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Reducing exhaust noise

edmondthx1138

edmondthx1138

Member
Does anyone have experience or an opinion on modifying the muffler or exhaust pipe to reduce noise? My research indicates that a large amount of engine noise comes from the hot cases escaping and causing pulses.

The simplest solution I can think of is expanding the diameter of the exhaust pipe, and/or its length, and/or adding some kind of silencer baffles.

Any thoughts?
 
I'm not much of an expert of exhaust flow, but I do know that the you start monkeying around with either squeezing off or slowing down the exhaust gasses it can have some unintended performance losses.

That can occur either by restricting the exhaust or, even thought it seems counter intuitive, by increasing the size of the pipe so that the flow slows down too much.

Not saying that it's necessarily a bad idea, but just something to think about. I do know that there has been some success with turning the exhaust tips downward so that the pulses don't hit the belly aft of the firewall and turn it into a drum.

Hopefully somebody who's given it a go will chime in.
 
Most, if not all, commercially available exhaust systems built for RVs are tuned for optimum performance.
Not a good idea to mess with a good design without discussing it with the designer.
 
One of the best things about RVs is no muffler to choke performance and quiet our flying sports cars down.
 
Do you have any sound deadening on the floor?
And if not, before tossing nylon carpet in there... consider what happens should you get an engine fire (even on the ground). That floor can get hot fast. Not a place for anything that smokes or combusts.

The EAA Tony Bigelis book "Firewall Forward" shows a piccolo pipe silencer on page 101.
 
On the old continent, where there are certainly way more people squeezed in a square kilometer than in the US, the use of muffler(s) is very common.
Germany, Austria, and Switzerland, have almost all of their RV fleet equipped with mufflers.
It’s pretty easy here, once your project has become an airplane, you have to attend to a sound… or noise, test. Too noisy and you don’t get your final permit to fly, and are de facto grounded…
This measurement is done in flight by simulating a climb over a fixed point. A mic placed on the ground will measure the noise emitted.

My guess is the same will happen in the US in the long run… just a matter of population density ;)
 
edmondthx1138 said:
Does anyone have experience or an opinion on modifying the muffler or exhaust pipe to reduce noise? My research indicates that a large amount of engine noise comes from the hot cases escaping and causing pulses.

The simplest solution I can think of is expanding the diameter of the exhaust pipe, and/or its length, and/or adding some kind of silencer baffles.

Any thoughts?
Click to expand...

It is not really the heat or the pulses, although all exh has a pulsing rhythm to the noise. The noise comes from the cylinder and through the pipe. Only way to address is to absorb the noise. Glass packs (F/G insulation in outside shell with louvers in the inner pipe) flow well but don't cut down much noise. Chambered mufflers (which is what autos use) create some restriction but very effective in absorbing noise. However, their footprint is large due to the chambers (flow has to be reversed several times)
 
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I’m guessing from the OP’s profile that he’s probably talking about an RV-12 and a Rotax, but the general comments made so far are still valid - exhaust systems are pretty much optimized when they are designed and built.

I will say that one thing I have learned from building an electric airplane is that it is almost as noisy on the rap as a piston-powered machine. The noise from the prop is probably a much bigger signature than that from the exhaust on a piston engine. I need to quantify that someday….

Paul
 
I will say that one thing I have learned from building an electric airplane is that it is almost as noisy on the rap as a piston-powered machine. The noise from the prop is probably a much bigger signature than that from the exhaust on a piston engine. I need to quantify that someday….

True, as evidenced by a Bonanza takeoff, I suspect their props tips are approaching supersonic
 
Airplane Noise....

Tommy123 said:
I will say that one thing I have learned from building an electric airplane is that it is almost as noisy on the rap as a piston-powered machine. The noise from the prop is probably a much bigger signature than that from the exhaust on a piston engine. I need to quantify that someday….
True, as evidenced by a Bonanza takeoff, I suspect their props tips are approaching supersonic
Click to expand...

I remember taking an IQ test when I was in junior high school and one of the questions was: which makes more noise on an airplane: the engine or the propeller? I said propeller and got it wrong. :mad: I guess they had not been to OSH to see/hear the AeroShell team performing!!

I actually like the sound of RVs. :) It is always a distinct sound as they fly overhead or depart and doesn't seem that loud. I do not get the floor pulse but my exhaust are pointed down at about a 30 degree angle or so. I have no experience with what the -12s sound like.....
 
A very astute aircraft engineer once told me that there three sources of noise from an aircraft in flight: Engine exhaust, prop and airframe. Yes the airframe itself passing through the air makes a sound ...

So my ride has the Vetterman 2 pipe on an IO-360 and Catto three blade. Compared to a 206 it sounds like an electric plane, and the prop and exhaust sound noise is low and mighty nice.

Like the previous post mentioned, someday we'll be in the same boat as Europe with testing required (it already exists for commercial at some airports in the US) and mufflers will be more common. Honestly, I don't think our biggest offenders are the RV's, but the big engine constant speed machines taking off at max rpm (tip speed and all).

In the mean time, we should probably all do our best to keep the impact low and hope I time out before the regulators change the rules!

Cheers
 
Are these mufflers?

i-32DPqkX-S.jpg


Along these lines, I bought this RV-6A and it has these extensions on it, I was told they were to reduce noise, so muffler I guess. Anybody else used these or seen these before. I'm wondering if the drag they cause is worth any noise reduction.
 
Dale, yes, kindof... the end of the pipes have been applied the treatment as Tony Bingelis described in his book Firewall Forward on the chapter mufflers, starting page #92.

Not sure about their effectiveness though.
Mickey has a small article about the use of mufflers around here on his homepage: http://www.rv8.ch/aircraft-silencers-and-mufflers/
 

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as already pointed out, the biggest noise producer ain't the exhaust, but the prop. Therefore most RVs flying in central Europe have their RPMs restricted in order to meet emission requirements.
As an example, my own ship is restricted to 2'600, and the -8 I also get to fly to 2'500 RPM.

The RPM and mufflers restrictions only affect performance by some, thanks to the exceptional basic performance of the RV.
And nobody noted a lack of performance of my steed during my 2022 US tour :D

As an example, 2 pictures of mufflers as installed here. The first, the big pot (no aux tank ;)), is manufactured by Gomolzig in Germany (https://www.gomolzig.de/index.php?area=products&l_page=11)
Weight is about 6kg (13lbs), but the type is quite effective.
The 2nd if of Liese type (http://www.hliese.de), slightly less effective at noise attenuation, but much lighter at around 2kg (4.5lb) for both. This is by far the most popular as installed on our RVs.
 

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vanlew said:
i-32DPqkX-S.jpg


Along these lines, I bought this RV-6A and it has these extensions on it, I was told they were to reduce noise, so muffler I guess. Anybody else used these or seen these before. I'm wondering if the drag they cause is worth any noise reduction.
Click to expand...

Please inspect your exhaust pipe very carefully.
I managed to nearly destroy my Vetterman exhaust in 40 hours of running with a pair of these "piccolo" pipes as the extensions put a lot of stress on the junctions further up the pipe.
Google something called a "Swiss" muffler, also referenced (I think) by Tony Bingelis.
 
Southern Pete said:
Please inspect your exhaust pipe very carefully.
I managed to nearly destroy my Vetterman exhaust in 40 hours of running with a pair of these "piccolo" pipes as the extensions put a lot of stress on the junctions further up the pipe.
Google something called a "Swiss" muffler, also referenced (I think) by Tony Bingelis.
Click to expand...

Thanks, I will do that this WE !
 
Decades ago we had a pair of such perforated/squeezed "noise reduction" exhaust outlets on our Bonanza's tail pipes. The flame cone inside one muffler broke apart inflight and the pieces blocked off the exhaust flowing out the pipe. The resultant significant power loss dictated an emergency landing at a thankfully nearby closed but usable airport. No more perforated/squeezed tail pipes for me.
 
Ironflight said:
I’m guessing from the OP’s profile that he’s probably talking about an RV-12 and a Rotax, but the general comments made so far are still valid - exhaust systems are pretty much optimized when they are designed and built.

I will say that one thing I have learned from building an electric airplane is that it is almost as noisy on the rap as a piston-powered machine. The noise from the prop is probably a much bigger signature than that from the exhaust on a piston engine. I need to quantify that someday….

Paul
Click to expand...

Try this source Paul. There is an interesting old (1983) NASA study on sources of noise for piston powered aircraft. Located here: https://ntrs.nasa.gov/api/citations/19850022453/downloads/19850022453.pdf
Mind you it's about cabin noise, not how bystanders experience it, but the noise sources are what's relevant. A key quote from the conclusion confirms the contention about the prop being the primary source but also notes the other sources:
"The flight surveys also produced consistent evidence of propeller noise as a primary contributor to cabin noise in all types of aircraft, as well as revealing some evidence of engine and airflow noise being of considerable importance in most aircraft."
 
Here’s an example of the noise you get purely from the propellor (and maybe a little from the airframe). There is no combustion engine here - it’s all electric. And it still sounds like an airplane! (That’s about 50 kW, or 67 HP….)

https://youtu.be/JlBgnLMSvs4
 
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