What causes prop noise?

[holler1]

Does anyone know what causes the prop to make the noise it does at high rpm?
I've heard that the sound is produced when the tips of the prop reach the speed of sound, but I did some calculations and found that to get the tips of the prop moving at 743mph, on a 22in prop, the engine would have to be turning more that 11,000rpm. A 22in prop is usually run on a 50-60cc gas engine. I would guess that they wouldn't turn 11,000rpm. Any ideas?

[adrian]

wind

[A-Track]

The prop will make noise by the simple face that it's moving the air. I'm not an engineer so I don't have all the deets but I do beleive that the prop tip dosen't have to be going supersonic to make that noise, due to the blade accelerating the airmass you might have localized supersonic flow near the tips. Also the 743mph is the speed of sound at sea level on a standad day, the higher you are and with temps and dew points different from standard will cause the speed of sound to be lower.

[citrus]

	Quote: Originally Posted by A-Track
	The prop will make noise by the simple face that it's moving the air. I'm not an engineer so I don't have all the deets but I do beleive that the prop tip dosen't have to be going supersonic to make that noise, due to the blade accelerating the airmass you might have localized supersonic flow near the tips. Also the 743mph is the speed of sound at sea level on a standad day, the higher you are and with temps and dew points different from standard will cause the speed of sound to be lower.

Plus, as the air flows around the prop's airfoil the air is accelerated. You'd probably need one hell of a formula to figure out how fast the air is actually moving over the prop.
On airplane wings it's called critical mach. The plane is not above the speed of sound, often far from it but the air moving over the wing is.
I'm not saying this is the source of the prop noise, just adding to the author's original theory.

Chris

[wmat7039]

	Quote: Originally Posted by holler1
	Does anyone know what causes the prop to make the noise it does at high rpm? I've heard that the sound is produced when the tips of the prop reach the speed of sound, but I did some calculations and found that to get the tips of the prop moving at 743mph, on a 22in prop, the engine would have to be turning more that 11,000rpm. A 22in prop is usually run on a 50-60cc gas engine. I would guess that they wouldn't turn 11,000rpm. Any ideas?

Holler.....Take a look at research done for IMAC by George Hicks some 6 years ago when we were transitioning to cannisters/3 blade props...
http://www.mini-iac.com/Portals/57ad...cle_Jan_02.pdf

Hope this helps
Wayne

[Mithrandir]

The prop rip is from transonic flow over the tips of the blade. (A small "Sonic Boom")

Ever heard a Whip crack... same thing.. only 6000 per minute from a prop!!!

[3Dguy]

Ok, so... air?

[holler1]

Thank you. All of your explanations make sense. I forgot about the fact that the air is accelerated by the prop and that the speed of sound can be reduced with different conditions.

[RobertV]

There are many sources of prop noise and they have varying levels of significance. The most annoying is the very distinct "shriek" that comes from shocks forming at the tips. The above posts are correct in that the tip speed is a main factor in this noise source. The airfoil, planform shape, and thickness at the tip are also important since they define the Mach number at which compressibility effects begin to become significant. So the air does, indeed, accelerate beyond the rotational speed as dictated by these geometric factors and you will often find that keeping the tip speed below about Mach = 0.65-0.70 avoids the onset of shock waves and thus the noise associated with them for most hobby props.

So why does a shock make such noise? Well, sound is really just a pressure variation at a point in space that propagates to your ear. So, as a prop is turning through the air, it causes a perturbation in pressure that will ultimately be perceived as a sound. And this sound has a frequency that relates to the number of times a blade passes by that relative point per revolution. (we tend to be more annoyed by certain frequencies, too). Now consider that a shock wave is a really a large pressure rise. So, it really affects this sound pressure level and, thus, it's perceived intensity as you have heard at the field. The other parameters above (like thickness) and the aerodynamic loading of the blade itself also affect the pressure field and thus are contributors to sound, too.

By keeping your blade tips below some critical Mach number, you will certainly reduce or eliminate compressibility noise. But then the other noise sources become dominant, albeit less intense.

[aeroparts]

A nice spreadsheet there you can calculate and see the critical noise rpm on the propellers among other things. Its from Pe Reiverse
http://mvvs.nl/prop-power-calculator.xls
// Lennart

[Mithrandir]

	Quote: Originally Posted by RobertV
	There are many sources of prop noise and they have varying levels of significance. The most annoying is the very distinct "shriek" that comes from shocks forming at the tips. The above posts are correct in that the tip speed is a main factor in this noise source. The airfoil, planform shape, and thickness at the tip are also important since they define the Mach number at which compressibility effects begin to become significant. So the air does, indeed, accelerate beyond the rotational speed as dictated by these geometric factors and you will often find that keeping the tip speed below about Mach = 0.65-0.70 avoids the onset of shock waves and thus the noise associated with them for most hobby props. So why does a shock make such noise? Well, sound is really just a pressure variation at a point in space that propagates to your ear. So, as a prop is turning through the air, it causes a perturbation in pressure that will ultimately be perceived as a sound. And this sound has a frequency that relates to the number of times a blade passes by that relative point per revolution. (we tend to be more annoyed by certain frequencies, too). Now consider that a shock wave is a really a large pressure rise. So, it really affects this sound pressure level and, thus, it's perceived intensity as you have heard at the field. The other parameters above (like thickness) and the aerodynamic loading of the blade itself also affect the pressure field and thus are contributors to sound, too. By keeping your blade tips below some critical Mach number, you will certainly reduce or eliminate compressibility noise. But then the other noise sources become dominant, albeit less intense.

It seems that for the weight/displacement, our Gas motors are not nearly as much HP as a two stroke motorcylce motor... (Yeah. I know.. water cooling, tuned pipe and millions of research dollars.. ...)... but.. it is partly to blame on the apparent RPM limitation of say 6000'ish RPMs with a 32 inch'ish prop...otherwise transonic flow....

What if we could increase the rpms to a higher power level? Smaller diameter prop would be less efficient.. how about a prop with Transonic sections near the tip???
I dunno... something like;
http://www.ae.uiuc.edu/m-selig/ads/afplots/nl722343.gif
http://www.ae.uiuc.edu/m-selig/ads/afplots/nl722343.gif
http://www.ae.uiuc.edu/m-selig/ads/afplots/dsma523a.gif

????
And swept or sheared tips?? (Kinda like your current blades)
I wonder if anyone has done transonic/low RN research....
I expect it is less efficient.. but if we could get 35% more power and only loose 10%

I am no propellor/power plant guy.. just thinking out loud.....
efficiency on the prop.... (Just numbers I pulled out of thin air).. maybe we would be ahead???

[RobertV]

You have the right ideas here.

[aeroparts]

Maybe this will do .

[Nelvick]

WOW. Thats interesant for me. I has a post asking for prop noise too. But now thats clarify my answers.