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Help needed

DLE 20 RE silencer

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Bruce Collinson08/08/2018 19:10:55
179 forum posts

All,

My DLE 20, good in most respects, settling down to a nice idle after 6 tanks, plenty of power, starts reliably, etc is too loud, 86 dB without cowl on its makers exhaust.

I’m waiting for response from Just Engines about their in-House product (Duo?) but a clubmate tried one on his NGH and says it made no perceptible difference.

I know about the inserts which are available for not much money to stuff into the exhaust outlets but the consensus seems to be that they add temperature via backpressure, cost some power and lose very little noise. They are not winking at me.

Has anybody found a solution? Inverted mount. It’s a viable after-market silencer I’m looking for rather than helpful hints about larger props etc.

Bruce Collinson

TigerOC08/08/2018 22:15:13
299 forum posts
10 photos

I have just got a DLE 30 and ran it with the supplied exhaust. Really loud. Bought a Pitts type exhaust from HK and that has baffles and is a lot quieter. It has definitely stopped the bark. A lot of the noise is coming from the prop. We did some basic measurements and got 86db but it was next to a hedge row which is not ideal. Noise is not really a problem for us as we fly from a remote farm field with few neighbours who often come over and watch us flying.

A tuned pipe would be much quieter but you are going to pay some big money for a decent exhaust system.

Rob

alec james10/08/2018 20:28:08
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29 forum posts

Watching this with interest as our club works to the BMFA noise limit of 82dB from 7 meters, left, right, back and front etc.no pass no fly, (shouldn't we all?)

There does not appear to be any after market silencer/canister out in model land regardless of price, which will give these results and no one supplier would give any guarantee that their silencer/canister etc will get down to this noise level, saying there are to many variables to consider, which of course there are.

My winter project was to make some canisters for my Zenoah 20 and DLA 33 petrol motors and it has been a revelation how difficult it is to get down anywhere near 82dB. I am currently on my 5th canister for the Zenoah and cannot get anywhere near 82db from the front the rest is okay. One bought canister from Cosford (Chip Hyde and cheap)has been tried on the DLA, 96dB all round, but as I say it was worth a punt. You name it I've tried it, props revs soft mounts etc, so any input suggestions gratefully received. Even set it up on the Workmate to eliminate airframe noise.

Geoff Sleath10/08/2018 21:27:44
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3014 forum posts
247 photos

I bought a Krumscheid (sp) silencer for my Mackay 30cc petrol engine and it's subjectively very quiet (we aren't a very noise sensitive site (most fly electric anyway) so no noise tests). I don't know if they're made for other engines. It was quite expensive at £90 several years ago.

Geoff

alec james10/08/2018 21:41:13
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29 forum posts

It would be interesting if anyone has actually put a noise meter on one of these 'cans' as I'm afraid 'subjectively' does not 'cut the mustard' with our committee, thx for input though.

Denis Watkins10/08/2018 21:47:12
3196 forum posts
147 photos

The most success we had had at the club, as we approach this as a group

Was internal to the model, canister built, from aluminium, then welded, using the header cut from the provided muffler !

The best is an 8 inch length of 4 inch diameter aluminium THICK walled tube, end capped.

A close second was a 6 inch length of 3 inch pipe.

Basically the volume of the BANG of each second stroke is reduced by allowing the gas to expand on leaving the motor

You need to get the biggest chamber in the model space allowed.

I know many modellers do not favour soft mount for many reasons, but there is a couple of decibels reduction there.

Our friends with IC boats, mount the motors in a soft mounted cradle. We can learn from this

Edited By Denis Watkins on 10/08/2018 21:49:11

Percy Verance10/08/2018 21:49:38
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7098 forum posts
142 photos

I'd second Geoff's suggestion of a Krumscheid silencer. I heard one a few years back on a Zenoah 20cc in a Glens Models Maule, and it seemed to me to be no noisier than the glow engines being flown on the same day. Not cheap of course, but how much would you be prepared to pay to keep your flying site in these noise sensitive times? Particularly if your site might be under threat.....

And Denis has a valid point re: soft mounting. There are ways out there to effectively isolate the vibes from the hollow parts of the airframe structure. It's here much of the racket you hear stems from. Some soft mounts allow too much movement, but there are some good ones out there........

 

Edited By Percy Verance on 10/08/2018 22:00:00

Bruce Collinson10/08/2018 23:24:35
179 forum posts

We’ve metered the standard setup to BMFA standards, that’s how we know it’s at 86 dB. There is now a chance that the JE Duo might be better than first thought and objective tests are about to begin with both the DLE and the NGH, plus a larger prop, then the cowl and eventually I suppose a 3 blade.

The main issue is going to be the lack of space inside and just outside the cowl.

Also, one of my generous club mates is challenged by this conundrum and is determined to fabricate a can with baffles which will reduce the noise significantly.

Oddly, a Zenoah 20 with stock pressed steel exhaust does not sound excessive, subjectively, although we haven’t metered it.

I’m grateful for the responses and will keep you posted as the experiments progress. It seems faintly ridiculous that this huge hobby has failed to provide a robust solution in the after market. Will also check Krumbleshield.

BTC

Engine Doctor11/08/2018 09:24:00
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2080 forum posts
20 photos
Posted by Geoff Sleath on 10/08/2018 21:27:44:

I bought a Krumscheid (sp) silencer for my Mackay 30cc petrol engine and it's subjectively very quiet It was quite expensive at £90 several years ago.

Geoff

yes I had that combination some years back I found it very quiet without any apparent power loss. weather they make one suitable for the DLE ? A good effective silencer will cost a fair bit but worth it in the long run.

ken anderson.11/08/2018 09:42:41
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8226 forum posts
762 photos

amazes me when we see the council lads using their strimmer's etc...how quiet they are, and they get a lot of full throttle use.... a lot of them are Zenoah I believe...……...

ken anderson....ne...1.... council lads dept...

bert baker11/08/2018 10:59:44
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1234 forum posts
267 photos

A fair few of the chain saws have thick walled multi chambered alluminium exhausts the rest have multi chambered sheet steel box's, i think the key is being multi chambered with a large expasion chamber at the manifold end

Kim Taylor11/08/2018 18:03:22
222 forum posts
53 photos
Posted by alec james on 10/08/2018 20:28:08:

I am currently on my 5th canister for the Zenoah and cannot get anywhere near 82db from the front the rest is okay.

I am aware of, and support, the need for effective silencing of all our models, but isn't it a bit silly to fail the noise test, when that test is being skewed by the noise from the stalled prop, which we know will disappear when the 'plane is in flight? There is no reason why our engines should be any noisier from the front than from any other angle, is there?

Or is it just me!!!!!!!!!!!!devil

Kim

p.s. To get my NGH GT17 below the 82dB mark, I've had to prop it down and stuff the 'silencer' with stainless steel scourer material. I also used wing seating tape (could feel movement without it) and made sure that the wheels were a snug fit on the axles (these two things were 1dB).

This was from the front - from sides and rear, it was mid to high 70's (can't remember exactly)

Kim

Denis Watkins11/08/2018 20:16:02
3196 forum posts
147 photos
Posted by bert baker on 11/08/2018 10:59:44:

A fair few of the chain saws have thick walled multi chambered alluminium exhausts the rest have multi chambered sheet steel box's, i think the key is being multi chambered with a large expasion chamber at the manifold end

Spot on Bert, multi chambered has been the quietest pipe I have made.

8" long, 1" sided Hex bar

Bored Six times equally with an 8mm drill right through

Then bored down the centre with a 10 mm driil, breaching the 8mm holes 6 times

This leaves thick heavy curved flutes along the whole length of the muffler which is end capped and welded

Difficult to describe this ! Resulting in pseudo multi chamber, makeing a very quiet pipe

alec james11/08/2018 21:27:38
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29 forum posts

Kim. I agree about the stalled prop, only thing is in our club until you pass the BMFA noise test on the ground I can't find out what it's like in the air. No one in the club fly's petrol.

I agree that the canisters need to be as big as possible, I have been working on 8 times the volume of the engine i.e. 20 cc engine can should be 160cc but this is far to small I think. Homemade cans are 300x50 mm plus a header pipe made from 22mm s/s and approx. 500mm long soft mounted outside the fuselage as this is a retrospective fit..

Bruce Collinson12/08/2018 09:32:53
179 forum posts

My clunmate has a similar NGH 25 with similar noise issues. He bought a replacement from a prominent UK maker and thought it was an improvement so when we finished at Elvington yesterday we tried both silencers. Admittedly the acoustics in my drive are impure but the gist is that both original and aftermarket cans scored 92db at 7 m.

Propping up an inch made not rhe blindest (deafest?) difference.

We’re starting a home made one, biggest we can get inside the Yak cowl, before saving up for a Laser.

BTC.

alec james12/08/2018 17:46:48
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29 forum posts

Exactly my experience except my tests have been done in the middle of a 14 acre field.

Good luck with your home made one though!

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