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Thread: How rotational fan blade movements push the air forward?

  1. #1 How rotational fan blade movements push the air forward? 
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    Fan blades are moving in a rotaional way. The blades are oblique. When propeller blade is going down (mostly down) it collides with air. Why is the air going forward (red arrow) so we feel breeze in front of a fan instead of going in the way the blade moves (green arrow) ?

    (forum doesn't allow me posting image directy so I give a link without http at the beginning)
    i.imgur.com/j5jn7Od.png


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  3. #2  
    exchemist
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    Quote Originally Posted by bgfbgf View Post
    Fan blades are moving in a rotaional way. The blades are oblique. When propeller blade is going down (mostly down) it collides with air. Why is the air going forward (red arrow) so we feel breeze in front of a fan instead of going in the way the blade moves (green arrow) ?

    (forum doesn't allow me posting image directy so I give a link without http at the beginning)
    i.imgur.com/j5jn7Od.png
    Think of a cricket ball coming into contact with an angled bat. Or the direction of light reflection from an angled mirror. If something rebounds from a surface at 45 degrees to the direction of its travel it will do so at right angles to its former direction.

    In this case the obliquely angled blade hits the air obliquely and causes it to bounce off in a direction perpendicular the the direction of its motion.


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  4. #3  
    Forum Cosmic Wizard
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    Quote Originally Posted by bgfbgf View Post
    Fan blades are moving in a rotaional way. The blades are oblique. When propeller blade is going down (mostly down) it collides with air. Why is the air going forward (red arrow) so we feel breeze in front of a fan instead of going in the way the blade moves (green arrow) ?
    Because it acts like an airfoil instead of a piston. The motion of the blade causes relative wind around the blade. This generates lift, which displaces air towards the "bottom" of the airfoil. (There's also some motion in the direction of the green arrow, but in a well designed fan, most of the air comes from the lift effect described.)
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