Rotor hubs are used to mount the rotor blades of tiltrotor aircraft in predetermined geometric configurations, and also serve to oppose centrifugal forces acting to pull the spinning blades away from a centerline of rotation. Generally, hubs having a radially compact arrangement about the centerline of rotation experience lower relative loads, as the moment arm between system forces and the center of rotation is reduced. Lower loads allow components and other structure to be leaner, resulting in reduced system weight. Additionally, compact hub designs may have lower drag coefficients due to having smaller profiles. However, the extent of compact radial packaging may be limited by physical interferences between components, especially in systems having multiple spinning elements. In such systems, hub components may be arranged at a radius from the centerline of rotation sufficient to provide circumferential space to physically accommodate each spinning element and its given range of motion in a given plane, thereby limiting the compactness of the design.
In addition, rotor blade diameter on tiltrotor aircraft often results from a design compromise between desired vehicle performance in “helicopter mode” (primarily vertical takeoff/landing, hover, and low speed flight) and “airplane mode” (primarily high speed forward flight). Generally speaking, larger diameter rotors offer favorable performance in helicopter mode, but may degrade performance in airplane mode, and vice versa. Improvements in rotor efficiency in helicopter mode may provide for a smaller diameter rotor to be used, thereby potentially improving performance in airplane mode, resulting in overall improved vehicle performance.