Peter Wierach

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.

Characterization of Lamb wave attenuation mechanisms

Structural Health Monitoring (SHM) based on Lamb waves, a type of ultrasonic guided waves, is a promising technique for in-service inspection of composite structures. This study investigates the attenuation mechanisms of Lamb wave propagation fields. The attenuation of an anisotropic plate is experimental measured with air-coupled ultrasonic scanning techniques and analytical modeled using higher order plate theory. Based on the experimental and analytical data the various attenuation mechanisms are characterized for the fundamental Lamb wave modes.

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.

Design of a high-lift experiment in water including active flow control

This paper describes the structural design of an active flow-control experiment. The aim of the experiment is to investigate the increase in efficiency of an internally blown Coanda flap using unsteady blowing. The system uses tailor-made microelectromechanical (MEMS) pressure sensors to determine the state of the oncoming flow and an actuated lip to regulate the mass flow and velocity of a stream near a wall over the internally blown flap. Sensors and actuators are integrated into a highly loaded system that is extremely compact. The sensors are connected to a bus system that feeds the data into a real-time control system. The piezoelectric actuators using the d 33 effect at a comparable low voltage of 120 V are integrated into a lip that controls the blowout slot height. The system is designed for closed-loop control that efficiently avoids flow separation on the Coanda flap. The setup is designed for water-tunnel experiments in order to reduce the free-stream velocity and the systems control frequency by a factor of 10 compared with that in air. This paper outlines the function and verification of the systems main components and their development.

Carbon nanotube array actuators

Experimental investigations of highly vertically aligned carbon nanotubes (CNTs), also known as CNT-arrays, are the main focus of this paper. The free strain as result of an active material behavior is analyzed via a novel experimental setup. Previous test experiences of papers made of randomly oriented CNTs, also called Bucky-papers, reveal comparably low free strain. The anisotropy of aligned CNTs promises better performance. Via synthesis techniques like chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD), highly aligned arrays of multi-walled carbon nanotubes (MWCNTs) are synthesized. Two different types of CNT-arrays are analyzed, morphologically first, and optically tested for their active characteristics afterwards. One type of the analyzed arrays features tube lengths of 750–2000 �m with a large variety of diameters between 20 and 50 nm and a wave-like CNT-shape. The second type features a maximum, almost uniform, length of 12 �m and a constant diameter of 50 nm. Different CNT-lengths and array types are tested due to their active behavior. As result of the presented tests, it is reported that the quality of orientation is the most decisive property for excellent active behavior. Due to their alignment, CNT-arrays feature the opportunity to clarify the actuation mechanism of architectures made of CNTs.

Carbon nanotubes as actuators in smart structures

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 Youngs 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.

Poling Effects in Melt-Spun PVDF Bicomponent Fibres

This research shows the successful functionalisation of bicomponent fibres, consisting of a conductive polypropylene (PP) core, doped with carbon nanotubes (CNT) and a piezoelectric sheath (polyvinylidene fluoride, PVDF) by draw winding and poling. These steps lead to the usability of the PVDF’s piezoelectric capabilities. The PP/CNT constitutes the fibre core that is conductive due to a percolation CNT network. The PVDF sheath’s piezoelectric effect is based on the formation of β phase crystals (all-trans conformation), caused by draw-winding of the fibres. This β phase eventually has to be poled for the uniform alignment of polymer chains. The material’s behaviour in high electric field is analysed recording the poling voltage during the poling process. The outcome is hysteresis curves for different β phase contents, which verify a successful material poling.

Damage Assessment in Composite Structures Based on Acousto-Ultrasonics - Evaluation of Performance

This work focuses on the damage detection and assessment of barely visible impact damages that occur after impacting a composite stiffened structure through the acousto-ultrasonics technique. Delaminations and debondings have been introduced in two stiffened panels and afterwards interrogated with an integrated structural health monitoring (SHM) network within the project. In order to perform the damage assessment, a methodology for the identification of damages along with a graphical user interface is developed. The performance of the developed methodology is evaluated and a damage assessment based on several damage indices and actuation frequencies is carried out. The most suitable parameters for the given structure and damage types are finally reported.

Experimental and finite element analyses of multifunctional skins for morphing wing applications

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.