Fan Blades 7 to 5

[Bryan W.]

[Don H.]

Re: Fan Blades 7 to 5

I can't answer your question but I believe that only air conditioned 1965 (& possibly '64) Corvettes had the seven blade fan. Don H.

[Bryan W.]

[Gary B.]

Re: Fan Blades 7 to 5

Bryan,

And don’t forget the possibility of GM only asking, does a given fan pass some air flow or cooling test, and is it cheaper than another fan type? The change from 7 blade to 5 blade could be as simple as that.

Gary

[Bryan W.]

Re: Fan Blades 7 to 5

I would like to think the reason/s are deeper than that of cost. Maybe.

[Mark F.]

Re: Fan Blades 7 to 5

...What I would really like to understand is the thinking and reasons for the change from the larger sized fan used in 64/65 model year back to the smaller size for all models in 66? Was the 18” five blade fan better than the 18” seven blade fan? Better at what? Or, some sort of compromise?

Hi Bryan,

Let's say for the sake of argument that GM designed the 5-blade fan to have a nearly equivalent (or better?) cooling capacity as a 7-blade fan (maybe by increasing blade pitch and/or spacing, I don't know). If they were able to maintain acceptable cooling capabilities by doing that, an added benefit is that the Blade Pass Frequency (BPF) drops by nearly 29%. BPF is the rate at which a given blade passes by a fixed position and it is equal to the number of blades times the rpm of the motor driving the blades.

So, for a 7-blade fan at 2,000 RPM, the BPF is 14,000 passes per minute. For a 5-blade fan at 2,000 RPM, the BPF is 10,000 passes per minute. Converting both of those frequencies to Hertz (Hz), which is cycles per second (or BPF/sec, rather than BPF/minute), you get a "tone", or "sound" that is audible to the human ear on the low end of ~233 Hz for 7-blade and ~167 Hz for the 5-blade.

So, who cares and how is this relevant to a design change?

Did 7-blade fans have more noise complaints than 5 (I don't know). If so, shifting the frequency down (away from the speech frequencies) is the way to go if that were the case.

Another thing that may have come into play is resonant frequency. Shifting frequencies caused by a fan (in this case) away from the natural resonant frequency that all objects have can reduce the potential for vibration-related mechanical failures. Example: remember that Ella Fitzgerald commercial when she hits that High note coupled with her ability to provide plenty of volume and the wine glass shatters? Same thing can happen with car parts - they won't shatter like glass, but they can get shaken so much that they fail in other ways...bearing wear; casting cracks; excess heat; etc. https://www.youtube.com/watch?v=1-IvTF0xUxM

Anyhow, this all speculation on my part, but these things can have an impact if their studies (or customer complaints) pointed them toward a design change to address them. Can't wait to find out what the REAL answer is thx, Mark

[Bryan W.]

Re: Fan Blades 7 to 5

Thanks for your thinking Mark. I had considered sound ( Im not capable of the maths as you are) But discounted that in my mind as I read somewhere that the offset blades counters that and if sound was the issue why did they go back to seven blades in 67 and beyond.
Was the pitch of the blades different on a five blade to a seven blade? Did they both pull the same amount of air at an idle or at 3000 RPM?
Anyone else out there have the answers?

[Ronald L.]

Re: Fan Blades 7 to 5

I think you guys are mixing apples and oranges, generally you see 7 blade fans on cars with AC. A base car no AC still have 5 blade fans up to about 1970 or so, you need to make a matrix of options, fan diameters, etc. and GM part numbers from the AIMS and you might get to a trend. Then again maybe not.

[Leif A.]

Re: Fan Blades 7 to 5

The '67 L79 with A/C had a 5-blade fan from the factory.

[Richard G.]

Re: Fan Blades 7 to 5

Mark sounds like he has been trained in vibration. A subject I know a little about.
Nice analogy Mark.

However, the turbine wining sound is mitigated with the missing or offset blade.
I do not understand exactly how this works and have never heard an explanation of why this works so well.

Never underestimate the power of reducing costs within GM.
I thinking of the infamous Crankshaft Rope packing seal reborn in 1980 in the Citation.
The smart guy must have retired. LOL
Rick

[Mark F.]

Re: Fan Blades 7 to 5

Mark sounds like he has been trained in vibration. A subject I know a little about.
Nice analogy Mark...Rick

The noise problem due to the increased fan radius is a serious matter especially as the cabin noise becomes quieter for sedans. Of the fan noise components, discrete noise at BPF's (Blade Passing Frequency) seriously degrades cabin sound quality. Unevenly spaced fan is developed to reduce the tones. The fan blades are spaced such that the center of mass is placed exactly on the fan axis to minimize fan vibration. The resulting fan noise is 11 dBA quieter in discrete noise level than the even bladed fan system. https://saemobilus.sae.org/content/2008-01-0569/

The table below gives some perspective on the range and intensities of noise levels. Also note that decibels are measured on a logarithmic scale. Example: OSHA permissible exposure levels for noise equate an increase from 90 dB to 95 dB as being twice as loud; 90 dB to 100dB as four times as loud. The point being that the 11 dBA reduction mentioned in the abstract above is pretty significant. Sorry, but I can't seem to import Excel Tables and have columns at sensible widths very well...thx, Mark

Noise Source	Decibel Level	Decibel Effect
Jet take-off (at 25 meters)	150	Eardrum rupture
Aircraft carrier deck	140
Military jet aircraft take-off from aircraft carrier with afterburner at 50 ft (130 dB).	130
Thunderclap, chain saw. Oxygen torch (121 dB).	120	Painful. 32 times as loud as 70 dB.
Steel mill, auto horn at 1 meter. Turbo-fan aircraft at takeoff power at 200 ft (118 dB). Riveting machine (110 dB); live rock music (108 - 114 dB).	110	Average human pain threshold. 16 times as loud as 70 dB.
Jet take-off (at 305 meters), use of outboard motor, power lawn mower, motorcycle, farm tractor, jackhammer, garbage truck. Boeing 707 or DC-8 aircraft at one nautical mile (6080 ft) before landing (106 dB); jet flyover at 1000 feet (103 dB); Bell J-2A helicopter at 100 ft (100 dB).	100	8 times as loud as 70 dB. Serious damage possible in 8 hr exposure.
Boeing 737 or DC-9 aircraft at one nautical mile (6080 ft) before landing (97 dB); power mower (96 dB); motorcycle at 25 ft (90 dB). Newspaper press (97 dB).	90	4 times as loud as 70 dB. Likely damage in 8 hour exposure.
Garbage disposal, dishwasher, average factory, freight train (at 15 meters). Car wash at 20 ft (89 dB); propeller plane flyover at 1000 ft (88 dB); diesel truck 40 mph at 50 ft (84 dB); diesel train at 45 mph at 100 ft (83 dB). Food blender (88 dB); milling machine (85 dB); garbage disposal (80 dB).	80	2 times as loud as 70 dB. Possible damage in 8 hour exposure.
Passenger car at 65 mph at 25 ft (77 dB); freeway at 50 ft from pavement edge 10 a.m. (76 dB). Living room music (76 dB); radio or TV-audio, vacuum cleaner (70 dB).	70	Arbitrary base of comparison. Upper 70s are annoyingly loud to some people.
Conversation in restaurant, office, background music, Air conditioning unit at 100 feet.	60	Half as loud as 70 dB. Fairly quiet.
Quiet suburb, conversation at home. Large electrical transformers at 100 feet.	50	One-fourth as loud as 70 dB.
Library, bird calls (44 dB); lowest limit of urban ambient sound	40	One-eighth as loud as 70 dB.
Quiet rural area.	30	One-sixteenth as loud as 70 dB. Very Quiet.
Whisper, rustling leaves	20
Breathing	10	Barely audible

Source:

https://www.iacacoustics.com/blog-full/comparative-examples-of-noise-levels.html