# Fans : How many blades?

I was shopping for a ceiling fan at a light store (not a big box no nothing store), and was told by the salesman that a three bladed fan pushed more air than a four or five bladed fan. Didn't make sense to me so I asked why that was and was told he didn't know why, but that is the way it is. I asked if a two bladed fan pushed even more air, but he didn't know and of course didn't have any. I bit my tongue and didn't ask about a one bladed fan.
Is it true that a three bladed fan pushed more air than a four or five bladed fan, and if so, WHY???
I sure would appreciate any info you guys in the know can offer.
Bob-tx
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the amount of air pushed is proportional to the surface area of the blades, all other things being equal.
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Bob-tx wrote:

Single is the most efficient. The reason is additional blades disturb the air which makes the trailing blade less efficient. The main reason for multi blade is balance and aesthetics.
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that's why we always see airplane propellors only having one blade.

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_________________________________________________________
Wrong!
See the below URL.
www.aerospaceweb.org/question/propulsion/q0039.shtml

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wrote Re Re: Fans : How many blades?:

Interesting article. Thanks.
--
I filter all messages from google groups.

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charlie wrote:

The question was what was the most efficient. Many early propeller implementations had single blade props with a counterweight. Sometimes the most efficient isn't the most practical so you have design tradeoffs.

The question was what was the most efficient.
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wrote:

It was which pushed more air. That might be one meaning of more efficient, but ...
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3 blades is most efficient for wind turbines (electricity windmills), same might be true for fans.
Dave
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XR650L_Dave wrote:

And twenty blades is most efficient, or at least the most quiet, for nuclear submarines. Go figure.
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HeyBub wrote:

Multi blade is definitely not the most efficient. The overriding design criteria in a sub is stealth.
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I was going to upgrade to the 20 blade prop, but the dealer wanted too much for it and the after-market ones looked too flimsy.
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XR650L_Dave wrote:

No, the two-bladed fan is most efficient, but *only* when you coat both blades with WD-40.
--
PB
"I suspect you\'re an arrogant little pissant who grew up in the
Red Bull generation." - CJW
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Bob-tx wrote:

I have 2 20" box fans. The one with 3 8" blades blows a lot more air than the one with 5 2" blades. That's because the 3-blade fan has more blade area.
Depending on total blade area, a particular 5-blade fan may push more air than a 3-blade. Anyway, the 5-blade fan is likely to have narrower blades, which can make it quieter and more efficient.
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E Z Peaces wrote:

Andy writes:
Anyway, a 5 blade fan looks better -- more elegant...
Little 3 blade ceiling fans are cheapies --- less blades is cheaper to produce.....
Go with the five blade. You won't regret it.....
Andy in Eureka, Texas
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The salesman is an idiot. The number of blades is only one of many factors determining air flow. Diameter, pitch, blade with, and speed all come into play. To make a statement that one number of blades is better than another is pure lack of knowledge.
Buy the fan that looks best to you and it will move plenty of air. Be sure to get one that is very quiet.
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I once bought a 24" box fan at he HD. It had about 5 narrow blades and it looked like it would throw a lot of air.
I returned it after 20 minutes. It just made a lot of noise and did nothing but churn air. Too much turbulence and not enough air movement.
After many years of experimentation I have settled for 3 blades, low speed, high angle of attack, like shoveling snow with a biiig shovel.
Ceiling fans are also low speed because you want to keep the noise and vibration down. They, too, benefit from the same principle: Wide blades (3 or 5), low speed, high angle of attack (slope).
Air plane propellers are faced with different dynamics: Small planes universally have a two bladed propeller. Makes a lot of noise and shoves a lot of air because there is no churning. But, you don't want your ceiling fan to rotate at 4000 rpm.
--
Walter
www.rationality.net
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wrote:

No of course not. This is all nonsense. Keep the fan still and rotate the house.
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He was right, especially at higher speeds. But at extremely low speeds the 4 or 5 blade might be better because the tubulence has more time to infill again. It all has to do with turbulence. Planes have 2 blade props, can blow you over, but need to run high speed. If you look at the big industrial ceiling fans they install in gyms, factories and big box stores, they are all 3 blades and have a large diameter.
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Planes have 2 blade props, can blow you over, but need to run high speed.
If you look at the big industrial ceiling fans they install in gyms, factories and big box stores, they are all 3 blades and have a large diameter.
************************************************************************************ So much wrong information you have.
Many planes do have two blade props, except the ones that have three or four blades. Like the four blade Piper Meridian or the three blade Mirage. Want to check out some of the WWII fighter craft? http://www.newpiper.com/home/pages/MiragePhotoGallery.cfm
Now, for industrial fans, just take a look at the offerings McMaster Carr has and you see many different number of blades.