Coning is an upward sweeping angle of the rotor blades as a result of lift and centrifugal force.
Centrifugal force is caused by blade rotation. This force pulls the rotor blades horizontally and provides rigidity to the blades. The faster the rotation of the blades, the more centrifugal force. In contrast, lift acts perpendicular to airflow or resultant relative wind. The lift generated by a rotor blade increases from the root to the tip. The coning angle increases when more lift is generated as compared to centrifugal force.
Conversely, the coning angle decreases when the centrifugal force increases as compared to the lift generated. When a helicopter transitions from the ground to a hover, the increase in coning angle is easy to see. There are several flight conditions that effect the coning angle. Lower rotor RPM reduces the centrifugal force, which results in an increase in coning angle if the lift requirement remains the same. If the centrifugal force remains the same, the coning angle will increase with an increase in lift. High gross weight and high-G maneuvers require more lift.
With low rotor RPM, a dangerous situation can result when the blades cone due to the inadequate centrifugal force. The blades can cone to a level where it is unable to support the helicopter’s weight.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-15
Principles of Helicopter Flight, 2nd Edition, pg. 54, 85