dB Levels and Props

ben_beyer

Junior Member
Ok, here's a question for the engine gurus, specifically ones running the 170cc motors. From my searching, it seems all 170cc engine break the 100 dB mark except for the ones running a 3 blade prop. Based on what I read, it seems a significant amount of noise comes from the speed the prop tips are traveling. Also, prop ripping comes from an engine being under-propped.

1) Is there any evidence that suggests that 170cc engines running 2 bladed props break the 100 dB mark simply because they are turning bigger (I.E. 31+") props?

2) What is different between a prop just being loud because of size and prop rip?
 

Jace

Junior Member
Not sure on the differences in 2 blade vs 3, but wood props seem to make more noise than C/F props.
 

kwilson

Gold Member
I'm not an engine guru, but do have a lot of IMAC experience and therefore a lot of experience on this subject. The prop rip is that of the airflow over the tip of the prop (remember it is airfoiled) creating little sonic booms. So the faster the prop tip travels, the more noise it will make. The shape of the prop tip also is a factor and is why 2 different props of the same diameter can have different sound levels at the same rpm. The reason a 3 blade is quieter is the diameter is smaller so the tip speed is less and the same rpm. Increasing pitch and decreasing diameter will also lower the tip speed which is why the 30x13 and 30x14 2 blades have become popular. The quietest prop I have run is the Mejlik 29x12n 3 blade. It has narrow tips which I believe helps as well.
 

Tired Old Man

Staff member
The propeller is a much larger noise generator than the exhaust. In many cases you could run without a muffler and still hear the prop over the engine. Kwilson had it right when he mentions tip speed--and shape. A square tipped prop will be a screamer at high RPM while a prop with a swept tip can generate a lot less noise when done right. A thick tip is worse in sound and efficiency than a thin tip. Larger props spinning at lower RPM can be achieving higher mach numbers at the tip than a smaller prop running more RPM.

I don't recall the formula but you can do the math to obtain the speed of the prop at the tips for a given RPM. Once a tip hits about .4 mach they start getting loud. 3 blades generate less noise but they do so with some consistent and negative impacts on performance compared to a 2 blade. Thrust response is slower and down line braking is generally less. If one is using an engine that is marginal on power with a 2 blade that situation will get worse with a 3 blade. The concept behind a 3 blade prop is to add blades when an engine makes so much power a 2 blade of correct size is impracticable but that seems to work best with full scale. With our stuff we go to a 3 blade sized to achieve lower noise levels, not to take advantage of greater power availability.
 

Jedijody

Proud father of Soldiers
You have to use the right oil, fuel/oil ratio, and radio, if not your props will be breaking windows 20 miles away.

My planes sound like a gentle breeze.
 

BRUTUS

Plank Junky
Lifetime Supporter
I want the loudest 3 blade prop I can find for my next plane...:thumbsup:
 

Jedijody

Proud father of Soldiers
See if you can get Robert Vess to start making a 3 blade. It would be all but guaranteed to be the loudest 3 blade prop on the planet.
 

Classclown

Well-Known Member
And if his props are not noisy enough for you, engage him in conversation... That guy will talk your ears off........ Even more so than me! Vess is one of the nicest guys out there, I'm not into his props, but had fun talking to him at dinner, one of those nights during FlyForTots.
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
Someone that talks more than the Clown........ I have to tell him I have to take a dump to get him off the phone with me! Seriously!!

But in reality, wood props flatten out, to where the CF props dont. If you wamt a stiff wood prop, Xoar laminates. But they do get noisy too. 27x10 on my 106. Not as bad as a 150 running a 32x10 MenzS, but close!
 
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