Stinger 26cc: Noise reduction for Galaxy Mystic

[CorradoMatt]

Evening all,

 

I recently finished an old Galaxy Models Mystic that I have powered with an RCGF Stinger 26cc rear exhaust engine. The model has had 2 flights and performs well but unsurprisingly the engine is rather loud with the standard silencer. I have made a new silencer from aluminium using Durafix Easyweld to join the parts together. The new silencer is twice the volume of the standard silencer and uses "stinger tubes" to act as baffles. I have run the engine with the new silencer but have not taken dB readings yet. There are a few things that need modifying (plug cap is just touching the surface of the silencer for one) so i will need to make an improved mk2 version. I have found loads of information with regard to effective canister silencer design but there appears to be very little for the pitts style of box silencers. I was wondering if anyone has any good advice with regard to effective silencer design or could recommend something that has worked well for them? I was wondering if fitting a couple of perforated baffle plates between the exhaust inlet and stinger tubes would make a big difference? I've attached a few photos of the model with the standard silencer and home made item.

 

Thanks,

 

Matt

[Paul De Tourtoulon]

I will be following your experiments, I have bought 'pitts' silencers for my 20cc Rcgf engines, and none has done more than 2 flights before they have cracked up from the vibrations, I ended up making one up from steel and brazing it.

 

It's the bottom one in the picture.

[Engine Doctor]

CorrodoMatt .Half the battle in making a silencer that works is using material thick enough to prevent it resonating which you obviously have👍Did you fit anti crush tubes to stop mounting bolts squashing the case ? The pics look good . Have you tested it yet , what were results ?

Ive made a few exhausts over the years in alloy and steel. The alloy joined using techno-weld or durafix and similar alloy brazing rods. It worked ok for me on glow engines but has failed on petrol engines due i think to higher exhaust temp. Steel brazed or welded has been fine, but care has to be taken to get required strength without too much weight

Paul . My go to material now for petrol engine exhauhsts is steel. Easily brazed or welded into custom shapes. 

At many shows in years gone by there were a few traders selling nicely cut steel parts like exhaust flanges for various popular engines for making your own exhausts that little bit easier. They all seem to have packed up now as very few of us have either the skills , equipment or interest to make stuff.

Edited July 29, 2023 by Engine Doctor

[Paul De Tourtoulon]

Also my only choice, Steel brazed, even some silver soldering seems to fall to bits, I did make up my own restrictors they do work, but you also lose power.

 

Here is a really messy attempt on one of my Rcgf 20cc engines.🤢

[Cuban8]

Many years ago I built an add-on silencer from a design based on how a gun silencer is constructed. Although used on a glow two stroke it worked very well indeed and took the crackle away completely. The bonus was that it caused very little back pressure and hence minimal power loss, which seems to be an issue with restrictive baffle designs.

 

[Paul De Tourtoulon]

15 minutes ago, Cuban8 said:

Many years ago I built an add-on silencer from a design based on how a gun silencer is constructed. Although used on a glow two stroke it worked very well indeed and took the crackle away completely. The bonus was that it caused very little back pressure and hence minimal power loss, which seems to be an issue with restrictive baffle designs.

 

That's OK for the small methanol engines but look at the size of a 20cc canister,,,

I did that with my Dle 20ra on my P 47.

[CorradoMatt]

Thanks for the replies. I've run the engine with it fitted but haven't taken any db readings yet as i need to do it at the field where there are no obstructions etc. I suspect that the heat of the exhaust may crack the joints as the parts are only butt jointed and then aluminium brazed over the top (not much thickness to the braze material). I think next time I will extend the end plates past the edge of the box section and then run a fillet of braze along the joint. I didn't fit anti-crush tubes to this one but I will with the new version. I have bolted it using Norlock washers and high temp silicone. The alternative is to have larger access holes opposite the exhaust port that i can pass the bolt heads through and only bolt up on the exhaust mating face. I'll then have to fit some kind of blanking plate to cover the access holes.

 

I've got a few ideas about fitting extra baffle plates  either side of the exhaust port or fitting another perforated tube that the exhaust feeds into first.

 

 

[Engine Doctor]

3 hours ago, Cuban8 said:

Many years ago I built an add-on silencer from a design based on how a gun silencer is constructed. Although used on a glow two stroke it worked very well indeed and took the crackle away completely. The bonus was that it caused very little back pressure and hence minimal power loss, which seems to be an issue with restrictive baffle designs.

 

Yes worked on noise  absorbtion rather than baffles like the old motorcycle silencers stuffed with fibre glass wadding . Some were sold on ebay a couple of years ago very cheap and listed as boat silencers .I fitted two to my 50cc eurobat very similar to Paul De Tourtelons pic They took out the crackle out of the exhsust note well .

Edited July 29, 2023 by Engine Doctor

[Paul De Tourtoulon]

Yes mine were boat silencers probably 10€ each,,,

[jeff2wings]

A very neat job there Matt ,brazing ally is not the easiest of things to do particularly when joining thick/thin materials, I will stick to TIG welding my silencers! I think my approach would be take rpm db readings with the std setup and then experiment with different baffling/outlets maybe starting with blocking one outlet, one thing  I do know from past experience back in the '70's when the club I  was in then and looked into this issue ,there's no substitute for volume. 

[CorradoMatt]

Thought I should post an update on this. I took the Mystic to the field today and ran the engine with the new silencer. We took sound level readings using the club meter and achieved an average of around 86db which was 2 to 3db less than the standard silencer (Club limit is the usual 82db at 7m). I was allowed to fly it to see how it sounded in flight and although the bark had certainly reduced, it still sounded pretty loud. I'm now going to make a mark 2 version with an added baffle around the exhaust port and fill the inside with a stainless steel pan scourer. I can also adjust the dimensions for better clearance on the ht lead and change how the parts are joined for a stronger joint. I'll post some pictures once i start making it.

[CorradoMatt]

Photos of the new silencer below. I've added a baffle around the exhaust port that will also stop the stainless steel scourer entering the engine. The baffle also adds support to stop the silencer being crushed when the bolts are tightened. Just need to fit and seal it on to the engine and carry out some testing. If this doesn't work i'm not quite sure what to do next other than experimenting with props or possibly fitting a header and cannister.

[Engine Doctor]

looks good keep us posted how it performs .

[CorradoMatt]

Fitted now - I was pleased to see that i have clearance for the ht plug! I'll take reading with the new setup before I change anything else. I am currently running a wooden Menz 17x8 prop. I understand that using an APC prop may help reduce the noise and i could also experiment with the size. I am not sure whether to increase the diameter or pitch (or both) to reduce the noise. Does anyone have any advice on this?

[jeff2wings]

Do you have a base line rpm with the standard silencer? Otherwise it makes it difficult to assess the effects of the silencer and/or prop changes, 

[CorradoMatt]

Took the Mystic to the field today to test the mk2 silencer design with the additional baffle and stainless steel wadding. The noise level has now dropped from around 86 to 84db so I am pretty close now to the 82db club limit. The engine has certainly lost the typical 2 stroke petrol engine exhaust pop and bark. I'm now going to change from the wooden Menz 17x8 prop to an APC prop. I'm not sure whether to try a 17x8 APC, 17x10 APC or 18x8 APC. Props at this size are starting to get a bit pricey so I don't really want to buy a few different ones to try! I think i'm going to try an APC 18x8 unless anyone recommends anything else.

 

I've also been looking for alternative silencer designs and saw some threads on the LMA forum about a Sid King silencer design that is based on a gun silencer. I've registered for the LMA forum and will ask for more information. Does anyone here have experience of this design?

 

Matt

[Doug Campbell]

The outlet area looks very large to me. Have you tried blocking one of the outlets-simple to do and easy to undo if it doesn’t work 

[stu knowles]

My first thought was to either neck down both or block off one outlet.

 

On a zenoah 26 I would normally use a  17 x10 or 18x 8

 

 

[Engine Doctor]

36 minutes ago, Doug Campbell said:

The outlet area looks very large to me. Have you tried blocking one of the outlets-simple to do and easy to undo if it doesn’t work 

Thats a very valid pointa as its 9nly a 26 cc . Try blocking each pie in turn and compare . Also some flexi  /silicone tube on the ends with holes small punched through the side will also help. Dont ask me why the holes through the sides work but yhey do. Use sharpened brass tube to cut them.

[CorradoMatt]

Thanks for the replies. As a starting point I made the outlets the same diameter as the original silencer. I have a couple of those pepperpot add on inserts which might be worth a try.

[john stones 1 - Moderator]

Peperpots will definitely reduce the noise. Prop I'm with Stu K, 17x10 will quieten it some as will the 18x8.

[Paul De Tourtoulon]

4 minutes ago, john stones 1 - Moderator said:

Peperpots will definitely reduce the noise.

And the power.

[john stones 1 - Moderator]

1 minute ago, Paul De Tourtoulon said:

And the power.

 

Yes, but better than being grounded coz it's failed noise test.

[Paul De Tourtoulon]

1 minute ago, john stones 1 - Moderator said:

 

Yes, but better than being grounded coz it's failed noise test.

I tried a few 'stoppers" as he will find out getting down to 82dB will probably cost him at least 20% of power.

[J D 8]

 Something I have noticed. I recon that exhaust's who's exit points down at 90 degrees to the airflow do seem louder than those that exit with the airflow. I have Two Enya 90's and the one with a remote down ward exit is louder.