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Baffling rubber to cowling interface

stringbender

Member
Patron
Recent purchase RV6A 0360 carburetor, running high CHT’s.

Critical observation reveals a difficult task of evaluating where air is leaking.
I’ve 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.
I’ve 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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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
 
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.
 
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.
 
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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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.
 
These are all good suggestions.
Thanks!

I’m a bit concerned on the contours behind the starter ring.

I’d like to see another 0360 carburetor top inside cowl contour.

I look at the distorted rubber and think “that can’t be right. Thinking something needs to be a better fit, the top cowl possibly needs some fill work and reducing the sharp contours?

High CHT’s are on takeoff..
I’m going to change the main jet to enrich it.
I’m never pulling the mixture back but about 1/2 ‘’ at the most in cruise.
I’m going to do the Mooney mod that dates to 1962, basically turns a MA4-5 10-3878 Into a MA4-5 10-4164.
 
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. . I’m guessing you have a fixed pitch prop. What are its specs and static rpm? What is your wot climb out speed? I’m never slower than 100 kts on initial climb out at WOT. I’VE NEVER tried the POH ( mine and others I’ve seen) best rate/angle. Pretty sure things would heat up and quick. Since plane is new to you, I’d look at flying style before changing out parts on carb, baffling, etc. Not insinuating anything about ability, skills etc. Enjoy you RV.
 
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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With the bottom cowl off, lay the top cowl in place and have someone hold the forward end at the normal position relative to the spinner. You can then easily use a flashlight to determine what type of air gaps you have between the seal and the cowl top. the other week, I did some strategic seal replacements for a guy around here. Replaced two 6" pieces of seal that were not done well. Temps on that side dropped 25 degrees.

Also need to use a flashlight to determine where RTV needs to go, paying attention to the Inter Cyl baffles.

Larry
 
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Dan
thanks for your reply.

flying technique is in the learning stage, this fliver is very different than what I've spent most of my time in. In a way I find myself reaching back many years when I worked as a crew chief in flight test (Sikorsky a/c) and working with engineering. Attempting to re-aquaint myself with the RV and my mechanic skills of years ago. Challenging. I feel like I'm in a high level college course, a bit scary and forces your mind to adapt, sink or swim.

I believe this a/c will be quite a joy when I get the problems behind me.

Thanks to the support I've received I think I can adapt, and much faster.

Ok, today is going to start out installing the cowling and taking a look at the sheet metal, possibly I will just start with removing all the rubber/pop rivets and have a good look at the cowling.
The gloss white approach sounds like good feedback on optic's, which is challenging for anyone older and wearing glasses.

Question, if you were to have overlapping rubber pieces, would the overlap start in the front or up wind, and down wind lays under the up wind?

It may be difficult to observe in the pictures I provided, but I still have great difficulty imagining a upper cowl with such sharp contour changes on the inward inlet close to the ring gear. As exhibited the rubber seems to be very distorted.

Also: the outward inlet area rubber, as it comes in contact with the upper ramp, and continues to the front of the opening, the picture supplied above, it appears to me that the incoming air could blow it up and spill air into the cowling outside of the intended cowl area.
Could someone remark on this.

A picture of someone's cowl and rubber on a 0360 carb, would be helpful.

I'll check back later, right now heading to the hangar and start my day.
1. paint the upper cowl gloss white
2. take pictures before
3. remove rubbers
4. install cowl and remove metal interferences.
5. check back on vans
6. attempt the rubber install.

Regards
Doug Bent
Stuart, FL
 
Picture

I replaced all my seals (rubber). All were correct, just a bit worn. I saw no change afterwards, just a bit better seal. This picture is what things looked like before new rubber. I used the old as a template, I drilled out the pops and replaced one piece at a time…I recommend this approach so to keep track of things. Zoom in on pics you’ll see overlaps on rubber. I’ll say this, but sure you already know….pressure push all seals upwards, if fit has seals touching, pressure makes it touch more. Any other RV owners at your airport?
 

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There is a RV 6 owner but has a 0320, and no ramps on the inlets.

Can you provide a photo of the contour area of the top cowl underneath where the ramps are and where the front baffle comes in contact with it.
 
I removed the baffle rubber, painted the cowl area, and made up a model of my front metal baffle out of cardboard with the similar angles and simulated the placement.
From the pictures taken "in the mirror" it indicates that the left side is tight against the baffle aluminum and therefore caused the distortion of the old rubber as indicated in the photo.
The mirror photo on the right side show's clearance of I guess 3/8'' to 1/2''.

The contours of the top cowling where it comes in contact of these two sides, inward of the "inlet ramps" are different on mine, on either side, as mentioned above. I have great difficulty accepting the cowl in this state, it appears something was started and could not be compared to a finished example in the plans. My opinion; I cannot find in the plans, I cannot find examples on VAF forums.
My experience tells me I need to do some glass work, possibly work with foam and glass and move the left one to create more room for the rubber, possibly just flatten the sunken in area to create more surface area for the rubber.
I'm even thinking of adding a tubular hose to the top area and include the rubber flap also. (just seems quite difficult to seal in this area.
The right side might work ok with the spacing? But flatten out the contour.
Also thinking of sealing or foam and glass over the outside of the ramps.

I would like to see how the rubbers on the inlet area with cowl's installed appear on the outside inlet area.
My friend that has a RV7 has his rubber on the outside of his ramp, mine as shown in the beginning of the photos of this post shows different.

Suggestions

pictures

all welcome.

I have not figured out how to correct the picture posting orientation, sorry.
Pictures: Left to right.
1. right hand contour
2. left hand contour
3. distorted old rubber from left side
4. mirror shot of left side interference
5. mirror shot of right side spacing
6. tool showing placement of baffle.
7. close up of angle to contour.

Regards
Doug
 

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my baffles

Mine is an RV-4, but I tried to be very meticulous in closing gaps and angling material to form the seal when the cowling becomes "pressurized". I used black RTV over safety wire stitching in some of the lap-overs to prevent the pieces from reversing. I also finished the inner surface of my cowlings with epoxy silver primer so I don't have exposed fiberglass which will eat through baffling as well as wear the inner cowling surface.
 

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Mine is an RV-4, but I tried to be very meticulous in closing gaps and angling material to form the seal when the cowling becomes "pressurized". I used black RTV over safety wire stitching in some of the lap-overs to prevent the pieces from reversing. I also finished the inner surface of my cowlings with epoxy silver primer so I don't have exposed fiberglass which will eat through baffling as well as wear the inner cowling surface.
That is very clean. The baffle behind the ring gear looks awesome!
 
Bill
that is an impressive airplane you have there. I like how you cut that rubber and made it float across your baffling.

OK RV 6 builders, show us yours!
Please.

Could you please post pics of the contours of your inner front top cowling up close.
 
3 examples which 2 are dogs (that will be groomed soon)

The top cowl is off my current 6A. Note the smudge marks where the baffle contacts the fiberglass. The black puffies are where air is leaking, not many, these areas could use a bit of silicone applied to the rubbers to get a better seal. The 6A front inlet shots show how the rubber material was notched to go around curves and not buckle the sealing areas.

The baffles on the floor are off a 9A that I'm rebuilding, the engine just came back from Aerosport Power fresh, I wouldn't dare screw those baffles on this nice clean engine! - there is so many problems with these baffles that I am going to start from scratch and fabricate new ones...

The green baffles are on a future 6 rebuild project, I doubt those baffles did anything but sit on top of the engine & look ugly.

Both the old 9A (on floor) & 6 (green) baffles show signs that the vertical baffle walls were trimmed too close to the cowling. The rubbers were either pinched or just folded over at 90 degrees or more resulting in next to non existent sealing.
 

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Thanks for posting!

That front inlet area, appears to be challenging.

I finally have a picture of the ramps on the inside of the top cowling thanks dto you!

I’m still undecided how to route my rubber And on the outboard inlet areas.
 
Basically there are 2 ways to treat the sealing over or around the ramps.

The 6A has the baffle walls sealing around the outside of each ramp, the walls extend up to the cowl & seal underneath the ramps. You see how I had to trim a slot in the ramp where the baffle rubber curls up against the cowl outter skin. The front central baffle rubbers near the flywheel also extend up the the cowl skin & take a bit of effort to seal properly.

The other way is to trim the baffle walls so the rubbers actually seal to the ramp face. This calls for a concaved formed rubber seal, tricky to get a good air seal but with care can be done, than you have to figure out a good method for sealing the sides of the air inlet... Have done it this way, but never liked that approach.
 
Ramp Pics

While at my hangar last eve, I snapped a couple pics of my ramps with cowl on. Although mine's a -4, the -6 should be similar. When I built my cowls, I bonded the cheesy fiberglass ramps in place, then fully closed out the outboard side and smoothed it up. The inboard side (not visible) is left open adjacent to the flywheel. The baffle behind the ring gear pretty well seals up that area. You can see in the pics, the ramp sealing to the cylinder side baffles, and the lower cowl seal comes over the top of the cylinder front baffle.
 

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I've been an A&P mechanic for about 45 years. Fitting baffle seals properly is always a challenge. I see a lot of good advice on this thread and want to add that if you are not familiar with McFarlane's Cowl Saver, its really worth looking at. It is a silicone, fiberglass cloth reinforced material with Teflon bonded to one side. the Teflon side of the baffle seal is made to contact the cowl and prevents the transfer of motion and vibration from the engine to the cowl and hence, the rest of the airframe. A huge bonus is that the reduction in vibration also reduces wear on the cowl. I also find that with the silicone being like rubber ahs memory/stiffness which make it hug the cowl better giving a better seal. I noticed in many of the baffle seal photos in this thread that any memory from the material was gone which definitely hurts seal performance. Over time, I have noticed that one of the failures of baffle seals is when the edge of the seal material is not in contact with its mating surface. This provides a starting place for the high pressure air to go around the baffle seal instead of through the cooling fins.
This material is unique and has been patented.
 
McFarlane Cowl Saver

SkyJunkie said:
This is good stuff and I used it on my RV-10. It is a bit stiffer than the Van’s cowl seal material and the other fabric reinforced silicone products. It would not comfort to the forward section of the baffles behind the ring gear very well at all. I ended up removing that portion and replacing the Cowl Saver with the Van’s seal, and attaching the Cowl Saver material to the cowl in the mating area so I would still have full benefits of the anti friction/vibration benefits using the material.
 
Those are really good pics.
Fine workmanship too.

I notice the large countersunk washers for cowling appear to be used under your rivet heads, adding to more surface area holding more rubber vs no washers.
I have heard of the Teflon sided rubber, I saw a tool sold with it to scarf or cut the first of three layers in the contact area, like a crosshatch of cuts, I gather to make more favorable for sealing?

I looked at my cowl and found no appreciable amount of contact scaring, wearing, just the color change.

So I decided to go with the high temperature rubber that aircraft spruce sells, however the washers included are the same diameter as the rivet head?
A waste.
I prefer the extra grip as shown on your cowl rubber.

I have decided to fill the ramps with urethane foam and extend the ramp to the outside cowling.

After fitting the cowl several times I felt I could only be assured of sealing the ramp areas unless I re-glass the inboard surface areas to match the aluminum baffle wall contour.
The dimension appeared to be 1/4” in one area so I used foam vs filler.
Then I applied Kevlar/graphite cloth.
Attempting to reduce weight, add strength vs glass.

I have to install the rubber yet and the inboard ramp areas appear challenging.
We shall see.

I appreciate the help people have expressed!
Thanks
 
Doug , AS sells a balling rivet kit. I wouldn’t use counter sunk/tinnerman washers, the larger surface does nothing since baffling sheet metal isn’t countersunk.

My setup is …. Metal baffles, rubber seal, backing strip. Pop rivet, through outside sheet metal, through rubber, backing strip, washer on shop head. Nothing rivet to rubber in my install

Just how mine is installed….pictures show inside and outside front left.
 

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