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Aeroelastic Concepts for Flexible Wing Structures AEROELASTIC CONCEPTS FOR FLEXIBLE WING STRUCTURES Sebastian Heinze Aeronautical and Vehicle Engineering Royal Institute of Technology SE-100 44 Stockholm, Sweden TRITA/AVE 2005:22 ISSN 1651-7660 Universitetsservice US AB, Stockholm 2005 Aeroelastic Concepts for Flexible Wing Structures 1 Preface The work in this thesis was performed between April 2003 and May 2005 at the Department of Aeronautical and Vehicle Engineering at the Royal Institute of Technology. Papers A and B were financially supported by the European research project Active Aeroelastic Aircraft Structures (3AS), funded by the European Commission under the Fifth Framework Programme. Paper C was financially supported by the Swedish Defence Materiel Administration (FMV). I would like to express my special gratitude to my advisor, Dan Bor- glund, who guided me into my life as a researcher, and who made me enjoy this profession. His technical understanding and enthusiasm were of great support at all times, and last but not least I appreciate him as a colleague and friend. I also would like to thank my supervisor, Pro- fessor Ulf Ringertz, for his support and guidance, and for trusting in me as a graduate student. Furthermore, in alphabetic order, special thanks to Carin, David, Fredrik, Marianne, Martin, Martin, Ulrik, and all the other guys at Far & Flyg for being - or having been - great colleagues and friends making me enjoy every day at work. Also thanks to all involved in the 3AS research project. Working with you was a great experience for me, I was able to see a variety of enthu- siastic people and innovative ideas, making me feel comfortable with choosing aeroelasticity as my field of research. In particular, I would like to express my gratitude to Professor Moti Karpel for his visionary way of thinking and for his contribution to my work. Last but not least, thanks to my family and friends back in Germany, who supported and encouraged me in all my decisions. Deepest grati- tude also to all my friends in Stockholm, in particular those at F18, for making me feel at home here in Sweden during the past years. Stockholm, May 2005 Sebastian Heinze Aeroelastic Concepts for Flexible Wing Structures 3 Abstract This thesis summarizes investigations performed within design, analy- sis and experimental evaluation of flexible aircraft structures. Not only the problems, but rather the opportunities related to aeroelasticity are discussed. In the first part of the thesis, different concepts for using active aeroelastic configurations to increase aircraft performance are consid- ered. In particular, one study deals with the minimization of the induced drag of a highly flexible wing by using multiple control surfaces. An- other study deals with a possible implementation of a high-bandwidth piezo electric actuator for control applications using aeroelastic amplifi- cation. The second part of the thesis deals with the development of an ap- proach for modeling and analysis of flexible structures considering un- certainties in analysis models. Especially in cases of large structural vari- ations, such as fuel level variations, a fixed-base modal formulation in robust flutter analysis may lead to incorrect results. Besides a discussion about this issue, possible means of treating this problem are presented. Aeroelastic Concepts for Flexible Wing Structures 5 Dissertation This licentiate thesis is based on a brief introduction to the area of re- search and the following appended papers: Paper A D. Eller and S. Heinze. An Approach to Induced Drag Reduction with Experimental Evaluation. To appear in AIAA Journal of Aircraft. Paper B S. Heinze and M. Karpel. Analysis and Wind Tunnel Testing of a Piezo- Electric Tab for Aeroelastic Control Applications. To be presented at the EADS/CEAS/DLR/AIAA International Forum on Aeroelasticity and Struc- tural Dynamics, Munich, Germany, June 28-July 1, 2005. Paper C S. Heinze. and D. Borglund. On the Influence of Modeshape Varia- tions in Robust Flutter Analysis. TRITA/AVE 2005:23, Department of Aeronautical and Vehicle Engineering, KTH, 2005. To be submitted for publication. Aeroelastic Concepts for Flexible Wing Structures 7 Contents Preface 1 Abstract 3 Dissertation 5 Introduction 9 Aeroelastic phenomena . 9 Analysis . 11 Design . 13 Active aeroelastic concepts . 14 Future work 19 References 21 Division of work between authors 25 Appended papers Aeroelastic Concepts for Flexible Wing Structures 9 Introduction Aeroelasticity concerns the interaction of flexible structures with the sur- rounding airflow. Since aircraft structures are particularly flexible due to weight restrictions, aeroelasticity is commonly addressed by aeronau- tical engineers. As an aircraft moves through the air, loads will act on the structure and on the surrounding air, leading to a perturbation of the flow field, but also causing deformations of the flexible structure. These deformations change the geometry of the structure, which in turn leads to a change of the flow field and the aerodynamic loads, resulting in a closed loop of loads and deformations. This loop can develop in differ- ent ways. Under most flight conditions, the aerodynamic loads and the internal elastic loads in the structure will converge to some equilibrium, i.e. a statically deformed structure in steady airflow. There are cases, however, where the loop becomes unstable, causing increasing defor- mations with or without oscillations, finally leading to structural failure of the aircraft. Even though the basic physics behind most aeroelastic phenomena were understood very early, the research on this topic is still very active, aiming at higher accuracy in the predictions and increased efficiency of the analysis tools. Aeroelastic phenomena The aeroelastic phenomena considered as problems in current aircraft industry are similar to those at the very beginning of flight. In general, two classes can be defined: static and dynamic aeroelastic phenomena. Static aeroelasticity concerns all phenomena that do not involve oscilla- tions, and that are independent of the mass properties of the aircraft. One of these phenomena is static aeroelastic deformation, that char- acterizes the case where the airloads and the elastic forces of the struc- ture converge to an equilibrium of constant structural deformation. This phenomenon is always present to some extent as the aircraft is subject to airloads. Figure 1 shows a glider aircraft in flight, where considerable wing bending is present. Another well-known phenomenon in static aeroelasticity is the de- crease of control surface efficiency at high airspeeds. For example, as an aileron is deflected downwards, the lift is increased. At the same time, however, the wing experiences a nose-down moment due to the lift pro- duced in the trailing edge region. This moment twists the entire wing, causing negative lift. In fact, depending on wing stiffness and geometry, 10 S. Heinze Figure 1: The eta glider aircraft. there is a certain airspeed, called the reversal speed, where the positive lift of the aileron deflection is compensated by negative lift due to wing twist, making any control input on the aileron ineffective. At airspeeds exceeding the reversal speed, the aileron efficiency will have a negative value, i.e. the airplane rolls in the direction opposite to the pilot input. A problem often experienced in the early days of aviation was wing divergence. Divergence characterizes the phenomenon where an initial deformation of the wing leads to aerodynamic loads that increase the deformation further, finally leading to failure of the structure. Even though the deformation increases with time, this phenomenon is com- monly classified as a static phenomenon, since there are no oscillations involved, and since it is independent of the mass properties of the wing. One of the most important dynamic phenomena is flutter. Flutter oc- curs when the unsteady aerodynamics cause forces that tend to increase the total energy involved in the motion of the structure and the sur- rounding air. In other words, flutter is a fluid-structure interaction with negative damping, leading to oscillations with a magnitude increasing with time. Virtually all aircraft structures will suffer from flutter at some airspeed, and this phenomenon is one of the greatest concerns related to aeroelasticity in the aircraft industry today. The flutter phenomenon can be particularly difficult to predict due to the complex physics in- volved. Factors such as control surface freeplay, structural imperfections or slight changes in the mass distribution may be enough to cause flutter. Other subjects, closely related to the flutter phenomenon, are gust response and vibration. Especially when aircraft operate in the vicin- Aeroelastic Concepts for Flexible Wing Structures 11 ity of the flutter speed, the damping of the fluid-structure interaction may be very low, making the aircraft very sensitive to turbulence in the air. Even though stability may be guaranteed, the ride quality may not be acceptable and the structure will be subject to larger deformations leading to higher loads and reduced lifetime due to structural fatigue. Analysis Historically, aeroelastic problems were first encountered when airplane design aimed at higher airspeed and lower weight, a development that progressed especially during World War I and II. As a result of this, aeroelastic problems occurred repeatedly during test flights, and the need for analysis tools was established since it was both expensive and dangerous to investigate the aeroelastic behavior of aircraft by flight testing only. In many cases, rules of thumb were applied due to lack of knowledge. For example, swept wings were partly introduced because they were found to prevent divergence. In the 1930s, scientists started to research the theory behind many aeroelastic phenomena [1, 2], and simple analysis tools were estab- lished. Any analysis was based on the so-called equations of motion relating elastic, inertial, damping and aerodynamic loads to describe the motion of the system. The theoretical model made it possible to understand the physics, but it was hard to apply the theory to real air- craft. To do this, a numerical model of the aircraft structure was coupled with forces from an aerodynamic model.
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