Brodak 40 - noisy!

Hi

Has anyone had any experience with lessening the sound of a Brodak 40 with original muffler? My club is worried because it is just on the 80db limit - and we don't want any complaints that could put our field usage in danger

Charles

>Has anyone had any experience with lessening the sound of a
>Brodak 40 with original muffler? My club is worried because it
>is just on the 80db limit - and we don't want any complaints
>that could put our field usage in danger

80 db? At what distance? That doesn't seem like it's very loud to me.

Just for reference, most current piped planes are around 92-93 at ~9 feet. Maybe it's 80 at 100ft, but the background at most places is in the 65-70 range.

Brett

George, our club chairman measured it at whatevever the distance should be. I'm not disputing the measurement, just asking if anyone has experience / suggestions on making it quieter.

The problem is that it definitely IS noisier than the other motors at the club, some are 4-strokes (always quiet) and others Irvine 40's etc.

I think I may have to make another of my home-made scent can mufflers as they are fairly quiet. I don't want to lose performance and most important don't want to lose the 2-speed properties of the motor. I've only used my home-made mufflers on RC motors and diesels so far.

Charles

>George, our club chairman measured it at whatevever the
>distance should be. I'm not disputing the measurement, just
>asking if anyone has experience / suggestions on making it
>quieter.
>
>The problem is that it definitely IS noisier than the other
>motors at the club, some are 4-strokes (always quiet) and
>others Irvine 40's etc.

The reason I was asking was that without the distance it's hard to determine how it compares to other situations. For instance, we noted that some 4-strokes read significantly louder than some other piped 2-strokes. An 0s52 running in the pits, for instance, drowns out the sound of my PA61 in flight.

I'm surprised that the Brodak is noticably louder than any other engine. There have been a few people around here with those and they definitely didn't stand out from the rest as far as noise goes.

Obviously the quick answer is to put a stinger on the exhaust. That can have a surprising effect on the noise without any noticable effect on the power or backpressure. I don't know for sure what the mechanism is, but the soft rubber seems to damp out some of the high frequencies. Not to mention that it points the sound at the ground.

Brett

Ah!

That's a good idea. The Brodak is not such a noisy engine - just compared to the others. Also it is "constant speed" more or less, whereas the RC guys are very seldom flying Full Bore.

Has anyone tried a LONG piece of silicone tubing on the end of the muffler? I mean like about 18"?

Charles

>Ah!
>
>That's a good idea. The Brodak is not such a noisy engine -
>just compared to the others. Also it is "constant
>speed" more or less, whereas the RC guys are very seldom
>flying Full Bore.
>
>Has anyone tried a LONG piece of silicone tubing on the end of
>the muffler? I mean like about 18"?
>

I've seen that done a lot. They used to do that all the time in WAM endurance. All the extra backpressure cut the power so it would use less fuel. Windy did something like that on one of the Cardinals, as well, with a 40VF. With similar results - very soon after it was determined that the 40F wasn't very powerful and burned up quickly.

The after-muffler trick is probably your best bet, but it would be worth trying a stinger.

Brett

Charles,
I'm sure you're aware that R/C aircraft fly much higher than CL airplanes and therefore don't have the ground, trees, etc. to absorb sound like CL models do.
While to the modeler standing on the ground beneath them, they may sound quieter, the noise they make travels much farther from the flying site than the CL model closer to the ground. This is especially true down wind of the flying site.At our flying site in Tucson the traffic on Silverbell road a few hundred feet away is louder than airplanes flying even when there are as many as five airplanes in the air and some of those are unmuffled combat planes. There are bushes and trees (yes we do have a few trees in Tucson and lots of bushes), around the flying field and they do a pretty good job of absorbing the sound.
Noise is a very subjective subject, sound can be measured.
The distance at which the sound is measured and obstacles between the sound (like bushes and trees) make a tremendous difference in the readings.
Noise is totally dependent on the listener and if that listener has a separate agenda it will always be irritating.
Tough problem!

What seems to annoy the Old Ladies, retired colonels etc etc is the wha, wha, wha as we fly round and round. This seems to upset them far more than the less regular noise of the RC guys.

Long, long ago I had a Yamaha YDS-2 250cc motorcycle. VERY fast for those days, but the exhaust sound was something like a trombone - not offensive at all. A twin 2-stroke running at about 6,000 rpm obviously has about the same frequency as us with single cylinder 2-S running at 10,000 to 12,000

Charles

Charles,

Brett may recall an "after-muffler" that was used some, I understand, in the Los Angeles, California, area perhaps 20 years ago.

As I recall, it consisted of an alumin(i)um tube about 1" outside diameter, three pieces of 1/4" plywood cut to fit tightly in the alu tube, and drilled for short pieces of smaller diameter alu tube.

Two of the ply pieces served as end-caps, and connection to the muffler (at inlet) and to the atmosphere (at outlet). The third drilled ply piece had a pass-through tube from the front chamber it formed when stuffed halfway through the outer tube, to the aft chamber, thence through to the outlet. The short connecting tubes were about 1/2" outer diameter, again, as I recall. I've seen a few of these after-mufflers, with main tube some 10 to 12 inches long.

All that length is not likely to be necessary, as the function appears to be additional expansion/cooling volume with the two chambers providing some 'cushioning' for the sharper pulses... ...Never heard this rig run, but it seems promising, and less likely to stifle engine power. I think an old 'technical' term for such added muffler (silencer?) volumes in automotive application was 'resonator?'

Brett?

>Charles,
>
>Brett may recall an "after-muffler" that was used
>some, I understand, in the Los Angeles, California, area
>perhaps 20 years ago.
>
>As I recall, it consisted of an alumin(i)um tube about 1"
>outside diameter, three pieces of 1/4" plywood cut to fit
>tightly in the alu tube, and drilled for short pieces of
>smaller diameter alu tube.

Uh, no, I don't recall that. But there have been various after-muffler designs over the years. OS used to sell them as accessories. Ted even had a home-job after-muffler built in to the aft end of one of his 40VF airplanes, although ultimately it didn't work out so well.

I have tried them on the bench, made from cut-up tuned pipes, It was as you described, basically - an acoustic capacitor. All you need is to smooth off the pulses and ideally you would get down to a constant output flow.

Brett

I believe i've found what you are looking for:

http://www.flyquiet.com/

I have heard the Brodak 40 run quit and also Loud and I think it depends on how it is muffled and how fast you run it. When you run it fast it seems to run a little harsh. It is not as noisy as some of the older engines we used to fly even when they have been muffled. You just cannot get any better than the low growl of a good Supre Tigre 60. It's a sweet sound.

"Windy did something like that on one of the Cardinals, as well, with a 40VF. With similar results - very soon after it was determined that the 40F wasn't very powerful and burned up quickly."

Brett

Sorry for the big snip, but I simply cannot be the only one who thinks the above is a hugely funny comment.

Dan