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Nice data Steve.
Your speed change correlates closely with what I measured on my RV-4
with and without the Spintech and Edelbrock RPM mufflers (3 - 4 mph
reduction with the muffler). The smaller Hushpower II that I use now
only drops it by a barely measurable 1 mph but it is mounted closer to
the belly in the turbulent air behind the cooling outlet. I used a
.007" thick SS shield pop riveted to the belly also and this worked
well even with no insulation between SS and aluminum. When the SS
sheet heats up, it expands slightly creating an air gap between it and
the fuselage which nicely isolated the heat.
The Hushpower II looses some but not all of it's effectiveness as it
ages (loosing packing probably) so I'm still searching for the optimum
muffler. You might be on to something here. Wish it was available
off the shelf.
Tracy
On Fri, Jan 16, 2009 at 12:57 PM, sboese <sboese@uwyo.edu> wrote:
The following muffler description is provided purely as another data point
in the muffler discussion. I would suspect that not many of you would be
interested in it due to its large size and external mounting. I have had
good service from it so far and it fits somewhat into my loosely interpreted
Navy F6F Hellcat inverted paint scheme as a bomb.
The materials of construction are 0.032" thick 304 SS and the design is
based on data in NACA Report 1192. The weight is 9.6 lb and the diameter is
6.25 inches. A SS heat shield is also installed between the muffler and the
fuselage belly. I have about 100 hours on this installation with no signs
of deterioration of the muffler. The engine is NA with exhaust splitters
and is operated normally at density altitudes above 7000 ft which limits max
power, although several hours of max power operation at DA lower than 5000
ft have not damaged this muffler so far.
I have been told that from the ground, the primary source of noise for my
plane is the propeller. I have landed with a C172 following me in the
pattern and observers told me that they did not know I was there until I
taxied in since they only heard the C172. I have no quantitative data on
the noise characteristics of my muffler system. Inside the cockpit, the
noise level goes from reasonable to unbearable if the muffler is removed.
This is purely subjective, of course.
Not shown in the "muffler parts" image are three 1" x 1" angles made from
the same material as the shell that connect the rear of the internal center
tube to the front of the rear cone section. The round flange in the center
of the internal tube is a push fit in the shell and is not fastened directly
to the shell. A down turn also has been added to the outlet of the rear
cone section.
From data in the "muffler test chart" image, the speed penalty of the
muffler is about 3 kt.
The "muffler in flight" photo may be of interest in that it shows the heat
distribution in the muffler with the dark areas being hotter. This image is
not current since the lower cowling has been modified and the engine cooling
air outlet is now much smaller than shown in this photo.
Steve Boese
RV6A, 1986 13B NA, EC2, RD1A
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