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I will admit up front that the topic of insulation is controversial. It seems that when the topic of noise comes up, most people tell you to get better ANR headsets! While that will certainly work, and adds little weight, I wanted to at least take a stab a reducing noise.
Talking with other builders who are now flying their planes identified the main source of noise as vibrating skin panels in the aft fuselage (i.e. tail cone rattle). Looking at my own fuselage and drumming on it with my fingers confirmed this tendency. The aft fuse has relatively large, flat panels that vibrate. The is also true of the stainless steel firewall. My course of action was to reduce the tendency of these panels to vibrate by using some type of stick-on insulation.
Ideally, the insulation would stiffen the panels to the point that only low frequency noise would make it through. I am by no means an expert (as I know only a little about noise attenuation), but I am convinced that by attacking the most problematic areas, I can achieve some level of noise reduction. I guess I'll find out next year some time :-)
Having said all that, keeping the added weight to a minimum was mandatory, as was keeping costs low. I settled on an insulation material sold by Aircraft Spruce named, aptly enough, "The Insulator". I purchased 8 running feet of 48" wide single layer material (32 square ft). Total cost was $34, and the whole roll weighed in a just under 6 lbs, so that should be about the same weight that actually goes into my plane once it is trimmed to fit and adhesive is added. To investigate the effects of adding this weight behind the C.G., I modified a typical Sonex W&B spreadsheet by adding the whole 6 lbs to the tail wheel weight. The shift in C.G. was minimal, and certainly within limits.
I cut the panels to fit tightly against the sides in the forward 3 bays, and used 3M Super 90 spray adhesive to affix the insulation to the skin. This stuff is really strong! I was worried about potential fire hazard, so I made up a test piece and turned my propane torch loose on it. It proved to be very resistant to burning.
Now, having all the insulation panels in place in the tail cone, I can definitely report MUCH less noise when drumming my fingers on the outside. Hopefully that will prove effective in the air as well.
As for the firewall, the rudder pedals sit very close to the inside of the firewall, and don't leave much room for insulation. So, I chose a thin (1/8") foam sheet from Sound-Ex Products. It is kind of a cross between neoprene and open cell foam. I cut a panel to fit on each side of the firewall stiffener tube. Again, hopefully this will reduce drumming and add very little weight. This stuff is sold by the sheet (3 ft x 4 ft), and runs about $8 per sheet.
The last three pictures are of my burn test sample. I glued some scrap insulation to some scrap aluminum to serve as a test subject. I then applied a direct 6" flame from my propane torch on the insulation in various areas. I kept the flame in contact for about 5 seconds, and then checked for damage. In each case, the material did not actively burn. It merely curled up and turned crispy. There was very little odor present, suggesting that it would not fill the plane up with toxic fumes should something attempt to ignite it. Overall, I was pleased with the results of my burn test.
NOTE: photos link to full size image
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