Archive for the ‘Filtered Air Box’ Category

Making the FAB Intake Seal

May 27, 2013 Leave a comment

Monday May 27, 2013

Making the rubber seal that couples the filtered air box to the cowl intake is tricky business. The shape of the FAB opening is such that the seal must be tapered toward the front so a straight strip of seal material does not fit well. I needed to determine a good shape for the seal strip that fits my air box so I made a paper template first. It took several iterations but I got to a shape that lays down against the side walls and overlaps the air box by 0.7 inch. I am using a 0.4 inch gap between the air box and the cowl intake and another 0.7 inch overlap with the cowl intake for a total span of 1.8 inches. Here is the paper template taped in place.


The shape turned out to be an arc which is not a big surprise. Ignore that little chamfer in the upper left corner. That was a mistake.


I could not use the stock rubber strip from Vans because it is a straight strip, 1.8 inches wide, not a arc. I order a 12 x 12 piece of .062 thick Nitrile rubber from McMaster-Carr as a replacement. I cut the arc strip using my paper template and taped it into the air box for a trial fit. It looks like it will make a good seal if I make some RTV fillets in the top corners.


I put the cowl on to see how it looks from the outside where the cowl intake will mate to it. I debated attaching the seal to the air box but that had two major drawbacks. First it would leave the seal hanging out on the air box when taking the cowl off which will make it harder to clear the cowl. Also, it would put the unattached edge of the seal strip upstream in the air flow so it could curl up and block the air flow (not good). So the seal will be securely attached to the air intake and the internal pressure will force the seal against the internal walls of the air box for a good seal.


The next step was to make the seal fit to the air intake. I don’t have pictures of the process but I can say that it took several iterations of trimming, shaping, and laying on more fiberglass to get the seal to fit well. The original tunnel I made using the Vans process did not fit well so I cut off the last 0.7 inch and formed a new one. And I contoured it internally to provide a smooth tunnel for the air flow while externally it is shaped to sit flush to mate flush to the conical shape of the seal. Here is the modified air intake with a new seal strip I cut with some tabs that I will explain later.


This is how the seal strip fits onto the air intake. I drilled for rivets but I am thinking about other ways to attach this that don’t require rivet heads inside the tunnel.


Categories: Filtered Air Box

Attached Alternate Air Door – 2 hrs

May 12, 2013 Leave a comment

Sunday May 12, 2013 (Mother’s Day)

I mostly took a break today from the build because it is Mother’s Day but I did do one thing and that was to install the alternate air door on the filtered air box and attached the cable. To make sure I had the right location for the Adel clamp on the air box I first finished the routing of the cable along side the mixture cable. I used zip ties to lace them together along the firewall. Then the point on the FAB for the Adel clamp became clear. I installed a #10-32 nut plate inside the air box. To cut the cable I pulled the wire back out about 18 inches then cut the outer sheath using a dremel with a cut-off wheel. Then I pushed the wire back through, wrapped it around the screw three times and trimmed off the excess length. When I operated the door from the cockpit I was pleased to find that it opens smoothly and it even closed properly when I push the knob back in, something that it is not guaranteed to do. In most cases they say you have to remove to cowl to reset the door.


Various Tasks – 5 hrs

May 11, 2013 Leave a comment

Saturday May 11, 2013

Today I worked on a variety of small projects, trying to wrap up loose ends. My first activity was to make some custom parts for the P-Mag blast tubes. I saw a thread on VAF about how to route and secure blast tubes from the engine baffles to the P-Mags. An interesting idea was to wrap safety wire around the P-Mag body and route it though the blast tube into the engine baffle compartment. That keeps the tube pointed right at the point of the P-Mag where the wire wraps. I gave it a try and it worked well but there was nothing to attach the safety wire to in the baffle compartment. So I designed some interesting little parts to secure the wires to. They are made of two round pieces of .025 aluminum with a 1-inch hole in the center. Two of these pieces are riveted together on one side and the other sides are bent outward to make a little bridge that goes over the blast tube hole without blocking it significantly. Here is one during a check of the fit over the blast tube.


I made two of these parts, one for each P-Mag blast tube. I primed them and painted them red to match my baffles. I also primed and painted the alternate air control bracket while I was doing paint.


So here is the right blast tube with the safety wire wrapped around the P-Mag and routed through the blast tube.


I put the new bridge part on, pulled the safety wire tight, and wrapped the it around the bridge. That all there is to it. The parts is not riveted to the baffle or attached in any way. The tension in the safety wire keeps it tight against the baffle.


Next I  riveted the alternate air control knob bracket to the forward bulkhead and re-installed the control cable. I put a short piece of edge grommet material on the bracket in case I have a tendency to hit it with my leg.


This picture shows that I also finish primed the fiberglass air box, painted it and the top cover, and riveted the subassembly together. Black goes with the AFP servo.


Here is the filtered air box installed on the servo. This is almost done. I just need to attach the alternate air cable and finish the rubber seal strips on the front opening.


Last but not least I drilled a hole and installed the manifold pressure line in the firewall. I used two Adel clamps to hold the line to the engine mount.


Installing Alternate Air Control Cable – 2.5 hrs

Tuesday May 7, 2013

I started out to install the control cable for the alternate air door on the FAB. The obvious question was how to route the cable and where to mount the knob. I don’t really want this control to be accessible to the right seat passenger because the cowl has to be removed to reset the door if the knob is pulled. If the alternate air door needs to be opened, the pilot should be the one to do it. So I decided to put the knob under the subpanel on the left. It can be easily reached by the pilot but is not anywhere close to the cabin heat knob which looks almost identical.

The cable routing is trickier. I could not find any information on the forums for my configuration so I had to figure this out myself. After considerable head scratching I decided to route it through the firewall recess directly above my mixture cable. Then the cable turns downward along side the mixture cable and then turns forward to the bottom of the air box. Once I settled on that I became concerned that it might interfere with the VPX-Pro control box so I had to temporarily mount that to check for interference. The photo below shows the VPX on two .063 angles approximately 19-5/8 long. This is just held to the ribs by cleco clamps right now but it gave me enough confidence to proceed with the cable installation.


I made a small bracket out of .040 aluminum to mount the knob to the left bulkhead just beneath the cool air vent scat tube. Then I punched a hole in the firewall recess directly above the location where the mixture cable feedthru was placed. Installed there an eyeball feedthru with a .188 diameter hole which fits the alternate air cable perfectly. In this photo you can see how the cable routes in an S bend above the rudder pedals.


Here is a view from above after I added a couple of zip ties to hold the cable down below where the VP-X will mount. The bends are gentle and the cable knob moves freely. This routing is good with the stock 6 foot long cable.


Molded FAB Intake – 6 hrs

Sunday May 5, 2013

The filtered air box connects to the air intake scoop with a fiberglass tunnel that is custom molded. I started by installing the FAB to the fuel servo and making sure that the front opening was pointed directly at the air scoop by looking in from the front. Then I removed the lower cowling and filed a slight taper to the scoop opening where the fiberglass will overlap. This is to give the fiberglass a surface to bond to without reducing the inside diameter of the scoop.

Then I fit the foam block into and against the opening from the inside of the cowl. This required me to carve away the corners of the block and taper the front edges a bit to get it flush with the inside surface of the cowl. After confirming it was flush I bonded it in with four big globs of bondo which cures up in only 30 minutes.


Then I put the lower cowl back onto the fuselage and began cutting out the tunnel. I started by drilling a 1-inch hole with a hole saw. Then I used a hack saw blade and a file with a curved face to carve out the foam back to the inlet of the air box.


It was pretty easy to carve the foam and get the shape to blend from the oval opening at the front to the more rectangular shape at the rear. The file was better for the finish work.


This is how the carved out tunnel looks from the inside. There is a slight taper as the opening gets larger as it goes aft.


I mixed up some epoxy and laid three layers of 8 oz glass around the inside of the tunnel. I wet the fiberglass between two sheets of plastic drop-cloth film and rolled it out to remove excess epoxy and that allowed me to put the layers down smoothly on the foam which was first wet with a thin layer of epoxy to help the glass stick. Then I put down peel ply to smooth it out and help minimize sanding later. I did not use a balloon as described in the plans.


Late in the day I pulled off the peel ply and sanded the inlet area a little. It looks pretty good. I still need to paint on a layer of epoxy to fill the weave but it looks very promising so far.


I chipped away the foam block and the bondo globs and sanded the outside of the tunnel to get the last of the foam off. This looks like some kind of weird fish orifice. Its not shown here but I also applied two more layers of fiberglass around the outside of the tunnel at the fillet with the cowl as recommended in the instructions to strengthen that joint.


I also worked on  installing the alternate air door on the bottom of the FAB. First I sanded the fiberglass shell because I found lots of cracks in the surface of the primer. I don’t know why that happened but I sure didn’t want to paint over that so now the box looks kind of ugly. Regardless, I drilled a 2-11/16 hole in the bottom of the box for the alternate air door bezel. The blue tape is the direction I plan to run the cable which is what I used to orient the bezel. You can also see the two 1/8 inch holes I drilled for drainage; one inside the filter and one outside the filter.


After match drilling the holes I put in machine countersinks for the rivets. I bent the little tab on the bezel which acts as a catch and a stop for the door. I mixed up some ProSeal and riveted the bezel to the box with ProSeal in the joint. THe I temporarily installed the door with the supplied hardware. Now the ProSeal needs 3-5 days to cure before I paint this thing.


More Filtered Airbox Work – 8 hrs

April 28, 2013 Leave a comment

Sunday April 28, 2013

I worked mostly on the filtered air box this weekend but I also made a small doubler plate for the front left engine baffle section because I realized the gap to the engine there is too large to seal with RTV. I used a piece of scrape aluminum from the baffle trimmings and painted it red like the rest of the baffles. It is riveted on in this picture with the four unpainted rivets in the lower right corner.



So then on to the airbox again. I made the six aluminum retainers for the filter, aligned them to be flush with the filter cutout, drilled, dimpled and installed nut plates. This photo shows you how they fit with the filter.



I went out to Corona on Saturday to take my biennial flight review (I passed) and I stopped at ACS to pick up a few things including a 2 x 2 foot sheet of .032 alclad aluminum 2024-T3. From that stock I made a doubler plate for the .062 thick top mount for the FAB. You can see it on the left in this picture after I scuffed up the surfaces of the plates for bonding.



I want a uniform thin layer of ProSeal between these two plates so I made some small shims out of .016 thick aluminum stock and epoxied them down to the .062 thick plate as a first step for bonding. I allowed the epoxy to cure about 5 hours and removed all the excess with MEK and fine sand paper. Then I mixed up a batch of ProSeal and spread it out thin over the plates (but not on the shims). I put the two plates together and aligned them with clecos, then I removed the clecos and stacked a piece of plywood on top and added several gallon paint cans to press it down. After about 10 minutes I removed the paint cans and plywood and found that the two plates had shifted relative to each other. The ProSeal is like a lubricant and it allowed the top plate to flow because the table is not perfectly level. It was clear I need a way to maintain plate alignment while this cures.



I drilled holes in the plywood to allow clecos to pass through with clearance around each one and I drilled six holes in the table top so the cleco tips would not touch the table either. Now the clecos align the two plates in plane and the plywood just presses down on the plates for uniform pressure. You can see I put wax paper on each side of the plates stack to keep it from sticking to the table and the plywood.



Here you can see four gallon paint cans and a 10 lb bag of cat litter on top squishing it all down to apply pressure while this cures. It will take a couple of days to set up so I won’t find out until at least Tuesday if this was a success.



I also drilled the airbox top cover flange to the fiberglass shell this weekend – except for the part in front of the upward bend that is. The plans say to wait on that part until the vertical alignment of the top plate is set with the cowl opening. The bare fiberglass shell is kinda ugly so I sanded it and sprayed on a coat of white primer. An hour later I found a bazillion pin holes so I put on an additional coat of UV smooth prime because it fills the pin holes better than Stewart Systems primer.



I also carved the airbox foam block enough to fit it into the front air intake opening. This is in preparation for carving out a hole and laying up a fiberglass tunnel to the air box. I have to wait until the air box is assembled and back on the engine and aligned before I bond this plug in place and start carving.



This photo shows the approximate line around the intake scoop that I am considering cutting to make the scoop removable for easier cowling installation. The line will be at the top of the tape leaving just enough room for a ring of screws (probably six).


Filtered Air Box Construction – 1.5 hrs

April 24, 2013 Leave a comment

Wednesday Apr 24, 2013

A couple of days ago I cut out a piece of .025 thick alclad sheet to fit into the bottom of the filtered air box fiberglass shell. That is because I have heard of cases where vibration caused the filter to literally rub through the fiberglass. The aluminum sheet just provides a harder surface against which the filter contacts so hopefully, if anything, the filter will wear and not the aluminum. I glued the aluminum sheet into the shell using ProSeal and tonight it was pretty well set up.

The evolution of the filtered air box continued tonight as I fit the top plate to the fiberglass shell. The top plate needs to bend upward near the air inlet so I cut two notches in the side walls and made a slight bend between those two notches so the top plate generally follows the angle of the fiberglass shell.


I flipped the top plate over and drilled four additional holes to attach to the top mount as well as hold the clips that restrain the position of the filter. I cut the raw stock for the clips and match drilled them to the top plate in the positions required to hold the filter just inside the cutout in the top plate. The filter actually sits flush against the top mount.


Here is the quasi-assembled air box with the filter installed.


Finished Riveting Engine Baffles – 6 hrs

April 21, 2013 Leave a comment

Sunday April 21, 2013

There were lots of chores needing attention around the house this weekend so I only had time to do a few things on the airplane. At least I finished riveting all the rubber seal strips to the engine baffles with a little RTV on the surfaces to seal up air leaks. I’m glad to have this done. My hands are sore from squeezing all the pop rivets.


This view shows the aft side of the spinner area.


I also did some more trimming of the filtered air box top. It is now a little over 2 inches from the front edge to the inside edge of the inlet. That should be enough that the cowl can drop straight down without hitting the top cover. I am mentally struggling with how much gap to put between the air box and the inlet scoop. I am concerned about making the gap too small because of the threads I have read about FAB top plate cracking, perhaps caused by forces put on the FAB at engine startup due to relative motion between the FAB and the cowl. But a large gap may be harder to seal effectively. I also want to make the cowl as easy to install as possible and the FAB seal can be a major factor in that. I had a few questions I posted on the VAF forums so I am waiting to see what responses I get before proceeding.


I also sanded the exterior of the top cowl this weekend in preparation for applying a seal coat of thinned epoxy. No pictures of that.

Started Filtered Air Box – 4 hrs

April 14, 2013 Leave a comment

Sunday April 14, 2013

Today I started working on the filtered air box, also known as FAB. I am hoping this does not turn out to be as big of a project as the engine baffles but it didn’t take long to encounter the first problem. The Airflow Performance fuel injection kit came with a nice gold anodized FAB adapter plate that is pre-drilled to fit the clamping collar that attaches it to the servo. They even made it thicker (.090) than the standard top plate – I assume because of the many reports of the adapter plate cracking. I clamped the adapter plate to the air box top plate and mounted it to the servo for a check fit to the cowling.


First I found that the air box top plate is too long and interferes with the cowl but this was to be expected. So I cut an inch off the front end and tried again. This time nothing interfered and I was able to see the impact of the offset of the fuel servo from the centerline of the cowl. I could rotate the air box top plate to point directly at the cowl air inlet but there was only about 3/8 inch clearance between the air box top plate and the inside surface of the cowl along the side of the air box. I was not comfortable with that little clearance because of the relative motion that occurs at engine start up and shut down. So I began investigating options to slide the adapter plate sideways a bit to increase clearance. I want at least an additional 1/4 inch or a total of about 5/8. Unfortunately when I tried to slide the AFP adapter plate that far to the side two of the six holes that will mount it to the air box top plate have less than acceptable edge distances. After looking for options for a while I finally concluded that I need to make a new adapter plate that has the center hole offset by at least 1/4 inch and is wider to maintain hole edge distances. I used the AFP part as a template. In this photo you can see the new plate underneath the gold AFP adapter. I roughed out the new adapter with extra material on the near side for the mounting holes.


I sanded about 1/8 inch off the side of the AFP plate so I could slide it a little further and transferred the center holes for the fuel servo to the new adapter plate. Here is the new adapter plate mounted to the air box top plate. I used two holes to connect them temporarily with clecos. You can’t see it in this photo but I also trimmed about 1/4 inch off the inside edge of the top plate center cut-out on that same side to allow the air filter to sit flush against the adapter plate all around. This means the shape of the air filter will be slightly elongated on the left side compared to the standard oval race track shape. I don’t see any way this can cause a problem.


When I put the new setup on the engine for another fit check I measured a clearance right at 3/4 inch which is the thickness of the middle knuckle on my right middle finger (precision gauging). This is much more comfortable to me than 3/8 inch. This is a lousy picture looking in from the exhaust exit of the cowl but you can get the idea of the clearance along the side of the silver top plate. The clearance will decrease about 1/16 inch when I install the fiberglass air box.


As far as the new adapter plate is concerned, I made it from .062 alclad 2024-T3 stock. I considered using .090 stock like the AFP part but there are reports of even those cracking. I suspect the problem is also related to the stiffness of the rubber coupling between the FAB and the cowl inlet which puts loads on the FAB every time the engine is started but that is really a hypothesis. Another theory is the vibrations of the engine are causing fatigue failure. So my approach is designed to address both theories.  I plan to make a doubler out of .032 thick 2024-T3 alclad and bond it to the new .062 thick adapter with a thin layer (~.010) of Proseal. That will give me a total aluminum thickness of .095 for strength and the Proseal will add some damping to the plate to attenuate vibration response. I will also make the rubber coupling between the air box and the cowl snout as compliant as I can. Of course I will inspect the new adapter part regularly to look for evidence of cracking but I think this design approach gives me a good chance of avoiding the problem. We will see.