Proc. SPIE. 10164, Active and Passive Smart Structures and Integrated Systems 2017
KEYWORDS: MATLAB, Data modeling, Matrices, Control systems, Signal processing, Partial differential equations, Solids, Finite element methods, Aluminum, Chemical elements, Optimization (mathematics), Acoustics, System integration, Systems modeling
In most aviation applications, a major cost benefit can be achieved by a reduction of the system weight. Often the acoustic properties of the fuselage structure are not in the focus of the primary design process, too. A final correction of poor acoustic properties is usually done using insulation mats in the chamber between the primary and secondary shell. It is plausible that a more sophisticated material distribution in that area can result in a substantially reduced weight. Topology optimization is a well-known approach to reduce material of compliant structures. In this paper an adaption of this method to acoustic problems is investigated. The gap full of insulation mats is suitably parameterized to achieve different material distributions. To find advantageous configurations, the objective in the underlying topology optimization is chosen to obtain good acoustic pressure patterns in the aircraft cabin. An important task in the optimization is an adequate Finite Element model of the system. This can usually not be obtained from commercially available programs due to the lack of special sensitivity data with respect to the design parameters. Therefore an appropriate implementation of the algorithm has been done, exploiting the vector and matrix capabilities in the MATLABQ environment. Finally some new aspects of the Finite Element implementation will also be presented, since they are interesting on its own and can be generalized to efficiently solve other partial differential equations as well.
As a consequence of operational eﬃciency because of rising energy costs, future transport systems need to be mission-adaptive. Especially in aircraft design the limits of lightweight construction, reduced aerodynamic drag and optimized propulsion are pushed further and further. The ﬁrst two aspects can be addressed by using a morphing leading edge. Great economic advantages can be expected as a result of gapless surfaces which feature longer areas of laminar ﬂow. Instead of focusing on the kinematics, which are already published in a great number of varieties, this paper emphasizes as major challenge, the qualiﬁcation of a multi-material layup which meets the compromise of needed stiﬀness, ﬂexibility and essential functions to match the ﬂight worthiness requirements, such as erosion shielding, impact safety, lighting protection and de-icing. It is the aim to develop an gapless leading edge device and to prepare the path for higher technology readiness levels resulting in an airborne application. During several national and European projects the DLR developed a gapless smart droop nose concept, which functionality was successfully demonstrated using a two-dimensional 5 m in span prototype in low speed (up to 50 m/s) wind tunnel tests. The basic structure is made of commercially available and certiﬁed glass-ﬁber reinforced plastics (GFRP, Hexcel Hexply 913). This paper presents 4-point bending tests to characterize the composite with its integrated functions. The integrity and aging/fatigue issues of diﬀerent material combinations are analyzed by experiments. It can be demonstrated that only by adding functional layers the mentioned requirements such as erosion-shielding or de-icing can be satisﬁed. The total thickness of the composite skin increases by more than 100 % when required functions are integrated as additional layers. This fact has a tremendous impact on the maximum strain of the outer surface if it features a complete monolithic build-up. Based on experimental results a numerical model can be set up for further structural optimizaton of the multi-functional laminate.
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 <i>c<sup>p</sup></i> 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.
Smart fiber placement is an ambitious topic in current research for automated manufacturing of large-scale composite structures, e.g. wing covers. Adaptive systems get in focus to obtain a high degree of observability and controllability of the manufacturing process. In particular, vibrational issues and material failure have to be studied to significantly increase the production rate with no loss in accuracy of the fiber layup. As one contribution, an adaptive system has been developed to be integrated into the fiber placement head. It decouples the compaction roller from disturbances caused by misalignments, varying components’ behavior over a large work area and acceleration changes during operation. Therefore, the smart system axially adapts the position of the compaction roller in case of disturbances. This paper investigates the behavior of the system to compensate quasi-static deviations from the desired path. In particular, the compensation efficiency of a constant offset, a linear drift with constant gradient and a single-curved drift is studied. Thus, the test bed with measurement devices and scenarios is explained. Based on the knowledge obtained by the experimental data, the paper concludes with a discussion of the proposed approach for its use under operating conditions and further implementation.
Former research on morphing droop-nose applications revealed great economical and social ecological advantages in terms of providing gapless surfaces for long areas of laminar flow. Furthermore a droop-nose for laminar flow applications provides a low noise exposing high-lift system at the leading-edge. Various kinematic concepts for the active deployment of such devices are already published but the major challenge is still an open issue: a skin material which meets the compromise of needed stiffness and flexibility. Moreover additional functions have to be added to keep up with standard systems. As a result of several national and European projects the DLR developed a gapless 3D smart droop-nose concept, which was successfully analyzed in a low speed wind tunnel test under relevant loads to prove the functionality and efficiency. The main structure of this concept is made of commercial available glass fiber reinforced plastics (GRFP). This paper presents elementary tests to characterize material lay-ups and their integrity by applying different loads under extreme thermal conditions using aged specimens. On the one hand the presented work is focused on the integrity of material-interfaces and on the other hand the efficiency and feasibility of embedded functions. It can be concluded that different preparations, different adhesives and used materials have their significant influence to the interface stability and mechanical property of the whole lay-up. Especially the laminate design can be optimized due to the e. g. mechanical exploitation of the added systems beyond their main function in order to reduce structural mass.
Natural laminar flow is one of the challenging aims of the current aerospace research. Main reasons for the
aerodynamic transition from laminar into turbulent flow focusing on the airfoil-structure is the aerodynamic
shape and the surface roughness. The Institute of Composite Structures and Adaptive Systems at the German
Aerospace Center in Braunschweig works on the optimization of the aerodynamic-loaded structure of future
aircrafts in order to increase their efficiency. Providing wing structures suited for natural laminar flow is a step
towards this goal. Regarding natural laminar flow, the structural design of the leading edge of a wing is of special
interest. An approach for a gap-less leading edge was developed to provide a gap- and step-less high quality
surface suited for natural laminar flow and to reduce slat noise. In a national project the first generation of the
3D full scale demonstrator was successfully tested in 2010. The prototype consists of several new technologies,
opening up the issue of matching the long and challenging list of airworthiness requirements simultaneously.
Therefore the developed composite structure was intensively tested for further modifications according to meet
requirements for abrasion, impact and deicing basically. The former presented structure consists completely
of glass-fiber-prepreg (GFRP-prepreg). New functions required the addition of a new material-mix, which has
to fit into the manufacturing-chain of the composite structure. In addition the hybrid composites have to
withstand high loadings, high bending-induced strains (1%) and environmentally influenced aging. Moreover
hot-wet cycling tests are carried out for the basic GFRP-structure in order to simulate the long term behavior of
the material under extrem conditions. The presented paper shows results of four-points-bending-tests of the most
critical section of the morphing leading edge device. Different composite-hybrids are built up and processed. An
experimental based trend towards an optimized material design will be shown.
Shape adaptive systems and structural configurations are necessary to fulfill the demands of a future unmanned aerial
vehicle structure. Predominantly the present approaches are based on a passive load-bearing structure having smart
actuation systems deforming the passive structural configuration elastically in the wanted shape. Therefore the actuation
system can be based on discrete actuators, like electrically driven motors using gearing systems to transform the
displacement into the structure or on smart material configurations placed on the load bearing passive structure,
deforming the structure within the elastic region into the wanted shape.
Using smart materials within load-bearing structures, elastic and static strength properties vary between passive and
active structures. Matching these properties is a great challenge for future structural configurations. This is a successful
approach for certain applications, e.g. smart rotor blade.
The availability of two-dimensional smart actuator configurations with distinct actuation orientation allows the definition
of a distinct load bearing active structure. Therefore the so called "web" of a spar-equivalent configuration was
substituted by such a smart material actuator also known as macro fiber composite (MFC). Activating the web of the
active cantilevered spar-configuration is resulting in a free end displacement. The main advantage lies in the fact that this
approach will allow larger active displacements in comparison to a passive structural configuration with applied smart
Within the paper the process of developing the shear web based actuation system with configuration details will be illustrated and future steps will be proposed.
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.
Carbon Nanotubes have diameters in nanometer scale, are up to tens of microns long and can be single- or multi-walled (SWNT and MWNT). Compared with carbon fibers, which typically have a Young's modulus of up to 750 GPa, the elastic modulus of Carbon Nanotubes has been measured to be approximately 1-2 TPa. The strength of Carbon Nanotubes has been reported to be about two order of magnitude higher than current high strength carbon fibers. Additionally especially SWNT show excellent actuator behaviour. Electromechanical actuators based on sheets of SWNT show to generate higher stress than natural muscles and higher strains than ferroelectrics like PZT. Unlike conventional ferroelectric actuators, low operating voltages of a few volts generate large actuator strains. Thus, this paper will give a brief overview of the current activities within this field and show some recent results of the Carbon Nanotube actuator development at the DLR-Institute of Structural Mechanic suggesting that optimized SWNT sheets may eventually provide substantially higher work densities per cycle than any previously known material.
The following paper presents the development of an adaptive aeronautical structure at the example of a high lift device, a so-called fowler flap. It shows the passive optimization of the flap with a complex finite element model and illustrates the necessity to develop an active structure, due to the limited possibilities to increase the efficiency of the passive flap. Different kinds of active measures are discussed and it is shown, why an activation of the structure with shape memory alloy (SMA) actuators is preferred. Additionally, the results and findings of experiments with an active spar that was developed, built, and tested at the Institute of Structural Mechanics (ISM) at the German Aerospace Center in Braunschweig are presented.
The development of a new technology for the manufacturing of adaptive structures on the basis of thin monolithic peizoceramic wafers is an important goal of the German industrial project 'Adaptronik'. Partners from automotive-, space-, medical-, engineering- and optical industry participate in this project to enable new adaptive solutions for their applications. Due to the extreme brittleness of the piezoceramic material the manufacturing of these structures is still very demanding. Very often cracks in the piezoceramic material make the structure useless. This problem becomes serious when large scale structures with many actuators and sensor are considered. To come to more reliable results the use of encapsulated piezoceramic actuators and sensor came into focus. With respect to the great variety of different requirements given by the industrial partners the use of standardized solutions was not feasible. The goal was to develop new elements with improved performance parameters that can easily be adapted to different applications. Due to a modular concept, the developed multifunctional elements can be designed to meet a great variety of different structures was developed. A first step to adapt this technology to prototype structures has been done by the development of special encapsulated patches for an adaptive lightweight satellite mirror.
In 1997 the BMBF announced a highly paid competition for future oriented key technologies and their industrial utilization. 230 proposals from industrial enterprises and research establishments were submitted. An independent group of experts selected altogether only 5 projects which were proposed to the BMBF. One of these projects was the major project ADAPTRONICS which is funded from 1998 to 2002 with a total volume of 25,000,000 EURO. This project is under the direction of the DLR and focuses on the integration of piezoelectric fibers and patches into lightweight structures aiming at active vibration and noise reduction, shape control and micro positioning. The main project target is the implementation of this technology in different industrial branches like the automotive industry, rail technology, mechanical engineering, medical engineering, and aerospace technology. This paper will give an overview of the recent progress and the next steps in the various tasks.
Civil transport airplanes fly with fixed geometry wings optimized only for one design point described by altitude, Mach number and airplane weight. These parameters vary continuously during flight, to which means the wing geometry seldom is optimal. According to aerodynamic investigations a chordwide variation of the wing camber leads to improvements in operational flexibility, buffet boundaries and performance resulting in reduction of fuel consumption. A spanwise differential camber variation allows to gain control over spanwise lift distributions reducing wing root bending moments. This paper describes the design of flexible Fowler flaps for an adaptive wing to be used in civil transport aircraft that allows both a chordwise as well as spanwise differential camber variation during flight. Since both lower and upper skins are flexed by active ribs, the camber variation is achieved with a smooth contour and without any additional gaps.