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thats simple (as compared to your other questions) wind blows from high pressure to low pressure. When the blades of the helicopter rotate , it pulls air above it and pushes it down creating high pressure below it as compared to the pressure above. the helicopter starts flying as the wind blowing from down to up is enough to take its weight. After taking off it contiues the same process. Pushes air in the direction opposite to the direction of motion.
With Sriram's answer, consider Newton's 3rd law. That should hopefully make sense.
i have a doubt in ur explanation sriram... if the blades pulls the air above and pushes it down to create the necessary pressure to lift the helicopter.. the air that is pushed down wil not strike the body of helicopter ???..
Don't think of helicopter lift any differently than an airplane's lift--the only difference is how they get air to go over the wing surface. For an airplane, the engine propels the plane forward and thereby allows air to flow; in a helicopter, the engine powers the blades (essentially a wing) to spin to generate airflow over its surface. Now, for the purpose of simplification, consider the wing of an airplane and a helicopter to be equal for the duration of the explanation. The shape of the wing is designed to split the airflow, one flowing over the top, is faster and thus has "less-pressure", the second, flowing underneath is slowed and thus is "high-pressure" (the design of the wing is based on Bernouli's principle concerning fluid dynamics). So, you have a curved wing that has a low-pressure column of air above and a high-pressure column of air below: enter Newton! Newton's third law says that for any reaction there is an equal and opposite reaction, therefore, when the "wing" presses down on the air below it, it receives an equal and opposite reaction; however, since the column of air above the wing has less mass, the wing "lifts" upward. In summary, all you need for lift is airflow over a winged surface designed to split that airflow into a lower pressure body above and higher-pressure body below the wing. The airflow is taken care of by a motored-driven linear movement in an airplane or a motored-driven rotational movement in a helicopter. The lower/higher pressure part is taken care of by the wing's design. That's lift, it's a large text for explanation but the idea is so, so, so simple. P.S. When you punch a ball, the ball actually responds back at you with an equal force that you struck the ball, but, since you weigh so much more than the ball, your "backward" movement is unnoticed. Think of the wing as "punching" down on the air below, but, the air below is so much "heavier" and generates an opposite reaction back up which the wing does easily because the air above is so much "lighter".
thanks a lot for this explanation... its been a while i come to this site.... anyway nice to get a convincing answer....