Design and analysis of variable-twist tiltrotor blades using shape memory alloy hybridcomposites

Abstract

The tiltrotor blade, or proprotor, acts as a rotor in the helicopter mode and as a propellerin the airplane mode. For a better performance, the proprotor should have different built-intwist distributions along the blade span, suitable for each operational mode. This paperproposes a new variable-twist proprotor concept that can adjust the built-in twistdistribution for given flight modes. For a variable-twist control, the present proprotoradopts shape memory alloy hybrid composites (SMAHC) containing shape memory alloy(SMA) wires embedded in the composite matrix. The proprotor of the KoreaAerospace Research Institute (KARI) Smart Unmanned Aerial Vehicle (SUAV),which is based on the tiltrotor concept, is used as a baseline proprotor model.The cross-sectional properties of the variable-twist proprotor are designed tomaintain the cross-sectional properties of the original proprotor as closely aspossible. However, the torsion stiffness is significantly reduced to accommodatethe variable-twist control. A nonlinear flexible multibody dynamic analysis isemployed to investigate the dynamic characteristics of the proprotor such as naturalfrequency and damping in the whirl flutter mode, the blade structural loads ina transition flight and the rotor performance in hover. The numerical resultsshow that the present proprotor is designed to have a strong similarity to thebaseline proprotor in dynamic and load characteristics. It is demonstrated thatthe present proprotor concept could be used to improve the hover performanceadaptively when the variable-twist control using the SMAHC is applied appropriately.