# 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

• 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. 