The neighbours are complaining

[Kiwi 1]

By chance do those plastic exhaust extenders decrease the amount of noise from i/c models?

 Thanks

[Tim Mackey]

Nope...just meant to help direct the gunk away from the model, with very limited success I may add. Answer ? Well to start with........try
A bigger prop, to lower the revs, secure installation, minmum rattles from airframe/cables/ rods etc. Decent silencer ( some after market versions are a little better than standard ).

PS some makes of prop are also noisier than others....experiment. 

[Erfolg]

Do not worry about the noise!Tell the neighbours,” its my new powered lawn mower". Say, wife wants me stop using it and start with "late evening barbecues with disco". Ask neighbours which they prefer.If they do not see sense, ask for help in building disco stage. Casually enquire, do they think that 1000W amplifier will be enough.

Agony Aunt

Erfolg

[Tom Mann]

Wotcha Kiwi

My best result was a new motor mount which features a metal plate bolted inbetween backplate and cylinder case, with said plate attached to firewall via three rubbery things.  This limited transmission of vibes to the airframe.  Half the noise comes from the airframe hitting a nice resonant frequency and actually acting as a drum.

 Oi Timbo you're everywhere pal.  I've seen things called 'actuators' in full size rattly helis... they 'read' vibrations and actively produce equal and opposite vibrations to cancel out the frequency.  Anyone played with these on a small scale?

tom

[Grant Darnley]

Remind them that their sons ford fiesta with a bean-can exhaust makes more noise than all your planes running at once

[Erfolg]

Most of us (when I used smelly, inconvienant, noisy) glow motors, would run the motor for a maximum of 5 minutes at a time. Surely your neighbours have no trouble with this!

The suggestion that the airframe is the cause will have some truth (I think most is still the engine). Engineering type solutions to resonation, is to alter the frequency of body.  Fill with foam, add additional bracing, change the covering, remove some of the bracing, change the shape (alters frequency), alter the input frequency (the motor mount idea). The amplitude of the input effects the movement in the body rather than if it resonates (enough energy and destruction will be assured).

As an aside all bodies resonate, it is the frequency that this occurs, which most seek to control (with the amplitude doing the damage).

Erfolg

[Tom Mann]

Jetsome that's a cool idea.  How big was your exit from the fuel proofed wooden box?  Was the power loss related to this obstruction to exhaust?

tom

[Martin McIntosh]

Don`t try that with a four stroke- my Spiteful caught fire because a 4T exaust is much hotter than a 2T! Prop blade tips make a huge difference. If you have rounded tips such as the shape of a typical wingtip (not talking about APC shape-more the Super nylon), then squaring the tips as if you had done it with sandpaper on the edge of a spinning prop can save you 3dBa which is half the previous noise level. Try it. Martin Mc.

[Erfolg]

I find the discussion of propeller noise interesting.

I fly electric, the most powerful motor I use on a reregular basis is drawing 350w, the revs/min is some 10,000. The noise is from everything is that of a fan room heater. It is almost silent.

I have noticed that flying wings, using 480 motor running as pushers are noticably noiser than those runinning as tractors, I assume it is the masking effect of the wing, loading and reloading the propeller. 

The motor must be the equivalent to a 0.30 ts, ic. The revs are lower 2,000 ish. I do wonder what makes an ic propeller so noisey.

Questioning

Erfolg

[Bravedan]

I fly electric too, and my noisiest plane by far is a wing with prop in slot mounting.......this is SOOOOOO noisy I don't fly it early or late or when a lot of people are around (legal park fly site).

It's brushless and has so far had two differing motors as one failed...........no difference noticed.

Even spinning the prop by hand transfers a drumming into the EPP airframe, which acts as a giant sounding board. 

I have the SAME motor/prop combination in a conventional (but still EPP) airframe.......nearly silent............... 

Both rigidly mounted, etc, etc. 

What does that tell you??!! 

Now to 2 stroke IC................

Many silencers have the gas exit path from a point of high pressure. 

However, a double cone silencer with the output from the centre widest part (so point of least pressure) is inherently quieter.............  (old two stroke racing motorcycle trick!!!)

[Martin McIntosh]

Interesting about the pushers. I have a couple of Rutan foamies, which of course are pushers, and the noise is considerably greater than wnen the same motors are used in tractor type models. Regarding pipes, if you Techno weld an ali aerosol can of suitable size to the rear of a standard, old fashioned none disc tuned pipe, then fit say a 6mm ali tube or just `pepperpot` the end with 1.5mm holes, you will end up with an amazingly quiet set up. Been using this for years. MM.

[Tom Mann]

On the pusher/puller question, I have a guess.

 A pusher setup will drive the prop towards the motor and airframe, so every vibration/pulse will be shoved into the airframe, making it resonate hard.

A puller will drive the prop+shaft away from the airframe, with the airframe getting tugged along by its prop shaft.

Clearly this is layman's speak and pure fantasy but I think there might be something in it.

tom

[Evan Pimm 2]

Nah, it's actually the blades cutting through the wake turbulance off the trailing edge of the wing or slot in front. Even if you have a pusher mounted on a pylon over the wing the turbulence off the strut/s will have the same effect. Tractor props don't usually have bits of airframe in front of them...

Evan 

[Dave Bran]

SIDE LOADING 

Don't think so....as spinning the prop by hand won't cause enough prop force to push the motor against end float hard enough to create suitable conditions for noise transfer by side load......yet the noise is there.

WAKE TURBULENCE

Again, would this be evident at low "hand spin" speeds??  I think not............

Both may influence the overall effect, but they are not the root cause. Spin the prop by hand, and there's a considerable drumming in the airframe..........  

Some brushless motors seem to have much more magnetic strength/effect, creating a "knock knock" magnetic grab effect during rotation which creates rotational noise............  the Typhoon Blitz range seem particularly bad, the small GWS range particularly good.  In fact the Blitz 1650 will hardly spin by hand as the magnets "grab" the poles and stop it.........and it's this frequency noise that's drumming in the airframe.

Anyway, this is way off topic, so sorry, didn't mean to hijack the thread.........my original post did reply on topic, too............... 

[Mike Rolls]

If you can put up with the revolting looks and increased drag, a very effective add on silencer - 'invented' by the late Duke Fox - is to lead your normal silencer outlet into a soft plastic fuel tank like the Sullivan - size dependent upon motor size, IIRC Duke used a 4 oz. tank with a 35 - in which you have drilled (literally) a couple of hundred 1/16" inch holes. I tried it on the bench and it seemed pretty effective, but I could never bring myself to strap such an ugly object onto the side of a model. I suppose with suitable forethought it could be located inside the fuselage.
Mike

[Derek Small]

Going back to original question

 Just Engines do (or did) a rubber exhaust extension which has a small expansion chamber that does (measurably) drop the dbs. I have a couple and they have made a (small) difference on Pitsts style exhausts - 2St OS 90, and a Weston mini pipe used on an Irvine 53

[Tim Mackey]

Thats interesting....not seen those gizmos.

[Peter Miller]

J.Perkins do that exhaust extension with the extra chamber.

The most effective way that I found was to mount the engine mount on those rubber pillars. IT wobbles a bit at low rpm but not otherwise and it really does cut the noise down. Nexus Model Supplies sells them among others. Add a larger prop and you have a really quiet model.

I had a neighbour (Just moved) who would spend Saturday afternoon mowing miles of grass. They didn't complain about me so I didn,t

Best laugh: At one of our flying sites someone came over and complained about the jet model. When we were driving away there was the pillock, sitting on a b****y great ride on mower with ear defenders clamped to his head.

[Bob Howard]

Re. the reason why I/C props are noisier than electric props for the same size & rpm, I have seen this hypothosis put forward :-

 The thrust from the prop blade will always tend to deform by bending forward. On an electric model, this is no problem, as the speed throughout each revolution is constant, and thus the blade shape remains stable.

 On an I/C model, the the engine will slow down as the piston approcaches Top Dead Centre (TDC). The thrust from the blade is therefore reduced, and the blade is able to return closer to its original shape.

 As the engine fires, the piston accelerates down the bore, and the increased speed produces greater thrust, which deforme the blade by bending it further forward.

 In a nutshell, the blades  are flapping backward and forward once per revolution on a 2 stroke. It is a bit more complicated on a 4 stroke, but the same principle applies. This action is similar to that of of a loudspeaker diaphram.

Stiffer propellors will help to reduce the blade. Also, increasing the flywheel effect with a heavier prop will damp out the speed variation through each cycle.

 A simple anser is to use a 3 blade prop. The additional blade greatly increases the flywheel effect, and the shorter blades are much stiffer. They may, however, reduce the overall performance.

  I have found Graupner props to be good, and they have a wide range of sizes.

[Kiwi 1]

Hi Guys, thanks to everyone who replied to this. As "luck" would have it the "noisy"  engine was written off in a horrible high speed crash. No fire just balsa. Subsequent plane and engine were over propped using an APC which doesn't flex. On failing the sound check at the club one of those rubber exhaust extenders was added and the model actually passed. The sagely advice from the more knowledgeable fliers was that with the exhaust turned well away from the airframe the sound was less because there was less echo. I'm not complaining, just glad to be flying again. Apart from last Saturday where the fuel line expanded.....see other forum...Happy straffing......

[Peter Miller]

Neil Tidy of Laser engine fame said that the acceleration and deceeleration in the four stroke cycle could actually flex the blades of a nylon prop back and forth until it snapped.

 Many years ago, in the days when Fox silencers were a joke (Don't know what they are like now) Duke Fox actually suggested taking a clunk tank and drilling lots of holes in that and fitting it over the end of the silencer.

It did work, I tried it. But then anything would made a Fox silncer of the day better. IT also looked stupid.

[Erfolg]

The observations you make on the Fox silencer, were true for most manufacturers.

We modellers were/are reluctant to strap a dirty great can to the outside of our treasured replicas of full size or facsimlie models. That is unless we can get more power.

My Enya 20, Merco 35 all had the smallest volume/size silencers, to satisfy (visually) the clubs noise official. They did little for the noise. From time to time some would go the extra mile, often being diappointed by the drop in performance, which could be drag, drop in output power, or little apparent improvement in noise 

Erfolg

[Peter Miller]

The main problem with the Fox silencers was that they had about a 1/2" outlet. Some even had an open front end as well.

Plugging the outlet down to 1/4" made them much quieter but didn't lose too much power on the .25 but it wasn't a really powerful engine. It was a nice engine for its day though.

[David Martin 2]

Hi Kiwi,

Glad you've found a cure...if a bit drastic...

My 2 strokes all run well and quietly on Graupner props, when I snapped one last year, the only replacment in my box was a cheap black one with white tips.....the noise increase was horrendous, enough for me not to even take off until I had another Graupner on board.

If you need to reduce noise in the future, the Just Engines quiet exhausts work well, I've seen others use them with great results...

http://www.justengines.unseen.org/sils_std.htm

I tend to only run my engines at the field or my Dad's place (4 strokes in a 'big' garden), I always run in a new engine in the plane, this in the past has shown up the odd nut or joint etc that would otherwise have come loose in flight on either the engine or new model and any cooling issues. Plus it saves any hassle setting up a test bench and has the engine fitted in the place it was intended straight off.

Just my way but it works for me and the neighbourhood.

[Andy Gates]

You could always try a slightly higher pitch prop to bring the rpm's down.

 I had a play with the previously mentioned black props with light tips, a 12 x 6 on a .52 engine.

It was OK, but trying a 12 x 4 of the same brand made a horrendous noise. Changing to a all grey 12 x 6 prop (non Graupner), the noise was far better than before. Adding second additional silencer box connected by silicone tubing means the engine noise can't be heard above the prop noise.