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RV-7 N117TR

I admire your tenacity...it's turning out great!

It's true, sometimes you just need to take three days off and stare at what you've done for a while.
 
I admire your tenacity...it's turning out great!

Thanks for the positive reinforcement!

I was able to add the soric and 1 ply of 8.9 oz glass to the inside of the plenum then covered with peel ply. I used a wet fillet of epoxy/flow in the flange to top transition of the plenum to join/blend the soric at the edges. The 1 layer of glass covered the soric and flanges of the plenum. Of course I had enough epoxy to finish it without opening the new batch that arrived today.

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In 30 minutes my 7 month old and I were able to clean up the plenum flanges and excess pour foam, I would defenitely use the pour foam again as a "mold" reinforcement.

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There were two large air bubbles I found between the glass and the soric, I guarantee they were not there when a I left the layup for the night. The layup was rolled and stippled until all air bubbles were gone, not sure what happened. Oh well, I'll sand them out and patch with glass, I would really like to piddle with a vacuum bag setup.

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I built in an extra 1/8-1/4" of clearance on the front of the plenum, this should give me 5/8-3/4" clearance between the front of the plenum and the cowl. The original metal baffling cleared the cowling by 1/2" or slightly less with no signs of contact. Not sure if the lightweight prop or rubber baffle material pushing on the cowl had anything to do with less than 1/2" clearance being enough.

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Whew, real life got in the way the past couple weeks and it took an hour to find my bookmark. My last couple build sessions have been filled streaming Merle Haggard, good stuff! Plenum filled and sanded, just awaiting the pinhole process.

I used the router to make some plywood disk to hold the front of the intake boot plug concentric with the inlets, the jigsaw cut some angle chunks of foam.

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The cowling on the airplane, at least it still fits!

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The whole glue some foam together and you have your own plug/shape thing takes more time than my usual double the estimated time and the add 25% estimate. Slow going getting the foam to fit and the glueing together with dry epoxy/micro and coarse wood screws. The very rough shape that needs to dry before I hack it up.

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One of the hardest thing of this project for me was cutting my custom 7 hinge/cowling covers off, I had weeks of work in those things. So I know I wanted to carry over weeks of....eeerrr ahhhh......I mean those cool 7 covers. The new hinge pin geometry really likes a little more room so I made them slightly bigger, I'm going to try to shape some aluminum 7's this time. I made some aluminum ones last time but ended up making them from glass. I've only got a couple hours in this cardboard 7 template, it still took 4 variations of different radius's to get the 7 I liked.

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One flat aluminum 7 waiting for me to final shape the cowling so I can hopefully beat it into the correct shape.

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Looks like you're getting close to being back in the air, Andy!

Lots of work. You've got more patience than I, that's for sure.
 
Thanks guys, I'm hoping to be back in the air in 5 weeks.....

Roughly sanded to fit, I think the bottom of the diffuser boot is going to take the most imagination.

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That is looking real GOOD! I may need to be doing something similar to what you have going on here. I like how you moved the center baffle back, is that to enhance air diffusion once inside the ducts?
 
That is looking real GOOD! I may need to be doing something similar to what you have going on here. I like how you moved the center baffle back, is that to enhance air diffusion once inside the ducts?

Brett, I've seen your paint pics and those look GOOD!

And all of my posts are coming from the mind of an amateur.

I also saw your baffling post, although the Van's baffles may seem like a lot of on and off trimming etc they are light years ahead of making your own. It takes lots of headscratching and building to even get to the point where you start fitting like Van's baffling. On the other hand if you have the time and the "want to" it's kinda fun, like your paint job.

Looks like you are using a SJ cowling so the standard Van's baffle seal material is out the window, mixed reviews on the performance/installation of the SJ plenum so making your own isn't too far fetched.

Again I am an amateur so it's more my perception than fact. Yes, the curved center baffling gives me a longer diffuser length and eliminates a square corner that is hopefully more efficient at turning the velocity of the incoming air into the highest pressure possible. It is an entire system concept though, larger cooling air inlets with more external diffusion that is more tolerant of sins aft of the inlets and mass flow throttled by the exit. DanH does a much better job explaining it than I do.

I'm not sure what theories are involved with the straight tube design of the SJ plenum.

Lots of work but I would use Van's baffling for the perimeter of the engine, make your own plenum and inlets. I really wanted to add tabs to my existing perimeter baffling and lay a plenum on top, but I don't think it is the best strength wise and # of fastener wise. I think it's stronger with less fasteners to use flanges on the plenum and fasteners in"shear." I can forward some old articles/studies that influenced my inlet/diffuser shapes if you like.
Good Luck!
 
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inlets

I'm running SJ cowl with 5.25 inlets and I have a problem with cylinders running to cold and of course high oil temps on the ground.What are you planning for material between inlets and plenum for engine movement?
Bob
 
I taped some cardboard around the mounting flange for the inlet ducts to simulate the thickness of the rubber.

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I fit the top cowl several times and made sure the plywood discs were flush with the inside face of the rings.

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I couldn't get a fit I liked with the foam alone, so I over sanded the flange area on the foam and used a thick bead of dry micro to make an exact shape or so I hope.

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I spent some time shaping the foam inlet plugs, I'm not exactly an artist but this plug is close to final shape.

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I'm playing around with Sheetrock joint compound for finishing my intake plugs and pinhole filling on my plenum, so far it's not drying any faster. It is was easier to apply to a mostly finished surface by just squishing it around with gloved hands.

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A "rough" size comparison of my stock Van's inlet area to my new round inlet area. Everything was roughly estimated based on guesstimated measurements. My previous stock Van's inlet on a O-360 vertical induction cowl:

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A "rough" measurement of the new inlet diffuser:

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Andy, a few notes...



You'll need to extend your plugs further to the rear if you want to form the necessary rubber surface for an overlap seam.

I notice some negative draft angle here and there. Be sure to remove it during the shaping process, or you won't be able to remove the plug from the cured rubber/fiberglass sleeve.

Plug finish has to be perfect, polished paint or similar. Unlike the typical homebuilt epoxy/glass layup, there is no opportunity to improve the surface of the finished part after it is removed from the plug. As-molded is what you'll live with. Take your time.

You're actually building permanent tooling, not one-shot throwaway forms. We don't yet know the lifespan of the urethane rubber inlet ducts, so the ability to easily make another set sometime in the future is worth considering.

Last, the performance of the particular rubber I used has been entirely satisfactory except for one detail...it is sensitive to water. If left saturated or submerged long term, I'm pretty sure the ducts would mimic the Wicked Witch of the West. It's not been a practical problem, but it bugs me. Were I to build another set (yes, I have the plugs stored away), I'd investigate rubber compounds to see if that detail could be eliminated.

Plugs after epoxy/micro for final shape, brushed with System Three's Clear Coat epoxy for sealing. Here you can see it dripping off the plywood. Next will be skim sanding at 220 grit, then high build primer, then 400 wet sanding, then finish paint.



Rubber surface, as removed from the mold. As you know, paint on three or more coats of mixed rubber, like a gel coat application, before laying any cloth.



Hanging in there, now about 525 hours and 5.5 years since first flight.

 
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Nice work Dan and Andy.

Can't help but wonder, is a plenum better than standard baffle system?

Local RV-4 pilot is in the process of removing a plenum and going back to baffle system. Not sure why but it is happening.
 
Can't help but wonder, is a plenum better than standard baffle system?

It's about sealing. Sealing relates to two issues, cooling and drag, which although related, are not quite the same thing.

Cooling mass is forced through the cylinder fins and oil cooler by the difference in pressure between the upper and lower cowl volumes. Given the typical large exit, leakage past baffle seals lowers that differential pressure, resulting in less mass flow, thus less cooling. The brain-dead "fix" is to increase exit size rather than fix the leakage.

Why brain dead? Because although it may fix the cooling problem, any additional air passing through the cowl which doesn't pass through a heat exchanger is just pure drag. The old Mississippi State NASA reports showed that typical GA flap baffles allow a lot of leakage. The typical GA installation works with crappy baffling because the cowls generally have huge inlets and exits proportional to the HP cooled. They are also very draggy. Our RV cowls have inlet and exit sizes which don't offer much margin for bad sealing.

I can pretty much guarantee the system you see above has less than 1% leakage, and I'll stop those leaks when I see them. The goal is to pass less air through the cowl and heat it more, while maintaining residual pressure in the lower cowl volume in order to increase exit velocity.

BTW, measurement says the big round inlets are not a lot better than standard RV-8 inlets at converting dynamic pressure to increased static pressure. However, the leakage rate is far lower than standard flap baffles.

A plenum with a bad inlet is going to perform worse than standard inlet in terms of pressure recovery. IMHO, there are quite a few plenum installations with crappy inlets. The automatic advantage to a large, low velocity inlet is that it is hard to do it badly; they are tolerant of error. Look close at some recent GA designs with low Vi/Vo inlets. They work fine with no diffusers inside the inlets...zip.

Which brings us to a logical question; why one-piece rubber ducts rather than the common hard fiberglass snout with a tubular flex connector? First, with high engine torque (meaning at low airspeed, just when max cooling is needed) most of them are misaligned; the end of the snout doesn't line up with the inlet ring. The result is a huge lip inside the inlet, which trips flow. If inlet velocity is low (the tolerant low Vi/Vo inlet), it won't make a huge difference. If velocity is high (a small ring, high Vi/Vo inlet with an internal diffuser), tripping the flow hurts pressure recovery.

Second, in my own application (RV8, big motor, metal CS prop), I did not want a prop extension, as it would have created a CG problem. So, the inlet ducts needed to be short. A low velocity inlet doesn't need length for internal diffusion...and the rubber duct doesn't need length for the straight sections required for clamping a flex coupling to a hard duct.

Working out the idea, early 2009:

 
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Brett,
they were covered in Sheetrock mud as an experiment that turned out poorly. I ended up baking them in the oven to cure it enough to sand the mess off.

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Dan,
Most of the plug surface had a decent overlap (1") with the plenum and baffle aluminum, a few areas the epoxy/micro left holes I have to fill. I'm also working with the previous baffles which leaves a 1/2" lip on the vertical baffle in front of the #1&2 rocker cover. The negative draft angle on the front was on purpose to make sure the edge was formed with epoxy/micro, I'll take care to look for a negative angle elsewhere. The plugs with a new layer of epoxy/micro rough sanded and ready for the next coat. The white area is the flange area under the plenum and baffling.

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David,
I'm told it is a cooling SYSTEM. Inlets, shapes, sealing, baffling, RTV, exits and many other items comprise the system. One item in the system inlet, exit, sealing etc can have dramatic effects on the performance of the system. I spent 2 OSH visits wandering around looking at how others had tackled the inlet sealing problem. I was shocked at how many RV's lacked baffle rubber or had backwards baffling rubber around the inlets, such that the incoming air pushed the baffle strip inward to the lower cowl bypassing the cylinders. It makes removing the cowling easier but not good for cooling. More than 90% of the RV's with the stock square inlets and a plenum had no sealing between the inlets and the plenum. There were very few examples of well done stock or roll your own inlet sealing, a couple plenum to inlets sealed very well as well.
 
Dan,
I'm not sure the PMC 870 is still available, I've ordered the Smooth On 780 urethane. I'm under the impression that the cured product does not stick well to itself, from your previous post it looks like you molded the flange onto the cured intake boot.

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Your method would be preferable as long as the cured rubber sticks to the new flange, I also have the recommended glue. Perhaps some experimentation is in order.
 
Andy, I guess I should have warned you about joint compound. It sounds good then won't dry, then it goes in the oven, then it cracks or melts the exposed pink stuff. Yes, I tried them all. The best way, as you have found, is to sand the insulation to perfection a super smooth surface, then use micro and a resin layer. If you had continued, the poor adhesion of the joint compound would have pulled chunks. Wore out that t-shirt too.

Call smooth-on, as it was hard to find the right elastic material. They do have it, the last time I checked. I thought DanH had the numbers crossed, but what he used was not common on the website. I contacted them and it was available.

Good progress Andy, the elastic inlets are perfect for a standard cowl length with a tight expansion distance. I did some crude blower experiments and it is likely that there is flow separation 3" aft of the inlet ring. I will do some flight tests before making any hard conclusion, as I think the pressure recovery will be excellent anyway. Your larger holes will help that greatly.
 
Call smooth-on, as it was hard to find the right elastic material. They do have it, the last time I checked. I thought DanH had the numbers crossed, but what he used was not common on the website. I contacted them and it was available.

Bill, did you happen to ask about the water sensitivity?

This from Lars, way back when:

http://www.vansairforce.com/community/showpost.php?p=344305&postcount=19

In retrospect, getting oil on the inlet ducts has never been a problem. However, they get wet on a regular basis It's interesting when they do; even though the rubber compound is tinted black with a pigment at mixing time, cured rubber left in contact with pooled water turns white when wet. Wipe off the water, allow to dry, and it turns black again. The wiping rag will show slight black staining, which tells me I wiped away a few rubber molecules with the water. The damp surface will feel slight tacky, but returns to normal when dry.

Just an oddity, really. The 870 is doing fine in service.
 
Bill, did you happen to ask about the water sensitivity?

This from Lars, way back when:

http://www.vansairforce.com/community/showpost.php?p=344305&postcount=19

In retrospect, getting oil on the inlet ducts has never been a problem. However, they get wet on a regular basis It's interesting when they do; even though the rubber compound is tinted black with a pigment at mixing time, cured rubber left in contact with pooled water turns white when wet. Wipe off the water, allow to dry, and it turns black again. The wiping rag will show slight black staining, which tells me I wiped away a few rubber molecules with the water. The damp surface will feel slight tacky, but returns to normal when dry.

Just an oddity, really. The 870 is doing fine in service.

Hmmmm, no I did not. That is interesting, though. I would have thought the part that wipes off might be chalking on the surface, but turning white is interesting. It must be some absorption. I have no specific information.
 
Didn't know Smooth-on had quit listing PMC-870.

Looking at the other urethanes, PMC-780 Dry might be a reasonable choice. Mixed viscosity is 2000cps, which is a little high for saturating cloth; just have to try it and see. 25 minute pot life is fine. The interesting detail is that it's used for concrete and gypsum casting, and they recommend a water based release agent. Hardly sounds like a water-sensitive rubber.
 
I hope to have some test samples of PMC780 dry made up in the next couple days. I should've got a sample kit of the PMC790, it's mixed viscosity is 3000 cps.
 
I made some samples with some Smooth On PMC780 dry, I also used black so strong color tint. I kinda wanted to do grey by mixing black and white but I'm glad I didn't, I don't think I could be consistent on the color ratios in different batches. I waxed and brushed a couple layers of PVA onto a Tupperware cup, apparently it was my wife's FAVORITE Tupperware cup which I returned unharmed. I applied 3 coats of rubber at 30 minute intervals with a brush, it was too thin and mostly slid down the sides of the cup. The consistency worked well to saturate 1 ply of 8.9 oz glass which I applied on top of the previous 3 coats of rubber followed by 1 coat of rubber over the glass.

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The rubber copied the "rough" outside surface of the cup perfectly, a perfect little round squishy rubber tube.

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More "testing" needed, the tube was too flexible with just one glass layer but most of the rubber didn't end up on the tube. Smooth On also sells:

"URE-FIL? 9 - fumed-silica based thickening agent primarily used with liquid urethane and silicone rubbers for brush- on moldmaking applications. URE-FIL? 9 is added in small amounts to the mixed rubber until the desired viscosity is achieved. URE-FIL? 9 can also be added to liquid urethane resins to create casting e ects and to thicken the material for trowelable casting applications."

It sounds a lot like "cabosil," but there are warnings about the silica absorbing moisture from the air causing issues while the urethane rubber cures. Baking in the oven at low temps removes excess moisture. I think I'm going to make another sample thickened with dry cabosil and 2 layers of glass. Meanwhile the first batch of rubber should be fully cured and I can soak them in water, oil and fuel.
 
I don't know why I didn't see this before, there is no way a formed rubber duct is sliding off my right side I let duct plug. The shorter side requires a sharper transition to the front of the cylinder creating the flat spot on the bottom. Bottom side of the plug:

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Right side of the plug:

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Top side of plug:

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I'm not sure there is a better shape to try and make the plug. I could extend the metal baffling down or perhaps slit the rubber duct enough to remove the mold and use ure-bond glue and another strip of rubber to glue it back together. Or pour acetone over the foam and dissolve the plug.
 
Fitting check of the plugs "in assembly." I'm pretty close, the time to switch to guns errr ahh I mean 80-120 grit. You can see the 1/8-3/32 gap between the plenum/baffling and the plugs for the proposed thickness of the rubber duct.

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I thought more about extending the lower baffle aluminum on the right side, maybe a curved pice like this.

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I'm still brainstorming, I could leave the existing baffle in place and trim the new curved piece to the size of the existing blocking plate in front of the cylinder and radius the inboard portion back towards the barrel of the cylinder and leave flanges to bend around the intake boot.
 
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Probably don't need the "potbelly" in the bottom of the right duct, or at least as much as you have now. Our engine/cowl geometry is similar.



 
More "testing" needed, the tube was too flexible with just one glass layer but most of the rubber didn't end up on the tube.

A note on "too flexible". Recall I originally made a set of ducts with multiple plies of 8.9 oz 8-harness cloth. Unsupported in the hand, they hold shape wonderfully.

However, remember than every ounce of force necessary to flex them in service (with the engine moving in torque reaction) transfers to the baffle tin and cowl. Stiff is not so good. We want a flexible duct.

Made another set after thinking about it, using a single ply of 9.0 oz plain weave cloth. They're pretty floppy when off the airplane. As a glass reinforced tube, they firm up when both ends of the tube are fixated to something stiff, here the intake reinforcing ring and the baffle/plenum lid. Still, given that the engine mounts will sag a little over time, and the cowl mount points wear, the alignment of the engine and cowl won't remain precisely the same over time. The floppy duct will have some wrinkles when static in the hangar.



It doesn't matter very much. Realize that when in flight, the rubber intake ducts are balloons, held fully inflated to the limits of their glass fiber reinforcement by the static pressure delta between the upper and lower cowl volumes. If you assume a small cowl exit and relatively high lower cowl pressure, ballpark roughly half the available dynamic pressure for that airspeed. At an RV-typical 165 knots and 3500 feet, that would be about 8" H2O, or 0.3 psi, or 42 lbs per square foot. The pressure delta would be higher with a standard size cowl exit. I've never bothered to put a video camera in there to prove it, but I'd bet money that there are no significant wrinkles in the duct when flying.

Fabric alignment also plays a major role in reducing stress due to engine movement. The primary large motion is torque reaction, in particular when starting, as the engine body rotates around its crankshaft in reaction to accelerating the prop mass. The result is a vertical oscillation at the ducts.

When doing the duct layup, use a plain weave like 7500 or 7520, and align the fibers (as much as possible) at 0/90 degrees to the long axis of the duct, as viewed from the side. That way, vertical motion merely distorts the little shear webs of rubber; there is little tension or compression stress applied to the fibers, limiting stress applied to the attach points.

 
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But wait, there's more . . .

And to add to DanH, if you lay the fabric at a 45 degree angle to the main axis, it will tend to shorten with pressure, so it will definitely have no wrinkles left.
 
I sanded some of the "beer belly" off the bottom of the right inlet, I think the new shape will let the rubber boot slide off of the mold. I also added 3 layers of epoxy to the inlet plugs and the plenum.

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I also made an aluminum strip to clamp the bottom edge of the intake boot to the stock Van's baffling that also makes use of the stock block off plate to provide a curved path for airflow. Right side:

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Left side:

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I'm getting closer!
 
I soaked some room temp cured samples of the PMC780 dry in water, oil (chainsaw bar is oil) and 2 stroke mixed gas for the last several days. I visually compared and stretched/twisted etc the samples reference a piece of un-soaked urethane rubber. I couldn't detect any difference between the samples soaked in water or oil versus the un-soaked piece.

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The piece that was soaked in 2 stroke mixed gas looked and felt normal at first, the bending and twisting seemed to start small cracks on the edge much like an old alternator belt or radiator hose.

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Hey! did you make those boots yet? I'm at this juncture and him haw ing around trying to decide on the transition.
 
Hey! did you make those boots yet? I'm at this juncture and him haw ing around trying to decide on the transition.

So pushy........just kidding as the airplane has been low on the priorities.

I'm going to roughly follow DanH's proven design, instead of the aluminum support ring on the front I'm invisioning an epoxy/flox molded ring embedded in the rubber lip for support. I like the aluminum ring support better but have to work within my current tools/skill sets.

If your thinking/build process when going "off plans" is anything like mine it starts with what seems like a solid plan but in the end I have to just start building pieces and work out the details later.
 
I primed my glass with a DPLF epoxy primer followed by a high buildprimer, of course a few pinholes showed up where I got over zealous sanding the epoxy skim coat. Next time I'm going to shoot the epoxy primer only, fill remaining pinholes with epoxy/micro sand then shoot the high build primer.

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I wet block sanded the plenum, I really like the contrasting colors of the high build versus epoxy primer. I'm getting closer.

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I'm thinking my way through the front of the intake boot, I routed a 1 inch wide by 1/2 inch thick circle in some plastic to act as a mold for the front rubber lip.

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I also routed a 3/8 inch wide by 1/4 inch thick ring to act as a mold for an epoxy/flox reinforcement ring. This reinforcement ring will sit in the intake boot front lip and be covered with rubber......in theory anyhow. The reinforcement ring is still curing, I ran out of fast hardener and had to use slow. I clamped a flat piece of plastic over the mold to flatten the back side.

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off plans comment...

So pushy........just kidding as the airplane has been low on the priorities.

I'm going to roughly follow DanH's proven design, instead of the aluminum support ring on the front I'm invisioning an epoxy/flox molded ring embedded in the rubber lip for support. I like the aluminum ring support better but have to work within my current tools/skill sets.

If your thinking/build process when going "off plans" is anything like mine it starts with what seems like a solid plan but in the end I have to just start building pieces and work out the details later.

Yep,
What at first may see like an "improvement" usually turns out to be lots of time and engineering to get where you are going.

Daddyman
2016 dues paid gleefully
 
Version 1 of the reinforcement ring seemed strong enough yet too big, not enough room for rubber in the next mold. I made another ring 1/4 inch wide X 10/32 thick, I also drilled 3 holes in the mold to act as feet to hold the ring up into the second mold.

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The new ring is definitely more flexible, though once covered with rubber I think it will be a good compromise. In the next mold it has just over 1/8 inch on either side for rubber.

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Thanks Brett!

The plenum turned out pretty good for an amateur!

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Dang look at that! Awesome! I found that in my final paint prep for tinny missed pin holes, Bondo worked great, Just got to get it in there fast while it is wet and goopy, wet X pattern 360 and glossy black paint did not show anything. Bondo on an aircraft? are you Crazy? Yip!
 
I precut the straight weave 9.66 oz (7500 from spruce), I was only able to line up the fabric perpendicular as recommended earlier on the outside surface and keep the fabric 1 piece.

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I used 3 coats of wax followed by 3 coats of PVA on the plugs. I did bake my cabosil to remove any moisture as smooth on recommends doing for their ure-fil 9 silica based thickening agent. I used 2 tablespoons of cabosil per 25 grams of PMC-780 part B to thicken it slightly. I did 3 coats of rubber with a brush alternating right side up and upside down while the rubber dried to the tacky state, gravity did slide the rubber as it dried. I used unthickened rubber for the glass layer, then one more layer of rubber over the glass applied with a foam roller.

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I used water poured in from the backside to slide to boots off the plugs, I'm pretty happy with how these turned out. Now to trim/fit and mold the inlet lip!

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The untrimmed boots fit pretty dang well, I got a little aggressive sanding the corners of my plug and will have to glob some silicone in the corners. I need a couple days off work and I could be flying again!

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