Resonant Passive Energy Balancing of Morphing Helicopter Blades with Bend-Twist Coupling

Abstract

With increasing demand for rotor blades in applications such as wind turbines, helicopters, and unmanned aircrafts , improving the performance of such structures using morphing blades has received considerable attention. Resonant passive energy balancing is a relatively new concept introduced to minimize the required actuation energy. This study investigates resonant passive energy balancing ( RPEB ) in morphing helicopter blades with lag-twist coupling. The structure of a rotating blade with a moving mass at the tip is considered under aerodynamic loading. The aeroelastic behaviour of this structure includes potentially significant nonlinearities arising from the nonlinear elements of the structure and nonlinear aerodynamic loading. These nonlinearities make the design process complicated, and hence it is important to fully understand this system’s nonlinear dynamic behaviour. A reduced order model of the structure with three degrees of freedom ( 3DOF ), including the pitch angle and lagging of the blade, along with the motion of the moving mass, is used to analyse the dynamics of the structure. First, a single-degree-of-freedom ( SDOF ) model for the pitch angle dynamics of the blade is studied to examine the effect of important parameters on the pitch response. In this SDOF model, the harmonic excitation due to moving mass and the aerodynamic forces are considered. The results demonstrate that the coefficient of lag-twist coupling and the direction of aerodynamic moment on the blade are two parameters that play important roles in controlling the pitch angle, particularly the phase. Then, neglecting the aerodynamic forces, the 3DOF system is studied to investigate the sensitivity of the dynamics of the structure to changes in the parameters of the system. The results of the structural analysis can be used to tune the parameters of the blade in order to use the resonant energy of the structure and to reduce the required actuation force. A sensitivity analysis is then performed on the dynamics of the 3DOF model of the blade in the presence of aerodynamic forces to investigate the controllability of the amplitude and phase of the pitch angle using control parameters. The results show that the bend-twist coupling and the distance between the aerodynamic centre and the rotation centre (representing the direction and magnitude of aerodynamic moments) play significant roles in determining the pitch dynamics.