System Level Benefits of a Variable Diameter Rotor System for a Conceptual Tiltrotor Design

This work presents a conceptual variable diameter tiltrotor sized in the NDARC conceptual design code. Both the maximum allowable diameter change and the weight increment due to morphing were varied with the intent of investigating the system level benefits (improved weight, fuel use, etc.) associated with a variable diameter rotor system. Generally, for a 0% increment in rotor weight due to morphing, diameter morphing reduced vehicle gross and component weights, as well as mission fuel weight. As the rotor weight penalty for morphing was increased, the entire system saw a corresponding increase in weight. For these increased rotor weight conditions, diameter morphing was again able to reduce total system weight back to, or lower than, the baseline design. At very high levels of morphing however, (large reductions in rotor radius for cruise) weight was seen to drastically increase. This was traced to the rotor approaching stalled conditions during cruise due to increased blade loading. A conceptual approach for estimating cruise blade loading for a variable diameter or variable speed tiltrotor was then presented. In general, this work shows that even for fairly substantial increases in rotor weight, variable diameter morphing can have a system wide benefit in the form of reduced gross weight and reduced fuel weight for the given design mission.