a lot of details to finish up, but now at least I have the air filter mounted in its brackets, and the retaining ring that will hold it in place is mostly built.

By the way, this is what the plans for the inlet air duct and filter look like… xeroxed pencil drawings! The best thing to do is to use them to understand what the end goal is, then put them aside and figure out on your own how to build the parts you need to make things work.

Cold day in the garage with the air filter and a pile of aluminum bits. After a lot of careful measuring, I came up with a basic plan of attack, starting with making a new set of brackets. Then I bent a 90-degree flange on the front of the baffle, right at the forward edge of the air filter.

The filter butts up against the new flange at the front, and is held at the back by a piece of W-shaped bracket material. Actually it is more like VW shaped, since I had to put another bend in it to match the shape of the underside of the baffle.

I cut some new side brackets and clamped everything in place on the airplane:

Then I match drilled the side brackets to the inlet ramp baffle. Laying out the holes on the outboard side was challenging due to edge distance issues. The air filter is almost the same size as the inlet ramp!

Once all the brackets could be clecoed to the inlet ramp, I was able to reassemble the whole contraption on the workbench and see underneath. This is what it looks like on the bottom side:

I marked cut lines on the baffle, even with the edge of the filter on the sides and back, and somewhat back from the edge on the front face.

To do most of the cutting, I used a carbide grinding tool mounted in my drill press as a sort of crude milling machine, then followed up with files and scotchbrite. The hole has to be square at the back to admit the filter, but I left the corners round on the front side where the filter fits under the baffle overhang.

Here it is again for a round of test fitting:

The filter tucks under the front lip, then lays down into the cavity formed by the brackets. Of course it is just sitting in there, so I will have to build some kind of retaining device to keep it from falling out.

The air filter takes up pretty much the whole baffle, leaving no clearance between the baffle and the cowl. I marked a new trim line on the cowl:

I cut and sanded the cowl lip until I had an even 3/8" of clearance along the entire front edge of the inlet ramp. There isn't much cowl material left, but there should enough to attach the air seal material.

I don't have the air filter totally licked yet, but I think I am over the hump at last. Now that I know it is actually possible to make it fit – barely! – it's all detail work from here onwards.

I know, no updates for quite a while… not only has there been the usual round of holidays, travel, garage coldness, and winter illness, but also the following fact: even though I've been doing a lot, I don't feel like I've gotten anything done in a while. That is, in the sense of (adjective) completed; finished; through. The engine baffles and air inlet duct are each a series of interlocking mysteries, and I haven't completely cracked either one. Still, I have a bunch of pictures on my camera, so I guess I should post them so you can see I've been making some amount of progress.

I worked on fitting the left inlet ramp to the cowl, starting by drawing a line from the outboard corner of the baffle to the point where it touches the lower cowl:

I put an upwards bend along the line, a little at a time with plenty of trial and error fitting in between:

With the correct amount of bend in the middle, the inlet ramp now touches the lower cowl all along its length. Well, almost touches – I want an even 1/16" gap between the two parts to accommodate the rubber airseal that will go there later on. You can also see I've trimmed the outboard edge to bring the #2 cylinder baffle in line with the cowl.

I cut this baffle attach plate to fit, and mounted it on the engine case:

Drilled and clecoed the associated angle piece:

Here's an end-on view showing how the various parts are arranged. This locks in the angle of the left-side inlet ramp.

Per the plans, I cut a round plug out of some scrap wood. It kind of looks like Jupiter. Or maybe something else.

Using thin packing tape, I attached it to the throat of the fuel injector metering servo:

The plug centers the engine air duct on the engine air inlet while it's clamped in place for fitting.

This is how the air duct fit straight out of the box. Lots of trimming and grinding awaits.

It took lots of hours of tediously iterative work to improve the fit, and I didn't take many pictures along the way. Here's one I found on my camera that documents my use of some special-purpose tape that's specifically designed for taping ducts.

You can't control the fore-and-aft position of the duct on the baffle, but you can adjust the side-to-side fit by grinding away obstructions and rotating the duct around the wood plug on the fuel servo. It's made extra challenging by all the stuff that's in the way, and the fact that the inlet ramp has a kink in it.

I stopped trimming once the duct was centered side-to-side on the inlet ramp and the fit was pretty good all around. It will get trimmed back later to allow space for the air filter.

I had to grind off quite a bit of material to fit the duct around the alternator – in fact I'll have to open up this hole later on to provide some extra clearance.

The starter also has two protruding lugs on it, just visible in this picture, that hit the duct. I ground away more fiberglass to make it fit.

Skipping ahead a bit, here's what it looks like after being fitted. The extra holes will be glassed over once I'm done with everything else, thus making it a custom-fit part.

After looking at the documentation for the starter, I discovered that it's permissible to cut off the unused lugs on the starter as long as you don't cut into the case itself. Good news, since I was worried about how much the airflow in the duct would be constricted if I had to use fiberglass to mold in "dimples" for clearance.

So, I removed the starter from the airplane in order to perform some elective surgery:

Hah, not so tough now, are you?

He was no match for Señor Hacksaw and Mister Grindy:

Not a great picture, but you can sort of see that the starter lugs no longer intersect the surface of the duct. Next time I get out the glass and glue, I'll patch these two holes. That's the one nice thing about fiberglass – especially for nonstructural applications, you can usually put back on whatever you accidentally took off.

I marked a line on the duct, 3/8" from the underside of the inlet ramp:

Then I cut and sanded to the line. You can see the odd shape required to match the bottom of the bent-up baffle.

I drilled holes to mount the duct to the fuel servo, which was harder than it ought to be due to the lack of room to work in. Then I trimmed the excess material off the duct flange.

Bolted up for a test fit. Two of these bolts are very difficult to put in – too bad this piece still has to go on and come back off many more times.

I traced the outline of the duct onto the inlet ramp baffle. The plans say to cut to the line, but I'm not sure I trust them on this just yet.

Instead, I used the air nibbler to rough cut a hole about a half inch back from the line. That will give me room to adjust the hole size once I figure out the air filter mounting scheme.

Placing the baffle, filter, and duct on the engine for a trial fit:

At least the duct/filter/baffle intersection is a good fit. But, there's not much room between the filter and the cylinder, or ahead of the filter – especially once the air ramp gets trimmed to its final length. I'm going to have lots of fitment and edge distance problems here.

I made these filter brackets by following the dimensions in the plans. Silly me! Maybe I'll be able to use them as templates for how not to make the correct brackets – once the replacement material arrives, that is. Argh.

Really, the secret to the baffles and air duct is to use the plans only enough to understand the desired outcome, and then improvise the actual construction as necessary. Come to think of it, I think I saw this in a movie once.

Oh, I also finally broke down and bought myself a real work stool to sit on while I'm messing with the engine. For the last fifteen years I've used this milk crate as a chair while doing who knows how many projects, and I was starting to get really tired of it. Judging from its appearance, I'd say the feeling is probably mutual.

Slow progress on the baffles… I got the left-side baffles mostly fitted. By the way, the CB-706B spacer I made per the plans was way too short and I had to remake it from leftover material. Do yourself a favor and verify all dimensions given in the baffle plans before you start cutting!

The two rear case baffles didn't initially fit together very well with the center baffle brace:

A lot of the trouble was that they needed to be trimmed to fit better around the various bumps and bulges on the engine case… a process that took a lot of on-and-off fitting.

Closeup view of the same area after improving the fit:

On to the inlet ramp baffles… I did the usual steps of match drilling and deburring, and also drilled a one-inch hole as specified in the plans:

I had to disconnect the factory-installed prop governor oil line, which eventually ends up going through a grommet in that big hole:

Initial trial fitting of the right inlet baffle:

Closeup of the prop oil line; the yellow square is a bit of foam rubber to keep the pipe from getting all scratched up. Also note that the baffle needs to be trimmed to keep it from grinding against the ring gear.

I removed the dummy prop hub and flywheel for easier access:

Further work on left and right inlet ramps. You can just see where I ground away some of the right-side one to clear the ring gear:

The plans have you rivet "temperature risers" to the inlet ramps, which are simply a couple squares of aluminum that block some of the airflow to cylinders 1 & 2 (the front ones) so it can be used by cylinders 3 & 4 (the back ones). Problem is, the sizing of temperature risers is somewhat peculiar to each particular installation, and the plans method of attaching them requires you to remove the baffles and drill out rivets in order to modify them. Not a great idea if you ask me. Instead, I put some nutplates on the inlet ramps (screws visible in this photo) so I can experiment with differently sized temperature risers as needed.

Both inlet ramps fitted to the engine:

I was getting frustrated trying to decipher the next page in the plans, so Mary brought some refreshment down to the garage. I tell you what, the atmosphere in this place isn't so great, but the service is excellent.