For some time now, I have noticed that one of the
main sources of noise in the cockpit seems to be the carburetors.
I considered using the regular Rotax Intake Silencer kit, but
wanted to see if I could come up with something that would not
require rejetting the carbs, because to my mind, if you have to
rejet, then you are moving less air and therefore making less
power. Here is how it turned out.
Here is how it was framed up, 1/4" aircraft
plywood cut to fit around the stock air filter.
The frame is sheathed front and back with 1/16"
5 ply model aircraft plywood, epoxied in place, and with epoxy/flox
along the inside seams. The outside edge & the intake scoop
is sheeted with 1/32" 3/ply plywood. 2-lug K1000-3 locking
nuts are bolted to the inside of the box to secure the 3/32"
plywood retaining sheet that fits over the filter and holds it
in place, while letting the flanges poke through. The whole box
is fabric covered using Stits 2.7 fabric with all edges overlapped
for strength. Finished off with three coats of polybrush, three
coats of polyspray, and then Almond Gloss Krylon.
The air enters the square opening and then has to
turn 180 degrees to go down another short passage before it swirls
around the air filter. A brace is attached between the airbox
and the engine, using a small curved bracket at the engine end
retained by one of the water outlet flange studs. The 5/16"
rubber hose at each end lets the whole thing flex a bit but it
really can't go anywhere. Weight? Fairly light, it weighs about
half what the K&N air filter weighs.
Results are mixed. It is quieter in the cockpit,
but not any quieter standing beside the runway listening to the
airplane take off or fly by. It has restricted airflow a bit,
rpm's are down by 75 during full throttle take off and climb out,
and egt's are down 75 degrees both at full throttle and cruise.
This is acceptable during the winter, since the cold dense air
was going to require me to adjust the carbs anyway, however now
everything is about where it ought to be, egt-wise. Once the air
heats back up this summer, I will either have to rejet or take
the silencer box back off, or else figure out how to get it to
flow more air. For the moment, it seems a marginal success.
Update: Got a good deal on a standard Rotax dual intake
silencer. Experimenting is fun, but sometimes it's tough to do
better than the factory gurus...
