Untwist of a turbine blade is caused by?

• A. Gas loads on the blade surfaces
• B. Thermal gradients
• C. From the gas loads upon the surfaces
• D. Centrifugal forces on the blade root

Answer: (C)

Untwist is driven by the torques that arise from the gas pressures acting on the blade surfaces. The aerodynamic loading on the suction and pressure faces creates a moment about the blade root, changing the blade’s twist during operation. This direct moment from the gas loads on the surfaces is what reduces or alters the pre-set twist—the untwisting effect.

Thermal gradients can cause distortion due to differential expansion, but they are not the primary cause of untwist. Centrifugal forces mainly influence radial stresses and blade bending rather than the twist along the blade’s length.