Purple Bird

My Feedback: (24)

At my local flying field, the sound limit on 2 stroke is 92 db and on 4 stroke it's 94 db. I think this follows national AMA guidlines actually. We're on a nature preserve as well so we can't afford to be too loud.

I would eventually like to move up to flying 160 sized aircraft but my concern obviously is the NOISE. Are there any alternate ways to keep the noise down other than going electric?

-Brad

micsrcfn

Hi Purple Bird,
With the lack of info provided, it is hard to tell you which way to go. But you could start out with going up in pitch on your prop, say a 2" increase, or more. Another would be to muffle with a different muffler. That is just a starting point.

Mic

sbd-5

My Feedback: (4)

actually the larger engines usually turn slower than smaller displacement engines .the 3 blade prop was one way to reduce engine noise as it loads the engine more than a 2 blade.if they have a sound meter at your field measure your current engine with a 2 blade then swith to a 3 blade and re tune engine and take a reading it should be a bit quieter.when using a tree blade pitch or diameter should be reduced 1 inch over the 2 blade.also a vibration mount will get reid of some noise and a muffler that muffles will help.some one made a super quiet muffler at one time.k&b had real quiet mufflers on their sporster engines as long as the internal baffles were left in place.just remember there is a power loss with the quieter mufflers

rcuman

Wow.. there is a noise restriction on airplanes? I am glad we do not impose that rule because I would have to redesign my whole fleet.

Rcpilot

My Feedback: (78)

I've actually DONE sound testing on airplanes with a calibrated dB meter.

You'll have a hard time getting 92dB out of an airplane at 3 meters. Trust me.

They aren't as loud as you think.

We tested just about every kind of airplane imaginable. From trainers with 40 engines turning 9x6 props at 15,000RPM allllll the way up to 40% gassers with DA150 engines. We tested a LOT of different airplanes with different props, engines, airframes.

We only found 1 or 2 airplanes that were over 93dB--93 was the limit at our field.

It was found that the following would help keep your plane quieter than worrying about your engine, muffler, prop:

Airframe vibration accounts for an awful LOT of the sound. Keep the covering tight.

Loose screws and clevis' vibrate and make a LOT of noise. Use good hardware and ball links instead of metal clevis'.

Just go fly and don't worry about it. The "sound rule" is just in place so that we can "SAY" that we have a "RULE". I believe that the sound limit of 92dB was implemented because they KNOW it's hard to get beyond that.

You wouldn't BELIEVE how loud 93dB really is. I'm talking friggin louder than hell.[X(] You'd have to actually work at it to get an airplane that loud. Trust me on this. I've done sound testing.

red head

Can't get over 93 db, that's hard to believe and I've done some testing also. ENJOY !!! RED

50+AirYears

Can't find an engine over 93dB at 9 meters? Over grass, we have seen numerous engines pushing 96 to 98 dBa at the 9 meter distance. In fact, the noisiest I ever measured was an airborn Quadra on a "Muffler Mount" that measured 103 dBa at 75 feet. We also had a Stinger with a Zenoah that was "Muffled" with a 90 degree manifold. Didn't get it measured, but if the plane passed within 100 feet of anybody, they complained of pain in the ears and body.
The sound level meter was a Radio Shack meter that I check occasionally against our lab standard. After 11 years, it still measures within 1/2 of the needle's width of the correct level.
A lot has been published about controlling sound level. Generally, a good muffler, maybe a filter on the air intake, and use a slightly larger efficient prop to keep the RPMs down. At higher prop speeds, the prop noise can be louder than the engine generated noise with a good muffler.
The AMA's recommendations are principally for competition, but are also a good pattern to follow if you fly gas or glow near residential areas.

B.L.E.

On big planes, it's the prop that makes the lion's share of the noise and even electrics are not that quiet anymore, in fact, watching a high performance large electric plane fly is a great way to see for yourself just how much noise a prop makes. Pusher props are particularly loud.
If it were only a matter of muffleing the engine, it would be a piece of cake to make a quiet plane, look at how quiet cars and (unmodified) motorcycles are.

kdheath

We did a fair amount of testing when the 90 dBa rule was being implemented, years ago. Prop noise was clearly the main issue. Then we were running 60s and 11-7.5 props (16,000 RPM!) and calculated that the prop tips were going transonic. Lowering engine RPMs with larger props helped the most and good mufflers were next. Isolating the engine from the airframe helped some, too. So: low engine RPM, mufflers with sufficient volumn and a complex path, and soft mounts really did a huge amount to make things very quiet. With modern engines, you shouldn't miss anything in terms of power.

The pitch of the noise plays a role, too. Four strokes operate at a lower frequency and most people subjectively find them quieter. But the measured sound pressure levels are as high or higher than two strokes.

And it isn't any problem to find something that will pump 96+ dBA. Ever heard an open exhaust Fox 59? Shattering noise. Everyone on the field needs earplugs.

B.L.E.

Anyone who thinks that four-stroke engines are quiet should go to a place that is about a half mile away from the flying site and listen. You will still hear the four-strokes even after you are far enough away to barely hear the small two-stroke planes.

Rcpilot

My Feedback: (78)

Guys,
I NEVER said that you CAN'T find an airplane over 93bD. NEVER said that.

I said that we only found a couple airplanes over 93bD. I also said that it was harder than heck to actually GET an airplane over 93bD.

Yeah, you run a Fox or any other engine without a muffler and ya got yourslef a 100bD airplane. Thats not surprising.

Maybe the guys at our field weren't putting out a lot of noise because they were already using good mufflers and larger props.?? Maybe the higher altitude had something to do with it. I think maybe our engines don't run as strong as you guys' engines at sea level---so we wern't getting the same RPM out of the engines as you guys are used to seeing. Lower RPM = Less noise.

I'm just saying that in my experience---loud airplanes were really few--and far between. We tested over 425 airplanes at various times over the course of 1 summer. I was involved in about 75% of that testing. The remainder of it was done during the week--while I was at work. The results were published and printed for all members of the club to see.

Your mileage may vary.