Helicopters lift generation

The weight of air displaced by a helicopter = air density x volume of the helicopter. Since the helicopter is nor air tight, this volume will be quite small. The buoyant force will be at the most a couple of kilograms, which is nowhere near lifting the 'copter.

The basic principle behind lift generation is Bernouli's principle. The shape of helicopter blades are designed in the form of aerofoil where the air has to traverse more area on top side compared to bottom side.

As per bernouli's principle, for a flowing fluid the total energy has to be constant. Accordingly, if the velocity of flow of fluid is more and pressure of flowing fluid should be low. Alternately, if the velocity of flow of fluid is less and pressure of flowing fluid should more on bottom side so that total energy remains same. Due to this pressure difference between top and bottom side, lift is created.

Now to generate this lift, fluid has to flow with respect to body or body has to move with respect to fluid. In airplanes, thrust force is generated by engine to move it forward thereby creating fluid flow over wings (aerofoil). A helicopter does not have wings due to which fluid flow analogous to airplanes is not possible. Helicopter is designed with blades shaped in the form of aerofoil. These blades are driven by engines to create sufficient pressure below it so as to lift its own weight.

Now as per Newtons third law, to every action there is an equal and opposite reaction. Accordingly, if the engine rotates the blades, to counteract, the whole helicopter body will rotate in opposite direction. To overcome this, tail rotors are provided which balance the gyroscopic forces created by main rotor and make the helicopter stable and balanced.

Direction control in helicopters is done by changing the pitch angle of tail rotor blades thereby changing the lateral force on its body.

In addition to this, the main rotor blade pitch is also adjustable and can move the helicopter forward, backward or hover.

P.S : Helicopter Lift generation are much more complex involving a number of other forces and principles. I have mentioned only the basic ones so that every lay man can understand. Comments appreciated!!