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I have noticed that most of the motor with blades and engines rotate anti-clockwise direction when viewed from front facing blade. Is there any specific reason for this? Is it because of any kind of force acting? Or simply coincidence?

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    $\begingroup$ Define "front." Your question is ill-formed. $\endgroup$ – Carl Witthoft Jan 27 '14 at 12:32
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    $\begingroup$ In addition to what Carl wrote, I suspect that it's just design coincidence. It's not like they can't make motors or fans that spin the other way. $\endgroup$ – DumpsterDoofus Jan 27 '14 at 13:43
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    $\begingroup$ On some two-propeller aircraft, the two propellers deliberatey rotated in opposite direction to cancel out gyroscopic forces when the aircraft turns. This is a very real effect in single-engine aircraft. $\endgroup$ – Olin Lathrop Jan 27 '14 at 14:07
  • $\begingroup$ possible duplicate of Direction of rotation of a celing fan. If it isn't, my answer to that question should also apply here. $\endgroup$ – Colin McFaul Jan 28 '14 at 3:35
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This is the same idea as the direction of rotation of a ceiling fan or a box fan. Motor fans work by pulling air in from one direction, and pushing it out the other direction. Which side is the "in" side, and which is the "out" side, is determined by the direction of rotation of the blades and by the direction of the pitch of the blades. Most fans have their blades pitched so as to follow a "right-hand rule." If you stand in front of the fan with the fan blowing towards you, hold your right hand in front of you, and curl your fingers so they curl in same direction as the rotation of the blades, your thumb will be pointing towards you. There is nothing physically inherent about this direction: if you reverse the pitch of the blades, you will reverse the airflow without reversing the rotation of the fan. For most fans, it's just a convention that they do this.

There is one situation where inherent physical considerations affect the choice, as Olin Lathrop alludes to in a comment. Motor fans can be quite heavy and can rotate quite fast. When they do, they produce a huge angular momentum, which can make steering difficult. Using two fans pitched in opposite directions allows you to have both fans push air the same direction, and rotate in opposite directions. You then have zero or very little net angular momentum, and easier steering.

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Sense of rotation of the fan is arbitrary. Consistency of sense allows one pitch of blade and one winding of the motor to be default compatible. Consider threads on nuts and bolts, lightbulbs and sockets. Viewed "going away from," right-handed is almost universal (e.g., left-threaded bulbs in the New York City subway to deter theft). On large scales (hurricanes, typhoons; Foucault pendulum, ring laser gyroscope) sense of rotation is not arbitrary because the spinning Earth (counterclockwise as viewed from the North Pole) is a biased non-inertial frame of reference. Flushing toilets are unaffected.

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Id say Its because they are designed to be used right handed so they throw sparks down or away from the user. Also when cutting downwards they pull away giving a constant pressure however when used left handed cutting down they jump and skip backwards towards the user.

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