Resonant passive energy balancing of morphing helicopter blades with bend–twist coupling

With increasing demand for rotor blades in engineering applications, improving the performance of such structures using morphing blades has received considerable attention. Resonant passive energy balancing (RPEB) is a relatively new concept introduced to minimize the required actuation energy. This study investigates 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. To this end, a three-degree-of-freedom (3DOF) reduced-order model is used to analyse and understand the complicated nonlinear aeroelastic behaviour of the structure. This model includes the pitch angle and lagging of the blade, along with the motion of the moving mass. First, the 3DOF model is simplified to a single-degree-of-freedom model for the pitch angle dynamics of the blade to examine the effect of important parameters on the pitch response. 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 its dynamics 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 in the presence of aerodynamic forces to investigate the controllability of the amplitude and phase of the pitch angle. 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.

Stroboscopic aliasing in long-range interacting quantum systems

We unveil a mechanism for generating oscillations with arbitrary multiplets of the period of a given external drive, in long-range interacting quantum many-particle spin systems. These oscillations break discrete time translation symmetry as in time crystals, but they are understood via two intertwined stroboscopic effects similar to the aliasing resulting from video taping a single fast rotating helicopter blade. The first effect is similar to a single blade appearing as multiple blades due to a frame rate that is in resonance with the frequency of the helicopter blades' rotation; the second is akin to the optical appearance of the helicopter blades moving in reverse direction. Analogously to other dynamically stabilized states in interacting quantum many-body systems, this stroboscopic aliasing is robust to detuning and excursions from a chosen set of driving parameters, and it offers a novel route for engineering dynamical n-tuplets in long-range quantum simulators, with potential applications to spin squeezing generation and entangled state preparation.

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

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.

Augmentation of low frequency damping via hydrogen-doping in a hybrid shape memory alloy composite

Carbon fiber reinforced polymer (CFRP) composites hybridized with hydrogen-doped NiTi wires can be used to design structures requiring high stiffness and high damping in the low frequency range, such as helicopter blades. The current work investigates aging and hydrogen-doping for high damping without hydrogen embrittlement. We establish a hydrogenation treatment that (i) results in a response that is repeatable in the martensitic phase and after exposure to composite processing temperatures and (ii) increases the loss factor in NiTi wires by nearly 470%. By embedding H-doped wires exhibiting the highest damping into the interlayers of a [0/±45]s carbon/epoxy laminate at a volume fraction of 0.1, the hybrid NiTi-CFRP composite loss factor increases by 170%. The measured dynamic properties were found to be close to micromechanical predictions based on the properties of the NiTi and CFRP.

Geometrically exact, intrinsic mixed variational formulation for smart, slender multilink composite structures

Flexible links are often part of massive aerospace structures like helicopter or wind turbine blades, satellite bae, airplane wings, and space stations. In the present work, a mixed variational statement based on intrinsic variables is derived for multilinked smart slender structures. Equations involved in the derivation do not involve approximations of kinematical variables to describe the deformation of the reference line or the rotation of the deformed cross-section of the slender links resulting in a geometrically exact formulation. Finite element equations are derived from weak formulation, which can analyze large geometrically non-linear problems. The weakest possible variational statement provides greater flexibility in the choice of shape functions, therefore reducing the associated numerical complexities. The present work focuses on developing a single integrated computational platform which can study multibody, multilink, lightweight composite, structural system built with both embedded actuations, sensing, as well as passive links. Validation of static mechanical and electrical outputs from 3D FE simulation and literature proves the efficacy of the computational platform. Dynamic results will be communicated in future correspondence. The computational platform developed here can be applied for monitoring and active control applications of flexible smart multilink structures like swept wings, multi-bae space structures, and helicopter blades.

Node-Dependent Kinematics and Multidimensional Finite Elements for the Analysis of Single/Double Swept, Composite Helicopter Blades

This paper deals with finite element analysis of helicopter blades with single and double swept geometries made by metallic and composite materials. First, classical and refined beam theories are combined at the element level via a node-dependent kinematic (NDK) concept, which was recently introduced by the authors. Such an NDK approach enables the accuracy/efficiency ratio of the solution to be tuned according to the level of fidelity required by the design phase. Second, one-dimensional NDK models are combined with the possibility to introduce solid elements in those regions of the blade with a sharp variation of the geometries. The numerical examples consider a swept-tip rectangular beam and a double-swept helicopter blade with a realistic airfoil. Natural frequencies and through-the-layer stress distributions are reported to demonstrate the flexibility and computational efficiency of the proposed methodology.

Effective Static and Dynamic Finite Element Modeling of a Double Swept Composite Rotor Blade

The paper concerns mechanical responses of helicopter blades made of composite materials. Structures with complicated geometries are modeled by using both beam and solid finite elements. The adopted one-dimensional kinematics only encompasses pure displacements; therefore, the connection with three-dimensional elements can be carried out with ease. Contributions to elastic and inertial matrices deriving from nodes shared by beams and solids are merely summed together through a standard assembling procedure. Stress, free vibration, and time response analyses have been performed on different configurations. A straight metallic rotating structure and a swept-tip blade made of an orthotropic material have been considered for verification and validation purposes. Current results have been compared with experimental data and numerical solutions available in the literature. Furthermore, a straight and a double-swept blade with a realistic airfoil have been studied. For the straight configuration, the one-dimensional results have been compared with finite element solutions obtained with commercial software. The methodology enabled complicated stress distributions and coupling phenomena to be predicted with reasonable accuracy and affordable computational efforts.