Abstract This paper discusses open-loop and closed-loop active control investigations of a full-scale Bo 105 helicopter rotor with active camber morphing. The potential of an active camber morphing concept to reduce non-rotating vibratory hub loads and rotor power using active control was investigated. The mechanism employed was a dynamically actuated airfoil camber morphing concept known as Fish Bone Active Camber (FishBAC) that smoothly deforms the camber over the aft section of the airfoil. A comprehensive rotorcraft aeromechanics analysis was used that modeled the blade elastic motion using one-dimensional finite beam elements combined with multibody dynamics. Aerodynamic forces were calculated with a free-vortex wake model together with lifting line theory for the blade aerodynamics. The open-loop investigation comprised of a parametric study of relevant control parameters that govern the active camber deflection cyclic actuation profile and their effects on rotor performance and hub vibration. It was found that active camber morphing using superimposed once-per-revolution (1P) and 2P control inputs was able to simultaneously reduce rotor power by 4.3% and overall vibratory hub loads by 27%. Additionally, a closed-loop adaptive multicyclic controller was used to identify the potential of this morphing concept for hub vibration reduction using multicyclic active control inputs. Active camber actuation using a sum of four control harmonic inputs, i.e. 1-4P, resulted in a maximum hub vibration reduction of 50%.

Abstract This paper presents a summary on the ongoing research and development of a solid-state piezoelectric composite rotor design for use in rotary systems. The paper focuses on the theoretical analysis of a two-bladed rotor with varying parameters such as flight speed, blade pitch angle, and rotational speed. XROTOR, a blade element method based software, is used for analysis. The two-dimensional aerodynamic characteristics are acquired from the previous research on a Macro-Fiber Composite actuated simply supported thin airfoil. A set of simulations are conducted to determine the best geometric configuration, so the piezoelectric increase in thrust is maximized. The proposed hub-rotor system has the potential to be implemented in unmanned-aerial-vehicles such as single-rotor, tandem-rotor, multi-copter, and ducted-fan rotorcraft, or other rotating systems such as wind turbines, turbine engines, and marine propellers. This paper presents a summary of previous findings on a solid-state rotor prototype, and a new investigation on the theoretical aerodynamic behavior.

Abstract Since helicopter rotors have different demands from different flight stats, the final design is always a compromise between flight stats such as hover and fast forward flight. Two of the design parameters are twist and chord length. This paper is giving some reasoning from rotor simulations on what twist and chord length should look like in order to increase performance in hover or forward flight. The result is, that the inboard chord length should be much larger for hover than for forward flight. This paper is presenting a structural concept, that can enable a helicopter rotor blade to change its chord length.

The aerodynamic foil bearing is a special type of air bearing in which the flexible foil structure between rotor and rigid housing supports the rotor bearing system with a greater robustness against thermal distortion and production misalignments. In such bearings, the generation of an aerodynamic pressure in the lubricating film after reaching the lift-off speed prevents the solid contact between rotor and foil structure. Since many static and dynamic properties of air foil bearings strongly depend on the inner contour of the bearing, the idea of an adaptive air foil bearing (AAFB) is developed to optimize the bearing’s performance at different operating points. This paper concentrates on a semi-analytical model based on plate theory using Ritz method for simulating the static shape control of piezoelectrically actuatable supporting segments for an AAFB under different loading conditions. The elastic suspension of the supporting segments and symmetries of the bearing are considered in the modeling. After validation by means of FEM analyses and experimental tests the influence of geometry and material is examined in a parametric study. Later on, the model is used for parameter optimization in order to achieve the most effective shape morphing.

The reduction of low-frequency noise transmission through thin-walled structures is a topic of research for many years now. Due to large wavelengths and the mass law, passive solutions usually gain low performance in the frequency range below 500 Hz. Active systems promised to fill the gap and to achieve significant reductions of transmitted sound. Nevertheless, experiments showed the outstanding performance of such specialized systems, but also demonstrated the computational and hardware effort of such solutions. The upcoming additive manufacturing technology enabled new multi-material designs of complex structures. Based on this technology, acoustic metamaterials emerged in the laboratories and in literature. Arrays of miniaturized locally resonant structures are able to change the noise transmission of thin walled structures beyond the limits of the given mass law in certain frequency bands. For future aircraft contra-rotating open rotor (CROR) engines are a promising technology to reduce their CO 2 footprint. Since the contribution of CROR engines to the cabin noise is higher than for jet engines, new strategies for the reduction of noise transmissions for frequency bands below 200 Hz are necessary. For the tonal noise of the CROR engines, acoustic metamaterials seem to be an appropriate solution. In this paper a 110 × 110 × 1 mm 3 thin-walled sample plate is presented. It is covered with a 5 × 5 array of multi-material resonant structures, which are printed as mass on a beam. The rubber-like beam material combines a low Young’s modulus with a high material damping, leading to a low eigenfrequency of the resonators. The design of the resonators using simulations and experimental data is shown. To explore the potential of the design, an acoustic test box is manufactured. Starting with all resonators unblocked the emitted sound intensity of the plate is measured. Sequential blocking of selected resonators proves the concept. Additional laser scanning vibrometer measurements give insights into the vibration behavior of single resonators.

The main goal of this study is the optimization of vibration reduction on helicopter blade by using macro fiber composite (MFC) actuator under pressure loading. Due to unsteady aerodynamic conditions, vibration occurs mainly on the rotor blade during forward flight and hover. High level of vibration effects fatigue life of components, flight envelope, pleasant for passengers and crew. In this study, the vibration reduction phenomenon on helicopter blade is investigated. 3D helicopter blade model is used to perform the aeroelastic behavior of a helicopter blade. Blade design is created by Spaceclaim and finite element analysis is conducted by ANSYS 19.0. Generated model are solved via Fluent by using two-way fluid-solid coupling analysis, then the analyzed results (all aerodynamic loads) are directly transferred to the structural model. Mechanical results (displacement etc.) are also handed over to the Fluent analysis by helping fluid-structure interaction interface. Modal and harmonic analysis are performed after FSI analysis. Shark 120 unmanned helicopter blade model is used with NACA 23012 airfoil. The baseline of the blade structure consists of D spar made of unidirectional Glass Fiber Reinforced Polymer +45°/−45° GFRP skin. MFC, which was developed by NASA’s Langley Research Center for the shaping of aerospace structures, is applied on both upper and lower surfaces of the blade to reduce the amplitude in the twist mode resonant frequency. D33 effect is important for elongation and to observe twist motion. To foresee the behavior of the MFC, thermo-elasticity analogy approach is applied to the model. Therefore, piezoelectric voltage actuation is applied as a temperature change on ANSYS. The thermal analogy is validated by using static behavior of cantilever beam with distributed induced strain actuators. Results for cantilever beam are compared to experimental results and ADINA code results existing in the literature. The effects of fiber orientation of MFC actuator and applied voltage on vibration reduction on helicopter blade are represented. The study shows that torsion mode determines the optimum placement of actuators. Fiber orientation of the MFC has few and limited influences on results. Additionally, the voltage applied on MFC has strong effects on the results and they must be selected according to applied model.

In order to reduce the “cost of energy” for wind turbines it is an ongoing trend to increase the rotor diameter, which increases fatigue loads in the blade root area. Thus, a critical prerequisite for increased rotor diameter is the reduction of loads, which can be utilized by passive and active measures. This paper is giving an overview of current research work towards the use of a flexible trailing edge for load reduction as it is being pursued in the German national SmartBlades project. The active trailing edge is designed to change the lift of the outer blade in a way to counteract sudden changes caused by gusts or wind shear. Areas that are covered include the simulation towards the load reduction potential of such flexible trailing edges, the structural design of the trailing edge itself as a compliant mechanism, its experimental validation and fatigue investigation as well as multistable approaches for the design of such trailing edge flaps.

Regional aviation is an innovation driven sector of paramount importance for the European Union economy. Large resources and efforts are currently spent through the CleanSky program for the development of an efficient air transport system characterized by a lower environmental impact and unequalled capabilities of ensuring safe and seamless mobility while complying with very demanding technological requirements. The Green Regional Aircraft (GRA) panel, active from 2006, aims to mature, validate and demonstrate green aeronautical technologies best fitting the regional aircraft that will fly from 2020 onwards with reference to specific and challenging domains: from advanced low-weight and high performance structures up to all-electric systems and bleed-less engine architectures, from low noise/high efficiency aerodynamic up to environmentally optimized missions and trajectories management. The development of such technologies addresses two different aircraft concepts, identified by two seat classes, 90-pax with Turboprop (TP) engine and 130-pax, in combination with advanced propulsion solutions, namely, the Geared Turbofan (GTF), the Advanced Turbofan (ATF) and the Open Rotor (OR) configuration. Within the framework of the Clean Sky program, and along nearly 10 years of research, the design and technological demonstration of a novel wing flap architecture was addressed. Research activities aimed at demonstrating the industrial feasibility of a morphing architecture enabling flap camber variation in compliance with the demanding safety requirements applicable to the next generation GRA in both open rotor and turboprop configurations. The driving motivation was found in the opportunity to replace a conventional double slotted flap with a single slotted morphing flap assuring improved high lift performances — in terms of maximum attainable lift coefficient and stall angle — while lowering emitted noise, fuel-burnt and deployment system complexity. Additional functionalities for load control and alleviation were then considered and enabled by a smart architecture allowing for an independent shape-control of the flap tip region during cruise. The entire process moving from concept definition up to the experimental qualification of true scale prototypes, characterized by global and multi-zone differential morphing capabilities, is here described with specific emphasis on the adopted design philosophy and implemented technological solutions. Paths to improvements are finally outlined in perspective of a low-term item certification and series production.

Helicopters suffer from a number of problems raised from the high vibratory loads, noise generation, load capacity limitations, forward speed limitation etc. Especially unsteady aerodynamic conditions due to the different aerodynamic environment between advised and retreating side of the rotor cause most of these problems. Researchers study on passive and active methods to eliminate negative effects of aerodynamic loads. Nowadays, active methods such as Higher Harmonic Control (HHC), Individual Blade Control (IBC), Active Control of Structural Response (ACSR), Active Twist Blade (ATB), and Active Trailing-edge Flap (ATF) gain importance to vibration and noise reduction. In this paper, strain-induced blade twist control is studied integrated by Macro Fiber Composite (MFC) actuator. 3D model is presented to analyze the twisting of a morph and bimorph helicopter rotor blade comprising MFC actuator which is generally applied vibration suppression, shape control and health monitoring. The helicopter rotor blade is modeling with NACA23012 airfoil type and consists of D-spar made of unidirectional fiberglass, ±45° Glass Fiber Reinforced Polymer (GFRP) and foam core. Two-way fluid-structure interaction (FSI) method is used to simulate loop between fluid flow and physical structure to enable the behavior of the complex system. To develop piezoelectric effects, thermal strain analogy based on the similarities between thermal and piezo strains. The optimization results are obtained to show the influence of different design parameters such as web length, spar circular fitting, MFC chord length on active twist control. Also, skin thickness, spar thickness, web thickness are used to optimization parameters to illustrate effects on torsion angle by applying response surface methodology. Selection of correct design parameters can then be determined based on this system results.

The present study concerns with the performance of a skid landing gear (SLG) system of a rotorcraft impacting the ground at a vertical sink rate of 5.0 m/s. The impact attitude is per chapter 527 of the Airworthiness Manual (AWM) of Transport Canada Civil Aviation and FAR Part 27 of the U.S. Federal Aviation Regulation. A single degree of freedom helicopter model is investigated under two rotor lift factors 0.67 and 1.0. Three Configurations are evaluated: a) A conventional SLG; b) SLG equipped with a passive viscous damper and c) SLG incorporated with a magnetorheological energy absorber. The non-dimensional solutions of the helicopter model show that the passive damper system could reduce the maximum acceleration experienced by the helicopter occupants by 21% and 19.8% in comparison to the undamped system for the above rotor lift factors, respectively. However, the passive damper fails to constrain the non-dimensional energy absorption stroke of the damper within the given 18 cm maximum stroke and a bottoming out of the damper piston was noticed. Therefore, the alternative and successful choice was to employ a magnetorheological energy absorber (MREA). To improve the MREA controllability and to resettle the payload with no oscillations, i.e. in one cycle, two different Bingham numbers for compression stroke and rebound stroke were defined in the non-dimensional solution. Several simulations were conducted for different values of Bingham numbers. Among these numerical simulation results, the solution that implemented the optimum Bingham numbers was found to be the only one feasible solution. In this case the MREA with optimum Bingham number for compression could utilize the full energy absorption stroke to attain soft landing. In the rebound stroke, the generated optimal on-state damping force successfully controls the bounce of the payload until the payload settles down to its original equilibrium position with no oscillations.

This paper presents a summary of the ongoing research and development of a solid-state piezo-composite rotor design for use in rotary systems. The proposed hub-rotor system can be implemented in unmanned-aerial-vehicles such as single-rotor, tandem-rotor, multi-copter, and ducted-fan rotorcraft, or other rotating systems such as wind turbines, turbine engines and marine propellers. The preliminary feasibility of the solid-state rotor concept has been previously demonstrated through flight-testing a prototype on a quad-copter vehicle platform. This paper presents a summary of previous findings, and a new solid-state rotor prototype with the corresponding experimental results.

Gas turbine engines are generally optimized to operate at nearly a fixed speed with fixed blade geometries for the design operating condition. When the operating condition of the engine changes, the flow incidence angles may not be optimum with the blade geometry resulting in reduced off-design performance. Articulating the pitch angle of turbine blades in coordination with adjustable nozzle vanes can improve performance by maintaining flow incidence angles within the optimum range at all operating conditions of a gas turbine engine. Maintaining flow incidence angles within the optimum range can prevent the likelihood of flow separation in the blade passage and also reduce the thermal stresses developed due to aerothermal loads for variable speed gas turbine engine applications. U.S. Army Research Laboratory has partnered with University of California San Diego and Iowa State University Collaborators to conduct high fidelity stator-rotor interaction analysis for evaluating the aerodynamic efficiency benefits of articulating turbine blade concept. The flow patterns are compared between the baseline fixed geometry blades and articulating conceptual blades. The computational fluid dynamics studies were performed using a stabilized finite element method developed by the Iowa State University and University of California San Diego researchers. The results from the simulations together with viable smart material based technologies for turbine blade actuations are presented in this paper.

The main objective of this work is to develop a non-contact, non-invasive, structural health monitoring technique for surface and sub-surface damage detection in structures such as composite helicopter rotor blades. In many cases, composite structures are prone to damage in the form of cracks, delamination, and manufacturing defects, which can propagate beneath structural surfaces and cause severe component or catastrophic structural failure. The damage detection technique in this study works on the principle of electrical capacitance tomography. Different patterns of electrical field are propagated in a pre-defined sensing area. Using measurements of electrical response along boundaries of the sensing area, the permittivity distribution within that space can be reconstructed. First, a series of numerical simulations was performed by altering the electrical permittivity at different locations to simulate damage. The shapes and locations of permittivity changes were captured by the proposed technique. Second, to demonstrate its validity, an experimental test setup was built with a set of boundary electrodes. The system was connected to a function generator that supplied an electrical signal and induced electrical fields between electrodes. Capacitance between pairs of electrodes were then measured, which were used as inputs for solving the inverse permittivity reconstruction problem. Various test cases with different objects placed in the sensing area were conducted for validating this technique. The preliminary results show that the system was able to reconstruct spatial permittivity distributions and detect the presence, shapes, and locations of objects, thereby suggesting potential for damage detection.

A direct adaptive control approach is used to track the tip speed ratio of wind turbine to maximize the power captured during the below rated wind speed operation. Assuming a known optimum value of tip speed ratio, the deviation of actual tip speed ratio from the optimum one is mathematically expressed as tip speed ratio tracking error. Since the actual tip speed ratio is not a measurable quantity, this expression for tip speed ratio tracking error is linearized and simplified to express it in terms of wind speed and rotor speed, where rotor speed can easily be measured whereas an estimator is designed to estimate the wind speed. Important results from stability and convergence analysis of the proposed adaptive controller with state estimation and state feedback is also presented. From the analysis it was observed that the adaptive disturbance tracking controller can be combined with adaptive state feedback to achieve other control objectives such as reducing the wind turbine structural loading. Hence, an adaptive state feedback scheme is also proposed to reduce wind turbine tower fore-aft and side-side motions.

Active Vibration Control (AVC) and Active Structural Acoustic Control (ASAC) gained much attention in all kind of industries in the past. Promising results have been achieved in controlling the vibration and the noise emission/transmission of single panel structures. Especially for aircraft applications, concepts for the reduction of the turbulent boundary layer, rotor or jet noise are presented in the literature. In most cases the contributed work is focused on a single panel or a section of the fuselage/lining. However, an AVC/ASAC system can only be effective for the passengers when it is expanded to the entire fuselage structure. This expansion inevitably leads to a large number of sensors and actuators and thus to a controlled plant of high dimensions. For model-based control approaches especially, the system identification and the proof of stability would be challenging and probably not realizable. In this paper a strategy for such large-scale problems is investigated. A decentralized control approach with collocated actuator-sensor pairs is proposed. Since adjacent control loops are highly coupled by the underlying structure, special attention has to be given to the global stability of the entire control system. Instead of proving local stability and setting a global master gain, a method for the tuning of the single collocated control loops is developed that takes the cross-couplings into account. Based on data of DLR’s experimental aircraft Dornier 728, it can be shown that the new method increases the performance of the control system compared to the master-gain method.

This paper deals with active vibration isolation of unbalance-induced oscillations in rotors using gain-scheduled H ∞ -controller via active bearings. Rotating machines are often exposed to gyroscopic effects, which occur due to bending deformations of rotors and the consequent tilting of rotor disks. The underlying gyroscopic moments are proportional to the rotational speed and couple the rotor’s radial degrees of freedom. Accordingly, linear time-varying models are well suited to describe the system dynamics in dependence on changing rotational speeds. In this paper, we design gain-scheduled H ∞ -controllers guaranteeing both robust stability and performance within a predefined range of operating speeds. The paper is based on a rotor test rig with two unbalance-induced resonances in its operating range. The rotor has two discs and is supported by one active and one passive bearing. The active support consists of two piezoelectric stack actuators and two collocated piezoelectric load washers. In addition, the rig is equipped with four inductive displacement sensors located at the discs. Closed-loop performance is assessed via isolation of unbalance-induced vibrations using both simulation and experimental data. This contribution is the next step on our path to achieving the long-term objective of combined vibration attenuation and isolation.

Current prosthetic legs rely on technology that is inefficient. Electric motors used today require high amounts of current to operate. Prostheses utilizing smart materials, including magnetorheological fluid (MRF), require less power to operate effectively. In this study, the MR fluid knee was designed for above the knee amputees. The design was in the family of MR fluid brake/clutch as it provides simple design and good MR fluid uniformity. However, this MR knee design is different from any existing MR brake, featuring a novel non-circular rotor. Three fourths of the rotor, considered circular, has the same radius, and the remaining quarter has a varying radius to provide a variable gap size between the rotor and stator. This feature allows the MR fluid to experience a stronger magnetic field as the knee rotates, resulting in a variable braking torque. At this stage of research, the first working prototype of the proposed MR fluid knee was built and tested to determine the braking torque profile. Corresponding theoretical models were also created. The calculated torque was used to predict the performance of the MRF knee. The experiments has been conducted to examine the MRF knee performance and to compare with the theoretical model. The knee was expected to use less power to operate and provided the necessary braking torque required by an average male human body. An improved design was planned for weight reduction and optimization to allow the commercialization of the MRF knee.

A hybrid torsional vibration damper is proposed in this study incorporating a conventional Centrifugal Pendulum Vibration Absorber (CPVA) and a Magnetorheological (MR) damper. While CPVAs are simple and reliable passive torsional vibration absorbers, their performance is limited to the designed tune conditions. MR dampers have recently received considerable attention due to their inherent fail-safe feature, low power requirement and capability to attenuate vibration under unpredictable environmental conditions. The present research aims at developing a novel hybrid torsional vibration damper combining conventional CPVA with the MR damper capable of suppressing torsional vibration at varying excitation frequencies. This research presents results from analytical investigation of rotor systems integrated with the proposed hybrid torsional vibration damper. The system under investigation consists of a rotor with attached hybrid torsional MR damper subjected to an external harmonic torque. The CPVA has been connected to the cylindrical housing of the MR damper. Different cases have been investigated including: 1- Rotor system without any damper, 2- Rotor system with only CPVA, 3- Rotor system with only torsional MR damper and 4- Rotor system with the hybrid torsional damper. Results for each case have been illustrated and compared against one another.

In the field of smart structures, piezoceramic actuators are wildly used for vibration reduction and acoustic manipulation of structures. Those applications typically run at frequencies between 10 Hz and 10k Hz. Prominent examples are the piezoceramic actuators implemented in helicopter rotor blades to twist them dynamically for higher harmonic control (HHC) or individual blade control (IBC). Once the actuators are implemented it would be a great benefit to also use them to statically change the blade twist (higher twist for take-off and landing — for higher lift; lower twist for high speed forward flight — for reduced drag). Staying with this example it can be found that sensing the twist displacement is not an easy task at all (see [1, 2]), so it would be most desirable, to use open loop control. In order to do that, the transfer function has to be known accurately. Unfortunately measurements show that the amplitudes for such very low frequencies behavior behave strongly non linear. This paper presents experimental results investigating the influence of the frequency on the amplitude — especially going for frequencies in the lower mHz region. A variety of piezoceramic actuators has been investigated: from stacks to patch type, d 33 as well as d 31 effect actuators. A second focus of this paper is the reaction of piezoceramic actuators on the application of a constant DC voltage. The drift that occurs has to be taken into consideration. A third focus of this paper is the dependency of a displacement output of such an actuator at a constant applied DC voltage on the voltages that the actuator had seen before. This topic is of special importance for aerodynamically effective surfaces that are driven by piezoceramic actuators and should be analyzed (generation of polars) in static conditions.

This paper presents the modeling and control of a Continuously Variable Planetary (CVP) transmission in a wind turbine system. The primary purpose of this paper is to evaluate its effectiveness for mechanically decoupling the variable speed turbine rotor from the grid tied induction generator. It is expected that a CVP controlled wind turbine can take advantage of the grid tied induction generator without the use of an inverter, while optimizing the blade speed aerodynamically. This system also expands its operating range making it possible to track the optimal tip speed ratio over a wider wind speed range, which allows higher power to be captured from the wind. System characteristics have been studied by simulating an 8kW horizontal axis wind turbine in a MatLab/Simulink® environment. Experimental results have been included to verify the model of the system. Analyses conducted show that the continuously variable transmissions are potential candidates for small wind turbine applications.