Is it possible to successfully land a heli, if engine fails at mid air/or during flight?
I read some article somewhere depicting its possibility but now i cant get it.
How?

there is something called as "autorotation"...!!

check this for full details....
http://en.wikipedia.org/wiki/Autorotation_(helicopter)

thanks vishvesh.. im not into helis, so cant get this simple thing so i am asking agan..

Does the main rotors spin in opposite direction in case of autorotation.In this case,doesnt airfoil create any hindrance?  .

what is collective pitch in case of helicopters?

Or is it possible with helicopters with inverted pitch (like used in marine helis to produce downward thrust to stuck firmly to unstable deck of small ships).?

its not "rocket science" its quite simple... but difficult for me to explain

Quotein normal, powered flight, air is drawn into the main rotor system from above and exhausted downward, but during autorotation, air moves up into the rotor system from below as the helicopter descends. Autorotation is permitted mechanically because of both a freewheeling unit, which allows the main rotor to continue turning even if the engine is not running, as well as curved main rotor blades such that when the collective pitch is fully down the inner part of the blade has negative pitch relative to the horizontal plane and can be spun up by the relative wind. It is the means by which a helicopter can be landed safely in the event of complete engine failure.

anwar bhai or some hardcore heli guy can explain in simple words.... thats all from my side and my understanding....

In simple terms

when heli has power Rotor disk of heli has positive pitch and the heli is upright, and the air is blown downwards as a result the heli moves up, now when the engine fails we tend to give full negetive collective pitch on heli so it comes down faster blowing air upwards (marine heli style) and this way the rotor gets more inershia power or kinetic energy and once very close to ground we give full positive and use all the power to stop and land the heli safe and sound. like air brakes. I hope that made some sense

@ Hellyflyer : Thanks for explaining it clearly

Hellyflyer is right, except for the "full negative pitch" part on RC helis. 

One of the things every heli flyer (both scale and RC) should practice is the autorotation maneuver. I have saved RC helis at least 10 times from serious trouble with autorotation, and in every one of those cases, without proper autorotation those helis would have sustained serious damage.

I also try to do at least couple of auto-rotations at the end of any heli flight on a 500 size or higher heli.

More on the pitch used for autorotation, at least for RC helis, see this thread :

http://www.rcindia.org/helis/learning-auto-rotations/

  all especially hellyflyer.. got it

thanks abhay

The link which Anwar sir provided has really good detailed information, please do go through it.