Petrol two strokes and 82db

[Martin Arnold 1]

It would appear that most models these days that require a 20cc or over are recommended to be powered by a two stroke petrol.

The problem we have found in our club is that most of this type of installation, will not pass the 82db at 7 metres noise test with the supplied silencer set up.

Most of our members (me included) are running 4 stroke glow anyway, but usually when somebody shows up with a largish two stroke petrol powered model, it won't pass the noise test.

I often wonder how the flyers we see at the shows (when they are on) running large petrols, get on at their local club.... If they fly them there anyway.

Edited March 28, 2021 by Martin Arnold 1

[Graeme White]

Following with interest as I have the exact same question. When I see models being sold with 30cc+ engines, where do people fly? I can't believe so many people have private land or non bmfa sites? I'm also interested in 40cc+ 4 stroke. I was looking at the ngh 38cc 4 stroke and it seems to be the point where 82db is a struggle, how do people fly bigger?

[David Davis]

I believe that some German companies (Zimmerman?) make excellent silencers for large engines but as a four-stroke glow man I bring a wealth of inexperience to this debate so I'll shut up! ?

[Jon H]

Work on the basis that you need to spend as much on the exhaust as you did on the engine. Once you agree to those terms its pretty easy to keep them quiet with a suitable after market job. 

 

It should also be noted that most of the noise our engines make comes from the prop and you can save loads of noise in this department by choosing a decent brand of prop. Again though, a biela prop is nice and quiet, but its 3 times the price of a noisy old master airscrew. 

 

One of the things i hope to work on this year are some new exhausts for Laser. Its on my long list of improvements i would like to make as even a big 4 stroke can get pretty loud, even if the noise its not as annoying as a 2 stroke, we are tied to dB so perceived noise is not really a consideration unfortunately. You do however start from a better position with a 4 stroke, so dont feel you have to use a 2 stroke petrol just because its shown on the instructions. Most of my warbirds are '50cc' models and usually attract DLE50's or similar. Obviously i am not using those ?  

Edited March 28, 2021 by Jon - Laser Engines

[Martin Arnold 1]

As Jon says. it's always prop noise I struggle with on four strokes... but I do like to let my Saitos rev !!

Really don't like the sound of a petrol two stroke in a model plane.

[Chris Walby]

First question Where does the noise actually come from?

 

All of my current models are balsa with some fully sheeted and some near fully open  covering and a lot of the noise comes from either the airframe "drumming" or off the pop in stalled air.

 

With glow its just a case of over propping to hold the revs down, but with electronic ignition engines how about rev limiters?

 

 

[john stones 1 - Moderator]

Firstly, flyers you see at shows, will not be running petrol engines with the supplied Silencer ? Because they're neither use nor ornament.

How do flyers get on at their clubs ? Well they've either got the noise down to 82db, or their sites not noise sensitive and they can fly above 82db.

[Nigel R]

 

 

 

14 hours ago, Jon - Laser Engines said:

One of the things i hope to work on this year are some new exhausts for Laser.

 

Jon do I remember you saying there is some variation in noise depending on the angle the exhaust is pointed?

 

 

 

14 hours ago, Jon - Laser Engines said:

most of the noise our engines make comes from the prop

 

agreed

 

prop noise is very closely tied to rpm, a few hundred rpm can significantly drop the noise, with an 11" or 12" prop anything much above 11.5k is a bit borderline in my experience

 

and tip shape, those knife like stiff APCs are about as good as we can get, I think

 

too much pitch can also cause a stalled blade at full throttle on the ground which is highly problematic for the test particularly from the front on - although the noise vanishes once the airframe is moving

 

[J D 8]

Prop noise, some electric types make quite a racket, little props going at silly rev's. Like this one.

[Jon H]

Chris, the noise is from the disturbed airflow. We are adding energy to the air, making it turbulent etc and this generates noise. Just think of how much noise you can make with a wooden stick just whipping through the air. Unfortunately there is a tendency to relate noise to power. If it really sounds like its going like a good'un, it must be more powerful that the quiet thing. This isnt true, as a quiet prop is only quiet as less of its energy is being wasted as noise and if that energy is not being lost to noise then its giving more thrust. 

 

You are right about model construction as well. I miss the old days of foam core wings and decks together with 1/4 balsa sides. These damped the sound from vibration wonderfully. Modern laser cut spider webs covered in film drum like crazy. 

 

 

Nigel, i do recall the thread where exhaust direction was mentioned and to be fair i disagreed with the point being made. In any event, our exhaust design is 30 years old and needs another look. 

 

on prop rpm it varies. an 12 inch prop at 12k is getting marginal with a tip speed of 430mph. However, an 18 inch prop at a reasonable 8000rpm has a tip speed that is nearly 430mph as well. This isnt exactly slow! Then you take a 21 inch prop to a reasonable 7500 and get nearly 470mph at the tip. 24 inch props are limited to 6500 to keep in 450mph ish trp speed range. The problem is, we already consider 430mph to be marginal on our 60 2 stroke with it 12x6, so 450-470 on the bigger engines? its not good is it. 

[Nigel R]

it's a difficult one for users of bigger motors, for sure.

 

5 minutes ago, Jon - Laser Engines said:

foam core wings and decks together with 1/4 balsa sides

 

For IC, that simple construction style had a lot going for it, provided the wood was not heart-of-oak grade. Laser cut lattice work is great for electric, but structural vibration can contribute a lot to noise.

 

13 hours ago, Chris Walby said:

With glow its just a case of over propping to hold the revs down,

 

I agree. With a two stroke glow, anything remotely recent seems to come with a 'reasonable' two (or more) chamber baffled silencer. Anything like that (including a good multi chamber tuned pipe) will push the exhaust noise well down below problems caused by props and airframes.

 

[Jon H]

most engines will tolerate much bigger props than we normally use. I have a 15x5 on my saito 45 and while that is an extreme example, it works really well. 

 

Glow engines are less tolerant of heavy loads due to their ignition. a petrol engine could have its timing retarded a smidge to help it out with a higher load prop. 

[stu knowles]

I have petrol engines from 17cc to 75cc.  I confess up front to never having used a noise meter but I honestly believe that most would fall into the 'quiet' classification.

 

All are over propped from the normal recommended sizes.  

I have two or three fitted with the oriental Pitts style silencers which have two massive outlets.  They have all been modified in house by cutting off and plating over one outlet completely.  The remaining outlet is sleeved down to 10 or 12mm.

 

This does restrict max rpm and power slightly. Most of my models tend to be scale   but the fighter types will loop with ease and the Moths bimble around beautifully and are super quiet.

 

It is a case of horses for courses and what balance between power and noise you are willing to settle on.  As an aside, I think that the standard zenoah 23 silencer has 2 x 8mm O/D outlets and the z26 two slightly larger, but compared to what is being supplied with many engines, they are chalk and cheese

[Frank Skilbeck]

7 hours ago, J D 8 said:

Prop noise, some electric types make quite a racket, little props going at silly rev's. Like this one.

A pusher with the prop close to a flying surface is often quite noisy.

[Nigel R]

3 hours ago, stu knowles said:

I have two or three fitted with the oriental Pitts style silencers which have two massive outlets.  They have all been modified in house by cutting off and plating over one outlet completely.  The remaining outlet is sleeved down to 10 or 12mm.

 

I think the actual, continuous, exhaust flow of most engines can fit through surprisingly small outlets. But it has to be "smoothed" out first, via one or two or three chambers. Once the gas has done it's expanding, and the flow is smoothed, the peak flow is much lower.

 

[J D 8]

           The above is why turbo engines are not all that noisy even with out a silencer fitted, turbo smooths the flow.

[Frank Skilbeck]

The exhaust noise isn't so much the exhaust flow but the pressure wave, you need to either allow it to expand or break down before it exits the exhaust, to reduce the amount of energy transferred to the surrounding air.

 

Look at gun silencers, they are a tube with lots of small expansion chambers to absorb the pressure wave (and a turbo is sort of a rotating baffled expansion chamber). Would be interesting trying one on an engine, but not keen on plod turning up at the doorstep asking why I an ordering firearm accessories ?

[Chris Walby]

Still no convinced that the majority of the noise comes from the exhaust and not the airframe drumming and the prop.

 

If the engine is over propped is the reduction in noise just that the engine is reving less or the fact that the prop is turning slower.

 

I suppose one of the electric boys could do a load test rig prop noise testing over different revs to single the effect out...they might not want to if  they find the results are bad.

[J D 8]

46 minutes ago, Frank Skilbeck said:

 

 

Look at gun silencers, they are a tube with lots of small expansion chambers to absorb the pressure wave (and a turbo is sort of a rotating baffled expansion chamber). Would be interesting trying one on an engine, but not keen on plod turning up at the doorstep asking why I an ordering firearm accessories ?

  I happen to have a sound moderator [ silencer ] [ they are never as good as the movies would have you think] Next time I am doing some test stand running may give it a go and see

 Sound moderators are available for air rifles [ which are not licenced below 12lbft ] however they are quite expensive.

[Graeme White]

Is there a point where it is not possible to get a prop below 82db? As a boyhood dream I have always wanted the classic 1/4 spitfire and hope one day to build it but I have always wondered about the 82db reg as not much point in building if it can't be flown anywhere. I was always assuming it would have to be electric which I'd rather it not be and have a 120/130cc 4 stroke but noise has always concerned me. Even if it was electric, would a prop for a plane that big get below 82db? I have spent many hours googling if anyone has ever done db readings on the larger planes but have never seen an example.

[Nigel R]

53 minutes ago, Chris Walby said:

If the engine is over propped is the reduction in noise just that the engine is reving less or the fact that the prop is turning slower.

 

Well yes. See Jon's tip speed post.

 

We do get lots of vibration from our engines. Airframes are pretty good at resonating; electric power eliminates a lot of that. 

 

 

[Bob Cotsford]

Don't forget carb noise while you're at it.  Depending on the engine layout and cowling that can be a significant contribution to the overall noise.  Ever ridden a big twin motorbike?  Inlet airbox noise can be louder than the exhaust!

As for prop noise, even electric setups suffer if the tip speed starts to get high or there's significant interference from disturbed airflow eg where a pusher prop is close to the wing trailing edge.

[Jon H]

side induction engines like zenoah are pretty bad for carb noise as the intake pokes right out the side of the cowl. Rear induction isnt so bad as you tend to not be looking straight at the source of the noise due to the cowl. 

[Nigel R]

Big two stroke glows can't do well on that front either, then, if they're the typical front carb layout.

 

Does an intake filter make much difference?

[Tim Flyer]

Resonance is particularly bad on many ultra light aerobatic style models with two strokes. It might be considerably reduced by filling compartments with foam.