Why So Noisy

[phil c]

Why are some engines so noisy?  Came to mind in the thread on mufflers vs. tongues.  Bob Z has an Orwick 64(I think) that makes a huge amount of noise.  A Fox Mk VI with an open exhaust sounds much louder and harsher than an OS 32 running similar rpms and props for speed limit combat.  The LA's are relatively quiet sounding, even with open exhaust.  A Stunt 35 seems much louder, esp. when it is 2 cycling on a smaller prop.

Doesn't seem like it is the port timing.  It runs the gamut from mild to wild for  both kinds of engines.

[Eric Viglione]

In your stated case, more likely ignition timing that port timing. Advanced ign timing will make more noise through the venturi but still won't reach levels of a highly retarded (like old SuperCyc's with the lever pulled back or low comp Fox 59's) engine which will make more noise out the exhaust. The other factor I would throw in is case wall thickness & temper.

EricV

[Scott Jenkins]

Noise, what noise, I gave my hearing up to a Pratt-Whitney J79, yes I was wearing ear protection, 30 years ago something about trying to set a fuel control while at 80% thrust and having your whole body shake in unison with the screw you are trying to turn will do strange things to your hearing, about the only one now that makes me run is a Dynajet, mostly out of abject fear.

Scott

[Joey Mathison 9806]

Noise, what noise, I gave my hearing up to a Pratt-Whitney J79, yes I was wearing ear protection, 30 years ago something about trying to set a fuel control while at 80% thrust and having your whole body shake in unison with the screw you are trying to turn will do strange things to your hearing, about the only one now that makes me run is a Dynajet, mostly out of abject fear.

Scott

   how could you be scared of a little ol dyna jet

[Elwyn Aud]

I've heard some Fox 35's that seemed to be noisier than others. I wonder how much difference the airframe makes on noise?

[john e. holliday]

I have had people complain more about my open face Fox Stunt 35 than any other engine.  I can still remember the first time I fired up my first Fox Combat Special in the back yard.  Couldn't get it shut down quick enough.  But, I also left the mufflers off the Honda 450 I was rebuilding.  A Harley with straight pipes is quieter.  I also think motorcycles are not noisy enough.  The Honda was shut down immediately and mufflers put on.  DOC Holliday

[L0U CRANE]

Phil,

No science, here, but something to ponder...

Brass orchestra instruments come in all shapes and sizes. Some, like the cornet, are small and fairly simple. Some look like a brass pipe sculpture of the world's largest bowl of fettucine...

They each have characteristic sounds.

I've observed that diesels sound loudest up close, but the noise seems to not carry far. Some - mostly diesels these days - seem louder than others. The old OK Cub "360°" porting style is still common on diesels, and much like the earlier Arden 09 and 19 engines, and seems louder for their displacement than you'd expect.

The comment, about spark engine loudness relating to where the spark advance is set, is interesting. As with diesels, the ignition 'timing' is controllable. Directly with the sparker's breaker points, indirectly by varying compression in a diesel. Both gasoline and kerosene have slower combustion flame propagation speeds than methanol, per SAE tests. A diesel, running smoothly, likely has its ignition tailored to extracting most of the heat before the exhaust opens. The sparker??

And as with the brass instruments in an orchestra or band, the variety of shapes for where the noise came out was much more varied before we settled into the near-universal current Schneurle 2 or 3 bypass standard. ...And the exhaust stacks seemed more like the bell on a band brass, too. Now, engines are more massively made, though not much heavier, which may also dampen sound and limit it to the usually deeper but narrower exhaust port and stack area.

Just some thoughts... and I do not care to be real near an open exhaust ANYTHING with huge RPM, power, etc., without protection for what hearing I have left (most of it, thanks be.)

[Dick Fowler]

Just thinking out loud here. What about the port shape? Height versus width affecting the sharpness of the sound. Wide ports expose a larger area more quickly as the piston uncovers the port than does a narrow width . It would seem to me this would lead to a larger volume of gas exiting rather abruptly when compared to a narrow port which would intially bleed off some of the gases when first opened. The LA series motors have tall, narrow exhaust ports while Fox .35's are wide and short. As someone pointed out, LA engines are not real sharp sounding as compared to a Fox.

Make sense?

[Paul Smith]

Apart from muffler theory, I believe that the difference in dB level relates to exhaust port timing.

Specifically, if the exhaust port opens while the charge is still burning, it makes a huge amount of noise.

Conversely, if the fire has burned out before the exhaust opens, it barely needs a muffler.

On both 2-stroke and 4-stroke engines, high performance is made by opening the exhaust early, wasting fuel, and making a lot of noise.

Engines like to OS FP and LA series have conservative timing, don't use much fuel, and are quiet.

[L0U CRANE]

Ty and Paul,

Good points!
(Ty, I was referring to the model airplane diesels., btw...)

Diesels (compression-ignition, model jobbies, that is) probably do get most of the kerosene burnt by the time the ports open, after enjoying a longish push of heat expansion. Less pressure difference at port opening, as Paul points out. The 'louder when closer' thing seems to do with a few shrill overtones or harmonics that don't actually pack gobs of power. The shrillness does the job for them.

Gasoline may burn even slower, and still make the big bang at Ex-O, but what about those running sparkers on glow fuel?

Early days with the squish and bowl heads, the 'lands' around the bowls were much flatter and the bowls smaller. Almost all of the combustion took place in the bowl, as could be seen after some running. The head or button, as case may be, showed dark charring in the bowl and nearly no coloration out onto the flat 'land' surface. (I occasionally refer to glow ignition as being like a hammer blow, while diesel ignition is more of an extended push. Fuel flame speed thing.)

Later days with squish/bowl chambers: smaller band of flat, larger cone or hemi shape around the plug. Yet some are loud, others seemingly less so. Sure, port timing has some effect, but with most new engines designed for RC "Sport" RPM, and with the OEM cited peak HP RPMs we read, I don't think that is as involved as might be.

As for brass instrument augmenter 'bell' effects, OS engines have opened the exhaust port into a triangular section stack volume since the earliest FPs. That's quite a large width and the shape should favor louder... hmmm...

Just another sweet mystery of life?

[Jim Kraft]

I have found on Fox 35's that the hemi head is louder than the stock head. Also, the stock Fox standard plug is much quieter than the idle bar plug. My Super Cyke seems quieter on fuel than on gasoline. My Fox 59 is loud no matter what. It makes a great deal of difference on spark engines what prop you use. 14-6 on a 60 size is much quieter than a 12-6. Ohlson's are the quietest of the sparkers that I have found, at least the side ports.