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Silencer recommendations

hitting 82 DBA with a 32cc petrol

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Adrian Smith 113/05/2018 21:22:49
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Each to their own, I guess. Don't have those problems now with electric glad to say, so I will disappear from this thread! wink

Martin McIntosh13/05/2018 21:29:57
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Sorry to hear that Percy. Who will be next?

Bob Cotsford13/05/2018 22:23:17
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YT's website is still apparently live and showing stock. I think their main supplier ModelTech went bump, maybe it's run-down stock?

Percy Verance14/05/2018 06:35:07
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Martin, PM sent.........

cymaz15/05/2018 06:53:09
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Maybe an aluminium scrubbing put inside the exhaust body would deaden the “ ringing “ of the body.. the other way is to place a perforated tube side a solid one. Anything to slow the shock wave of the exhaust gasses

Robin Colbourne15/05/2018 10:14:59
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What come out of the exhaust is a supersonic wave which is created when the exhaust port opens. This is the source of the noise. After that you get the subsonic gases and exhaust residue. The secret is to catch the supersonic wave, whilst providing as little restriction as possible to the gases and exhaust residue.

The way to achieve this is to have sweeping curves and straight lines in the exhaust pipe to direct the gases and residue straight out, whilst having one or more breaks in the tube to let the supersonic wave dissipate into a chamber surrounding the pipe. Think of a gun silencer as James Bond uses. The bullet needs to go straight on, but the noise needs to be 'caught'.

Here are a couple of sectioned gun silencers. I think the bullet would travel left to right in the one on the left, and right to left in the one on the right:

For the model engine silencer, the gaps between the inner concentric centre tubes should be no more than about 6mm, otherwise the exhaust residue will expand into the chamber and choke the engine. Of course some will get in there, so a small drain hole at the bottom of each chamber, leading to one at the back will stop it filling with oil. The outer tube should be as large as you can make it, and each chamber should be four or more inner pipe diameters long.

To prevent the exhaust from ringing, some people coat it in silicone bath sealant.

Once you have got the exhaust quiet, then your remaining noise sources will be the engine inlet (again a wave from the rapidly opening and shutting inlet), mechanical noise from the engine, and the airframe resonating (much worse if it film/fabric covered open areas than a foam veneer construction). In the case of the latter, antivbration mounts may be the answer, provided the front of the engine is supported.

Generally a silencer of this size will need to be mounted remotely, as its inertia will destroy any mount attaching it directly to the vibrating engine.  Whilst silicone tube is fine for glow installations, a corrugated metal tube will be necessary for petrol engines.  Toolstation do some whcih could be cut down, but I don't know what they weigh.  https://www.toolstation.com/shop/Plumbing/d20/Flexible+Connectors/sd2807/Flexible+Stainless+Steel+Hose/p92277

F3A aerobatics fliers put a lot of effort into noise reduction about 20 years ago, so its worth looking for the results of their research.

Edited By Robin Colbourne on 15/05/2018 10:33:30

Percy Verance16/05/2018 06:54:52
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Robin

Many F3a pilots went down the route of soft mounting their engines. It was efffective, but had to be done properly and carefully. The Parc Amber mount was very popular at one time. The Dubro isolation mounts are very good, reducing noise considerably, but not everyone is a fan of them. I first came across them some 15 or so years ago when a guest flew at our field. I initially thought his model was electric powered, but it turned out to have a .25 glow on a Dubro Quiet mount!

Edited By Percy Verance on 16/05/2018 06:55:38

Frank Skilbeck16/05/2018 08:15:31
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Some interesting thoughts there Robin, wonder if anybody has ever tested a gun silencer as an add on silencer. Although you might get a knock on the door if you tried to order one on-line wink

Percy Verance16/05/2018 18:07:30
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I should imagine you'd be right there Frank. They'd be on your case like a shot....

And if they felt you were hiding something, they'd be rifling through your stuff in no time......

john stones 116/05/2018 18:25:49
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Posted by Percy Verance on 16/05/2018 18:07:30:

I should imagine you'd be right there Frank. They'd be on your case like a shot....

And if they felt you were hiding something, they'd be rifling through your stuff in no time......

Scraping the barrel for puns there Percy, I'm not so easily triggered.

Percy Verance16/05/2018 18:47:16
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I'll need to take stock while I think of some more John......

I'll aim to get them posted as soon as I can.....

Edited By Percy Verance on 16/05/2018 18:49:52

cymaz16/05/2018 19:28:38
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Don’t go shooting your mouth off ! embarrassed

Robin Colbourne16/05/2018 20:00:50
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The engine's job is to turn a prop. Does that make it a revolver? smile p

Rocker cover, cowling, access panel...

🎩 🧥 🚪

Edited By Robin Colbourne on 16/05/2018 20:01:32

John Stainforth26/05/2018 11:30:40
201 forum posts
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I'm sure I've read that a major cause of model engine noise the rotational speed of the prop tips. But, I must say I've always been a bit doubtful of that because of the much lower noise of electric engines running at similar rpm.

Frank Skilbeck26/05/2018 11:37:24
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Posted by John Stainforth on 26/05/2018 11:30:40:

I'm sure I've read that a major cause of model engine noise the rotational speed of the prop tips. But, I must say I've always been a bit doubtful of that because of the much lower noise of electric engines running at similar rpm.

If the prop tips approach supersonic speeds they can be very noisy, but yes I have an electric plane turning a 18" dia prop and the DLA32 turning a similar prop and the electric plane is very quiet.

I'm going to fly the Maxilift at a few flyins with the DLA32 where there is not a noise limit and then think about my options, either better silencers or change it to a fourstroke (probably a laser on glow fuel and 10% oil).

Percy Verance26/05/2018 11:45:37
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It isn't just prop noise John, airframe vibration/resonance can add to the racket too. Effective isolation mounting of the engine can kill much of this, but iso mounting needs to be done correctly and carefully otherwise you'll end up with further problems. Even excessively loose wheels on an undercarriage can add a bit of extra noise. 

We saw and heard the difference quite markedly about 20 years ago in my club. At the time I had a Wot 4, built from a kit (they all were back then), powered by a Super Tigre .45. A fellow flyer had his Wot 4 clone ( he'd drawn round the kit parts, plus built up his wing). This model also had an ST .45 on the front. We both used identical props, but the clone model with it's built up wing was noticeably *thrummier* in the air.......

You'd be inclined to think that electric models ought to be a fair bit quieter as there shouldn't be engine vibes running through the airframe to the same extent as i.c. jobs. My old Ripmax Spitfire with it's Mega motor and Aeronaut carbon folding prop seems virtually silent in flight.

 

Edited By Percy Verance on 26/05/2018 11:49:46

Edited By Percy Verance on 26/05/2018 11:53:42

Martin Harris26/05/2018 12:55:45
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Posted by John Stainforth on 26/05/2018 11:30:40:

I'm sure I've read that a major cause of model engine noise the rotational speed of the prop tips. But, I must say I've always been a bit doubtful of that because of the much lower noise of electric engines running at similar rpm.

The worst noise generation can be at the worst time for our measurements. We have an F5b flyer at the club and when he operates the @6kW motor with its something like 22ish x 23 9000 rpm prop with the model static, the noise has to be heard to be believed. It echoes off the wood at the far side of the field and reverberates throughout the locality like a demonic chainsaw. Thankfully, once the model accelerates beyond the prop's stalled condition the noise subsides (and eacm motor run is less than 2 seconds) but this reproduces the noise test conditions as per the DoE guidelines...

Edited By Martin Harris on 26/05/2018 13:02:02

Martin McIntosh26/05/2018 14:16:10
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A model I have powered by a four stroke would send the prop tips super sonic when opening up at the bottom of a loop on an APC 16x6, but replacing this with an APC 15x8 cured it totally.

onetenor26/05/2018 17:11:15
1718 forum posts

If tips go supersonic why can we hear them?

J D 826/05/2018 18:06:40
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875 forum posts
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When prop tips go supersonic a mini sonic boom comes off each tip. A North American Texan/Harvard prop was direct drive from the engine and made a noise to behold [ well for a bit ]

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