6 Why Are Turbine Blades in Groups of 3?

Why are turbine blades in groups of 3?

  • A condition called chatter occurs when a turbine with two blades attempts to yaw.
  • This condition occurs because the moment of inertia of a blade is significantly greater when it is horizontal than when it is vertical to the ground.

 

Moment of Inertia for the blade when horizontal

  • The moment of a two-blade system when it is horizontal to the ground is given by

1/12 ML2 where M is the mass and L is the length of both blades.

  • If an AW-1500 turbine had only two blades, the moment of inertia when the blades are horizontal would be 1/12(11,560 kg)(80.6 meters)2 = 6.26 x 106 kg-meters.

 

Moment of Inertia for the blade when vertical

  • The moment of a two-blade system when it is perpendicular to the horizon is given by
  • 1/2 Mr2 where M is the mass and r is the radius of one of the blades.
  • If an AW-1500 turbine had only two blades, the moment of inertia when the blades are vertical would be 1/2(11,560 kg)(1 meter)2 = 5,780 kg-meters2.
  • Note the 1-meter radius of a blade is an estimate and the blade is not a true cylinder, but this is a useful model.

 

The Difference in Moment of Inertia

  • For a hypothetical 2 blade setup, the horizontal moment of inertia would be 6.26 x 10^6 kg-meters^2, and when vertical would be 5,780 kg-meters2.
  • The horizontal moment is over 1,000 times greater than the vertical moment.
  • This difference causes chatter

 

3 Blades to the rescue!

  • For 3 blades, the moment of inertia is always the same since the x and y components of all the blades balance out each other at any point in its rotation.

 

 

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