<P> Modern rotor systems may use the combined principles of the rotor systems mentioned above . Some rotor hubs incorporate a flexible hub, which allows for blade bending (flexing) without the need for bearings or hinges . These systems, called "flexures", are usually constructed from composite material . Elastomeric bearings may also be used in place of conventional roller bearings . Elastomeric bearings are constructed from a rubber type material and provide limited movement that is perfectly suited for helicopter applications . Flexures and elastomeric bearings require no lubrication and, therefore, require less maintenance . They also absorb vibration, which means less fatigue and longer service life for the helicopter components . </P> <Ul> <Li> Bell 407 </Li> <Li> Bell 430 </Li> <Li> Eurocopter AS350 </Li> </Ul> <P> Most helicopters have a single main rotor but require a separate rotor to overcome torque . This is accomplished through a variable - pitch antitorque rotor or tail rotor . This is the design that Igor Sikorsky settled on for his VS - 300 helicopter, and it has become the recognized convention for helicopter design, although designs do vary . When viewed from above, the vast majority of helicopter rotors turn counter-clockwise; the rotors of French and Russian helicopters turn clockwise . </P> <P> With a single main rotor helicopter, the creation of torque as the engine turns the rotor creates a torque effect that causes the body of the helicopter to turn in the opposite direction of the rotor . To eliminate this effect, some sort of antitorque control must be used with a sufficient margin of power available to allow the helicopter to maintain its heading and provide yaw control . The three most common controls used today are the tail rotor, Eurocopter's Fenestron (also called a fantail), and MD Helicopters' NOTAR . </P>

Rotor blades start to rotate at what velocity