Active Flutter Suppression of a Highly Flexible Swept Wing through Multiple Flap Control

Future commercial aircraft designs tend to light-weight wing structures for energy and cost saving reasons. The slenderness and flexibility of these higher aspect-ratio wings might show susceptibility not only to lower flutter flight speeds. Also, the danger of a larger number and variety (e.g. through sweep induced coupling) of potentially wing-dominated flutter modes is impending. The main objective of the submitted work is to depict a suitable control methodology, and show how dynamic stability of highly flexible wings can be achieved by actively influencing the aeroelastic behaviour of the overall deformed structure. On the prediction side, the design, build-up and analysis of aero-servoelastic simulation models become necessary. The incorporation of active control elements like sensors, actuators and controllers leads to closed-loop models. On simulation side, the strategic goal of this investigation is to reach the capability of completely erasing any kind of flutter occurrence, with no restrictions, neither in composition (e.g. heave, torsion, in-plane sway or flap dominated) nor in number (e.g. up to 4) of eigenmodes. The design of the controller transfer functions turned out to be crucial. In order to have the desired variety of eigenmodes at disposal, here a number of three configurations of the nominal generic wing design (baseline with five deflection-controlled flaps plus two modifications) underwent the analyses. By means of active flutter suppression (AFS), formerly unstable wing configurations could be transferred into aeroelastic stable flight conditions at any point of a potential flight envelope

Specifying the light-absorbing properties of aerosol particles in fresh snow samples, collected at the Environmental Research Station Schneefernerhaus (UFS), Zugspitze

Atmospheric aerosol particles like mineral dust, volcanic ash and combustion particles can reduce Earth’s snow and ice albedo considerably even by very small amounts of deposited particle mass. In this study, a new laboratory method is applied to measure the spectral light absorption coefficient of airborne particles that are released from fresh snow samples by an efficient nebulizing system. Threewavelength photoacoustic absorption spectroscopy is combined with refractory black carbon (BC) mass analysis to determine the snow mass-specific and BC mass-specific absorption cross sections. Fullerene soot in water suspensions are used for the characterization of the method and for the determination of the mass-specific absorption cross section of this BC reference material. The analysis of 31 snow samples collected after fresh snowfall events at a high-altitude Alpine research station reveals a significant discrepancy between the measured snow mass-specific absorption cross section and the cross section that is expected from the BC mass data, indicating that non-BC light-absorbing particles are present in the snow. Mineral dust and brown carbon (BrC) are identified as possible candidates for the non-BC particle mass based on the wavelength dependence of the measured absorption. For one sample this result is confirmed by environmental scanning electron microscopy and by single-particle fluorescence measurements, which both indicate a high fraction of biogenic and organic particle mass in the sample

Observation of enhanced chiral asymmetries in the inner-shell photoionization of uniaxially oriented methyloxirane enantiomers

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical, it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer - an effect termed Photoelectron Circular Dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects as the circular dichroism in angular distributions (CDAD). Accordingly, orienting a chiral molecule in space enhances the PECD by a factor of about 10

The aeroelastic impact of engine thrust and gyroscopics on aircraft flutter instabilities

Since more and more modern civil aircraft are equipped with UHBR-engines for reasons of fuel efficiency and environmental aspects, the need to tackle specific engine related dynamic problems has occurred. The request for UHBR-engines with high bypass ratio numbers and with their intrinsic advantages of economic fuel consumption and lower acoustic emission asks for enhanced vibration prediction capabilities. Beside the energetic benefits such engines add to the aircraft design their rotating large diameter fans can influence the dynamic behaviour of the complete elastic aircraft fuselage in a very unfavourable manner. Additional questions which arise with regard to structural dynamics and aeroelastic stability are treated in this work. Especially in the scenario when large rotating engine masses are to be combined with elastic wing structures the possible occurrence of specific structural vibration problems can be avoided by taking the gyroscopic effects into account. As another important engine related aspect the modelling and the impact of the engine thrust is highlighted by integrating the first order deformation induced terms into the dynamical simulation model. By introducing an increased coupling level between the degrees of freedom in the equation of motion (through additional off-diagonal terms) both eigenfrequencies and eigenmodes are affected. In the case of high engine participation in the structural deformation we can observe a lowering of the eigenfrequencies (in the test aeroplane up to 6[%]) and a loss in symmetry properties in the now strongly asymmetric eigenmodes. With the occurrence of flutter cases the critical speed had been experienced to shift about an amount of similar magnitude. Although in the presented cases the flutter speed moved to higher values, it was found indespensable to check every individual aircraft configuration with regard to the stability margin

Using Multibody Dynamics for the Stability Assessment of a New Rotor Test Rig

The secure entry into service of a new rotor test rig requires the assessment of the dynamic and aeroelastic rotor stability. To this end, a multibody dynamics based numerical model was developed and coupled with an unsteady aerodynamic model based on Wagner's function and related enhancements for the general motion of an airfoil section considering heave and pitch motion. The simulation model uses modelling techniques for the setup of a linearized model and allows both, the investigation of ground resonance and flutter for the rig with clamped and articulated rotor blades in frequency domain. With respect to ground resonance, the dynamic examination of the two- and four-bladed rotor configurations shows a mechanically stable behaviour for the clamped and articulated rotor blades with lead-lag hinge in the planned rotor speed range up to 65 Hz. The aeroelastic assessment shows a damped behaviour for the configurations with clamped rotor blades, whilst the articulated rotor with lead-lag hinge is unstable beyond rotational speeds of 25 Hz for the two-bladed rotor and requires additional damping measures

From FEM to MBS: Stability analysis of the elastic H/C-rotor

As a consequence of the variety of effects the rotation has on the vibrating structure it is important to take into account the complete shares of the gyroscopic influence in the equation of motion. One prerequisite will be the formulation of the mass terms for all three axis of rotational movement of the vibrating rotor even if it deals with slender beam like structures. This is the case as well in the in-house Finite Element Method code GYRBLAD (FEM) as in the commercial Multi Body System code SIMPACK (MBS), which both have been applied in this investigation. The numerical calculations of the eigenmodes and the stability behaviour of the rotor will be conducted by using two different modelling concepts: the advantage of the FEM code lies in the capability of describing the deformation of a flexible structure in an already linearised manner (Euler-Bernoulli beam), whereas the potential of the MBS code comes from the complete nonlinear formulation of the arbitrarily large movements of elastically interacting (rigid) bodies in an equilibrium or accelerated state. In order to take into account the characteristics of the flexible continuum also in MBS the code offers the special feature FEMBS for combining the FEM with MBS modelling. Thus the potential of a sophisticated, hybrid MBS code like SIMPACK as a powerful simulation tool for helicopter dynamics will be demonstrated with respect to the dynamics of the elastic rotor. For validation purpose the results of the FEM and the MBS code are quantitatively compared to the results of the analytical description of the dynamic behaviour of the rigid body rotor. Representing the case of fixed boundary conditions at a rigid hub the results of a single rotating blade are shown. The Princeton beam with its double symmetric cross section allows the focus on the DOF coupling as a result only from the rotation. For a single blade the pure gyroscopic coupling will be displayed for the flapping-torsion and the lagging-stretching movement. The investigation of the complete rotor (four and six blades) follows where all classes of rotor eigenmodes (collective, cyclic and reactionless) will be studied. As results for the eigenbehaviour the coupled complex eigenmodes and the variation of the eigenfrequencies with respect to the rotation speed will be shown (fan diagrams). Resonance phenomena in the eigenmodes occur at specific rotor speeds and frequencies where the pitch (torsion) amplitude rises over all measures. Although slim and slender bodies with a high aspect ratio are investigated not negligible coupling effects specially on the blade pitch movement have to be stated. For the aeroelastic stability analysis of the rotating elastic helicopter blade this can be highly hazardous

The structural dynamics of a free flying helicopter in MBS- and FEM-analysis

Since Multi Body System (MBS) codes have been proved to be potentially powerful simulation tools in the whole range of helicopter rotor dynamics, here the question of modelling the free flying helicopter in a pure MBS as well as in a hybrid FEMBS dynamical simulation model is highlighted. The objective of this research work are modelling techniques for decribing the dynamical behaviour and the struchtural interaction between helicopter rotors - main and tail rotor - and the nacelle of a free flying helicopter. Here the focus lies on the coupling of the rotating structure of the fully elastic main rotor with the non-rotating parts of the body structure via a flexible rotor-nacelle interface. As simulation platform the 9[to] generic model "Helicopter H9" has been developed. Representing the research object for this investigation it serves as a demonstrator model and as dynamic reference configuration for both the MBS and the FEM calculations. The "Helicopter H9" has a five blade main rotor with a diameter of D=16[m], a four blade tail rotor with a diameter of D=2.8[m] and a MTOW of 9118.4[to]. Investigated are modelling techniques for simulating the dynamics of the structural behaviour of the free flying helicopter in the frequency domain. On the MBS side the commecial tool SIMPACK is tested while on the FEM side the scientific rotor code GYRBLAD is used. For reasons of a better clarification of the rotor-cell coupling effects the center of gravity of the helicopter fuselage exhibits large offsets in all three coordinate directions. As a consequence we get a highly non-symmetrical dynamical system w.r.t the main rotor axis and a rotated principal axes system. By the fact that the main rotor axis does not coincide with any of the three inertial axes all three rigid body rotational modes will be coupled by the main rotor gyroscopic effect.\ud Concerning the specific dynamic coupling effects between rotor and nacelle a survey study with topics like the main rotor suspension (lateral and vertical) or the elasticity of the drive train had been conducted. In systematic variation of the respective stiffness values (over four decades) the results of different parameter studies are presented as numerical results for single constant rotor speeds as well as in frequency fan diagrams for the overall dynamical behaviour under the change of rotor speed. By applying different blade pitch angles the influence of the blade pitch positon on the rotor eigenbehaviour has being tested. By introducing different kinematical and dynamical boundary conditions, cases of stability loss due to ground resonance could be reproduced for the isolated rotor. Even cases of stability loss of the free flying helicopter concerning elastical eigenmodes of the coupeled rotor-nacelle-system - an air resonance type - could be detected in this work. The validation of the models finally was done by comparing the eigenmodes and the eigenvalue results produced with the two elasto-mechanical methods MBS and FEM. Thus different algorithms and independent tools have been used in the examination. It has been shown that for the non-rotating as well as for the rotating test cases the coupling effects will be reproduced without any restriction in both approaches. Thus the potential of a sophisticated MBS code like SIMPACK as a powerful simulation tool for helicopter dynamics has been demonstrated with respect to the dynamics of the free flying helicopter

Aeroelastische Untersuchung der ALLEGRA-Konfiguration (Version "S")

Dieser Bericht behandelt die aeroelastischen Stabilitätsuntersuchungen an der ALLEGRA-Flugzeugkonfiguration in der Entwurfsversion "S". Er beschreibt den Aufbau des Simulationsmodells, umreißt den Lösungsgang und enthält die Ergebnisse der Flatteranalyse für zwei ausgesuchte Beladungszustände (c01 und c09) für jeweils zwei Flughöhen (5400 um 11000 m). Diese Untersuchung hatte zum einen den Nachweis der Flattersicherheit der ALLEGRA-Konfiguration innerhalb der Flugenveloppe zum Ziel; zum anderen lassen sich durch die Ausweitung des untersuchten Geschwindigkeitsbereiches und das Auffinden instabiler Zustände auch bei höheren Fluggeschwindigkeiten Erkenntnisse über das aeroelastische dynamische Stabilitätverhalten im Falle von z.B. Strukturveränderungen gewinnen (Klassifizierung der Flatterzustände). Als Ergebnisse der aeroelstischen Eigenwertanalysen werden mit den Flatterfrequenzen, den Flatterformen und den jeweiligen kritischen Geschwindigkeiten die wesentlichen Kennzahlen der Flatterfälle präsentiert. Durch Erfassung des Einflusses des Triebwerksschubes und der Gyroskopie des Triebwerksrotors wurden Erweiterungen höherer Ordnung an dem Basis-Simulationsmodell vorgenommen. Die Auswirkungen dieser linearen Modellerweiterungen der Triebwerke unter Betriebsbedingungen auf die Flatterzustände wurden untersucht

The MBS modelling of structural blade offsets and its impact on the eigenbehaviour of elastic helicopter rotors

Since Multi Body System codes (MBS) have been proved to be potentially powerful simulation tools in the whole range of helicopter rotor dynamics, in this study the question of modelling the structural blade cross-section offsets in the MBS models is highlighted. The relative positions of the characteristic points of the cross-sections of helicopter blades like the shear center, the neutral axis and the cross-sectional center of gravity are formative for the dynamic behaviour of the rotating structure. Even the location of the reference point of the cross-section — which is defined by the radial connection between the hub and the cross- section, standing normal on the respective plane — plays an important role concerning the equilibrium of a rotating blade differential mass element. Although in helicopter blade design generally efforts are made to keep the offsets of these characteristic structural points small they can reach non-negligible extents and thus, together with the gyroscopic effects, contribute significantly to the coupling mechanisms between the motion components of the vibrating elastic blade structure like the flapping, lagging and torsional deformation. Also in terms of aeroelastic stability the cross-sectional offsets may have a dominant influence. Here the scope of modelling blade offsets in MBS is to capture the complete variety of offsets resulting in the full range of mechanical coupling mechanisms like the bending-torsion coupling, the bending-longitudinal coupling and the bending-bending coupling (with “bending” meaning the flapping or lagging motion respec- tively). It is shown how special joint modelling techniques and the usage of “pseudo” bodies with additional DOF are introduced into the “pure” MBS model to reach this aim. The two basic approaches for incorpo- rating elastic properties into a MBS model are addressed. The way of mapping the continuously distributed elastic properties of the blade beam structure on dynamic equivalent discrete spring stiffnesses of a “pure” MBS model is compared with FEM models which can be used as basis in the strategy of importing separately built upp elastic Finite Element models with modal substructure techniques (FEMBS), thus resulting in a hybrid MBS model. While modelling of structural offsets within the pure MBS model refers to the genuine characteristics of the MBS modelling approach, the hybrid FEMBS model inherits all the advantageous (or deficient) properties of the incorporated Finite Element substructure. Since for the rotating blade the equilibrium state is now not only defined by the longitudinal normal forces but is three-dimensional, it is important for the geometric stiffness matix Sg of the FEM formulation to contain the whole set of second order terms, while the cutting forces depend linear on the cross-sectional offsets and contribute linear to the geometric stiffness. Because the completeness of the FEM model constitutes the quality of the FEMBS solution by providing in particular the geometric stiffness matrix of the rotating blade, a separate FEM model has been analysed. On the MBS side the commecial tool SIMPACK has been tested while on the FEM side the inhouse tool GYRBLAD is used. As fully elastic single blade examples generic test beam cases with considerable offsets are used. The validation of the models is done by comparing the eigenvalue results produced with the two independent elasto-mechanical methods MBS and FEM

The aeroelastic behaviour of a forward-swept wing configuration with focus on engine gyroscopics and T-tail flutter

Since more and more modern civil aircraft for reasons of fuel efficiency and environ- mental aspects are equipped with UHBR-engines, the need to tackle specific engine related dynamic problems has occurred. The request for UHBR-engines with high by-pass ratio numbers and with their intrinsic advantages of economic fuel consumption and lower acoustic emission asks for enhanced prediction capabilities. Beside the energetic benefit such engines add to the aircraft design their rotating large diameter fans can influence the dynamic behaviour of the complete elastic aircraft fuselage in a very unfavourable manner. Especially in the scenario when large rotating engine masses are to be combined with elastic suspension structures the possible occurrence of structural vibration problems can be avoided by taking the gyroscopic effects into account. As another important engine related question the modelling and the impact of the engine thrust is highlighted by integration of the follower force induced terms into the dynamical simulation model. A further approach towards lower fuel consumption is the drag reduction of the airplane. This can be realized by keeping the flow field around the wing surfaces laminar as much as possible. With the ALLEGRA-S configuration a short and medium range aircraft has been designed with the aim of drag reduction by keeping the wing flow laminar as long as possible. Together with laminar aerodynamic wing airfoil sections the forward sweep of the wings has a favourable influence on the laminar character of the wing flow. The forward swept wings as well as the T-tail empennage and the backward position of the engine nacelles on both sides of the fuselage also have a formative influence on the flutter behaviour and thus the stability margins of the design. The flutter behaviour of several baseline mass configurations has been examined. Important questions with regard to the enhancement of the flutter model and the impact on structural dynamics and aeroelasticity are treated in this work. For example by introducing additional d.o.f. coupling into the aeroelastic model the component correction terms have influenced particular flutter eigenmodes and caused (minor) deviations in flutter frequencies and velocities