Quiet Flight

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Jan Carlsson

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Speed reading the NASA report give us the result that a more effective muffler, less exhaust noise and less restriction in back pressure, made the C-150 more noisy, but the climb was 100 fpm better.

Both of this i guess is from the engine is more powerful and can swing the 69" prop faster.

To reduce noise, first the loudest item have to be addressed first, then if over-all noise reduction is wanted every part of the package is needed to be looked over.

The O-200 turning the 69" prop at 2750 rpm will have a tip speed of 848 ft/sec Mach .76, going to a 3 blade will require a 63" giving a tip speed of 778 ft/sec or M. .70

For noise reason the tip speed should be under 820-790 ft/sec

A 4 into 1 system and a Schwarzpipe combined with glass or "foam" looks promising. A 4 to1 system will probably give 15-20% more power then the Cessna system, making the climb and noise pattern on the ground better.


Jan
 

berridos

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sorry dont read...already discussed

In bingelis book he talks about swiss mufflers. they were the answer to the strict swiss noise regulation. They are 3-5 feet long 3 inch diameter aluminium tubes with thinner perforated steel tube inside and inbetween rockwool.
They dont take power away and are very effective.
I have to see if they make sense for a turbo engine and see if i have enough space for a tunnel in the fuselage while keeping adequate distance between the muffler and the composite. Such a tunnel could serve as a stringer for the fuselage structure.
 

DarylP

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sorry dont read...already discussed

In bingelis book he talks about swiss mufflers. they were the answer to the strict swiss noise regulation. They are 3-5 feet long 3 inch diameter aluminium tubes with thinner perforated steel tube inside and inbetween rockwool.
They dont take power away and are very effective.
I have to see if they make sense for a turbo engine and see if i have enough space for a tunnel in the fuselage while keeping adequate distance between the muffler and the composite. Such a tunnel could serve as a stringer for the fuselage structure.
That rock-wool is a great sound deadening product. I was at a builder show once where a company was promoting spray in rock-wool for insulation and sound deadening in homes. They had a five gallon painters bucket lined on all side with the stuff, I think about four inches is all. They had a small 130db siren that they turned on, (it was loud!) and then they dropped it into the bucket and covered it. You could hardly hear it! Amazing difference.

GESchwarz...in a previous post you said: Although the piccolo by itself is a a form of muffler, in this design it appears as though the piccolo tube was used primarily for mixing with ambient air to get the temperature down.

That being said, what would change if you used a turbo charger? A turbocharger uses the exhaust to spin the turbine. How much would the turbo cool the exhaust afterward, and how would that affect those pulses?

DarylP
 

GESchwarz

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Turbochargers are very effective in lowering exhaust sound volume. Compare the sound of a Bearcat which is turbocharged with the sound of a Mustang which is supercharged. The Bearcat flying overhead sounds more like a Learjet than a piston-powered machine...Smooooooooth.
 

Bart

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That rock-wool is a great sound deadening product. I was at a builder show once where a company was promoting spray in rock-wool for insulation and sound deadening in homes. They had a five gallon painters bucket lined on all side with the stuff, I think about four inches is all. They had a small 130db siren that they turned on, (it was loud!) and then they dropped it into the bucket and covered it. You could hardly hear it! Amazing difference.

GESchwarz...in a previous post you said: Although the piccolo by itself is a a form of muffler, in this design it appears as though the piccolo tube was used primarily for mixing with ambient air to get the temperature down.

That being said, what would change if you used a turbo charger? A turbocharger uses the exhaust to spin the turbine. How much would the turbo cool the exhaust afterward, and how would that affect those pulses?

DarylP
And, rockwool will sustain temperatures of over 2,000 degrees before breakdown, about twice the breakdown temperature of fiberglass. Rockwool holds much less moisture than fiberglass, so less buildup of acidic condensate residue which corrodes the metal muffler housing and pipe.

Regarding Bengelis and the Swiss muffler: Not as good as hoped, per one homebuilder who made and installed one on his BMW R1100 boxer (a very quiet engine) powered plane. He wound up buying a slightly used aftermarket crotch rocket muffler for <$30 and using that, with less weight & noise and more performance. (Some such motorcycle mufflers are made of carbon fiber, if saving a bit of weight and avoiding corrosion is a priority.)

Since many of those motorcycle mufflers can be opened and rebuilt, why not use rockwool as replacement filler?
 
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berridos

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i have never understood what kind of epoxy they use for motorcycle carbon mufflers. I know of high temperature epoxys but not of resins that resist the temperature of an exhaust
 

Bart

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i have never understood what kind of epoxy they use for motorcycle carbon mufflers. I know of high temperature epoxys but not of resins that resist the temperature of an exhaust
See if this comes through:

BMW - Engine

This is the guy who put a BMW R1100S engine into his Cherry, tried the Bengelis Swiss muffler, found it to be too loud, so wound up using a Suzuki 1300 motorcycle muffler and later a titanium Honda Fireblade muffler for ~$50.
 
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GESchwarz

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Rock wool is like fiberglass or cotton batting used in making quilts. I can't imagine it lasting very long due to the velocity of hot gasses passing by or through it. I would think that it would all blow away in short order. It is also carcinogenic.
 

BBerson

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The Swiss muffler used wire mesh "hardware cloth" rolled into about 3" diameter by 36" tube, if my memory is correct.
 

DarylP

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I recently read that the muffler should go near the end of the tailpipe. They also say that if you do have a length of pipe on the exhaust side of the muffler, to pay attention to the color. If it starts to blue-out, that means that you developing back pressure at that point. I know that some of the aircraft exhaust companies use silencers at the end of the tailpipe.
 

rtfm

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All this talk of Swiss Mufflers... I for one can't wait for the Schwarzpipe. I think it's going to be a winner.

Duncan
 

Bart

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Rock wool is like fiberglass or cotton batting used in making quilts. I can't imagine it lasting very long due to the velocity of hot gasses passing by or through it. I would think that it would all blow away in short order. It is also carcinogenic.
Per Bengelis' Swiss muffler design, the batting is held in by stainless steel mesh (course grade version of window screen) and rolled into a tube, so apparently would prevent the exhaust pulses from blowing the rockwool out.

And, Googling keywords "rockwool carcenogenic" shows a slew of cites, perusal of the first few indicates Rockwool to be pretty safe and non-carcenogenic, although fiberglass has aroused some suspicion. In any event, I would not be doing much breathing from the tailpipe.
 

plncraze

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Be careful of muffler design. Peter Garrison sent a letter to Sport Aviation many years ago (70's)detailing an incident when his home made muffler came apart in flight and gagged the engine. The insulating material came loose inside the muffler and almost plugged the exhaust if I remember correctly.
 

GESchwarz

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I’ve done a little home work and I’m excited to tell you all that I think I’ve learned a couple of things.

Thing 1. There are non-traditional ways to eliminate the need for baffles and packing material.

Thing 2. It is much more difficult to push pulses of exhaust gas through a pipe than it is to push the same volume as a steady flow.

I have attached 6 photos of my test bed using the 1-cylinder Briggs & Stratton motor on the edger my Dad purchased back in the early 60’s. I’ve been operating that edger ever sense, on one oil change.

Photo 1: All of the pieces. The engine without a muffler of any type produces very definite, loud pulses...a popping-like sound.

Photo 2: Centrifugal Accumulator (CA) The design and name should speak for itself. There are no internal baffles. The exhaust pulses enter tangentially, thus the pulse energy is disipated along the cylinder wall in a circumferential path. The flow out the exit port located at the centerline of the cylinder is a steady, constant flow rate, and that means Quiet. I don’t have the sound meter yet but this device alone cuts the sound to just a little quieter than the stock muffler. The stock muffler produces a slight pulsing sound. The other key point about this device is that its large volume, located immediate to the engine, acts as accumulator, or in electrical terms, a capacitor which effectively flattens the highs and lows.

Photo 3: CA with Piccollo Pipe (PP). When installed, the PP is capped off at both ends. This assembly is nearly as quiet at the Schwarz Pipe Assembly.

Photo 4: PP, Reverse Flow Sleeve (RFS), and End Cap detail. Notice that this end gets an end cap.

Photo 5: Complete “Schwarz Pipe” Assembly (SP) installed. This device completely eliminates exhaust sound as far as I can judge. All I hear is what seems to be emanating from the engine block/head and drive train. This photo shows the exit end of the RFS.

Photo 6: PP Only. When this pipe was fit directly to the engine while running at medium speed of 1000 RPM, the engine stopped due to the excessive back pressure caused by the long pipe. This was a big surprise! Even though it had all of those holes (ten ¼” holes at the up stream end, twenty 1/8” holes at the down stream end, and the down stream end of the tube was open…no cap!). What does this say for standard exhaust manifolds and pipes? It says that they may be producing a lot more back pressure than we think, even without a muffler.

Photo 9: Exit port of stock Briggs & Stratton Muffler. Notice the 1/8 dia holes in the baffle. There are only 12 of them. The engine doesn’t quit even with this small of exit area because the volume of the muffler housing plus baffles has converted the exhaust pulses into a constant, even flow rate. At a steady rate, the volume has no problem getting through.

Note: One of the two blades of the edger is painted white for detection by the photo-tachometer.
 

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JimCovington

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Very interesting. I don't suppose you had any way to test power output?

Can you record mp3 files? I'm curious to hear the sound characteristics.
 

GESchwarz

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I may be able to record onto my laptop but I'm not sure how. The fact that the engine quit like it did is a strong indication of backpressure.

The biggest star of this show is not the piccollo tube, but rather the Centrifugal Accumulator. A big empty volume where the pulses can run around (disipate) the circumference, while the pressure is released out through the eye of the tornado, so to speak. I got this idea from some folks in the Mazda rotary powered aircraft group who have been using it. They connect a rather small CA to the stock exhaust manifold with great results. I'm taking it a step further...I'm cutting up the manifold and welding a much longer cylinder directly to the inlet ports of the manifold. By doing this the exhaust is going directly into the cylinder (CA) from the engine and I'm saving manifold weight and space. So the CA becomes the collector portion of the new manifold.
 
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