Not So Baffling?..

Baffle seals age slowly, and the effects of temperature and wear change CHT?s a little bit over a long time. Season after season passes, we go from winter to summer over and over again, and the overall rise in temperatures goes un-noticed until one day you realize that you?re compensating with increased climb speeds on every flight. Suddenly, you realize that the seals have been doing their duty for close to a decade and it is time for a re-do! Such was the case with the Valkyrie, my 1800+ hour RV-8?.so last weekend I decided it was time for new seals. As most builders remember, cutting and trimming baffle seals takes time and a lot of effort ? not something we remember fondly. But I?ve got good news ? replacing existing seals can be quick and easy! Here are a couple of notes from my project ? which really only took about five hours.

Crinkly! Yup ? these seals have taken a set and are heat checked ? ready to replace.


Use a center punch to pop the mandrels out of the pulled rivets ? a few clicks and they?ll come out with vigor!


After pulling off the old seal, use a sanding disk to grind off the shank ? sand it down flush to the baffle aluminum.


With the shank ground down, pull the tail off with a pair of dikes ? the holes will still be perfect!


Take each segment off one at a time. Lay them on the new material on top of a sacrificial wooden surface, then drill through the holes and cleco to the surface. Press down to flatten and cut with a new razor nice ? and the new piece will be perfect ? ready to remount.

All of the edge seals were done in about three hours ? only the center seals still to go!


Remember how complex those center seals were to figure out the first time around? The good news is that you don?t have to figure them out the second time ? just copy them!


Center section complete! If the cowl is hard to put back on, you?ve done it right ? the new seals need to compress and conform ? and that means you?ll get good results.


The results? Subjectively, duplicating a flight done the day before the replacement, with similar OAT?s (about 90 degrees F at 5,000?), I?d say that CHT?s ran about 20 degrees F cooler than with the old seals. The baffles themselves were in great shape, and although I went over the hole baffle, found very few things that needed additional caulking with RTV. Don?t be afraid to change seals if you seem to be running hotter than before ? it?s just not that hard.

(Look for a complete story on this in an upcoming issue of Kitplanes. I just figured this might help someone through the heat of summer!)

Paul

Nice job

Mr. Dye. excellent narrative too.

The red silicone baffle seals on my C-180 are 20 years old and have 610 hours. They show some wear but are as flexible as they day they were installed. There is no cracking where they bend.

Please bear in mind that it was 20 years ago or more, but I got the material from Denver Gasket and Packing, 10943 Leroy Dr., Northglenn, CO 80233-3615, (303) 452-4600.

I plan to use the same or similar material for my RV-3B, when I get that far.

Dave

I noticed in your pictures that the seals seem to have a nice inward bend. Once the cowl is replaced it will lay nicely and pillow outward as air is forced in. Is that the result of the cut, the material, the workmanship or does it "just work"? It would be unfortunate to do all the work, get to the last rivet and find out the material wants to lean outward.

Lemmingman said:

I noticed in your pictures that the seals seem to have a nice inward bend. Once the cowl is replaced it will lay nicely and pillow outward as air is forced in. Is that the result of the cut, the material, the workmanship or does it "just work"? It would be unfortunate to do all the work, get to the last rivet and find out the material wants to lean outward.

Click to expand...

Initial fitting is part of it, but you'll also see that I tack the rear corners togetehr to encourage the seals to tilt inward. Other than that....magic!

I use the black stuff that Van sells partly because it comes in sheets versus strips , and I find I can make bigger single pieces that way. I like the orange silicone stuff as well, but it usually is found in 3" rolls, which is harder to fit on some of the more extreme curves (and the forward baffle). I figure that close to a decade is good life anyway.

Baffles

Thanks for your post, Paul. My baffles are new but will need replacing sometime. How did you tack the back corners together? I might add to my current baffling even though it's working very well (just on taxi testing). Thanks in advance for any info and as well for the great post. Bill

Bill, I tack the corner with a baffle seal rivet, backed up with a washer. If the stack is too thick, use a long-reach pop rivet and a washer on each side.

......and if you'll cut the new rubber seal material across its narrow dimension, it will have a natural inward curve....the way it was rolled.

Best,

I called the company now known as Die Cut Technologies (formerly Denver Gasket and Packing) to ask about the availability of silicone rubber sheet. It is available is widths which would be convenient for us.

Their phone number is (303) 452-4600.

There's a minimum order of $100.

It's available in the orange we're used to, black or white. Some other colors might be available for more money.

The salesman, Francis, wasn't entirely sure what specific material I'd used twenty years ago so he couldn't give me actual prices and sizes. But we agreed that I could send him a sample if anyone is interested in getting it. I've some in my hangar and could cut off a piece easily enough. Please send me an email (click on my name above and then use that for this) if you are interested in it. Of course if you ordered it, it would come as a sheet - it would not be die cut and there would be no installation plans. It would simply be some raw material. You'd have to make patterns and do your own cutting and fitting.

He suggested that Viton would be a suitable alternative with higher performance but at a much higher cost. I don't know any more than that.

I don't have any connection to them except for having been a customer a long time ago.

And Aircraft Spruce sells it in 1/8" x 36" by the foot, with a fiberglass insert, but it's not exactly cheap.

Dave

Paul,

I seem to remember when you changed the cylinders for brand new Lycoming units you observed an increase in CHTs across all cylinders. Was this on Valkarie?

If so, do you think this could be attributed to the baffle seal condition (given you account a 20 deg F drop with the new seal)?

Did you observe these high CHTs drop anytime over the first 100 or so hours on these cylinders?

I am curious because I have had a hot cylinder that has now at 200 hours has dropped to be in line with the other that defied all attempts to modify the cooling arrangement in the early hours. I have a theory for this behaviour but would be interested to hear if you have observed something similar.

Doug said:

Paul,

I seem to remember when you changed the cylinders for brand new Lycoming units you observed an increase in CHTs across all cylinders. Was this on Valkarie?

If so, do you think this could be attributed to the baffle seal condition (given you account a 20 deg F drop with the new seal)?

Did you observe these high CHTs drop anytime over the first 100 or so hours on these cylinders?

I am curious because I have had a hot cylinder that has now at 200 hours has dropped to be in line with the other that defied all attempts to modify the cooling arrangement in the early hours. I have a theory for this behaviour but would be interested to hear if you have observed something similar.

Click to expand...

It might have been baffle seals, but I was also just beginning to experiment with P-Mags, and that raised temps a bit when I used the more advanced timing curve. As usual, fixes are usually a combination of little things. The initial high CHT's due to break-in dropped pretty quickly when I changed the cylinders of course.

We see higher CHT's out west in all our planes due to higher altitude and thinner air for cooling - plus more high power operations on take-offs at altitude. A combination of little things.

Paul

Though baffle seals don't get changed very often, I have found that there is benefit to installing them with screws and backing plates. I also try to keep the cuts to a minimum to eliminate the chance of leaks.

I just re did the Rocket baffle seals and went to some effort to modify the rear baffle so that it was straight across and could use a one piece seal, with no cuts. In fact, on this engine I'm one piece on each side as well, with zero cuts anywhere. Careful stretching and pulling in the right spots allowed the seal to follow contour, and also roll inward nicely.

The retainer plates are simply a piece of .75 wide strip of .016 aluminum bent into an "L" shape (.25x.50 legs) with nut plates riveted on (no countersink). Since I have a sheet metal shrinker/stretcher, I was able to form them to follow the contour pretty closely.

This setup allows a tight seal without the puckering found with pop rivets, and is removable without any drilling.

That's a very nice looking job Mike - and with the nutplates and screws, you can remove a few screws if you ned to take off one "corner" of the baffles to change one jug - the reason for the cut and overlap in the middle of most four-cylider side baffles.

I like your system.

Toobuilder said:

Though baffle seals don't get changed very often, I have found that there is benefit to installing them with screws and backing plates. I also try to keep the cuts to a minimum to eliminate the chance of leaks.

I just re did the Rocket baffle seals and went to some effort to modify the rear baffle so that it was straight across and could use a one piece seal, with no cuts. In fact, on this engine I'm one piece on each side as well, with zero cuts anywhere. Careful stretching and pulling in the right spots allowed the seal to follow contour, and also roll inward nicely.

The retainer plates are simply a piece of .75 wide strip of .016 aluminum bent into an "L" shape (.25x.50 legs) with nut plates riveted on (no countersink). Since I have a sheet metal shrinker/stretcher, I was able to form them to follow the contour pretty closely.

This setup allows a tight seal without the puckering found with pop rivets, and is removable without any drilling.

Click to expand...

Absolutely beautiful craftsmanship... Very Nice!

Thanks for the kind words gentlemen.

Because I was doing this as a retrofit and tried to pick up the existing pop rivet holes, I'm not real proud of the spacing (see the top picture). This will never win a Gold Lindy at Osh! I am proud of the functionality however. The seal edges are wearing in nice and evenly all around, and as the pictures show, there is only the possibility of the slightest leakage at the two back corners. I'd say I'm capturing almost as much air as any sealed plenum system, with far less maintenance headaches.

And as Paul observes, ongoing maintenance is simplified as well. While the backing plates appear to be one piece in the pictures, they either butt together or slightly overlap. Each individual baffle segment has its own corresponding backing plate which allows for relative movement without cracking, and maintenance. Just remove the screws and peel the seal back for access.

For those interested, the seal material is simply the 3/32 x 3 inch wide silicone stuff you buy for about 2 bucks per foot from Spruce. I didn't even have to trim it to follow contour- I just cut it to overall length and worked it into position. Stretching and bunching as required allowed it to form right the way I wanted it. Though the silicone material seems to last forever, it's nice to be able to use existing material "off the shelf" without too much custom fitting.

And one final word about the backing plates- don't let the fact that I used a shrinker to curve them scare anyone; they could be made in a few segments of perfectly straight sections and be just as effective. Very easy to do and well worth the effort - especially in an initial build.