14 CFR § 27.547

Each main rotor assembly (including rotor hubs and blades) must be designed as prescribed in this section.

[Reserved]

The main rotor structure must be designed to withstand the following loads prescribed in §§ 27.337 through 27.341:

Critical flight loads.

Limit loads occurring under normal conditions of autorotation. For this condition, the rotor r.p.m. must be selected to include the effects of altitude.

The main rotor structure must be designed to withstand loads simulating—

For the rotor blades, hubs, and flapping hinges, the impact force of each blade against its stop during ground operation; and

Any other critical condition expected in normal operation.

The main rotor structure must be designed to withstand the limit torque at any rotational speed, including zero. In addition:

The limit torque need not be greater than the torque defined by a torque limiting device (where provided), and may not be less than the greater of—

The maximum torque likely to be transmitted to the rotor structure in either direction; and

The limit engine torque specified in § 27.361.

The limit torque must be distributed to the rotor blades in a rational manner.