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As I mentioned in my first e mail concerning my
trip to Florida, my left EGT was running 125 F higher than normal and my rpm
choked at 6000 with a small amplitude irregular vibration present.
A bit of background for those who may have missed
some of the episode.
I inserted special configured discs in my two 2"
diameter "muffler" tubes and discovered that while they did a great job of
suppressing the nose, unfortunately it took only one 4 minute flight to clearly
demonstrate they suppressed power equally well. Could only get 5200 rpm in
the air.
After realizing (doing the calculations) that
2" dia mufflers did not provide sufficient airflow (with my discs inserted) I
switched to a 3" dia tube (after doing the math that showed the discs would have
more open area than my 2" tubes (that sans discs I have flow with for over 100
hours).
Well, I inserted 5 of these discs using a threaded
5/16" stainless steel rod (with nuts) to clamp the discs to the rod and maintain
the spacing between the discs. I then inserted this set up into the 3"
diameter tubes and closed them up.
I knew I had better exhaust flow immediately as the
static rpm increased to 5800 rpm. The noise suppression was not as good as
the 2" tube but considerably better than my open tubes. The open tubes (as
Tracy later reminded me) produced at least 125 dB of noise where as the new set
up produced only 104 dB at the same distance from the aircraft at WOT. So
some improvement.
In anycase, it was time to head to Florida and no
more time to mess with them. So I launched for one quick trip around the
pattern to make certain I had good take off power and then landed to check
everything out. One of my hangar neighbors mentioned that at WOT just
before launch, he would have sworn that I had a turbine due to the turbine like
sound he heard. Well, I didn't think anything of it as you know we
sometime refer to our rotary as a "poor Mans turbine" So I loaded up and I
launched for Florida on Wednesday last week.
Well, the launch was fine with 5800 rpm - however,
several things became clear once I leveled off.
1. My right exhaust EGT was 125F higher than
it had ever been (normally my max is 1600F), it was reading 1725F, The
right one was a bit higher as well but not nearly as much
2. While the exhaust flowed fine at 5800 rpm, the
flow choked the engine at 6000 rpm. Well, I can fly fine on 6000 so I
proceeded on down to Tracy's
3. Along the way I also notice a low amplitude
rapid vibration which did not to seem to be directly engine related (like
gears), but did seem correlated - I also notice my coolant temps were running
around 10F higher than normal compared to the oil temps.
Got down to Tracy's just fine but decided to take
the mufflers off and see what was happening.
1. It turns out that 2-3 of the discs in the
left and possibly 1 in the right tubes had been rotating at high rpms. Despite
the fact that each discs had been clamped in position by "Jam" nuts on
each side.
2. The tremendous speed of their rotation was
evident by the wallowed deformed bell shape of stainless steel metal around the
hole in their center (though which the stainless steel threaded rod had been
inserted). Also all thread had been eroded away from the rod where the
discs had rotated. In one case the stainless steel nut had a "flange" added to
it by the tremendous temperature and force.
3. Looking at the inside walls of the .065 3"
dia 304 tubes you could see where the tips of the discs had been contacting the
walls. The tips of the discs were burnished also testifying to that contact -
undoubtedly the source of the vibration I felt.
4. One of the discs had been shattered and various
other indications of violence inside the tubes.
So the "turbine" sound my neighbor had heard were
undoubtedly those spinning discs, that of course was also responsible for the
impeding the exhaust flow (like several windmilling props), which in turn choked
the power at 6000 rpm and caused the EGT to increase.
With Tracy's kind help, I stripped all but two
discs from each rod (making certain that we paired discs which would want to
rotate in opposite direction (hopefully stopping any - or most rotation).
Then Tracy welded the discs, nuts and rod together. I inserted that arrangement
back into the 3" tubes, closed them up and flew the rest of the flight with that
arrangement.
The trip to Lakeland and home would seem to
indicate I have found a satisfactory (for now) solution. I hit 6000 rpm
static on take off this morning and got up to 6600 rpm straight and level (which
is 200 rpm more than I got with the straight (empty) 2" tubes. I intend to
make one more modification by placing a cross feed tube between the entrance to
the two mufflers, the old "H" pipe hook up which according to many sources,
assists in lowering noise and improving power.
If indeed this bears out as it now looks then the
only other thing I may consider is using .035 3" dia tubing rather than the .065
tubing I had available. The .065 tube exhaust each weighed 9 lbs making it
18 lbs total, using the thinner tubing would approximately half that
weight.
All in all - I consider the experiment a
modest success but will undoubtedly continue to experiment with the exhaust
system {:>)_
Best Regards
Ed
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