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  #1  
Old 09-19-2021, 06:56 AM
stringbender stringbender is offline
 
Join Date: Dec 2020
Location: stuart, fl
Posts: 22
Default Baffling rubber to cowling interface

Recent purchase RV6A 0360 carburetor, running high CHTís.

Critical observation reveals a difficult task of evaluating where air is leaking.
Iíve compared the plans picture to the rubber install and it appears identical.
However I cannot find the upper cowling inner pictures.

There are two distinct areas, the ramp at inlet and then inward of those eyebrows the contour changes.
Iíve been looking for pictures of other cowls on vaf and cannot find postings.

I have the replacement rubber, but I do not want to install it if the contours are incorrect on the cowling.

I could use some help please.
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  #2  
Old 09-19-2021, 02:05 PM
Rv3Dave Rv3Dave is offline
 
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Location: Florida
Posts: 62
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High CHT can also be a engine timing issue or running too lean (intake leak). You might want to check these before reworking your baffle seals
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  #3  
Old 09-19-2021, 02:11 PM
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wcalvert wcalvert is offline
 
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Location: Anacortes Wa
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stringbender

Whoever did that work was on the right track, but those seals look a little long in the tooth. I just recently did some more tune up on my new seals, focusing on the are behind the flywheel, and noticed a marked drop in CHT's, so the result is worth the effort.

Couple of observations from my build, and since it's hard to really tell from your photos, these are general comments:

Take a close look at the seal and cowl mating surfaces as they are installed now. You should have wear patterns on BOTH, EVERYWHERE. Any shiny spot on the seal or dusty spot on the cowl is a leak. Gotta fix that.

Segmented seals are fine, and on some spots I used wire tie lacing cord to tie together an overlap spot to hold the seal in position. It's an ok technique but a properly attached and well shaped seal is better.

On the parts of the baffle where the edge is "curved" as viewed from the plan view (sides above the cylinder heads and behind the flywheel), your seal will pucker when bent over to the meet the cowl. These spots are you biggest challenges. Cut the seal in a curve and match it to the edge of the baffle, fastening it so when it folds over it retains a smooth surface... easier said than done. Notching the portion of the seal, where it attaches to the baffle, may allow you to pull the seal around into a shape that will not pucker.

With cowl in place, use an endoscope to look from the intakes and through the oil door at all the surfaces where the seal should be mating with the cowl. Put a bright light on the opposite side and look for light leaks. ACCEPT NO GAPS. If you see a pucker, or bump in the seal, that'll be a leak. Put your hand into the inlets and feel around, especially around the curved forward seal just behind the flywheel. All should be down and smooth. Do this on your old cowl and note the problem spots, then focus on them for your new seals.

When you build the new seals, imagine (or look with your scope) how the gap between the baffles and the cowl fits up. There are many other techniques for doing this (paperclips etc.) Take note of the size of the gap (hopefully uniform, this makes building the seals easier if the gap is uniform) and how the seal will fold over onto the cowl. This is challenging because you can't work from inside cowl. Consider what the fold will do to the shape of the seal when installed. So cowl on, take a look, cowl off, mark your gap etc.

Look at all your seals inside of the intakes holes. All seals should lay down like shingles, overlapping to cover the one downwind. This may sound obvious but it's easy to make something that looks good but blows up and leaks in flight.

Use a leather punch to make the holes in the seals. You'll get a better lay-down on the baffle edge where the rivets go and will be able to mark the baffle through the clean hole you made. Drill the baffle, then attach the seal with a cleco, reassemble and asses.

Once you get it done, fly it and look for evidence of shiny/sooty spots. Adjust and fly again.

This has been one of the longest iterations of the build, but it's paid off.

Now search the forum for some of the comments (Dan H has some good ones) about leaks around your cylinders. All air must be directed to do useful cooling or you're wasting your effort.

Good luck and keep at it!

Cheers.
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  #4  
Old 09-19-2021, 02:33 PM
Kyle Boatright Kyle Boatright is offline
 
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Quote:
Originally Posted by wcalvert View Post
Any shiny spot on the seal or dusty spot on the cowl is a leak. Gotta fix that.
I think this is substantially backwards. Look at the photos. The baffle seals are dull and a little oxidized in locations where they *don't* seal against the cowl. The shiny spots are where they are in contact and wearing (polishing) against the cowling.

The picture of the baffle seals near the flywheel is the clearest example of this.
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  #5  
Old 09-19-2021, 02:36 PM
BH1166 BH1166 is offline
 
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Open oil door lay a bright flashlight inside close door and go look in the cowl openings for light, throw a blanket over you and cowl and you can do in daylight. Light is thinner than air….you’re looking for beams=gaps. You may not want to do this. Spray inside top cowl with white paint, cowl up, go fly several times, look for rub marks. Unless you have huge gaps, a lot of them, likely fixing seals will not make a lot of difference. Egregious seal issues… big difference.

Is this hot cht in cruise at altitude or in WOT takeoff ?
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Last edited by BH1166 : 09-19-2021 at 02:39 PM.
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  #6  
Old 09-19-2021, 02:36 PM
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wcalvert wcalvert is offline
 
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Regarding the shiny/dull ... I was referring to the new seal material that starts out shiny. Any place I have a good seal has scuffed to dull on the seals.
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  #7  
Old 09-19-2021, 09:38 PM
stringbender stringbender is offline
 
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Location: stuart, fl
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These are all good suggestions.
Thanks!

Iím a bit concerned on the contours behind the starter ring.

Iíd like to see another 0360 carburetor top inside cowl contour.

I look at the distorted rubber and think ďthat canít be right. Thinking something needs to be a better fit, the top cowl possibly needs some fill work and reducing the sharp contours?

High CHTís are on takeoff..
Iím going to change the main jet to enrich it.
Iím never pulling the mixture back but about 1/2 Ďí at the most in cruise.
Iím going to do the Mooney mod that dates to 1962, basically turns a MA4-5 10-3878 Into a MA4-5 10-4164.
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  #8  
Old 09-19-2021, 09:54 PM
stringbender stringbender is offline
 
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Yes I have read Dan Hís info on cylinder baffling.
Very interesting.
I had no idea before.
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  #9  
Old 09-20-2021, 06:06 AM
BH1166 BH1166 is offline
 
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If all seems well at cruise, push the nose down, climb out @ faster speed and reduce throttle if necessary. I can push mine to over 400 degrees with aggressive climb, no matter the altitude, time of day, ambient outside temps. . Iím guessing you have a fixed pitch prop. What are its specs and static rpm? What is your wot climb out speed? Iím never slower than 100 kts on initial climb out at WOT. IíVE NEVER tried the POH ( mine and others Iíve seen) best rate/angle. Pretty sure things would heat up and quick. Since plane is new to you, Iíd look at flying style before changing out parts on carb, baffling, etc. Not insinuating anything about ability, skills etc. Enjoy you RV.
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  #10  
Old 09-20-2021, 07:21 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by stringbender View Post
Critical observation reveals a difficult task of evaluating where air is leaking.
Bill mentioned "dusty spots". They are one of many clues found in the top cowl. I've marked a few in the photo below. They are blackish soot/dirt deposits anywhere air has been blowing past a flap seal. They also one of several reason I recommend painting the inside of cowls gloss white.

Typically the area at the inboard and outboard ends of the inlet are poorly sealed, as well as the area behind the spinner, above the case spine. Soot tracks can be harder to see there because they get mixed in with rubber dust from the alternator belt and sometimes a bit of oil residue. Still, if the whole are is kinda black, it's a clue.

Sealing looks poor where marked, and I suspect the aluminum is rubbing the cowl.

Very often the seals at the inboard and outboard ends are installed backwards, apparently in the hope that proximity alone will make them work. It does, sorta, if close enough. However, the key is to install so air pressure blows the gap shut...not open.

Someone mentioned climb speed. Dynamic pressure more than doubles between 85 and 125 knots. For the vast majority of RV's, Vy is only used short term.
.
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