rotorradius

Rotorradius is a concept used in rotor dynamics and rotorcraft engineering to describe the effective radius of a rotor disk that predominantly contributes to the aerodynamic moment or lift distribution. It is not simply the geometric radius of the rotor; rather, it weights parts of the disk by their lift production to indicate where the thrust or torque is effectively produced.

The rotorradius is defined as a lift-weighted average radius of the rotor disk. If L(r) denotes the lift per unit area at radius r on a rotor of radius R, the rotorradius Ra is given by Ra = (∫ from 0 to R of L(r) r^2 dr) / (∫ from 0 to R of L(r) r dr). The integrals account for the area element and the torque contribution (lift times radius). In practical terms, Ra represents the effective location on the disk where the bulk of the aerodynamic moment is generated.

Special cases help illustrate the idea. If lift per unit area is uniform across the disk, Ra equals 2R/3, indicating the middle-to-outboard region dominates torque. If lift increases strongly toward the blade tips, Ra moves closer to R, reflecting greater contribution from outer radii.

Applications of the rotorradius include simplifying rotor design analyses, comparing different blade load distributions, and informing control strategies by characterizing how lift distribution affects torque and stability. It is related to concepts such as center of lift, rotor disc loading, and blade element theory.