Figure 1-97.  The blade tip speed of this helicopter is approximately  300 knots. If the helicopter is moving forward at 100 knots, the  relative windspeed on the advancing side is 400 knots. On the  retreating side, it is only 200 knots. This difference in speed causes  a dissymetry of lift. Figure 1-98.  Fully articulated rotor system. Figure 1-99.  Five-blade articulated main rotor with elastomeric  bearings.  Numerous designs and variations on the three types of main  rotor systems exist. Engineers continually search for ways to  reduce vibration and noise caused by the rotating parts of the  helicopter. Toward that end, the use of elastomeric bearings  in main rotor systems is increasing. These polymer bearings  have the ability to deform and return to their original shape.  As such, they can absorb vibration that would normally be  transferred by steel bearings. They also do not require regular  lubrication, which reduces maintenance. Some modern helicopter main rotors have been designed  with flextures. These are hubs and hub components that are  made out of advanced composite materials. They are designed  to take up the forces of blade hunting and dissymmetry of  down. This lead-lag, drag, or hunting movement as it is  called is in response to the Coriolis effect during rotational  speed changes. When first starting to spin, the blades lag  until centrifugal force is fully developed. Once rotating, a  reduction in speed causes the blades to lead the main rotor  hub until forces come into balance. Constant fluctuations in  rotor blade speeds cause the blades to “hunt.” They are free  to do so in a fully articulating system due to being mounted  on the vertical drag hinge.  One or more horizontal hinges provide for flapping on a  fully articulated rotor system. Also, the feathering hinge  allows blade pitch changes by permitting rotation about the  spanwise axis. Various dampers and stops can be found on  different designs to reduce shock and limit travel in certain  directions.  Figure 1-98  shows a fully articulated main rotor  system with the features discussed.  lift by flexing. As such, many hinges and bearings can be  eliminated from the traditional main rotor system. The result  is a simpler rotor mast with lower maintenance due to fewer  moving parts. Often designs using flextures incorporate  elastomeric bearings.  [Figure 1-99] Ordinarily, helicopters have between two and seven main  rotor blades. These rotors are usually made of a composite  structure. The large rotating mass of the main rotor blades  of a helicopter produce torque. This torque increases with  engine power and tries to spin the fuselage in the opposite  direction. The tail boom and tail rotor, or antitorque rotor,  counteract this torque effect.  [Figure 1-100]  Controlled  with foot pedals, the countertorque of the tail rotor must be  Antitorque System B l a d e r o $_{t}$a t $_{i}$o n B l a d e r o t a t i o n Relative wind Forward Flight 100 knots Relative wind Direction of Flight Advancing Side Retreating Side Blade tip speed  plus helicopter speed (400 knots) Blade tip speed  minus helicopter speed (200 knots) Pitch horn Drag hinge Flap hinge Damper Pitch change axis (feathering) 1-45