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Why does a plane's fan push air backward instead of forward?

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And how does fan push air in any direction? shouldn't air molecules be pushed in the same plane as that of fans blade instead of going in and out of the plane?

How do we determine which direction the air will flow?

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The blades are at an angle so as to push the air backwards. An easy way to feel/see this at home: next time you're in the car, put your hand out the window with your fingers pointing perpendicularly to the car (has to be a relatively high speed to feel this), and with your fingers together. Now twist your wrist and try having your hand at different angles. You'll notice that at some angles your hand is push upwards, and at different angles, it's pushed downwards. Since your hand is being pushed up/down then it is pushing air down/up (newton's law). This is the same principle as in the blades of the airplane, except they just turn really fast in a circle to produce this force, whereas your hand is moving linearly.

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