Which term describes the upward flexing of rotor blades?

• A. Blade twist
• B. Negative flap
• C. Blade feather
• D. Rotor blade coning

Answer: (D)

Upward flexing of rotor blades is called coning. When the rotor is producing lift, the blades bend upward from the hub because the root is fixed while the tips are free to move. This bending causes the rotor disk to take on a cone shape, with the blade tips extending above the plane of the hub. The amount of coning grows with increased thrust, since more lift pushes the blades upward, and is influenced by the blade’s stiffness and the rotor’s design. This phenomenon is distinct from blade twist, which is a deliberate change of pitch along the blade span, blade feather, which relates to reducing drag by aligning the blade with the airflow, and flap movements in the rotor plane, which are another mode of blade motion not characterized by the whole disk bending upward.