THE ROTOR

Rotors are shaped like airfoils in order to utilize lift from Bernoulli's principle.  By tilting the nose forward, some of the lift is converted into thrust, driving the helicopter forward; by tilting back, the rotor causes more drag and slows the helicopter down.

Dissymmetry of Lift

Helicopters fly because the rotor spinning is in the shape of an airfoil; Bernoulli's principle is the reason that there is lift on the rotor blades.  However, in helicopters, the rotors tend to be subject to different forces; for example, in a 10 mph wind, the forward moving rotor's speed is reduced but the rearward moving rotor's speed is increased.  Because of the difference in velocity, each rotor experiences different amounts of lift, the helicopter tilts and then it crashes.  To counteract this, helicopter rotors are mounted so that they can flap in the wind, either by mounting the blades so that they can flap on a set of hinges (Articulated rotor system), or so that the whole mount can flap (Semi-rigid rotor system).

TAIL ROTOR

The tail rotor controls the helicopter's lateral movement (left and right).  It works like the propeller on an airplane, pushing against off the air.  The tail rotor provides a great deal of torque, as it spins six times faster than the main rotor.

The tail rotor tends to cause the helicopter to drift in hover because it pushed in one direction; the helicopter must tilt itself in hover to diminish this force.  This is called hanging "Left skid low". 

TANDEM ROTORS

When two rotor systems are used on a helicopter as in a Chinook, the need for a tail rotor is eliminated.  The helicopter's maneuvering is based on how the rotors are tilted.  When both are level, the helicopter hovers.  If both are tilted, the helicopter moves in the direction of the tilt.  If only one rotor tilts, the helicopter pivots around the level rotor.  If they are tilted in opposite directions, the helicopter rotates around the center of the helicopter.

The rotors rotate in opposite directions; this is called counter-rotation.  The motors need to be in sync because the two rotors cannot mesh.

For a comprehensive source on helicopter flight, visit www.helicopterpage.com