1. INTRODUCTION Aeroelasticity concerns the interaction of flexible structures with the surrounding air flow. Since aircraft structures are particularly flexible due to weight restrictions, aeroelasticity is commonly addressed by aeronautical 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 different ways. Under most flight conditions, the aerodynamic loads and the internal elastic loads in the structure will converge to some equilibrium, ie. a statically deformed structure in steady air flow. There are cases, however, where the loop becomes unstable, causing increasing deformations 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. Aeroelasticity is the branch of physics and engineering that studies the interactions between the inertial, elastic, and aerodynamic forces that occur when an elastic body is exposed to a fluid flow. Although historical studies have been focused on aeronautical applications, recent research has found applications in fields such as energy harvesting and understanding snoring. The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity, which deals with the static or steady response of an elastic body to a fluid flow; and dynamic aeroelasticity, which deals with the bodys dynamic (typically vibrational) response. Aeroelasticity draws on the study of fluid mechanics, solid mechanics, structural dynamics and dynamical systems. The synthesis of aeroelasticity with thermodynamics is known as aerothermoelasticity, and its synthesis with control theory is known as aero-servo-elasticity.

In an aeroplane, two significant static aeroelastic effects may occur. Divergence is a phenomenon in which the elastic twist of the wing suddenly becomes theoretically infinite, typically causing the wing to fail spectacularly. Control reversal is a phenomenon occurring only in wings with ailerons or other control surfaces, in which these control surfaces reverse their usual functionality. Dynamic aeroelasticity studies the interactions among aerodynamic, elastic, and inertial forces. Flutter and Buffeting are common examples of dynamic aeroelasticity.

Fig 1.1 collar triangle

2. 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 oscillations, and that are independent of the mass properties of the aircraft. All the phenomena can be shown by Venn diagram as shown in Fig. 1

Fig 2.1 Three Ring Venn diagram

One of these phenomena is static aeroelastic deformation, that characterizes the case where the air loads and the elastic forces of the structure converge to an equilibrium of constant structural deformation. This phenomenon is always present to some extent as the aircraft is subject to air loads. Another well-known phenomenon in static aeroelasticity is the decrease of control surface efficiency at high airspeeds. For example, as an aileron is detected downwards, the lift is increased. At the same time, however, the wing experiences a nose-down moment due to the lift produced in the trailing edge region. This moment twists the entire wing, causing negative lift. In fact, depending on wing stiffness and geometry, 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. Eventhough the deformation increases with time, this phenomenon is commonly 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 occurs when the unsteady aerodynamics cause forces that tend to increase the total energy involved in the motion of the structure and the surrounding 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 involved. Factors such as control surface free play, 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 vicinity 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.

3. HISTORY Aeroelastic loads created by lifting surface distortion have been an important part of aeronautical engineering from the very beginning of controlled, powered flight. In the late 1890s and early 1900s, Professor Samuel P. Langley developed an airplane, the Aerodrome, capable of being launched from a houseboat anchored in the Potomac River near Washington, D.C. This airplane failed on each of two attempts. The first failure, on October 7, 1903, ( Fig. 2 ) the disaster was probably due to a front-wing guy post catching on the catapult launch mechanism. The failure of the second (and final) Aerodrome flight. For a number of years this failure was attributed to insufficient wing torsional stiffness that led to structural static divergence, an instability that leads to excessive torsional deformation of the wing.

Fig. 3.1 Langley Aerodrome Failure, October 7,1903 In 1901 the Wright Brothers used a tethered kite, to demonstrate wing warping. Wing warping uses controlled,anti-symmetrical bi-plane wing structural twisting displacement to create aerodynamic rolling moments. The wing warping concept had been tested by Edson Gallaudet as early as 1898. He did not pursue, publish or patent this idea. The Wright Brothers obtained a patent for wing warping control, creating a financial boon for themselves but retarding aeronautical design as a result. Warping depends on building torsionally flexible wing surfaces easily distorted by the pilot, but the wings are also easily distorted by the airstream that may produce self excited, unintended airloads. During the first decade of powered flight, airplane speeds were low enough and structural stiffness large enough that loads due to aeroelastic deformation were inconsequential for most airplanes. There were spectacular exceptions. Like the Wright Brothers Flyer, the effectiveness of Bleriots wing warping roll control required relatively low wing torsional stiffness. As engine power and airspeed increased, low torsional stiffness created aeroelastic problems that led to wing failures at high speeds. During World War I a self-excited, vibratory aeroelastic instability, later called flutter, occurred on the horizontal tail of the British Handley Page O/400 bi-plane bomber. Flutter is a dynamic, oscillatory structural instability enabled by interactions between unsteady aerodynamic forces and moments created by vibratory motion of lifting surfaces and the vehicles to which these surfaces are attached. Following World War I, engines continued to become more powerful and horsepower-to-weight ratios increased. As airspeeds increased, monoplane designs reappeared, this time as low drag, semi-monocoque designs. A new type of aeroelastic instability, called wing-aileron flutter plagued aircraft designs. Just as the wing warping type of control had led to wing divergence, the new aileron control surfaces led to dynamic aeroelastic failures. Wing-aileron flutter is a self-excited vibration that occurs when lift generated by the oscillation of an aileron creates wing bending or torsion deformation. The oscillation frequency depends on airspeed because the aileron acts like a weathervane; its rotational stiffness and natural frequency increase as airspeed increases. The aileron acceleration, as well as the airloads transmitted to the wing, force the wing oscillations and create interactive coupled vibration.

4. MAJOR AERODYNAMIC PROBLEMS Among the several aerodynamic problems like divergence, load distribution, buffeting and many others, flutter and control effectiveness forms the most severe concern to the aero industry today.4.1 Flutter Flutter belongs to a special class of mechanics problems called non- conservative problems. The flutter mechanism depends on flying at or above an airspeed and altitude to allow two or more aircraft vibration modes to interact or couple together. Flutter is categorized into at least five different areas, each with its own characteristic modes of motion: 1) classical flutter wing bending & torsion; 2) control surface flutter surface rotation and wing bending; 3) empennage flutter fuselage torsion and tail torsion; 4) stall flutter wing torsion; 5) body freedom flutter wing bending and fuselage pitch. Aircraft and missile resonant natural frequencies, depend on stiffness, mass distribution, airspeed, altitude and Mach number. Disturbances decay with time at an airspeed corresponding to point A (Fig.4.1.1) where the resonant natural frequencies are well separated. As airspeed is increased to point B, an initial disturbance produces (after some transient motion) harmonic oscillatory motion at a fixed amplitude. An attempt to operate at the airspeed associated with point C will lead to disaster, since the amplitude of the response to the initial disturbance grows rapidly with time. The motions associated with airspeeds at A, B, C are classified as stable, neutrally stable and unstable, respectively. Flutter is not forced resonant response. The airstream causing flutter is steady and non- oscillatory until the system is disturbed. In addition, without internal structural damping, resonance response amplitude grows linearly with time, while the flutter dynamic response has an exponential increase until the structure is destroyed or some nonlinear mechanism limits the response amplitude.

Fig.4.1.1 Aircraft Vibration modes couple together to allow airstream energy to be absorbed by the structure4.2 Wing flutter and Divergence The dynamic interaction of airflow with a flight vehicle is one of the more complex problems to be solved in the aerospace field. Most aerodynamics deals with flow around rigid objects but, in fact, a flight vehicle is relatively light and there is always a degree of flexibility that can lead to interesting modes of motion. Aerodynamic forces applied to a vehicle will not only cause it to change flight path according to the rules of aircraft performance and flight mechanics but will also cause flexure of aircraft components relative to each other. These forced structural modes of motion lead to a class of problems that fall under the heading of aeroelasticity. One of the simplest interactions that is found in a fixed wing aircraft is the flexure of the wing relative to the rigid fuselage. For aircraft with slender straight cantilever wings two typical modes of motion exist. The first is a bending mode where the wing tip flexes up and down relative to the fixed wing root. The second is a twisting mode where the wing rotates about its stiffness axis, which is typically the spar. Normally there is minimal effect of these two modes on structural behaviour, with only a slight vibration being seen for each motion. The bending mode shows up as a relatively low frequency flapping effect while twisting mode is found to be a much higher frequency vibration. However, with the application of high-speed airflow as a source of excitation energy, these two modes can produce motions with will severely distort or break the wing. The first effect is called divergence. In this case the moment produced by the air load is greater than the structural torsional stiffness of the wing and thus it will be twisted off the vehicle. The threshold speed for this type of failure to occur is called divergence speed and will hopefully be much higher than any normal operating speeds of the vehicle. Particular problems occur with swept forward wings as these have a relatively low divergence speed. The second effect is called flutter. In this case there is a synchronised interaction between both modes so that energy is absorbed from the airflow in one mode to increase the amplitude of the other. At this point the frequency of each mode has converged to the same value so that only one combined mode is possible. The wing will absorb energy from the airflow and will behave as an ever increasing bending and torsion flexure until sufficient displacement is reached and the wing breaks. When the airflow is increased to the critical point to cause this failure, it is called the flutter speed. Again flutter should only occur at speeds much higher than operating speeds of the aircraft, but may be induced by inappropriate ratio of wing torsion and bending stiffness, or by addition of wing mass at points a long way behind the wing spar.An estimate of the occurrence of these conditions and the interaction of airflow on a wing can be obtained using a simple 2-degree of freedom (2dof) dynamic model of the wing. 4.3 Control effectiveness In addition to aeroelastic stability, other aeroelastic response phenomena plagued aircraft in the 1920s. Control effectiveness is the ability of a control surface such as an aileron or a rudder to produce aerodynamic forces and moments to control airplane orientation and maneuver along a flight path. A symmetrical aileron rotation produces rolling acceleration and roll rate. The ability to create a terminal or steady-state roll rate is the primary measure of aileron effectiveness.

Fig.4.2.1 Control effectiveness declines with increasing airspeed

Consider Fig. 4.2.1 Without wing torsional flexibility, the terminal roll rate is a linear function of airspeed. Rotating the aileron downward produces an effective angle of attack to produce lift, but also twists the wing surface nose-down, reducing the local wing angle of attack and reducing lift that creates the rolling moment. The size of the nose-down twisting moment and nose-down twist depends upon the: 1) size of the control surface; 2) amount of aileron deflection; 3) structural stiffness; and, 4) dynamic pressure, q. The terminal roll rate becomes maximum and then declines rapidly as airspeed increases. At a special airspeed, called the aileron reversal speed, the ailerons will not generate a rolling moment even though there is substantial wing surface distortion and substantial aileron rotation. At speeds above the reversal speed, the aileron produces a roll rate, but in the opposite direction to that intended. The aileron is reversed.

5. SWEPT WING AEROELASTICITY In 1935 Dr. Adolph Busemann and his colleagues in Germany proposed sweeping wings to delay the onset of wave drag due to high speed flow compressibility near Mach 1.High speed swept wing aircraft designs appeared in Germany late in World War II. There are three reasons to sweep a wing forward or backward: 1) to improve longitudinal stability by reducing the distance between the aircraft center of gravity and the wing aerodynamic center; 2) to provide longitudinal and directional stability for tailless (flying) wings; 3) to delay transonic drag rise (compressibility). Although not the first swept wing aircraft, the American B-47 jet bomber, was the first to encounter and to address high speed swept wing aircraft aeroelastic issues ranging from control effectiveness to flutter. Sweptback wing bending displacement reduces wing section angles of attack, leading to three static aeroelastic problems: 1) flexible sweptback wings are lift ineffective because wing bending reduces the total wing lift for a given wing angle of attack; 2) bending deformation moves the wing center of pressure inboard and forward; and, 3) bending displacement reduces sweptback wing aileron effectiveness as well as sweptback tail/rudder effectiveness so much that ailerons are often replaced by spoilers. On the positive side, local angle of attack reduction created by swept wing bending counters the increased angle of attack created by torsion. This cancellation makes wing divergence unlikely if wings are swept back more than 10-15 degrees, but more likely if the wings are swept forward. Sweepback exacerbates control reversal. The incremental lift created by a downward aileron deflection not only causes detrimental nose-down twist of an aft swept surface, but also bends the surface upward. Upward bending of a sweptback wing amplifies the aileron effectiveness problem. As a result, the spanwise location and sizing of control surfaces on sweptback wings is crucial to the success of the design. In some cases, the use of ailerons at high speeds is abandoned altogether and lift spoiler devices used in their place.

6. NONLINEAR AEROELASTICITY High-Altitude Long-Endurance (HALE) aircraft have wings with high aspect ratios. During operations of these aircraft, the wings can undergo large defections. These large deflections can change the natural frequencies of the wing which, in turn, can produce noticeable changes in its aeroelastic behavior. This behavior can be accounted for only by using a rigorous nonlinear aeroelastic analysis. Results are obtained from such an analysis for aeroelastic behavior as well as overall fight dynamic characteristics of a complete aircraft model representative of HALE aircraft. When the nonlinear fexibility effects are taken into account in the calculation of trim and fight dynamics characteristics, the predicted aeroelastic behavior of the complete aircraft turns out to be very different from what it would be without such effects. The overall fight dynamic characteristics of the aircraft also change due to wing flexibility. Nonlinear aeroelastic analysis has gathered a lot of momentum in the last decade due to understanding of nonlinear dynamics as applied to complex systems and the availability of the required mathematical tools. The studies conducted by Dugundji and his co-workers are a combination of analysis and experimental validation of the effects of dynamic stall on aeroelastic instabilities for simple cantilevered laminated plate-like wings. ONERA stall model was used for aerodynamic loads. Tang and Dowell have studied the flutter and forced response of a fexible rotor blade. In this study, geometrical structural nonlinearity and free-play structural nonlinearity is taken into consideration. Again, high-angle-of-attack unsteady aerodynamics was modelled using the ONERA dynamic stall model. Virgin and Dowell have studied the nonlinear behaviour of airfoils with control surface free-play and investigated the limit-cycle oscillations and chaotic motion of airfoils. Gilliatt, Strganac and Kurdila have investigated the nonlinear aeroelastic behavior of an airfoil experimentally and analytically. A nonlinear support mechanism was constructed and is used to represent continuous structural nonlinearities.

Aeroelastic characteristics of highly fexible aircraft was investigated by van Schoor and von Flotow. The complete aircraft was modeled using a few modes of vibration, including rigid-body modes. Waszak and Schmidt used Lagrange's equations to derive the nonlinear equations of motion for a fexible aircraft. Generalized aerodynamic forces are added as closed-form integrals. This form helps in identifying the effects of various parameters on the aircraft dynamics.Linear aeroelastic and flight dynamic analysis results for a HALE aircraft are presented by Pendaries. The results highlight the effect of rigid body modes on wing aeroelastic characteristics and the effect of wing fexibility on the aircraft flight dynamic characteristics The present study presents the results obtained using a low-order, high-fidelity nonlinear aeroelastic analysis. A theoretical basis has been established for a consistent analysis which takes into account, i) material anisotropy, ii) geometrical nonlinearities of the structure, iii) unsteady flow behaviour, and iv) dynamic stall. The formulation and preliminary results for the nonlinear aeroelastic analysis of an aircraft has been presented in an earlier paper.The present paper is essentially a continuation of the earlier work and presents more results specific to HALE aircraft. The results obtained give insight into the effects of the structural geometric nonlinearities on the trim solution, flutter speed, and flight dynamics.

7. ANALYSIS Historically, aeroelastic problems were first encountered when airplane design aimed at higher airspeed and lower weight. 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. In the 1930s, scientists started to research the theory behind many aeroelastic phenomena and simple analysis tools were established. 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 aircraft. To do this, a numerical model of the aircraft structure was coupled with forces from an aerodynamic model. But due to simplifications in both the structural model and in the aerodynamics, errors were always present. Due to the complexity of the unsteady aerodynamics, most simplifications were made for the aerodynamic forces. One of the simplest models known as strip theory is still used today. In strip theory, the three-dimensional aerodynamics are approximated by section-wise two-dimensional flow. This method yields typically good results for long slender wings. Due to the two-dimensional aerodynamics, the aerodynamic forces are over predicted, in most cases leading to conservative results for stability. Aircraft configurations, however, have changed, and many aircraft wings became shorter with lower aspect ratios and could no longer be assumed to be slender, making the strip theory too conservative. Also, the structural and aerodynamic modeling capabilities were improved. In the recent years, entire software packages were developed for relatively user-friendly modeling and analysis of aircraft structures. ZAERO, for example, uses an aerodynamic method based on linear un-steady potential flow, with the possibility to model bodies. In modern aviation, properties of flight control systems are commonly included in the analysis as well, since the closed-loop nature of such systems can interact with aeroelastic phenomena. This research area, having the objective of analyzing control systems considering aeroelastic interactions, is commonly referred to as aero-servo-elasticity (ASE).8. DESIGN AND CONTROL MEASUREMENTS Any structure is flexible to some extent, and due to weight restriction, aircraft structures are particularly flexible. Aeroelasticity is therefore always a concern in aircraft design. Most importantly, the designed aircraft must not suffer from aeroelastic instability within the flight envelope. But also, flexibility has to be considered when optimizing aircraft performance, e.g. in terms of lift-to-drag ratio. Therefore, any aircraft operating today, where large deformations are apparent in flight, is most likely designed to feature the most favorable deformation under some specific flight condition. For real aircraft to guarantee stability within the flight envelope, several analyses are required since many different flight conditions (altitudes, velocities, weights) have to be considered. Thousands of computations for different configurations are not unusual in the design of transport aircraft, and the result is often that there are some critical configurations that may lead to instability in a certain velocity and/or altitude range. Possible means of dealing with such results are either to avoid operation in this region of the envelope, or to modify the aircraft. Mass balancing or modification of the structural stiffness are most commonly performed for stabilization. In fact, one of the reasons for placing the engines of modern transport aircraft upstream of the wing is flutter suppression. Despite the large number of investigations using high-fidelity models, aircraft may still suffer from aeroelastic problems during flight. One reason for that may be neglected details in the numerical model, and often those problems can be eliminated afterwards by modifying the structural or aerodynamic properties of the aircraft slightly. This is however in most cases related to higher aircraft weight or other drawbacks. In general, it is preferable to analyze and solve the problems in the early design stages, both for cost and performance reasons. In the past, aeroelasticity was often seen as a problem that had to be eliminated when designing aircraft. A recent trend in research, however, is active aeroelasticity. The objective is to exploit aeroelastic effects for improved performance. This can be done in several ways. Many active aeroelastic concepts aim primarily at reducing the structural weight, and compensate for the resulting flexibility increase by active means. Other concepts aim directly at increasing aircraft performance by means that would not be possible with stiff structures. A few examples are presented in the following. In the US, the Active Aeroelastic Wing (AAW) research project was initiated for investigation of a leading-edge control surface on a flexible wing. The aircraft considered is the F-18 ( Fig.5).The F-18 in its original configuration suffered from severe problems with low aileron efficiency due to a highly flexible wing structure. The solution to this problem was a stiffer, but also heavier wing. In the AAW project, however, the original wing was reused, equipped with a leading edge aileron. Leading edge ailerons usually are not very efficient when applied to stiff structures. Applied to flexible structures, however, leading edge ailerons cause aeroelastic twist that actually increases the aileron efficiency, hence improving maneuverability especially at high airspeeds. The main advantage of the active concept is a considerably lighter wing. Similar research is going on in Europe, where the Active Aeroelastic Aircraft Structures project focuses on different concepts for improving aircraft performance by means of active aeroelasticity. Different concepts in the areas of aerodynamic control surfaces, all-movable control surfaces and active and passive structures are investigated. The concepts are demonstrated in laboratory or wind tunnel tests using several demonstrators. The most frequently used demonstrator within the project is the European Research Aeroelastic Wind Tunnel Model (EuRAM) at the TsAGI Institute for Aeroelasticity in Moscow. The model is a 1/10 length scale model of a 57 meters span transport aircraft. Similar to the AAW concept, a wing-tip control surface was attached to the wing. To increase the efficiency compared to a conventional leading edge flap, however, the control surface was placed upstream.

Fig 7.1. AAW research aircraft with leading edge flap Due to the flexibility of the wing structure, the wing tip control features high roll efficiency especially at high airspeeds. Other studies were performed where the wing tip device was proven useful for gust load alleviation as well. Active vibration control on traditional aircraft may not lead to significant improvements in ride quality. When reducing the wing structural weight, however, the resulting flexibility increase may lead to more significant vibrations due to higher sensitivity to aerodynamic gusts and turbulence. Gust load alleviation devices are therefore most beneficial when reducing weight and stiffness. Another concept implemented in the EuRAM model was an all-movable vertical tail (AMVT), that was used instead of the conventional tail. The objective was to use the AMVT in order to reduce the weight compared to a fixed tail with rudder. Lightweight and therefore flexible vertical tails of conventional type suffer from efficiency loss at high airspeeds, similar to trailing-edge ailerons. Using a flexible attachment at a downstream position of the AMVT, the elastic deformation actually increases the efficiency of the tail, similar to the leading-edge control surface concept. With increasing flexibility, however, the risk for divergence increases, and it was found beneficial to use a variable stiffness attachment to obtain reasonable efficiency, without the risk of divergence, for any airspeed in the envelope. Aeroelastic tailoring introduces structural bending/torsion elastic coupling by rotating the laminate fiber direction off-axis of the wing sweep axis. Swept forward wings with the fibers aligned along the wing swept axis leads to deformation called wash-in and increased airloads. This reduces the wing divergence airspeed compared to an unswept wing. Substantial added structural stiffness (and weight) is required to provide aeroelastic stability. Orienting laminate fibers slightly off-axis changes bend/twist displacement coupling. As the wing bends upward it twists in the nose-down direction, creating wash-out . This reduces the local airloads and increases wing divergence speed without extra weight. Aero-servo-elasticity uses interactive, active flight control to modify aeroelastic dynamic response and stability. In the past few decades, aircraft active flight control has brought flight mechanics much closer to aeroelasticity than it has been in the past. Until a few decades ago, except in unusual cases, aeroelasticians isolated lifting surface aeroelastic response from vehicle response. Aeroservoelasticity began by improving XB-70 supersonic bomber ride quality. Later active flutter suppression using actively controlled ailerons was demonstrated on a B-52 test aircraft.

9. APPLICATIONS Flow induced vibrations appear in many circumstances in nature and in different engineering concepts. Trees and flowers move in the wind, and flags flutter. Wind harps give an enjoyable sound and is an example of "positive" flow induced effects. Civil engineering structures, such as bridges and tall buildings, are typical constructions where flow induced vibrations must be taken into account. Flow induced vibrations are of major concern in the design of modern tube and shell heat exchangers (the problem is especially critical in nuclear steam generators that often are designed to last 30 years or more). Fluid flow through a flexible pipe, submarine periscopes, oil pipe lines, television antennas and telephone wires often encounter vibration troubles of aeroelastic origin.

Fig.8.1 Failure of Tacoma Narrows Bridge in 1940

Sluices for the regulation of water flows in rivers and dams vibrate under some circumstances, and blades in hydraulic and thermal turbomachines (both axial and centrifugal flow machines) are subject to large time-dependent variations in the oncoming flow. Vibrations of measurement instruments or their supports, such as long tubes holding neutron flux and temperature sensors in nuclear power plants reactor cores, are of concern. Among other examples of structures where flow induced vibrations are of importance, harbor and marine piles, offshore drilling and production platforms, smoke stacks and chimneys, missiles on launch pads, heat shields in afterburners of jet engines, propellers of aircraft and rotor blades of helicopters, can be mentioned. In other cases unsteady flow effects and induced vibrations lead to high noise levels, which can today be of major environmental concern. From the above it is clear that flow induced vibrations can appear in any sort of fluid (such as, for example, air, water, oil), but also in mixtures such as, for example, two- phase flows.

10. FUTURE ENHANCEMENT Today, the significance of aeroelasticity is well understood, and modern aircraft design incorporates analysis of aeroelastic stability and performance. There is however a gap between the state-of-the-art in research and the actual application in industry. Potential improvements of aircraft structures have been shown in research for example within the area of active aeroelasticity. Most of the promising concepts, however, have only been implemented on wind tunnel models or in laboratory tests. The task is now to evaluate the feasibility of the promising concepts for application in real aircraft. Scaling possibilities of actuators and materials have to be considered, as well as energy consumption, reliability, and other details that often remain unaddressed in research projects. Researches shown that only very few control surfaces are needed to obtain a significant improvement in performance. For real aircraft applications, this could indicate that the same methodology may lead to improvements by using already available flaps and ailerons without the need of extra control surfaces. Implementation studies for the tab on real aircraft could be performed, as well as further investigation of possible applications. Application of aeroelastic concepts in real aircraft is benefited by available tools for the industry to perform efficient analysis. Developed methods have to be either easy to apply, or they have to be made available in user-friendly software packages. Potential of applying robust tools in flutter analysis of uncertain analysis models. The described approach is based on an existing aeroelastic model, and besides the uncertainty description, no additional modeling has to be performed. This feature makes robust flutter analysis simple to apply on existing models. A wind tunnel model with fairly simple uncertainties was considered for validation of the approach. The applicability to real aircraft with more realistic uncertainties will be investigated in the future.

11. CONCLUSION This thesis summarizes investigations performed within design, analysis and experimental evaluation of flexible aircraft structures. Not only the problems, but rather the opportunities related to aeroelasticity are discussed. Aeroelasticity concerns the interaction of flexible structures with the surrounding airflow. The two classifications, say, static and dynamic aeroelasticity causes instability to the various flight conditions and even the structural failure. Aeroelastic effects were affected since the time of invention of aerodrome by Prof. Samuel P Langley. Several analysis tools and numerical models coupled with forces from aerodynamic model also introduced. Later Strip theory introduced which could provide changes to many aircraft configurations. Thus, aircraft designing begins to incorporate the aerodynamic effects. Many researches shows several remedial measures to aeroelastic problems like restricted weight of wing, modifications of structural and aerodynamic properties, study on active aeroelasticity and many others. Even though the basic physics behind most aeroelastic phenomena were understood very early, scaling possibilities of actuators and materials have to be considered, as well as energy consumption, reliability, and other details that often remain unaddressed in research projects. And several researches on this topic is still very active, aiming at higher accuracy in the predictions and increased efficiency of the analysis tools.

12. REFERENCE D.H. Hodges and G.A. Pierce, Introduction to Structural Dynamics and Aeroelasticity , Cambridge University Press, 2002, ISBN: 0521806984. E.H. Dowell, E.F. Crawley, H.C. Curtiss, Jr, D.A. Peters, R.H. Scanlan, F. Sisto, A Modern Course in Aeroelasticity , Kluwer Academic, 1995, ISBN 0792327888. J.R. Wright and J.E. Cooper, Introduction to Aircraft Aeroelasticity and Loads, Wiley, 2008, I6SBN: 0470858400 R.H. Scanlan and R. Rosenbaum, Introduction to the Study of Aircraft Vibration and Flutter, Dover Publications Inc., 1968. R.L. Bisplinghoff, H. Ashley, Principles of Aeroelasticity, Courier Dover Publications. Y.C. Fung, An Introduction to the Theory of Aeroelasticity, Courier Dover Publications, 2002, ISBN 0486495051, 9780486495057.

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