A rotor operating at a speed higher than (or close to) its first critical speed is termed as a flexible rotor since it undergoes a significant transverse deflection at this speed.

• Unlike a rigid rotor, a flexible rotor cannot be balanced by adding two masses placed in two arbitrarily chosen planes.
  
  
• The principle used for balancing a flexible rotor is entirely different from that applied in the case of a rigid rotor. The objective of attaching the balancing masses to a rigid rotor, as already stated, is to neutralize the unbalanced forces and moments.
  
  
• In a flexible rotor, on the other hand, the balancing masses are attached to suitably modify the dynamic deflection characteristics of the rotor. The technique to do this is known as the modal balancing technique .

Congratulations! You have finished Lecture 6.