Silencer recommendations

[Frank Skilbeck]

Just putting a toe in the water with petrol with a 32cc DLA, I've fitted a Pitts muffler with internal baffles, but it still fails the club noise test, it's 90 DBA at 7 meters and needs to be 82 DBA.

It's running a wooden 18 x 8 prop at the moment and runs really well, any recommendations on a silencer I can fit that would get the noise level down to 82 DBA.

[Geoff S]

There was a guy flying models with Saito 4 stroke petrol engines today (the largest was a 60cc flat twin the other a single of around 40cc?) both sounded very civilised and when I asked him abiut the silencers he said he'd made them himself. That seems to be a common practice amongst users of those larger petrol engines both 2 and 4 stroke.

I have one model with a 30cc Mackay 2 stroke petrol which is acceptably quiet and it has a Krumscheid (or a similar name) Pitts type silencer which cost around £90 when I bought it about 6 years ago.

Geoff

[Peter Jenkins]

I've had good results with a JMB canister on a DLE 35 but the model has a pipe tunnel. JMB also do Pitts type muffler but I don't know how good they are compared with the canister. You can find them here.

Might also be worth trying a 17x10 prop to get the tip speed down from the 18" prop.

Also, worth installing a trumpet intake and getting the intake noise moved to inside the airframe.

Every little change may help to get the noise down.

[Rich too]

I agree, if you can use a cannister type silencer, they are very quiet - I have used cheap ones from HK with good results. And they can be fitted outside the fuselage.

[Former Member]

[This posting has been removed]

[Ron Gray]

You could try using ‘pepper pot’ baffle inserts in the exhaust outlets, they do help reduce noise but probably won’t get you down to the 82 db level.

As a general rule, the larger the silencer the more chance you have of it being quieter, Pitts type silencers tend to be somewhat small and act more like expansion boxes with little sound deadening. Of course I’m generalising as there may well be some that do work (up to a point)

As others have said, a can should get you the required silencing and with a bit of thought, and fuz re-engineering you may be able to fit one inside your ‘plane. Otherwise, as Rich has said, they can always go outside.

[Former Member]

[This posting has been removed]

[Former Member]

[This posting has been removed]

[Frank Skilbeck]

Thanks for all the suggestions, some food for thought. I haven't fitted the cowl yet, so may fit that and change the prop to an 18x10 which I have and see how much difference that makes.

It was a 2nd hand engine and although it runs really well it does have a couple of damaged fins, so I'm reluctant to spend a significant amounts on it, might be better putting the money to a petrol 4 stroke (come on Laser hurry up ) .

[Former Member]

[This posting has been removed]

[Adrian Smith 1]

I think you might have to accept petrol engines are always going to be difficult to achieve the 82 dB guidance at noise sensitive sites, Frank. From my own experience, I have tried a good few silencers from Pitts (really don’t think they can hack it), to canisters and tuned pipes. Of the canisters, the best I found where Hatori (up to 55-60cc) but to find them in the UK are like looking for hen’s teeth and expensive. Weston cans are good quality but I found them a bit noisy although lower in price. I have used Krumscheid tuned pipes too, but you really do need deep pockets and I am not convinced they will deliver the 82 dB you require for the cost involved. One of the reasons I converted to electric was the difficulty in sourcing Hatori cans. This coupled with the massive improvement in technology and quality of motors and batteries made the decision for me, particularly up to 60 cc equivalent. That said 60cc equivalent electrics are not quiet!

I don’t want to come over all negative on petrols because I really like them. Hopefully, the combination of the right prop size and canister exhaust will solve your problem, but it will be a bit of a journey.

[ozjimthree3]

Some years ago, Brian Winch wrote about silencers. I'm sure that if you look in the magazine's archive, you'll find the article. I used the advice to quieten a famous make .53 two stroke that was far too noisy on a small propeller. The silencer modification was done with two pieces of large bore silicone tube, slit and fitted inside the main body of the silencer. The power loss was minimal and the noise level was acceptable.

[Denis Watkins]

Hope am not too late

But most towns are within reach of an aluminium engine block repairer, the shop usually repairs aluminium wheels too

The point being that they weld aluminum substantially

The lads at my place acquired a long piece of 4 inch diameter thick walled aluminium tube.

The best cannisters made appear to be 8" and longer. The fabricators fit end caps and the motor bolt on made from the unused supplied silencer

These cannisters have worked

[Ron Gray]

Silicone tube inside a petrol silencer would melt wouldn’t it? Petrol engine exhaust gasses are a lot hotter than glow.

[Bob Cotsford]

A few years ago there was a contributor on the RCMF forum who got the noise down by making his own low-tech pitts silencer. The basis was a cast aluminium instrument box of the type that used to be sold by Maplin. I think he drilled the base to bolt onto the engine and drilled either the side or the top for exhaust pipes, with aluminium baffle plates making the base of the box into a multi chamber unit iirc. The top screwed onto the box once it was bolted to the engine.

[Antony Redding]

Frank, a few years back when I was running ST3250’s at Marcle I managed to get them under 82 running Beila 18x10 carbon props and large canister style silencers from just engines, I don’t remember if the canisters were made to my spec or off the shelf but they were around 60mm diameter baffled with 2 outlets, simple 90 manifold and high temp silicone joiner,they were steel though as I had one of the mounting brackets fail and had it welded locally, also used Dubro soft mounts and never had a problem, had many comments about how quiet the big st’s were and also how fuss free they were when everyone else said they were rubbish.

[Adrian Smith 1]

I would have said PTFE joiner instead of silicone joiner, Antony. Purely because I don't think silicone can stand the heat and most petrol engine manifolds, with accessories, all come with PTFE joiners + clips.

Edited By Adrian Smith 1 on 13/05/2018 16:53:34

[Martin McIntosh]

Tried a Teflon joiner from a well known supplier but before I could even open the throttle it vapourised and this was only to connect the existing so called silencer to a home made add on.

Thought I would try the Brian Winch idea of cutting the end off a Pitts type, stuffing it with a steel pan scrubber then bolting the end back on. Unfortunately that one has an internal smoke coil so not possible.

Tried restrictors inside the silencer outlet but this just reduced power.

Interestingly, a few years ago YT International did a demo at my club field and their 50cc models were really quiet. They seemed to have just restricted the outlets to 6mm or so, so why did this work for them? Might be worth a phone call to discover the secret.

[Former Member]

[This posting has been removed]

[Antony Redding]

Posted by Adrian Smith 1 on 13/05/2018 16:53:08:

I would have said PTFE joiner instead of silicone joiner, Antony. Purely because I don't think silicone can stand the heat and most petrol engine manifolds, with accessories, all come with PTFE joiners + clips.

Edited By Adrian Smith 1 on 13/05/2018 16:53:34

It was supplied by just engines with the canister and manifold and listed as high temp silicone. Whatever it was the whole set up worked fine with no failures. It fact I was so impressed I bought the same setup for another 2 models and again no problems over many gallons until I sold the models.

[Adrian Smith 1]

Each to their own, I guess. Don't have those problems now with electric glad to say, so I will disappear from this thread!

[Martin McIntosh]

Sorry to hear that Percy. Who will be next?

[Bob Cotsford]

YT's website is still apparently live and showing stock. I think their main supplier ModelTech went bump, maybe it's run-down stock?

[Former Member]

[This posting has been removed]

[cymaz]

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