I would like to fly my OS 91 powered float plane on a lake which is
surrounded by summer cottages , which I have done and it seems to be okay
with the cottage owners but its obvious that engine noise is a problem ,
especially first thing in the morning when the lake is like a mirror, has
any body figured a way of making a four stroke super quite ? Maybe adding
two stroke muffler with extra baffles ? Jim J
Has anybody here ever tried using a tank full of baffling material, such
as a tomato paste can full of coarse steel wool? I'm thinking of the
stainless steel wool sold for scrubbing pots. If the tubing and final
outlet aren't too small it wouldn't have to create too much back
pressure. I've always wanted to try this, but I never had an engine
noise problem around here.
Stanley Barthfarkle wrote:
On Tue, 19 Sep 2006 20:00:59 -0400, "Jim & Elizabeth"
wrote in :
You can probably get a muffler extension.
I know they are available for YS engines. I have
one for a YS 91 AC. They cut the noise considerably
and are or were used by pattern pilots to meet
One thing to be aware of; once you get the exhaust noise _way_ down,
other sources of noise become realatively more important and may need
to be addressed too.
These include prop noise (which can be a biggee), vibration transmitted
to the airframe thus making the airframe vibrate, and engine air intake
noise (can also be surprisingly loud)
On 20 Sep 2006 09:17:14 -0700, "Martin" wrote in
Put on a prop that will turn at RPMs low enough to
keep the prop tip from going supersonic. (Not
that big a problem with .91s, I suspect).
Or use a three-bladed prop.
There are some excellent soft mounts available.
They do make a big difference.
Cowl the engine? Put an air-filter over the
I use one on an OS 52 that has worked very well for me. It's a
resonator type that doesn't produce any negative effects on the
operation of the engine - in fact I often slope soar a Kadet LT50 with
the engine at a tickover idle for upwards of half an hour, and it has
no tendency to load up, or lean out due to the drop in fuel head over
that extended idle period.
Don't have a drawing, so I'll try to paint it in text. I wanted at
least 10:1 expansion ratio for quieting performance, so cross section
area of the chamber is 10X the cross section of of the exhaust header
pipe. Length is about 6", an somewhat arbitrary "TLAR' dimension that
intuition said would provide sufficient volume, and be near some small
ordered harmonic of the exhaust frequency. I think can presume from
what I wrote so far that there is nothing critical in the dimensions,
and you would be right! Inlet and outlet pipes extend the length of
the chamber and terminate embedded in the end plugs. They are spaced
evenly across the diameter of the chamber, i.e., each centered half
the radius out from the chamber CL. Both inlet and exhaust have holes
drilled over their length and circumference, I used about 20 in each
sized so the total of their area is at least 2X that of the exhaust
header. That was a 2nd iteration BTW - not enough holes used
initially, causing noticable increase in backpressure that affected
mix setting adversely. I hadn't made them big enough to account for
edge effects on exhaust gas flow. The connection is made to the
stock muffler with a short length of automotive fuel/vapor tubing.
This tubing has required replacement at about 1 year intervals as it
tends to harden and crack in time. The connection to to the short,
smooth stock muffler outlet is made more secure by cutting a groove
around it to fit a C-clip. This gets a bite on the connector tubing
and prevents slippage. When installed it is rotated with the outlet
pipe oriented down (bottom of chamber, so oil condensate gets blown
out wit hthe exhaust stream. FWIW, all material is scrap aluminum
tubing from a local salvage yard at 2 bucks/lb. The ends plugs were
machined by a buddy for prees fit. Never had come apart except used a
bench press to dismantle it for for the modification mentioned above.
That's about it - I had intended for it to replace the stock muffler,
but it worked so well in my test config as an aftermuffler that I left
it that way. Been working fine for more than 10 yrs now.
As for performance, it drops measured dBA level at 3 M by 5-6 dB at
WOT, depending on prop used. More than I expected to get, as the OS
52 is pretty darned quiet out of the box. At idle it's undetectably
silent during my slope soaring jaunts. Sometimes I blip the throttle
just to be assured that it is still running.
New tip just gleaned from the NSRCA pattern mailing list:
some of the guys are now gluing foam inside their cowls to
dampen the noise of the engine (probably the air-intake
noise from the carb).
So now your to-do list reads like this:
1. Enlarge muffler one way or another.
2. Soft mount engine.
3. Find optimum prop to avoid tip noise.
4. Cowl the engine and glue foam to the
inside of the cowl.
(Maybe you should consider going electric.)
Good points, Marty
Back when the pattern rules limited engine displacement to 10 cc,
geared engines with rear intakes and tuned pipes had their heyday. The
rear intake made damping intake noise easy, because it extended back
to a chamber in the fuselage where it was pretty well enclosed. The
gearbox kept prop rpm low and allowed a large diameter (more
efficient) prop to be be used while keeping tip speed down. The tuned
pipe is a large reactance (in analogous electrical terms, a big
capacitor to ground) that is an element in an efficient low-pass
filter in the acoustic path of the exhaust system. It all came
together to produce a very high (shaft power)/(noise power) ratio.
RE prop tip speed FWIW: There is a very high degree of correlation
with noise level throughout the range of prop sizes we use on models
(I collected a lot of data on these, but not for anything larger than
you might use on a FS 1.20). As a benchmark, if your objective is to
meet AMA's recommendation of 90 dBA at 3 M, the corresponding tip
speed limit is Mach 0.45.
Sure do. Got one that that's brand new, still sealed in the plastic
inner wrap of the package. Do you know of a collector that might
want to own what may the last one on the planet in that condition?