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----- Original Message -----
Sent: Tuesday, November 30, 2004 7:03
PM
Subject: [FlyRotary] Re: New Muffler
Design
At 04:02 PM 11/30/2004 -0500, you wrote:
Well, folks, got the new muffler design
installed today - and initially results are encouraging. I designed
the muffler based on what I believe I have learned concerning Finite
Amplitude Wave theory and applied it to this muffler
design.
Tell
us more about Finite
Amplitude Wave theory.
Bill, I would rather not reveal
just how little I understand about the theory - except to say that it is a
disturbance in the air (generally not caused by a vibrating medium) that
exceeds the strength of a 125 dB sound wave by 1000 of times. They are
formed in both the intake and exhaust systems of internal combustion engines
(as well as a number of other environments). They can be used to
enhance both intake and exhaust performance (If used properly) - when
they terminate they can produce localized pressures of over 15 psi (short
duration). The FAW can either support macro airflow in the direction
you want or adversely affect it depending on many variables.
The bottom line is these
waves interact in a different manner with their environment that what we
normally think of as sound waves. You can find a bunch on the internet
by searching for "Finite Amplitude Waves" - some so mathematically esoteric
that I certainly can't follow it - but other explanations are more
understandable. Try a search.
Here is a
starter
look down about half way
down the page for a layman's description of some of their
attributes.
The mufflers consider of two 2" dia tubes
4' long with sound attenuating discs inserted. Each muffler hangs off
of my two header pipes and extend back under the fuselage with the exhaust
exit dumping behind the cockpit area.
Pictures? Drawings? How many disks? What
spacing?
Sorry, I normally take photos of my projects, but the digital camera's battery
failed. I will come up with some drawings shortly - but, want to fly
with it first before doing so. The space between discs is variable in
order to make "traps" for different frequencies but is basically harmonics of
the basic rpm (so you have to sort of decide what rpm you want the effect to
be most dominate - but at least in my case it seems to be pretty effective
across the spectrum). I presume that is because the difference spacing
is doing a fair job of attenuating several different
frequencies.
For instance, if you combine
your two exhaust into one muffler you are dealing with 200Hz at 6000 rpm but
if you use two separate tubes (like I do) then with each exhaust port
feeding a separate tube the primary frequency (in each tube) is 1/2
200Hz or 100Hz. That calls for a different spacing of the discs.
I use two types of discs, one
is nothing more than a 2" dia washer with a 1" dia hole in the center.
The other type disk is a 2" washer with a 1/2" hole in the
center. This one is modified to (hopefully) present a solid disk to the
FAW causing it to bounce back and destructively interact with the next FAW
wave - but still permitting air molecules to pass with minimum
restriction. I use 3 of the 2/1 washers and 4 of the 2/1/2
washers in each tube with the spacing between discs starting off small (to
attenuate the higher frequencies) and incrementally increasing to a larger
distance between discs to attenuate the lower frequencies. I also place a 2"
dia SS wire mesh disc before each of the three 2/1 washers that has an
weave open area of 40%.
I really want to fly with this set
up to see if it survives. I discovered a couple of years ago that the
intensity of the rotary turbo housing exhaust could actually pulverize SS
tubing in a matter of a few minutes of flight. These were "Fishtails" or
squished down tubing ends. I came back from one flight with the ends of
the tubes looking like something had bitten big chunks out of them. This
due to the fatiguing of the metal by the powerful pulse (not the heat) in a
matter of minutes.
Besides, while there appeared
to be no adverse affect on my power at my static of 5600 rpm that may not
necessarily be true when I get airborne and attempt to reach 6400
rpm.
Subjectively, two hanger bums standing by the wing tips told
me that the exhaust was no louder at WOT than at idle and that the sound of
the prop drowned out the exhaust at WOT. I also
noticed that while sitting in the cockpit with the radio on I heard
ignition static over the radio receiver for the first time!
Did you do the test in the open or in the hangar?
The aircraft was in the
hangar, but the hangar has no door and the tail was pointed back outside the
hangar. There little doubt that I was getting some sound reflection
from the back and sides of my qonset hut type hanger - but I do not do run ups
on our exterior hard surface pad/runway due to its poor condition
and the fact that my last prop picked up two nasty nicks due to debrie lifted
by the prop during run ups.
For
you sound experts, I presume if a 3DB increase represents a perceived doubling
of loudness does that mean a 10 DB decrease represents an approx 3 fold
decrease in perceived loudness??
It is a log scale. 3 dB is
indeed 1/2 the sound intensity. The 9 dB is not 3 x 0.5 it is 0.5 x 0.5 x 0.5
Thus it is 1/8th the sound intensity.
Thanks, Bill.
Well, I guess that may well be why the individuals reported they
could not hear the exhaust over the prop noise. It also may mean I can
eliminate some of the discs and still have good sound supression if I do
run into adverse effects on power production at high
rpms.
I'm surprised
you haven't connected the two pipes after they leave the engine. In cars with
dual exhaust, they put a cross-over pipe just after the header (or manifold).
The cross-over pipe is typically smaller than the main pipes. The cross-over
effectively doubles the frequency and halves the amplitude of the sound pulses
in each side. The higher frequency is more strongly attenuated by the low-pass
filters (mufflers). The effect of adding a cross-over pipe is dramatic. The
exhaust note goes from painful to pleasant.
Sounds like reason for another
muffler design {:>). I am aware of the reported benefits of a cross
over tube - but attempting to keep the underside of the aircraft from looking
too cluttered I have not attempted it. Right now all I have are two 2
1/8" tubes running straight back from the bottom of the cowl separated by
approx 12". It does sound like it would be benefical to design one with
such a cross-over pipe. By doubling the frequency and halving the
amplitude that would appear to indicate the mufflers could be perhaps closer
to 24" rather than my 48".
If the
cross-over pipe placed properly, there is no loss of power. (Sometimes you get
a small increase.) You want the header to scavenge the combustion chamber
properly, so you have to leave enough header to do this. On a V-8 trying
to make good low-end torque, this is about 18" or so. I have no clue what
header a rotary would like to make max HP at high RPM.
Thanks for the questions and
suggestions, Bill
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
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