Untwist of a turbine blade is caused by which condition?

• A. Centrifugal force on the blade
• B. Thermal cycling
• C. From the gas loads upon the surfaces
• D. Mechanical wear

Answer: (C)

Untwist happens because the hot gas flowing over the blade surfaces applies a pressure distribution that does not act through the blade’s axis. This creates a torsional moment along the blade span. Since the blade is flexible, that moment twists the cross-sections and reduces (untwists) the blade’s built-in twist as it carries the gas load. In other words, the aerodynamic loads on both the pressure and suction sides generate a torque that changes the twist along the blade.

Centrifugal force from rotation tends to bend the blade radially but doesn’t produce the torsional effect needed for untwisting. Thermal cycling can cause distortion from uneven heating, but the direct cause of untwist under operating conditions is the gas pressure distribution on the surfaces. Mechanical wear affects stiffness and fit but isn’t the primary driver of twist change during operation.