Why air flow created by ceiling fan goes downward?

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Discussion Overview

The discussion centers around the mechanics and design of ceiling fans, specifically addressing why the airflow created by a counterclockwise rotating fan (as viewed from below) moves downward. Participants explore various aspects including blade pitch, motor design, and the intended effects of airflow on comfort.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that reversible ceiling fans can direct airflow either downwards or upwards, depending on the season and desired effect.
  • There is a discussion about the angle of attack of the blades, which some participants suggest is responsible for directing air downward when the fan rotates counterclockwise.
  • One participant mentions that the design of ceiling fans has historically favored downward airflow, comparing the blade pitch to that of a screw with a left-hand thread.
  • Another participant raises a question about whether the inquiry is about the mechanics of airflow or the purpose of moving air downward, suggesting that fans primarily serve to remove ambient heat from the skin rather than cool a room.
  • Several participants discuss the relationship between motor rotation direction and fan blade movement, with some suggesting that standard motors rotate clockwise and thus necessitate counterclockwise blade rotation to push air downwards.
  • One participant reflects on the implications of motor design on the direction of airflow, comparing it to other applications such as boat propulsion.

Areas of Agreement / Disagreement

Participants express a range of views on the mechanics and purpose of ceiling fan airflow, with no clear consensus reached. Some agree on the mechanics of airflow direction, while others question the underlying reasons and implications of design choices.

Contextual Notes

Participants mention various assumptions about motor design and blade pitch without fully resolving the implications of these factors on airflow direction. The discussion includes references to specific designs and comparisons to other mechanical systems, indicating a complexity in the topic that remains partially unexplored.

Who May Find This Useful

This discussion may be of interest to individuals exploring the mechanics of household appliances, those studying fluid dynamics, or anyone curious about the design principles behind ceiling fans and their operation.

ddnath
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Why air flow created by ceiling fan (which rotates counter clockwise (seen from beneath ceilling) ) goes downward?
 
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Mine are reversable, so the air could go either way. Look at the pitch on the blades and compare it to an airplane propeller.
 
ddnath said:
Why air flow created by ceiling fan (which rotates counter clockwise (seen from beneath ceilling) ) goes downward?

The instructions with mine (a reversible one too) are that Downwards is to provide a good, cooling draft onto your head on hot days and Upwards is to be used in cold weather to circulate warm air from a heater around without causing too much draft anywhere. (If that's what you mean by "why?")
 
The turbine blades have some angle of attack, so when they chop into the air at some angle, it pushes the air against the blade toward a direction.
 
ddnath said:
Why air flow created by ceiling fan (which rotates counter clockwise (seen from beneath ceilling) ) goes downward?

A quick search using google images shows that the majority (all?) ceiling fans are indeed designed to blow air downwards when they rotate counter clockwise (when seen from below). This is determined by the angle of the blades. There is nothing to stop someone designing one to rotate the other way. I have no idea why ceiling fans work this way other than that's the way it's allways been done?

Incidentally if you compare the blade pitch with the thread on a standard screw they are have oposite "hands". In other words a ceiling fan is similar to a screw with a left hand thread not the normal right hand thread.
 
I don't understand the question. Are you asking how does the fan move air downward, which some people have answered, or what is the purpose of moving the air downward. Since no one has answered this part I'll answer it.

Fans are designed to remove ambient heat, which is the heat on your skin. So think of it like air is moved to your skin, it absorbs thermal energy, and then more air is passed over moving that air away. This is the cooling effect that you feel, but it doesn't neccessarily cool down a room, which is a common misconception. Fans are only useful if you're under it. So turn them off if you aren't in the room.
 
CWatters said:
Incidentally if you compare the blade pitch with the thread on a standard screw they are have oposite "hands". In other words a ceiling fan is similar to a screw with a left hand thread not the normal right hand thread.
You got me thinking about that one. The reason, I think, is quite simple. A standard motor runs 'clockwise' and, if it is suspended with the shaft pointing downwards then the fan blades need to push away - which is the opposite way that most screws are operated. They wouldn't want to make a special set of motors for ceiling fans, would they?
It got me thinking about Pullers and Pushers for aircraft and boat propulsion. The same thing applies.
As it happens, I have an old Volvo Penta engine on my boat which is a 'clockwise' engine. If I want to replace it with another make, I shall almost certainly have to replace the propeller because all other makes (afaik) have counterclockwise engines. What a pain.
 
You got me thinking about that one. The reason, I think, is quite simple. A standard motor runs 'clockwise' and, if it is suspended with the shaft pointing downwards...

I think you have the answer.

It seems from this diagram that the shaft/armature is fixed to the ceiling and the blades are attached to the casing/field magnets which rotate..

http://static.ddmcdn.com/gif/ceiling-fan-3.jpg

So when the motor shart is rotating in the standard clockwise direction the casing and fan is rotating counter closkwise.
 
?
The motor is fixed and the shaft rotates, at least on mine. (You'd need slip rings for it to work the other way.) When it rotates in the way that most motor shafts rotate (clockwise, looking out of the motor and the way a drill goes), the blades are angled to push air away. There's your left hand screw.
 

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