Johannes Riemenschneider

Modeling of carbon nanotube actuators: Part I -Modeling and Electrical properties

The outstanding electrical and mechanical properties of single carbon nanotubes (CNT) are the motivation for an intensive research in various fields of application. The actuation effect constitutes the foundation for any application as a multifunctional material and within the field of adaptronics. The effect is in the majority of cases investigated by a CNT configuration of stochastically aligned CNT, so-called bucky-paper, in an electrolytic environment. The article presents an analytical model for a detailed understanding and investigation of the actuation process. The complete description and parameterization of the model is documented in two parts. In the first part the model is developed, the test setup for the model validation and parameter identification is elucidated, and the electrical parameters are determined. In the second part the mechanical system and the actuating effect will be examined. Finally the applicability of the model will be examined.

Development of active twist rotors at the German Aerospace Center (DLR)

Helicopter main rotors are characterized by complex unsteady aerodynamic conditions, which are causing vibrations and noise in and around rotary wing aircrafts. The aerodynamic conditions also cause increased drag, which leads to higher fuel consumption. Even modern helicopters still suffer from these drawbacks. This is why there are many efforts to influence the flow conditions by passive and active means. Active means have the advantage to adapt to varying demands, which can change significantly. A first attempt is to perform a sinoidal variation of the pitch angle of the blades with a frequency that is an integer multiple of the rotor frequency, the so called higher harmonic frequencies. This can be done by additional displacements of the swash plate or by a variation of the pitch link length using actuated pitch links. Since both designs have several drawbacks, one of the most promising approaches is the realization of a secondary control via the deformation of individual blades. Such an actuation can be realized by blade flaps or by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. At the DLR such twist blades have been investigated intensively. A series of blades has been built using thin skin integrated actuators. This Paper gives an overview of all active twist blades with skin integrated actuators that have been designed and manufactured at the DLR so far. Different design philosophies have lead to different geometrical setups. A comparison of the blades power consumption for given control laws for noise and vibration reduction is also given. Finally, a unique testing technique for nondestructive measurement of mass distribution is discussed in this paper. are characterized by complex unsteady aerodynamic conditions, which are causing vibrations and noise in and around rotary wing aircrafts. The aerodynamic conditions also cause increased drag, which leads to higher fuel consumption. Even modern helicopters still suffer from these drawbacks. This is why there are many efforts to influence the flow conditions by passive and active means. Active means have the advantage to adapt to varying demands, which can change significantly. A first attempt is to perform a sinoidal variation of the pitch angle of the blades with a frequency that is an integer multiple of the rotor frequency, the so called higher harmonic frequencies. This can be done by additional displacements of the swash plate or by a variation of the pitch link length using actuated pitch links. Since both designs have several drawbacks, one of the most promising approaches is the realization of a secondary control via the deformation of individual blades. Such an actuation can be realized by blade flaps or by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. At the DLR such twist blades have been investigated intensively. A series of blades has been built using thin skin integrated actuators. This Paper gives an overview of all active twist blades with skin integrated actuators that have been designed and manufactured at the DLR so far. Different design philosophies have lead to different geometrical setups. A comparison of the blades power consumption for given control laws for noise and vibration reduction is also given. Finally, a unique testing technique for nondestructive measurement of mass distribution is discussed in this paper.

Modeling of Carbon Nanotube Actuators: Part II -Mechanical Properties, Electro Mechanical Coupling and Validation of the Model

Carbon nanotubes have the potential to become one of the actuating materials of the future. Since the discovery of the actuating effect a lot of experimental data was collected to describe the electro-mechanical coupling. With this paper a model for the system behavior is available to enhance the understanding of the effect. An analytical model is introduced in the first part of the article. The electrical parameters of the model are determined by impedance spectroscopy. Furthermore, the mechanical parameters and the electro-mechanical coupling have to be examined. The main subject of the second part of the article is to validate the model and to compare measured and simulated responses to several excitations.

Compliant structures-based wing and wingtip morphing devices

Purpose The purpose of this paper is to provide an overview of the design and experimental work of compliant wing and wingtip morphing devices conducted within the EU FP7 project NOVEMOR and to demonstrate that the optimization tools developed can be used to synthesize compliant morphing devices. Design/methodology/approach The compliant morphing devices were “designed-through-optimization”, with the optimization algorithms including Simplex optimization for composite compliant skin design, aerodynamic shape optimization able to take into account the structural behaviour of the morphing skin, continuum-based and load path representation topology optimization methods and multi-objective optimization coupled with genetic algorithm for compliant internal substructure design. Low-speed subsonic wind tunnel testing was performed as an effective means of demonstrating proof-of-concept. Findings It was found that the optimization tools could be successfully implemented in the manufacture and testing stage. Preliminary insight into the performance of the compliant structure has been made during the first wind tunnel tests. Practical implications The tools in this work further the development of morphing structures, which when implemented in aircraft have potential implications to environmentally friendlier aircrafts. Originality/value The key innovations in this paper include the development of a composite skin optimization tool for the design of highly 3D morphing wings and its ensuing manufacture process; the development of a continuum-based topology optimization tool for shape control design of compliant mechanisms considering the stiffness and displacement functions; the use of a superelastic material for the compliant mechanism; and wind tunnel validation of morphing wing devices based on compliant structure technology.

Evolution of Active Twist Rotor Designs at DLR

Unsteady flow conditions in the rotor disk are causing intense vibrations and noise in rotary wing aircrafts. Even modern helicopters still suffer from these drawbacks. This decreases the on board comfort, causes material fatigue and reduces the public acceptance of helicopters. For this reason there are many affords towards an active manipulation of the flow conditions to decrease noise and vibrations. This could also lead to a reduction of fuel consumption. One of the most promising approaches to do so is a secondary control by deformation of the individual blades. Such an actuation can be realized by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. The German Aerospace Center (DLR) started to work with active twist rotor blades in the early 90. Ever since, the technology of actuation has evolved drastically, opening new ways to twist blades by means of actuators. This paper presents the evolution of blade design in the last years.

Experimental Investigation of an Active Twist Model Rotor Blade Under Centrifugal Loads

Individual Blade Control (IBC) for helicopter rotors promises to be a method to increase flight performance and to reduce vibration and noise. Quite a few concepts to realize IBC Systems have been proposed so far. Some of them have already been tested in wind tunnels or on real helicopters. A drawback of all systems that include discrete mechanical components like hinges, levers or gears is their vulnerability in a helicopter environment with high centrifugal loads and high vibration levels. That’s why the idea of using smart materials that are directly embedded in the rotor blade structure is very attractive for this application. Operating as solid state actuators they can generate a twist deformation of the rotor blade without any friction and wear. In the common DLR-ONERA project “Active Twist Blade” (ATB), DLR designed and build a 1:2.5 mach scaled BO105 model rotor blade incorporating state of the art Macro Fiber Composite (MFC) Actuators. The design of the blade was optimized using a finite element code as well as rotor dynamic simulations to predict the benefits with respect to vibrations, noise and performance. Based on these tools a blade was designed that meets all mass and stiffness constraints. The blade has been intensively tested within some bench- and centrifugal tests. The mechanical properties of the blade obtained within the bench tests showed a good correlation between measured and calculated values. The centrifugal test comprised a measurement of the active twist performance at the nominal rotation speed of 1,043 RPM at different excitation frequencies from 2 up to 6/rev. It was proven, that also under centrifugal loads the predicted twist amplitudes can be achieved.

System Response of Nanotube based Actuators

The electromechanical characterization of carbon nanotube (CNT) papers, which are immersed in an aqueous electrolyte, is the main focus of this paper. The experimental set up consists of an ordinary three electrode cell, filled with the liquid electrolyte and using the CNT paper as working electrode. The in plain strain of the paper is measured and its quasi static and dynamic response to various electrical potential excitations has been investigated in a series of experiments. Additionally, the influence of different electrolytes, pre-stresses and types of carbon nanotubes (SWNT vs. MWNT) was studied.

Design and Testing of a Compliant Mechanism-Based Demonstrator for a Droop-Nose Morphing Device

A demonstrator morphing leading edge was designed and manufactured as an intermediary step in preparation for wind tunnel testing of a droop-nose adaptive morphing wingtip (AMWT) as part of the European FP7 project NOVEMOR. This demonstrator features a flexible fiberglass skin and a monolithic aluminum internal compliant mechanism and support structure for lightweight design. The design process involves the design of the skin via a structural optimization tool, followed by continuum gradient-based topology optimization of first the compliant mechanism and then the support structure. The skin was manufactured using prepreg Hexcel HexPly® 913 plies, the aluminum internal structure was laser cut from stock plate material and the compliant region was driven by a linear stepper motor actuator. Displacements and strains were measured and compared with target values and that of finite element computations and overall show good agreement; however issues such as grey-areas and hinge-regions in the topology optimization need to be addressed for the final wind tunnel design for better post-processing and reduced stress concentrations.

Development of an active twist rotor blade with distributed actuation and orthotropic material

Individual blade control (IBC) as well as higher harmonic control (HHC) for helicopter rotors promises to be a method to increase flight performance and to reduce vibration and noise. For those controls, an additional twist actuation of the rotor blade is needed. The developed concept comprises the implementation of distributed piezoelectric actuation into the rotor blade skin. In order to maximize the twist within given constraints, as torsional rigidity and given actuator design, the concept takes advantage of an orthotropic rotor blade skin. That way, a combination of shear actuation with orthotropic coupling generates more twist than each one of these effects alone. Previous approaches with distributed actuation used actuators operating in +/-45° direction with quasi-isotropic composites. A FE-Model of the blade was developed and validated using a simplified demonstrator. The objective of this study was to identify the effects of various geometric and material parameters to optimize the active twist performance of the blades. The whole development was embedded in an iterative process followed by an objective assessment. For this purpose a detailed structural model on the basis of the BO105 model rotor blade was developed, to predict the performance with respect to rotor dynamics, stability, aerodynamics and acoustics. Rotor dynamic simulations provided an initial overview of the active twist rotor performance. In comparison to the BO105 baseline rotor a noise reduction of 3 dB was predicted for an active twist of 0.8° at the blade tip. Additionally, a power reduction of 2.3% at 87m/s based on a 2.5 to BO105 was computed. A demonstrator blade with a rotor radius of 2m has been designed and manufactured. This blade will be tested to prove, that the calculated maximum twist can also be achieved under centrifugal loads.

Groundtest of a Composite Smart Droop Nose

The future generation of high lift devices for transport aircrafts has to contribute to the reduction of noise during landing and a reduction of drag during cruise flight. Also it has to be compatible with affords for natural laminar flow on the wing. A smart gapless droop nose would be an alternative to today’s slats and promises to contribute to those goals. A consortium of Airbus, EADS-IW, CASSIDIAN and DLR developed such a smart leading edge in the framework of the fourth German national research program in aeronautics. This paper describes a 1:1 3D fiber reinforced flexible smart droop nose and its ground test. The results of these tests will finally be compared with the results of the finite element simulation.