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Why air flow created by ceiling fan goes downward?

  1. Nov 10, 2012 #1
    Why air flow created by ceiling fan (which rotates counter clockwise (seen from beneath ceilling) ) goes downward?
     
  2. jcsd
  3. Nov 10, 2012 #2
    Mine are reversable, so the air could go either way. Look at the pitch on the blades and compare it to an airplane propeller.
     
  4. Nov 12, 2012 #3

    sophiecentaur

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    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?")
     
  5. Nov 12, 2012 #4
    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.
     
  6. Nov 12, 2012 #5

    CWatters

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    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 thats 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.
     
  7. Nov 12, 2012 #6
    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 noone 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.
     
  8. Nov 13, 2012 #7

    sophiecentaur

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    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.
     
  9. Nov 13, 2012 #8

    CWatters

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    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.
     
  10. Nov 13, 2012 #9

    sophiecentaur

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    ???
    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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