Quiet Flight

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DarylP

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My god man....you have invented the flux capacitor! :roll:

And it's so quiet...
 

Bart

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Lemme get this straight: Exhaust valve opens and a pulse of hot gas puffs out through a short pipe, then tangentially into a cylindrical canister. Being tangential, the pulse flattens out and travels helically (rather than bouncing and reverberating around) along the inside curved wall of the can, spiraling toward the exit, where it comes out more uniform in flow and so pretty quiet. Piccolo pipe is called piccolo pipe because it's long and skinny like a piccolo, with a bunch of holes drilled in its wall. Gas from can goes out those holes rather than out the far end, which is plugged. Makes it uber-quiet. If you could radius the inner lips of those piccolo pipe holes, I bet it would flow even better.

If I got it right, it's less noisy because there's less turbulence inside the can since each new pulse is smoothly joining an internal flow already spinning in the same direction, just like when you pee into a flushing toilet in the same direction as the rotating water. (less shear noise)

I sure hope this works as advertised. Could be lighter, quieter, and more fuel-efficient with less back-pressure than normal mufflers.
 

rtfm

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Well done, Gary,
Now all we need to do is to determine if the Swarzpipe has any affect (positive or negative) on the power. And I'd be darned interested to hear the before and after sounds.

Nicely done, mate.

Duncan
 

GESchwarz

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Lemme get this straight: Exhaust valve opens and a pulse of hot gas puffs out through a short pipe, then tangentially into a cylindrical canister. Being tangential, the pulse flattens out and travels helically (rather than bouncing and reverberating around) along the inside curved wall of the can, spiraling toward the exit, where it comes out more uniform in flow and so pretty quiet. Piccolo pipe is called piccolo pipe because it's long and skinny like a piccolo, with a bunch of holes drilled in its wall. Gas from can goes out those holes rather than out the far end, which is plugged. Makes it uber-quiet. If you could radius the inner lips of those piccolo pipe holes, I bet it would flow even better.

If I got it right, it's less noisy because there's less turbulence inside the can since each new pulse is smoothly joining an internal flow already spinning in the same direction, just like when you pee into a flushing toilet in the same direction as the rotating water. (less shear noise)

I sure hope this works as advertised. Could be lighter, quieter, and more fuel-efficient with less back-pressure than normal mufflers.

I couldn't have said it any straighter Bart. Well put. But you did leave out the very last step, which is the effect of the RFS. This further extends the period of time that the individual pulse takes to exit, from begining to end. In electrical terms it acts like a resistor in a capacitor/resistor timing circuit, for you Electrical Engineers in the audience.
 
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GESchwarz

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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.

www.zelscope.com has sound measuring software that can be downloaded for a free 13-day trial period and $10 more to have it for a life time. I downloaded it but my computer is not able to run the program. So I sent an email telling them of the problem. So let's see what they can do to get me into the sound measuring business. You can even do screen prints of the freq/amplitude plot. Not bad for 10 bucks, if I can just get it to run.
 

Jan Carlsson

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An idea, a combo of the centrifugal and schwarze where the gaz travel from one chamber to next in the central pipe. where the inlet pipe should be is a question for experimental, at the forward end, central in the first chamber or closer to the mid baffel. ? ? ?

meaning you use 2 spray cans in the experimental version. starting with the centrifug, this to make it more compact.

I read that when making a 4 into 1 it should have an expansion chamber of 12-15 times in volume of one cyl, after the glove and before any silencer

Jan
 

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Bart

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I couldn't have said it any straighter Bart. Well put. But you did leave out the very last step, which is the effect of the RFS. This further extends the period of time that the individual pulse takes to exit, from begining to end. In electrical terms it acts like a resistor in a capacitor/resistor timing circuit, for you Electrical Engineers in the audience.

OK, try this: Instead of that cheap can, use an aluminum baseball or softball bat. It has a gently tapered shaft like a stretched Riesling wine bottle, and this smooth taper will allow the gasses to flow much better than that can. The handle part of the bat can be drilled for the piccolo tube, saving a step and some weight.

Actually, for the RFS reverse flow outer pipe, use another, larger aluminum bat. Use a small kid's T-ball bat for the can, and then a much larger diameter (4"?) adult softball bat fort the reverse flow section. So, narrower T-ball bat barrel gets tangential gas pulse from the engine, gas swirls around and down the neck to piccolo holes, out the holes into a plenum formed by the outer (concentric) fat barrel section of the bigger bat, where it reverses direction. Obviously, cut the knobs off the handles of both bats.

You can get these cheap from a thrift shop, or any local Little League team has dented bats no longer legal for use in sanctioned games, so those dented bats are discarded or only used in practice.

I don't know of bats are also made of stainless steel, but if temperature is a problem, this might be worth looking into.

And, since the internal flow seems happier as a helix or vortex, why not angle the holes in the picollo tube so those escaping gasses feed the vortex? You could test this by squirting a garden hose into the bat and see how the water comes out the holes. An ice pick inserted and used to bend the hole lips to orient the exit streams might work.
 

GESchwarz

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Bart,

For me, the design must be scaleable up to full size for my Mazda rotary engine. Because the the exhaust gas temp of the rotary is higher than a piston (1800F vs 1500F) I must use 321 stainless. There is no way I'll be able to produce the shapes you describe in an economical manner. Cylinders and tubes are readily available. The other thing about the piccolo tube and the reverse flow sleeve to work according to the principal, it has to have some length to it, because length equals time, and time is what we are trying to extend/stretch. So what we're making here is a Time Machine! :ban:

This down stream portion of the system is best located where you have a straight shot, like on the underside of the airplane. Although when I go to optimize this design, we may find that even a short assembly may be sufficient, because the CA seems to be doing the lion's share of the work. In other words, I think there is a point of diminishing returns regarding the length of the piccolo/RFS subassembly. I think that can only be determined after I get the real engine up and running.
 

Starman

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The fact that the engine quit like it did is a strong indication of backpressure.

A centrifugal accumulator will automatically increase back pressure, the higher the gas velocity, the higher the pressure. That's more likely with a steady flow though, with single cylinder pulses there may be another mechanism in action as well.

A centrifugal accumulator will also accumulate a lot of particulate deposits that will stick to the walls.
 
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DarylP

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CO
Gary,
Get that muffler (should I even call it that) finished and the airports would welcome you with open arms as many an airport have come under attack. The enemy? Those loud booming, droning, crackling thunderous, planes that fly over the Ziffles at green acres and upset Fred and Dorise's, Arnold the pig. Fred and Doris live next to the airport see, and well that Arnold, he can squeal like a____, and cause all sort of trouble.
:roll:

Gotta have fun...
 

GESchwarz

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In an attempt to reduce noise inside the cockpit I am considering the shape of the canopy cross-section. It makes sense to me that there should be no flat or otherwise planar surfaces as they would tend to vibrate or otherwise "oil can" in response to noise from outside. A curved surface is stiffer and therefore less likely to vibrate in response to exterior noise, and thus less noise would be transmitted inside.

Does this reasoning make sense to you vibration experts among us?
 

Rienk

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My god man....you have invented the flux capacitor! :roll:

And it's so quiet...

Geez - don't you know anything?

This is just a sub-atomic misgonemetric partial tasmicfan of the Rockwell Turboencabulator.

[video=youtube;Sn4WF7z8__4]http://www.youtube.com/watch?v=Sn4WF7z8__4[/video]
 
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Gnarly Gnu

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In an attempt to reduce noise inside the cockpit I am considering the shape of the canopy cross-section. It makes sense to me that there should be no flat or otherwise planar surfaces as they would tend to vibrate or otherwise "oil can" in response to noise from outside. A curved surface is stiffer and therefore less likely to vibrate in response to exterior noise, and thus less noise would be transmitted inside.

Does this reasoning make sense to you vibration experts among us?

It makes sense GES but it's all wrong I'm afraid. Pick the shape based on best aerodynamics because it sure isn't going to reduce the internal sound transmission (noise level).

You see a stiff structure MORE readily transmits sound energy. Said it before; there are only three physical properties of any material that govern sound transmission:


  1. Mass (heavier is better if you want to reduce transmission)
  2. Stiffness (limp is better here guys)
  3. Damping (well damped panels with no resonance better)

Obviously you need to reduce the noise levels at the source(s) as much as possible first and block any direct transmission paths (air leaks, gaps etc).
 

autoreply

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You see a stiff structure MORE readily transmits sound energy. Said it before; there are only three physical properties of any material that govern sound transmission:
A higher stiffness transmits less sound since it won't resonate as much as a lower stiffness panel. The best would of course be to get the eigenfrequency above the hearing limit (20 khz).

A 3D shaped canopy definitely decreases vibrations and thus noise and your thoughts are correct GES.

Here's a wealth of information about this very topic:
Oshkosh Presentation - Practical Noise Solutions for Aircraft Interiors - Pegasus Aeromarine Inc.
 

GESchwarz

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Nice link autoreply. I believe you are both right. All I would be changing is stiffness which as we can all agree, will increase the renosant frequency of the piece. Sound does travel better through stiff materials. Curving the piece increases the stiffness of the part, but not the material the part is made of.

Sound is a very complex thing. Unfortunately, I think the only way to know the net effect is through empirical testing. Most of us don't have the time or money to try both ways, so we have to go based on the information we can gather and feed that into the Biologic Common Sense Tasmicfan Turbointabulator and wait for the answer to spit out at the other end.

Aerodynamically I think that the 3D surface is better because the flat surface is going to be surrounded by sharper curves. Air doesn't like going around tight radiuses. Furthermore, turbulence itself is a source of noise.
 
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Gnarly Gnu

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It will be fascinating indeed to see this aircraft designed to invert the laws of physics!

A higher stiffness transmits less sound since it won't resonate as much as a lower stiffness panel.

In other advice offered water is not wet, the sun is very cold and gravity repels objects from the earth. :ponder:
 

GESchwarz

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I think what autoreply is saying is that the increased stiffness of a design will raise the resonant frequency of the part. A flat panel, being large, has a low frequency; giving it a curvature will drive up the frequency, thus eliminating the low frequency response. Which is all I'm trying to achieve by putting a curve in the cross section of the canopy, rather than have flat sides. I didn't want flat surfaces that would "oil can" to the beat of the prop blast, which is a relatively low frequency.
 

Dauntless

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Very interesting thread. Thanks for the great discussion and links! :)

I did notice that two areas seem incompletely addressed, which I know from personal experience can really help: coatings, and combining materials of differing densities.

Years ago the boating industry developed non-flammable sound and vibration coatings, which can work well when judiciously applied to airplane structures. Silent Running is one such coating; or search the web on 'marine sound absorbing coating' for lots of other sources.

Several years ago I became annoyed by the engine noise on my converted city bus RV. Its pusher diesel's droning and buzzing got to me, so I decided to do something about it. Aside from the coatings, I built a new door to separate the front from rear sections of the RV, which proved to be remarkably effective at cutting down the noise factor.

To make the door I decided to use several dissimilar materials, so I built a light-weight frame from 1x1 clear pine, which I filled in with 1" white insulating EPS foam. The rear facing skin was eighth-inch "white board", while the forward skin was eighth-inch luan mahagony. This sandwich construction worked really well to reduce the noise level in the forward cabin to a comfortable level. Weather stripping topped off the job.

This "sandwich" of dissimilar materials works so well because each time the sound wave penetrates from one material to another it requires disproportionate energy to transition from one material to the next, compared to continuing through one solid block of the same material.

On a related note, carbon fiber, kevlar and fiberglass each have very different sound transmission properties, which can be exploited for sound control purposes. I like to race sports cars, and we typically have to meet strict sound levels. Racers have found over the years that kevlar is a poor sound conductor, so its use around loud components if popular. Carbon fiber, OTOH, is an extremely efficient conductor, so is not the best choice where one needs sound attenuation. Fiberglass is somewhere between the two. Therefore, kevlar might be a good choice as the material for an engine cowling where sound attenuation is a goal, while carbon fiber might not be as good a choice for that application.
 

Gnarly Gnu

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Yeah Dauntless has kinda gotten it; reducing sound transmission by means of increasing the damping properties such as with composite layering. Flatten the resonance Q. After all of the 3 properties we can play with adding mass isn't popular on aircraft.

I'm suggesting it's perhaps not wise to ignore every text book ever written on acoustics and hope that just this once increasing the panel stiffness will actually reduce sound transmission coz it somehow worked for auto in an alternate universe.... :tired:

Still reckon I'd be picking a good aerodynamic shape first (a square-ish section may not be that bad in some instances) and working with that personally. Line the heck out of your engine compartment, use a good muffler, block / treat any gaps and use a composite sandwich for the firewall and doors + forward panels.

Do a simple experiment: compare a single layer of say 1.0mm aluminium sheet with two layers of 0.5mm sitting straight on top of each other. The two layers weigh the same as the single one but are much more flexible and better damped thus provide a vastly higher sound transmission loss. This is partly due to friction between the sheets, the sound energy is converted to heat. Glue them together with a rigid adhesive (i.e. stiffen them) and you loose the acoustic benefit, they will now act much the same as the single sheet.
 
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GESchwarz

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The exhaust manifold heat shield of my Mazda rotary engine is made of two thin layers of stamp-formed sheet steel, joined along the perimeter only. This is a sandwich of steel/air/steel. When you tap it, it makes very little sound in response, aka a dead sound, no ring to it like you would expect from a single piece of steel. So I agree with you on the disimilar sandwich idea.
 
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