Active Twist Rotor Research Articles

다중 사이클릭 헬리콥터 진동 제어

The vibration control performance of an active twist rotor using multicyclic control is evaluated and discussed. Artificial flight data for a descent flight with 6-degree flight path angle are generated using CAMRAD II. A linear, quasi-static, frequency domain system model with six multicyclic higher harmonic control inputs and 12 harmonic response outputs of nonrotating hub loads is identified offline by using the least squared error. The optimal control input for minimizing the quadratic performance function and the corresponding response to the optimal control are calculated. The open-loop control performance is simulated using CAMRAD II by applying the optimal control input. The nondimensionalized vibration index combining 12 nonrotating hub loads is reduced by 93 % using open-loop control. The gradient descent method is applied for closed-loop control, and MATLAB/CAMRAD II coupled analysis is performed to evaluate the closed-loop multicyclic control system. The closed-loop control using the gradient descent algorithm with the system model identified offline shows very good vibration reduction performance, and the reduced vibration level converges to the optimal solution.

Optimal Segment Control of Active Twist Rotor for Power Reduction in Forward Flight

The segment control of active twist rotor is investigated to evaluate the effectiveness in rotor power reduction. A numerical model for predicting the isolated rotor power and loads in steady level flights is deployed and validated. A parametric sweep of the amplitude and phase angle for uniform single-harmonic active twist control is conducted to demonstrate the mechanism of active twist control in rotor power reduction. The optimal control schedules and segment layouts of the segment twist control for power reduction while considering saturation limits are obtained using an optimization framework based on genetic algorithm. Up to 5-seg configuration is considered. The results indicate that the segment twist control reduces the rotor power more than the uniform twist control by applying divergent control schedules to each segment. The load distribution of the rotor disk is harmonized in both circumferential and spanwise directions. The 2-seg and 3-seg control configurations are appropriate, while the configurations with more segments yield limited benefits and they may be penalized with an increase in system complexity.

Optimization of an Active Twist Rotor Blade Planform for Improved Active Response and Forward Flight Performance

A study was conducted to identify the optimum blade tip planform for a model-scale active twist rotor. The analysis identified blade tip design traits which simultaneously reduce rotor power of an unactuated rotor while leveraging aeromechanical couplings to tailor the active response of the blade. Optimizing the blade tip planform for minimum rotor power in forward flight provided a 5 percent improvement in performance compared to a rectangular blade tip, but reduced the vibration control authority of active twist actuation by 75 percent. Optimizing for maximum blade twist response increased the vibration control authority by 50 percent compared to the rectangular blade tip, with little effect on performance. Combined response and power optimization resulted in a blade tip design which provided similar vibration control authority to the rectangular blade tip, but with a 3.4 percent improvement in rotor performance in forward flight.

Optimal deployment schedule of an active twist rotor for performance enhancement and vibration reduction in high-speed flights

The best active twist schedules exploiting various waveform types are sought taking advantage of the global search algorithm for the reduction of hub vibration and/or power required of a rotor in high-speed conditions. The active twist schedules include two non-harmonic inputs formed based on segmented step functions as well as the simple harmonic waveform input. An advanced Particle Swarm assisted Genetic Algorithm (PSGA) is employed for the optimizer. A rotorcraft Computational Structural Dynamics (CSD) code CAMRAD II is used to perform the rotor aeromechanics analysis. A Computation Fluid Dynamics (CFD) code is coupled with CSD for verification and some physical insights. The PSGA optimization results are verified against the parameter sweep study performed using the harmonic actuation. The optimum twist schedules according to the performance and/or vibration reduction strategy are obtained and their optimization gains are compared between the actuation cases. A two-phase non-harmonic actuation schedule demonstrates the best outcome in decreasing the power required while a four-phase non-harmonic schedule results in the best vibration reduction as well as the simultaneous reductions in the power required and vibration. The mechanism of reduction to the performance gains is identified illustrating the section airloads, angle-of-attack distribution, and elastic twist deformation predicted by the present approaches.

Synthesis of Active Twist Controller for Rotor Blade–Vortex Interaction Noise Alleviation

This paper deals with an efficient computational process for the synthesis of a low-frequency controller aimed at alleviating helicopter rotor blade–vortex interaction noise. It is based on an optimal, multicyclic, control approach that exploits distributed torque loads actuated by smart materials to twist rotor blades at the 2/rev frequency (active twist rotor concept). Aeroacoustic rotor simulations are based on aerodynamic and aeroelastic tools capable of accurately predicting wake–blade miss distance, which plays a crucial role in blade–vortex interaction noise emission. Its modification is the main objective of the proposed controller action. The control law is identified through a numerically efficient aeroacoustic solver based on analytical–numerical sectional aerodynamics modeling. Numerical investigations first examine noise emission sensitivity to active twist actuation, then identify control and output variables suitable for the closed-loop controller. Finally effectiveness of the proposed low-frequency feedback control law for blade–vortex interaction noise alleviation is assessed by application to helicopter rotor in descent flight.

Structural modeling and validation of an active twist model rotor blade

DLR has been researching on active twist rotor blade control for at least 15 years now. This research work included the design and manufacturing of model rotor blades within the blade skin integrated actuators. As a main subject, numerical benefit studies with respect to rotor noise, vibration, and performance were carried out with DLR’s rotor simulation code S4. Since this simulation code is based on a modal synthesis, it uses the natural blade frequencies and mode shapes to model the blade dynamics. Both, natural blade frequencies and mode shapes, are computed in advance employing a finite element beam model of the blade. Each beam element possesses certain structural properties that are derived from an ANSYS model for certain cross sections of the blade. Since model rotor blades are built for wind tunnel testing, they are highly instrumented with sensors and therefore vary in their structural properties along span. Modifications in the structural properties due to the instrumentation are not included in the ANSYS model. However, to account for these variations, two experimental methods have been developed. They allow the determination of the real values for the most important structural blade properties such that the structural blade model is improved. The paper describes the experimental methods, as well as the development of an advanced structural blade model for rotor simulation purposes. It shows a validation of the structural blade model based on the measured non-rotating and rotating frequencies.

On-Blade Control of Rotor Vibration, Noise, and Performance: Just Around the Corner? The 33rd Alexander Nikolsky Honorary Lecture

A concise review of the active control approaches for vibration reduction in rotorcraft is presented. Next, the evolution and status of higher harmonic control and pitch link actuated individual blade control is presented since these serve as the foundations of on blade control. Despite the success of these approaches, demonstrated by both full-scale wind tunnel and flight tests, higher harmonic control and pitch link actuated individual blade control have not managed to earn their way onto a production helicopter. An alternative, on blade control, is defined as a special implementation of individual blade control, where the control surfaces are located on the rotating blade and each blade has its own controller. A concise description of four on blade devices: (1) the actively controlled flap, (2) the active twist rotor, (3) the active tip rotor, and the (4) deployable Gurney flap, or microflap, is presented. An outline of an aeroelastic response modeling capability used to simulate active vibration and noise reduction using flaps or microflaps is presented. The simulation is a thread that links the various parts of the paper. Next, selected results from simulations and scale wind tunnel model tests on active flaps are used to provide insight on the operational and modeling aspects of these systems. Full-scale wind tunnel and flight tests are presented as culmination of the research effort invested in active flap rotors. Then, the evolution and application of the active twist rotor, and deployable Gurney flaps, or microflaps, is presented. The paper concludes with lessons learned and speculation about the potential implementation of on blade control on production rotorcraft.

New Optimization Strategy for Design of Active Twist Rotor

This paper presents the development of a mixed-variable optimization framework for the aeroelastic analysis and design of active twist rotors. Proper tailoring of the blade properties can lead to the maximization of the active twist and the control authority for vibration reduction under operating conditions. Thus, using mathematical optimization, the cross-sectional layout is designed using continuous and discrete design variables for an active composite rotor blade to maximize the dynamic active twist while satisfying a series of constraints on blade cross-section parameters, stiffness, and strength. The optimization framework developed includes the Intelligent Cross-Section Generator as the cross-section and mesh generator, University of Michigan/Variational Asymptotic Beam Sectional analysis code for active cross-sectional analysis, and Rotorcraft Comprehensive Analysis Software for aeroelastic analysis of the active twist rotor blade. The optimization problem is solved using a surrogate-based approach in combination with the Efficient Global Optimization algorithm. In this paper, the results with mixed design variables are obtained with three different techniques and are compared with the results obtained using continuous design variables.

A new concept to determine the mass distribution of an active twist rotor blade

In this paper a new method for an experimental determination of the mass distribution of rotor blades is presented. This mass distribution measurement method is developed for the testing of on active twist blades, but it is applicable to the mass property determination of any type of rotating blade. Usually X-ray computer tomography (CT) is used for non-destructive inspection, such as to find voids in composite components. In this research CT is used to get information regarding the spatial distribution of mass in a rotor blade. The CT-analysis of an active twist rotor blade and the implemented determination of the mass distribution are presented in this paper. Image reconstruction due to X-ray computer tomography makes possible to obtain a cross-section image of the rotor blade. Each pixel of this image has a CT-Number which is proportional to the attenuation coefficient of the material forming the blade. Due to the fact that the attenuation of X-ray is characteristic for each material, it is theoretically possible to correlate CT-Numbers with material properties. This method is “quasi”-experimental because the attenuation coefficient for each pixel is experimentally determined and then densities are assigned to these pixels. Once that correlation is done, calculating mass properties such as total mass, mass per length, local center of mass and local moment of inertia for each cross section is possible.

Measurement of twist deflection in active twist rotor

Since the early days of rotary wing aircrafts the unsteady flow conditions in the rotor disk and unbalanced masses are causing intense vibratory loads and noise. Even modern helicopters still suffer from these drawbacks. This diminishes the ride quality as well as the public acceptance of rotary wing aircrafts. Further on the vibrations present a physical load for the pilot which reduces his concentration and can therefore not be neglected. Hence in the past there were numerous attempts to overcome these drawbacks. One of the most promising approaches evolved from a secondary control of the swash plate to an additional pitch angle of individual rotor blades applied at the blade root and further on to the control of the individual deformation of blades. The last step in this evolution became an option when smart materials like piezoceramics were used as actuators in the passive structure of traditional rotor blades. Since the actuators and their field of application are quite new, their capability to perform under strong centrifugal loads has to be proven. Within this work one concept for the active deformation of rotor blades will be presented. The evaluation of active twist blades under centrifugal loads is quite a challenge. Ether the data has to be acquired in the rotating frame and transmitted outside or the data has to be captured from outside, which demands a great deal of the sensors due to the high speed of the rotation. This investigation focuses on the design and evaluation of different measuring concepts to deliver reliable data of the blade actuation in the rotating system under full centrifugal loads. Most interesting is the determination of the blade tip twist angle.

Identification and optimal control methodology for an active twist rotor

AbstractONERA — French Aerospace Lab — developed an innovative rotor concept, featuring two independent piezo-electric actuators embedded within each blade. This patented concept allows a dynamic modification of the blade twist during a rotor revolution. The main goal is to reduce drastically the vibrations transmitted to the helicopter mast and structure, thus reducing simultaneously the fatigue and maintenance needs of the structural elements in the long term, as well as the discomfort felt by pilot and passengers. In order to ensure the stabilization of the rotor, as well as the effective decrease of rotor vibrations, a closed-loop control scheme is currently developed and simulated by ONERA Systems Control and Flight Mechanics Department (ONERA/DCSD). An adaptive, Kalman-filter based, estimation of the open-loop transfer is made, then a linear quadratic optimal control gain is calculated in order to ensure the convergence towards the objective values. During the initial part of the project, this control law is simulated through a coupling with a rotor calculation code, and the numerical results show an effective reduction of the various vibration components. After these validation steps, the control law has been implemented in a real-time environment and will be used for the wind-tunnel test phase of the active rotor.

Modelling and dynamic stability of a hingeless active fibre composite blade

The main focus of this investigation was to investigate dynamic stability of what is now commonly referred to as an active twist rotor blade. With the ultimate intention of controlling helicopter blade vibrations, active or smart materials embedded in the rotor blade in various arrangements as opposed to the conventional passive isolation devices and absorbers are used. Structurally integrated interdigitated piezoelectric fibre composite material is considered in this work. This active composite may be used in constructing the blade in place of or concurrent with, composite blades. This fibrous composite is able to twist the blade, when subjected to an electric field, due to its unique arrangement of active fibres. The paper begins with outlining the design for lay-up sequence of the active blade. This is followed by developing a complete structural and dynamic modelling of the equations of motion of the smart blades for hover flight. These equations are used to investigate dynamic stability of the steady state motion of the smart blades under parametric excitation. In closing, as a preliminary step to determine the actuation authority of the active fibres, the open-loop response of the blade to an impulsive type of actuation is also investigated.

Design and Vibratory Loads Reduction Analysis of Advanced Active Twist Rotor Blades Incorporating Single Crystal Piezoelectric Fiber Composites

This paper presents design optimization of a new Active Twist Rotor (ATR) blade and conducts its aeroelastic analysis in forward flight condition. In order to improve a twist actuation performance, the present ATR blade utilizes a single crystal piezoelectric fiber composite actuator and the blade cross-sectional layout is designed through an optimization procedure. The single crystal piezoelectric fiber composite actuator has excellent piezoelectric strain performance when compared with the previous piezoelectric fiber composites such as Active Fiber Composites (AFC) and Macro Fiber Composites (MFC). Further design optimization gives a cross-sectional layout that maximizes the static twist actuation while satisfying various blade design requirements. After the design optimization is completed successfully, an aeroelastic analysis of the present ATR blade in forward flight is conducted to confirm the efficiency in reducing the vibratory loads at both fixed- and rotating-systems. Numerical simulation shows that the present ATR blade utilizing single crystal piezoelectric fiber composites may reduce the vibratory loads significantly even with much lower input-voltage when compared with that used in the previous ATR blade. However, for an application of the present single crystal piezoelectric actuator to a full scaled rotor blade, several issues exist. Difficulty of manufacturing in a large size and severe brittleness in its material characteristics will need to be examined.

Design and Manufacturing of a Model-scale Active Twist Rotor Prototype Blade

The design and manufacturing of an active twist rotor blade for vibration reduction in helicopters are presented. The rotor blade is integrally twisted by direct strain actuation through embedded piezoelectric fiber composite actuators distributed along the span of the blade. Highlights of the analysis formulation used to design this type of active blade are presented. The requirements for the prototype blade, along with the final design results are also presented. Detailed aspects of its manufacturing are described. Experimental structural characteristics of the prototype blade compare well with design goals, and bench actuation tests characterize its basic actuation performance. The design and manufacturing processes permit the realization of an active blade that satisfies a given set of design requirements. This is used to later develop a fully active rotor blade system.

Design and aeroelastic analysis of active twist rotor blades incorporating single crystal macro fiber composite actuators

The advanced active twist rotor (AATR) blade incorporating single crystal macro fiber composite (single crystal MFC) actuators is designed and the aeroelastic analysis is performed. The AATR blade is designed based on an existing passive blade and the NASA/ARMY/MIT active twist rotor (ATR) prototype blade. The AATR blade is designed to satisfy all the requirements and the properties of the AATR blade are compared with those of the ATR prototype blade. The aeroelastic analysis of the designed AATR blade for the hover condition is conducted. In order to predict the vibration reduction capability, the twist actuation frequency response of the AATR blade is investigated by the numerical simulation. As a result, although lower input voltage is used, the AATR blade can achieve higher twist actuation that is sufficient to alleviate the helicopter vibration as compared with the ATR blades using the AFC and the standard MFC.

지능형 헬리콥터 로터의 개별 블레이드 제어에 의한 진동하중 감소 해석

the present paper, a new version of DYMORE, which is an analysis to solve a nonlinear multi-body dynamics problem, is used to simulate an Individual Blade Control (IBC) algorithm in order to reduce vibration in helicopter rotors. The Active Twist Rotor (ATR), in which Active Fiber Composites (AFC) are embedded, is utilized for IBC. The main purpose of the present investigation is to compare the analytical results with experiments and previous version of DYMORE. The experiments are performed at NASA Langley Transonic Dynamics Tunnel. According to the present result, it is observed that the correlation regarding the vibratory loads is improved.

Induced shear actuation of helicopter rotor blade for active twist control

Induced shear based mechanism is used for attaining active twist in a soft-inplane hingeless rotor with a two-cell thin-walled airfoil section. The rotor properties dynamically represent a real rotor. A closed loop controller is developed to obtain the optimum voltage required to be given to the rotor blade for obtaining twist using strain rate feedback. Nonlinear relationship between piezoelectric shear coefficient and applied electric field is included in the controller design. Optimal placement of actuators lead to an overall vibration reduction of about 65%. Since thin-walled structures such as rotor blades are highly flexible and have strong aeroelastic effects, these effects on the loads and stability are thoroughly studied to evaluate the feasibility of the active twist rotor concept. No aeroelastic instabilities are found to occur due to active twist.

Analytical development of single crystal Macro Fiber Composite actuators for active twistrotor blades

A Macro Fiber Composite (MFC) is a piezoelectric fiber composite which has aninterdigitated electrode, rectangular cross-section and unidirectional polycrystallinepiezoceramic (PZT) fibers embedded in the polymer matrix. A MFC actuator has muchhigher actuation performance and flexibility than a monolithic piezoceramic actuator.Moreover, the single crystal piezoelectric material exhibits much higher induced strainlevels, energy density and coupling than those of polycrystalline piezoceramic materials.Thus, the performance of an MFC can be improved by using single crystal piezoelectricfiber instead of polycrystalline piezoceramic fiber. This study investigates theanalytical modeling, material properties and actuation performance of an MFCusing single crystal piezoelectric material (single crystal MFC). For single crystalMFC, the mechanical properties are calculated by the classical lamination theory,and the uniform fields model (UFM) is adopted to predict piezoelectric strainconstants. In addition, the actuation performance of the single crystal MFC withthe active twist rotor blade is studied. The material properties and actuationperformance of single crystal MFC are compared with those of standard MFC.

Closed-Loop Control Test of the NASA/Army/MIT Active Twist Rotor for Vibration Reduction

Closed-Loop Control Test of the NASA/Army/MIT Active Twist Rotor for Vibration Reduction

Wind-Tunnel Evaluation of a Sikorsky Active Rotor Controller Implemented on the NASA/ARMY/MIT Active Twist Rotor

Wind-Tunnel Evaluation of a Sikorsky Active Rotor Controller Implemented on the NASA/ARMY/MIT Active Twist Rotor