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#16
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' 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...
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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#17
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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.
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![]() "Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#18
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Quote:
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...
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TC Current stable: - 2004 Mazda RALLYWANKEL - 2007 Saturn sky redline - 2004 Explorer...under surgery. Past: 135i, GTI, 300E, 300SD, 300SD, Stealth Last edited by tbomachines; 07-07-2011 at 12:56 PM. |
#19
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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.
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Never a dull moment at Berry Hill Farm. |
#20
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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
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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/showthread.php?threadid=10414 http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#21
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Quote:
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
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![]() 1982 300CD Turbo (Otis, "ups & downs") parts for sale 2003 TJ with Hemi (to go anywhere, quickly) sold 2001 Excursion Powerstroke (to go dependably) 1970 Mustang 428SCJ (to go fast) 1962 Corvette LS1 (to go in style) 2001 Schwinn Grape Krate 10spd (if all else fails) |
#22
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On some turbos the intake fan has an uneven blade pattern and no turbo wine. My 84 300SD is like this.
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1984 300SD turbo 126 "My true love" God made me an atheist and who am I to question His wisdom |
#23
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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.
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#24
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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.
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#25
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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?
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#26
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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.
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#27
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Quote:
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? ![]()
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RIP: 2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver) 1964 Unimog 404.1 (wrongfully towed by the city) 1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss) 1982 300D, Silberblau under a blue repaint 256k (sold for parts) 1995 E320 Polarweiss, 131k (sold for parts) 1995 E300D Polarweiss, 287k (totaled by drunk driver) |
#28
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UH... no one has mentioned stringed instruments and the way length/ratios relates to these harmonics ?
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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/showthread.php?threadid=10414 http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#29
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Quote:
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TC Current stable: - 2004 Mazda RALLYWANKEL - 2007 Saturn sky redline - 2004 Explorer...under surgery. Past: 135i, GTI, 300E, 300SD, 300SD, Stealth |
#30
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Quote:
"Nonuniform event spacing in multi-event cyclic tonal noise components." processes results in reduction of the amplitudes of 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 |
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