Home made exhaust for petrols....

[john stones 1 - Moderator]

I think all agree the supplied exhausts for petrols ain't up to much, and yes 4 strokes are generally quieter. Not all models are suited to cans, so if you were to design and build your own silencer, what would you be thinking of to achieve lower noise. One or more outlets, bigger chamber, thicker walled material, length ? What's the advice ?

[Nigel R]

Shame really isn't it? After all 2 stroke glows went through this whole mill of noise reduction.

Thicker walls help a bit. Having several different size chambers seems to most useful.

[Ron Gray]

Large volume (size) plus baffles.

[Denis Watkins]

The best we ever did at my field was fitted inside the model, a big 8ft wingspan Maule with a huge cabin.

4" thick walled aluminium pipe was sourced, and 8" of this used to make a cannister, electric welded ends and header, and two internal strips running longitudinally.

This appreciably changed the more irritating noise and came in at 82db, the silencer strap soft mounted inside the cabin bottom

Edited By Denis Watkins on 03/09/2018 16:39:49

[Don Fry]

Now, I pontificate with a caveat. I fly off a field next to a motocross site. We a silent flight next to them when they get going. We don't own noise measuring kit, or care about noise.

But I used to do and see noise reduction efforts. What used to work was to use those o ring sealed alloy project boxes, the electronic component people sell. A plate was installed on a diagonal, with most of it drilled away. On to was laid green plastic green scrubber. Onlet one side of the noise moderator, exit on the other. Use standard plumbing components for the plumbing in and out of the box, or boxes.

[alec james]

Denis, interesting stuff as I believe this is the first time I think I have seen anyone saying they have achieved the BMFA 82db, just one question please what was the make and capacity of motor.

[Nigel R]

The 90s style pattern planes had 10cc two stroke glow noise down to a bare minimum, propping for sub 11k rpm, soft mount motors, with at first silenced, and then multi chamber pipes, using APC's scimitar shape props.

This was an environment where noise was measured and a bonus awarded for the competitors score.

Anyway

Back to the OPs situation, can you fit one of those project boxes to the firewall inside the cowl? It would be an easy solution for a second set of chambers.

If your finance will stand it, Weston can build you a custom solution to fit.

[David Davis]

When I was a member of Shropshire Model Flying Club, one of our members built a superb 1/4 scale Fokker D VII powered by a Zenoah 38. He had made up a silencer which fitted inside the fuselage made from an old Calor Gas cylinder but I don't know any of the details.

[john stones 1 - Moderator]

I could build an exhaust myself Nigel, was curious about the characteristics needed to make an effective one.

[Nigel R]

Fair enough.

https://www.justengines.co.uk/acatalog/Silencers.html

Have a look at the 'super quiet' picture.

Note;

With baffle in "quietest" position, the first chamber looks like about the same volume as the entire cast exhaust. Then there is the second chamber, about a third of the size.

I'd start there. Take existing silencer, no matter how effective, make a thicker box with the first chamber about that big. Then add on another chamber or two. Either by starting with a big box with baffles, or by adding boxes. This is pretty much all that car exhausts do, multiple chambers of differing sizes, without too much flow restriction.

They mention tuning the baffle for noise if desired, or reducing the cross section of the out pipe. Packing with wire wool would be another option. Just watch the back pressure doesn't start making the motor run hot.

[Gordon Whitehead 1]

Checking the volumes of the various Krumscheid silencers for large petrol engines here they are all in the range 14 to 16 times the capacity of the engines to be silenced. This, then, is probably a decent guide as to how big to make a home-made silencer for a given 2-stroke single.

Gordon

[Rich too]

If you are going to the trouble, you should also make sure the intake is sealed inside the fuselage.

[Denis Watkins]

Posted by alec james on 04/09/2018 15:43:16:

Denis, interesting stuff as I believe this is the first time I think I have seen anyone saying they have achieved the BMFA 82db, just one question please what was the make and capacity of motor.

45cc Zenoah Alec

Silencer soft mounted internally

8" of 4" Thick walled ally tube

Entry by pipe approx 5" in, and exit opposite side pipe

This is not the only way to achieve the 82db

But most petrol heads do not like loosing the top end

[bert baker]

Got my Zenoah G45 to 82 Db,,, using the larger G 62 standard box exhaust ..good choice of prop and carb bends over stuff

[john stones 1 - Moderator]

Tis true, some don't like losing some rpm, also noisier it is, some think it's got more grunt, and don't start me on about a good prop that'll quieten it some....how much.

[Bruce Collinson]

I had this issue with the Seagull Yak with a DLE 20 which I think I recounted on the Seagull has Landed thread. It was at about 86 dB with the standard exhaust box. A pal made a replacement from 4x2 alloy box section with one internal baffle and two outlet pipes, all alloy, welded construction. Down to 82 dB.

Weighs about 300 gr from memory and will protrude a good inch below the cowl, but ....also lost maybe 200 rpm although it’s still a bit tight anyway. It had to be angled away slightly to clear the plug cap. The Mk 2 would be lighter, maybe glued (LMS guru suggested JB Weld (?) epoxy as very heat resistant) an dpossibly more baffles.

It all gets tight around the back with the plug cap, throttle and choke. Mines a rear exhaust. We concluded that it’s probably the best compromise short of a large can which wouldn’t go inside this plane.

If of interest I’ll see if the maker, Jeremy “Sheep” Harmer, will post the details and drawings.

[john stones 1 - Moderator]

Any pictures or comments would be welcome by me, we've talked about the noise often enough, and cans aren't suitable for most of what we fly.

[Engine Doctor]

Posted by Rich too on 04/09/2018 18:23:46:

If you are going to the trouble, you should also make sure the intake is sealed inside the fuselage.

Agreed . Many of us go to considerable lengths with the exhaust but don't think about the inlet. This most noticeable in side port engines with carb sticking out the side. I demonstrated this by flying my Eurobat ( hopefully soon to be re-named Brexitbat ) powered with a Crrc Pro 45 cc side port petrol along the flight line in both direction. Carb away from us it was an acceptable noise level,it's fitted with a Zenoha 62 silencer. Flying past with carb facing us it was really noisy proving it's intake cycle really adds to the problem. An air filter or encasing the intake in a cowling will undoutedly reduce this horrible sound.

[Former Member]

[This posting has been removed]

[Bruce Collinson]

John,

I’ll try to smuggle some pics onto rye site so you’ll get a better idea. I lack the IT skills to do it for myself and I’m feeling too disinclined (old) to learn. I know people who can.

About to visit Diana Rigg in the outside Batcave to fix the batteries and c of g, hopefully to fly again this weekend.

BTc