This work presents the lessons learned from wind tunnel tests of a droop-nose morphing wingtip as part of the EU project NOVEMOR. The design followed a sequential chain and was largely driven through optimization tools, including a glass-fiber composite skin optimization tool and a topology optimization tool for the design of internal super-elastic and aluminium compliant mechanisms. The device was tested in the low speed tunnel at the University of Bristol to determine the structural response under aerodynamic loading. Measurements of strain from strain gauges show that the structure is capable of handing the aerodynamic loads though also show an imbalance of strain between the components. Measurements of surface pressures show a small variation of cp with the 2° droop morphing variation as per the target. The wind tunnel testing showed that further developments to the design chain are necessary, in particular the need for a concurrent as opposed to sequential chain for the design of the various components. Considerations of other problem formulations, the inclusion of nonlinear finite element analysis, and ways to interpret the structural boundary of the topology optimization results with more confidence are required. The utilization of super-elastic materials in morphing structures may also prove to be highly beneficial for their performance.
Since carbon nanotubes (CNTs) have been discovered in 1991, worldwide scientific research reveals excellent properties. Most of the found properties refer to almost defect-free, single-walled carbon nanotubes (SWCNTs) with nano-scale dimensions. However, scientists try to incorporate CNTs into applications to transfer their features in order to push the specific performance. Typically the results are comparably lower than expected because of the varying quality of used CNTs. This paper presents results of research using CNTs as actuators. In contrast to published paper which analyzed architectures of entangled CNTs as active components, like papers or yarns, to measure their bulk-strain this paper focuses on scattered, highly aligned CNTs. This approach promises to clarify the effect of actuation, whether it is a quantum-mechanically, or rather an combined electrostatic volume-transfer effect. Two experimental set-ups are presented. The first experiment is carried out using highly aligned multi-walled CNTs (MWCNT-arrays). Their orientation is investigated intensively in comparison to other analyzed CNT-arrays. Furthermore their substrate consists of electrical conductive carbon. The CNT-array is optically analyzed along the longitudinal geometry of the vertically aligned MWCNTs. The interfaces of the set-up, which may influence the measurement, have been analyzed in order to avoid second-order effects like thermal swelling or chemical degradation. The results reveal comparable high deflections starting at an activation-voltage of ±1.75V. The ionic liquid is tested within a voltage-range of ±2V due to time-staple performance. The second presented results are found by using Raman-spectroscopy to analyze single SWCNTs. This paper presents the first results of the challenging test campaign to analyze single-walled nanotubes within an electrolyte during charging. A shifting of Raman-peaks according to the wavenumber can be directly attributed to a geometry-change. Thus, the two presented experiments uses aligned CNTs a driving actuation mechanism of single CNTs may be identified.
The various excellent properties of carbon nanotubes (CNTs) are in the focus of researchers since years. Moreover
architectures, built of CNTs, show active behavior in terms of deflections. Therefore they have to be set up like
a capacitor within an electric field and covered by a matrix of free movable ions. The mechanism behind this
phenomenon is still discussed obsessively rather to be an quantum-mechanical elongation of the C-bonds or to be
caused by electrostatic repulsion of charged agglomerated CNTs. Formally investigated paper-like architectures,
known as Bucky-papers, consist of randomly oriented CNTs. This paper presents several experimental approaches
and results documenting doubts about the ability to clarify the active mechanism by investigating the electro-mechanical
properties of those paper-like architectures. In contrast a novel test set-up for analyzing specimen,
providing highly vertically aligned CNTs, is presented. This high resolution test set-up is designed to analyze
CNT-specimen in thickness-direction optically. The vertically aligned CNT-architectures, also called CNT-arrays,
consist of multi-walled CNTs (MWCNTs). The MWCNT-arrays are highly hydrophobic and can only
be moistened by polar liquids like ionic liquids (ILs). The latest results of the electro-mechanical system as
well as further challenges dealing with ILs and different kinds of CNT-arrays are presented. The presented
measurement method allows an even more precise investigation of the electro-mechanical behavior of a single
MWCNT and the strain-mechanism simultaneously. Furthermore this configuration points out an efficient mode
of future CNT-based actuators.
Excellent properties like low density, high mechanical stiffness as well as an outstanding thermal and electrical
conductivity make researchers focusing on carbon nanotubes (CNTs) since years. Beside that it is found that
structures made of CNTs can be actuated when they are set up like a capacitor. Usually two dimensional (2D)
CNT-papers with randomly oriented CNTs, called Bucky-papers, are used. They are charged and divided by
an electrically insulating but ionic conductible electrolyte. Experiments demonstrate low voltages for actuation
(±1V). Although the mechanism of CNT-actuation is still an open issue theoretical studies suggest a charge and
ion induced lengthening of the C-bonds, which predict theoretical strains up to 1%. These characteristics make
CNTs a potential candidate for lightweight and powerful actuators of future adaptive aerospace applications.
The presented work gives an overview of possible CNT-actuator configurations. Comprehensive analysis
tools for 2D mats of randomly oriented CNTs have been developed to guarantee a consistent data base for the
comparison of different CNT-configurations. It is focused on the electro-mechanical properties with respect to
the processing and configuration of CNT-actuators. For a more efficient use of the mechanical advantages of
the CNT-geometry a new aligning manufacturing approach is presented, to get highly oriented 2D CNT-papers.
Their properties are compared with randomly oriented CNT-papers. Finally a new test set-up will be introduced,
which enables deflection measurements directly on the top of vertically aligned CNTs (CNT-arrays). The buildup
and necessary prework are shown, as well as results of the first experiments. The method of measuring along
the axis of aligned CNTs qualifies this set-up to get a deeper understanding about the actuation mechanism
of CNTs. Vertically aligned CNTs promise to be a more efficient actuator configuration because of their high
stiffness in direction of actuation.
Future adaptable applications require electro-mechanical actuators with a high weight-related energy. Among
modern multi-functional materials carbon nanotubes (CNTs) have some special characteristics which give them
the potential to solve this demand. On the one hand raw CNTs have excellent mechanical properties like their
low density (1330kg/m3) and very high estimated stiffness of about 1TPa. On the other hand CNTs have the
ability under presence of ions, wired like a capacitor and activated by a charge injection to perform a dimensionchange
(length of C-C bondings). Calculations and experiments present achievable active strains of 1% at low
voltage of ±1V what qualifies CNT-based materials for leightweight powerful actuators.
In this paper the former work done with actuators using CNT-containing mats and Nafion as solid electrolyte
is evaluated by analyzing the two main-components in more detail. On the one hand the CNT-based modelmaterial
SWCNT-mats called Bucky-paper (BP) and on the other hand ion donating electrolytes in liquid-phase
like a NaCl-solution and its solid equivalent Nafion as thin-foils are tested. Additional methods of fabrication,
preparation and characterization of the CNT-powder and the manufactured BPs containing randomly oriented
single-walled carbon nanotubes (SWCNTs) are presented which provide a deeper system-understanding. Both
materials (BPs and Nafion-foils) are intensively investigated in different deflection-test-rigs due to their structural
assembly. This paper presents a method for electro-mechanical measurements of BPs in an in-plain test set-up
which avoids sensing secondary effects like thermal expansion or mass-transport and confirm that BP-deflection
should only be a capacity-driven effect. Nafion as solid electrolyte will be tested in an out-of-plane facility
to measure its possible actuation within the lamellar-direction. With this approach the dependencies of each
component and their individual characters on the deflection can be estimated. The active response can be
referred to the internal structure of both components as well as of the whole structural assembly.
The results give a certain direction to a BP-optimization referring to active strain, density, structural integrity
and conductibility. In addition to these facts the active character of BPs using CNTs of different suppliers and
Nafion is analyzed. These investigations are of particular importance for detection of global dependencies and
using both materials in a hybrid-assembly like solid actuators which are needed for structural applications.
Actuators based on carbon nanotubes (CNT) have the potential to generate high forces at very low voltages. The
density of the raw material is just 1330 kg/m3, which makes them well applicable for lightweight applications.
Moreover, active strains of up to 1% can be achieved - due to the CNTs dimensional changes on charge injection.
Therefore the nanotubes have to be arranged and electrically wired like electrodes of a capacitor. In previous
works the system's response of the Nanotubes comprising a liquid electrolyte was studied in detail. The major
challenge is to repeat such experiments with solid electrolytes, which is a prerequisite for structural integration.
In this paper a method is proposed which makes sure the expansion is not based on thermal expansion. This
is done by analysing the electrical system response. As thermal expansion is dominated by ohmic resistance the
CNT based actuators show a strong capacitive behavior. This behavior is referable to the constitution of the
electrochemical double layer around the nanotubes, which causes the tubes to expand. Also a novel test setup is
described, which guarantees that the displacement which is measured will not be caused by bending of a bimorph
but due to expansion of a single layer of nanotubes. This paper also presents experimental results demonstrating
both, the method of electrical characterization of CNT based actuators with implemented solid electrolytes
and the novel test setup which is used to measure the needed data. The actuators which were characterized
are hybrids of CNT and the solid electrolyte NAFION which is supplying the ions needed to constitute the
electrochemical double layer. The manufacturing, processing of these actuators and also some first experimental
results are shown. Unfortunately, the results are not as clear as those for liquid electrolytes, which depend on
the hybrid character of the analyzed devices. In the liquid electrolyte based case the CNTs are the only source
of stiffness, whereas in the solid electrolyte case electrodes and electrolyte contribute to the overall stiffness and
damping as well. Since the introduction of solid electrolytes is a major stumbling block in the development of
such actuators, this work is of particular importance.
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.