Why are the fan blades uneven? - Page 2 - PeachParts Mercedes-Benz Forum

leathermang · #16 07-07-2011, 11:56 AM

' The number of blades determines the number of spaces ' ????
when I first used my fingers to count up this morning... I was thinking that it was going to be x+1 or x-1.... so I was thinking that 6 blades were going to give an odd number of spaces... I am going back to bed and start over later...

Jeremy5848 · #17 07-07-2011, 12:27 PM

The forward edge of the blade also contributes to any generated sound. For example, owls fly silently because of the special feathers at the leading edges of their wings. I would imagine that duplicating such a structure in cast plastic is an aeronautical engineer's dream.

BTW, both of my cars have 9-blade fans; 9 is not prime, at least not since I last checked.

tbomachines · #18 07-07-2011, 12:39 PM

Quote:

Originally Posted by lutzTD

number of blades determines number of spaces, but its the trailing edge of the blade that makes the noise

primes are 3, 5, 7, 11, 13, 17 etc

heres why.

take an 8 blade fan, it will generate frequencies at multiples of 8, 4, 2, and 1

a 12 blade fan will generate frequencies at multiples of 12, 6, 4, 2, 3, 1

To elaborate just a tad on this point, keep in mind the way that sound waves behave. If you have a set of sound waves in phase, their amplitude (i.e. volume) multiplies at those intersecting points. A series of waves completely out of phase essentially cancels the net resulting energy, thus producing no volume (this is how noise canceling headphones work, btw). By having a prime number of blades, the sound waves coming off the fan blades will never have the opportunity to be in phase and not have a net resulting amplitude higher than the initial blade/air compression (at least in theory at a constant speed). By altering the spacing I would imagine this creates even more of an out-of-phase condition and more easily adapts to the changing speed of the engine. You should be able to have non-prime blade count with spacing that still allows for an out-of-phase condition to occur.

Edit: Actually I'm wrong on the prime part. Having them evenly spaced would create the same exact wavelength between each blade (same pitch and depth) and therefore amplify the noise dramatically. Having them unevenly spaced creates a different amount of length between each pulse, making for different wavelengths at each blade, each being out of phase with one another. Sophomoric physics mistake on my part but hey, I'm not an engineer...

R Leo · #19 07-07-2011, 12:41 PM

Greg,
Did you ever see any of those YO-3s when you were in SEA?

Moons ago, my uncle was telling me about aircraft he saw when he was in Da Nang on Monkey Mountain...what he described was unlike any military airplanes I'd ever heard of. Turned out they were the QT-2 and YO-3.

leathermang · #20 07-07-2011, 12:53 PM

No, I was there all of 1969... it looks like it arrived in 1970... I saw an article in Popular Mechanics later about it...that is why I thought about it..

I actually thought I remembered that YO as having Seven Blades... thus qualifying for this discussion based on that.. but as to noise...what they did was put a lot of blades on so as to reduce the speed of the prop...

This is a good ref...

http://www.answers.com/topic/propeller

lutzTD · #21 07-07-2011, 02:09 PM

Quote:

Originally Posted by Jeremy5848

The forward edge of the blade also contributes to any generated sound. For example, owls fly silently because of the special feathers at the leading edges of their wings. I would imagine that duplicating such a structure in cast plastic is an aeronautical engineer's dream.

BTW, both of my cars have 9-blade fans; 9 is not prime, at least not since I last checked.

sorry I should have said prime number of blades is important if they are evenly spaced. that entire paragraph assumed even spacing, sorry

also, if you look at the quiet fans in the industry they have serrated trailing edges to break up the trailing edge pressure wave

The Gears · #22 07-07-2011, 03:01 PM

On some turbos the intake fan has an uneven blade pattern and no turbo wine. My 84 300SD is like this.

spdrun · #23 07-07-2011, 03:18 PM

Essentially what you have is two (or more) even bladed fans superimposed on one another -- for example, a 2-blade fan on top of a three.

MBeige · #24 07-07-2011, 03:19 PM

A few observations:

Some of the auxiliary fan blades have tiny balancers attached to some of the blades to balance it out. I'd guess the main engine fan won't have these.

Both the auxiliary fan blades and the main engine fan blades are unevenly spaced out. Some of the main engine fans are evenly spaced out, or grouped together (the original 6-bladed OM617 engine fan is like this), while the updated 9-bladed plastic fan for the same engine is not spaced out evenly unlike its 6-bladed metal version.

tangofox007 · #25 07-07-2011, 03:21 PM

Quote:

Originally Posted by lutzTD

take an 8 blade fan, it will generate frequencies at multiples of 8, 4, 2, and 1

a 12 blade fan will generate frequencies at multiples of 12, 6, 4, 2, 3, 1

Why does it matter that the multiples are whole numbers versus fractional? Is a 12 blade fan going to be louder than an otherwise identical 11 or 13 blade fan?

rscurtis · #26 07-07-2011, 05:05 PM

Power equipment also uses modulated blades for noise reduction. Flywheel cooling fan blades are unevenly spaced and so are the blades on higher-end leaf blowers. The cheapest models sold at the big box stores may not have this feature, that's why they sound like fire sirens.

okto · #27 07-07-2011, 05:44 PM

Quote:

Originally Posted by tangofox007

Why does it matter that the multiples are whole numbers versus fractional? Is a 12 blade fan going to be louder than an otherwise identical 11 or 13 blade fan?

That's how harmonics work. Harmonics are basically amplification of oscillation where two or more oscillation periods coincide. They can't occur at fractional ratios, because the oscillations have to be in-phase, IE, whole-number multiples of each other.
Think about making waves in a bathtub: when you splash at a regular frequency, the waves get bigger because each new impulse coincides with the crest or trough of the existing waves.
I just realized harmonics are really difficult to explain.

Our hypothetical 12-bladed fan produces a harmonic oscillation for every balanced pair of blades. Essentially, axes of symmetry on a rotating mass create oscillations at regular intervals. Non-prime numbers of blades will result in more axes of symmetry.
So the 12-blade fan oscillates at w(rate of rotation)/1, w/2, w/3, w/4, w/6, and w/12.

A 13-bladed fan will only oscillate at w/1 and w/13, and so will seem much quieter. The two oscillation frequencies do not have any common factors, so no harmonic amplification, so less vibration, so less noise.

Clear as mud?

leathermang · #28 07-07-2011, 06:17 PM

UH... no one has mentioned stringed instruments and the way length/ratios relates to these harmonics ?

tbomachines · #29 07-07-2011, 06:54 PM

Quote:

Originally Posted by okto

That's how harmonics work. Harmonics are basically amplification of oscillation where two or more oscillation periods coincide. They can't occur at fractional ratios, because the oscillations have to be in-phase, IE, whole-number multiples of each other.
Think about making waves in a bathtub: when you splash at a regular frequency, the waves get bigger because each new impulse coincides with the crest or trough of the existing waves.
I just realized harmonics are really difficult to explain.

Our hypothetical 12-bladed fan produces a harmonic oscillation for every balanced pair of blades. Essentially, axes of symmetry on a rotating mass create oscillations at regular intervals. Non-prime numbers of blades will result in more axes of symmetry.
So the 12-blade fan oscillates at w(rate of rotation)/1, w/2, w/3, w/4, w/6, and w/12.

A 13-bladed fan will only oscillate at w/1 and w/13, and so will seem much quieter. The two oscillation frequencies do not have any common factors, so no harmonic amplification, so less vibration, so less noise.

Clear as mud?

Except, regardless of how many blades it has, if they are all equal in length and spaced out evenly they will amplify like crazy. That's because each one would have the same wavelength relative to the last and following blade, they would all be generating identical pulses.

Yak · #30 07-07-2011, 07:49 PM

Quote:

Originally Posted by okto

Just curious, does anyone know why the spacing of the fan blades round the hub on Mercedes cooling fans is non-regular?

Because

"Nonuniform event spacing in multi-event cyclic
processes results in reduction of the amplitudes of
tonal noise components."

http://www.google.com/url?sa=t&source=web&cd=5&sqi=2&ved=0CDkQFjAE&url=http%3A%2F%2Fminds.wisconsin.edu%2Fbitstream%2Fhandle%2F1793%2F10406%2Ffile_1.pdf%3Fsequence%3D1&rct=j&q=asymmetric%20blade%20spacing%20and%20fan%20noise&ei=bUYWTsmxF-Lf0QHI7uRn&usg=AFQjCNGZJAA8R1hsIg7FAd-Dfb0uSXRRvg

Or more on Fan Noise

For small fans such as those used on electric motors the noise generated by their centrifugal cooling fans can be controlled by:

Spacing the blades irregularly around the fan disc to improve the subjective quality of the noise radiated by fans.

http://www.diracdelta.co.uk/science/source/f/a/fan%20noise/source.html

Or:

The influence of irregular blade spacing of car alternator radial fans on the total sound pressure level (SPL) and the noise spectrum has been investigated. For this purpose, the SPL and spectra were computed theoretically and the values were compared to measured results for several types of fans with various blade spacing. As theoretical background, the theory describing discrete frequency sound radiated by axial fans in open space was adopted. This was done in order to model theSPLand the spectra of alternator radial fans, placed inside a casing. Furthermore, due to the low blade tip Mach number, blades were modelled as dipole point sources. It was found, in accordance with previous results from known literature, that alterations in blade spacing do not significantly alter the total SPL nor the cooling capacity of the alternator radial fan. However, significant dispersion of the sound power over several harmonics was found with irregular fan blade spacing, thus allowing for a reduction of the siren effect. This phenomenon was predicted theoretically and confirmed experimentally.

http://www.sciencedirect.com/science/article/pii/S0022460X98917072