Super quite OS 91

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
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Adding large silicone tubing to your muffler extension will quiet it significantly, but will also steal some power.
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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:

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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 noise requirements.
            Marty
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Jim & Elizabeth wrote:

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)
Martin
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Right.
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 intake?
            Marty
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On Tue, 19 Sep 2006 20:00:59 -0400, "Jim & Elizabeth"

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.
Abel
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On Tue, 19 Sep 2006 20:00:59 -0400, "Jim & Elizabeth"

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.)
            Marty
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On Thu, 21 Sep 2006 09:07:14 -0400, "Martin X. Moleski, SJ"

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.
Abel
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wrote:

Remember the OS .61VF with gear reduction unit? The straight cut gears made more noise than any other component on the model. Very loud and annoying.
Sparky
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wrote:

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?
Abel
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Ha! I used to do that with a Kaos 40! I also used to slope a 1/2A Pacer. Those were on the days when there was enough lift to fly a cinder block.
Dave
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