Composite Engine Baffles
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I first want to than Jim Price, who at Rough River in 2005 showed me his composite up draft baffling. Jim you were the inspiration that started me thinking about glass baffling for my down draft conversion.
I made my baffles out of composite materials for two reasons. One is I thought it might be more durable, and two because I felt I could have more control over the cooling of the engine. No more cracks, light weight and they have never been done before. Experimental aviation... gotta love it! The nice thing about laying up your own baffling is you can control air passages and divergent areas, to provide additional cooling to parts of the cylinder where necessary. I used a layer of glass UNI, one layer of carbon BID (45deg bias to the glass) and a layer of glass UNI. I made a number of test blanks to see what would work the best as I was primarily interested in tear/crack resistance than overall strength. I would recommend the same lay-up schedule again, but also add one additional layer of carbon BID in the center. The top edges are reinforced with 2 layers of carbon. The epoxy is MGS 235 as it had the highest TG I could find (about 230F). Where the attachment screws areas are reinforced with a square of AL in between the layers of glass to reinforce the hole and to ensure if the glass is squished out because of the heat and screw pressure, I would still have a strong attachment point.
Here is are some of the pictures of my process to make the baffling. I used my old cylinders which I mounted to my overhauled case..
I mounted the engine vertically tilted and glassed the back end (1, 2 cylinders) flipped the engine 180 deg and glassed the front end (3, 4 cylinders).
Here shows how I took some very thin Al (roof flashing) formed into a curved teardrop shape to fit the cylinders then I taped the end together with Al tape (heating and air type). While laying up the side baffling, I took 1 1/2" strip of 2 UNI and 1 Bid to wrap around the form and layer it on the cylinder.
The schedule I used was 1 vertical glass UNI, 1 carbon BID at 45 deg, and 1 vertical glass UNI. Put an extra horizontal layer of glass UNI (or BID) around the edges to reinforce the lips.
After forming the cylinder baffles, I made some dams for the pour foam to build the air inlets.
You can also see in the picture above how the baffling is split (one baffle for each cylinder) to assist with individual cylinder replacement.
I made 2 ea AL rings 1.5" wide in an oval shape for the flex boot for the
engine. It is slightly oval so I can
maintain constant pressure on the boot seal with a hose clamp. After glassing, I cut out 1/2" from the center of the for flex tube clearance leaving 1/2" of material on each side for clamping. The flex duct was made out of 1/16" High temp silicon which I cut into a 1 1/2" strip and sewed together with some nylon string (I just punched a series of holes in it and fished the string through the holes.)
After foaming the bottom area, I sanded the bottom of the ramp to a nice
shape, and glassed the ring into place.
After cure, I tapped the cured glass on the bottom ramp filled the duct with foam and shaped the upper part of the ramp. After final cure, I took all the foam out of the duct, put a layer of glass on the transition ring (outside
bottom, and inside top) to seal it in place.
Marking the ring for cutting after glassing.
I also tried making some snorkel snouts for air inlets... wasted effort don't do it. The main issue with this mod was trying to get the oil dip stick to seal. Extremely difficult...
I ended up cutting them off and reshaping the cowl into its present shape.
Many hours down the drain....
What I like about my present design is all for cylinders have their own baffle section so I can remove any one of them in about 10 minutes for full and easy access to remove the cylinder. Other than a slight discoloration and just a little softening after shutdown (due to the high heat soak) they are normally very ridge and I am sure in flight with the cool air there is no softening at all.
The following pictures is the way I originally made the cover.[photogallery/photo26962/real.htm]
***About 6 months later after completing the above design***
The pictures above is NOT recommended as a method of downdraft cooling. The open area above the cylinders caused a tremendous amount of pressure on the upper cowl to the point it started cracking the paint in some spots. I calculated over 140 lbs of pressure was pushing up the cowl in flight and I was nervous it might actually detach in at some point.
So I decided to enclose the top to make a engine plenum air box. The mod reduced the stress on the cowl, and reduced my CHT 30 deg. Now I have NO baffle seal material in my plane at all.... My temps run 330-359f in cruise (2500 rpm), 280-338f at 2400 rpm and amazingly 270-290f at 2000 rpm. I climb I usually get up to 390f at 125 kts climb speed.
When viewing them you will in this final mod, note I extended the dipstick, removed all the red baffling material and enclosed the top. I am VERY pleased with the composite baffle concept. The mod was very easy and only took about 10 hrs. If I had made the down draft cooling this way from the start, the added time would have only been about 3 hrs or less.
For some strange reason #1 and #3 cylinders run cooler than the starboard bank even with the dip stick faring in the way of the inlet. #2 and #4 has a shorter expansion inlet so maybe that is why they run 20 deg hotter...? I guess I need to measure the pressure in each inlet to see what's happening.
You can see the dip stick
I made a removable cover to service the plugs...
the dipstick goes through, I made a faring on the inside of the plenum box.
This means I have
no sealing issues with the dipstick since it just pass through the opening..