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Aeroelasticity is the science which studies the interactions among inertial, elastic, and aerodynamic forces. It was defined by Arthur Roderick Collar in 1947 as "the study of the mutual interaction that takes place within the triangle of the inertial, elastic, and aerodynamic forces acting on structural members exposed to an airstream, and the influence of this study on design." In more simple terms, it is the same set of conditions causing a flag to flutter in a stiff breeze or a reed to tremble in fast-flowing water. Flutter may occur in any fluid medium.
Airplane structures are not completely rigid, and aeroelastic phenomena arise when structural deformations induce changes on aerodynamic forces. The additional aerodynamic forces cause an increase in the structural deformations, which leads to greater aerodynamic forces in a feedback process. These interactions may become smaller until a condition of equilibrium is reached, or may diverge catastrophically if resonance occurs.
Steady aeroelasticity 
Steady aeroelasticity studies the interaction between aerodynamic and elastic forces on an elastic structure. Mass properties are not significant in the calculations of this type of phenomena.
Divergence occurs when a lifting surface deflects under aerodynamic load so as to increase the applied load, or move the load so that the twisting effect on the structure is increased. The increased load deflects the structure further, which brings the structure to the limit loads and to failure.
One such example case where this occurs is the divergent loading of a wing whose span-wise stiffness increases across the wing chord. In this case the trailing edge of the wing is stiffer along the wing span than the leading edge. When the wing is loaded aerodynamically, under lift, the leading edge deflects faster than the trailing edge yielding an increased angle of attack. This, in turn, increases the coefficient of lift resulting in increased lift load which further increases the wing loading. Failure to arrest this divergence is likely to result in structural failure of the wing.
Control surface reversal 
Control surface reversal is the loss (or reversal) of the expected response of a control surface, due to structural deformation of the main lifting surface.
Dynamic aeroelasticity 
Dynamic Aeroelasticity studies the interactions among aerodynamic, elastic, and inertial forces. Examples of dynamic aeroelastic phenomena are:
Flutter is a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion. Flutter can occur in any object within a strong fluid flow, under the conditions that a positive feedback occurs between the structure's natural vibration and the aerodynamic forces. That is, the vibrational movement of the object increases an aerodynamic load, which in turn drives the object to move further. If the energy input by the aerodynamic excitation in a cycle is larger than that dissipated by the damping in the system, the amplitude of vibration will increase, resulting in self-exciting oscillation. The amplitude can thus build up and is only limited when the energy dissipated by aerodynamic and mechanical damping matches the energy input, which can result in large amplitude vibration and potentially lead to rapid failure. Because of this, structures exposed to aerodynamic forces — including wings and aerofoils, but also chimneys and bridges — are designed carefully within known parameters to avoid flutter. In complex structures where both the aerodynamics and the mechanical properties of the structure are not fully understood, flutter can only be discounted through detailed testing. Even changing the mass distribution of an aircraft or the stiffness of one component can induce flutter in an apparently unrelated aerodynamic component. At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or lead to the destruction of the aircraft, as in Braniff Flight 542.
In some cases, automatic control systems have been demonstrated to help prevent or limit flutter-related structural vibration.
Flutter as a controlled aerodynamic instability phenomenon is used intentionally and positively in wind mills for generating electricity and in other works like making musical tones on ground-mounted devices, as well as on musical kites. Flutter is not always a destructive force; recent progress has been made in small scale (table top) wind generators for underserved communities in developing countries, designed specifically to take advantage of this effect. Peter Allan Sharp (of Oakland, California) and Jonathan Hare (of University of Sussex) demonstrated, in March 2007, a linear generator run by two flutter wings. The wind energy industry distinguishes between flutter wings, flip wings, and oscillating tensionally-held sweeping membrane wings for wind milling.
Dynamic response 
Dynamic response or forced response is the response of an object to changes in a fluid flow such as aircraft to gusts and other external atmospheric disturbances. Forced response is a concern in axial compressor and gas turbine design, where one set of aerofoils pass through the wakes of the aerofoils upstream.
Buffeting is a high-frequency instability, caused by airflow separation or shock wave oscillations from one object striking another. It is caused by a sudden impulse of load increasing. It is a random forced vibration. Generally it affects the tail unit of the aircraft structure due to air flow down stream of the wing.
Transonic Aeroelasticity 
Flow is highly non-linear in transonic regime dominated by moving shock waves. It is mission crtical for aircraft that fly through transonic Mach numbers. Role of shock waves was first analyzed by Holt Ashley. A phenenenon that impacts stability of aircraft known as 'transonic dip' where the flutter speed can get close to flight speed was reported by Farmer and Hanson, May 1976 of Langley Research Center.
Other fields of study 
Other fields of physics may have an influence on aeroelastic phenomena. For example, in aerospace vehicles, stress induced by high temperatures is important. This leads to the study of aerothermoelasticity. Or, in other situations, the dynamics of the control system may affect aeroelastic phenomena. This is called aeroservoelasticity.
Prediction and cure 
Aeroelasticity involves not just the external aerodynamic loads and the way they change but also the structural, damping and mass characteristics of the aircraft. Prediction involves making a mathematical model of the aircraft as a series of masses connected by springs and dampers which are tuned to represent the dynamic characteristics of the aircraft structure. The model also includes details of applied aerodynamic forces and how they vary.
The model can be used to predict the flutter margin and, if necessary, test fixes to potential problems. Small carefully chosen changes to mass distribution and local structural stiffness can be very effective in solving aeroelastic problems.
These videos detail the Active Aeroelastic Wing two-phase NASA-Air Force flight research program to investigate the potential of aerodynamically twisting flexible wings to improve maneuverability of high-performance aircraft at transonic and supersonic speeds, with traditional control surfaces such as ailerons and leading-edge flaps used to induce the twist.
Flutter induced by dry friction 
It may seem surprising that aeroelastic flutter is a phenomenon sharing similarities with instabilities due to friction. This was recently experimentally discovered by Bigoni and Noselli (2011), who have produced a follower force through dry friction at the contact between two sliding bodies, see the animation on the right and watch a movie for more details.
See also 
- Collar, A. R. (1978). "The first fifty years of aeroelasticity". Aerospace. 2 5: 12–20.
- "Visual demonstration of flutter which destroys an RC aircraft" (Video), YouTube, Google.
- The adequacy of comparison between flutter in aircraft aerodynamics and Tacoma Narrows Bridge case is discussed and disputed in Yusuf K. Billah, Robert H. Scanian, "Resonance, Tacoma Bridge failure, and undergraduate physics textbooks"; Am. J. Phys. 59, nr 2, s. 118-124, February 1991
- "Windbelt, Cheap Generator Alternative, Set to Power Third World". Popular Mechanics.
- Windbelt technology, Humdinger Wind Energy.
- FlutterMill, UK: Creative Science.
- Flexor Energy Company, Energy kite systems.
- Holt Ashley. "Role of Shocks in the "Sub-Transonic" Flutter Phenomenon", Journal of Aircraft, Vol. 17, No. 3 (1980), pp. 187-197.
- Farmer, M.G. and Hanson, P.W., “Comparison of Super- critical and Conventional Wing Flutter Characteristics,” NASA TM X- 72837
- Bigoni, D. Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability. Cambridge University Press, 2012 . ISBN 9781107025417.
- D. Bigoni and G. Noselli, Experimental evidence of flutter and divergence instabilities induced by dry friction. Journal of the Mechanics and Physics of Solids, 2011, 59, 2208–2226.
Further reading 
- Bisplinghoff, R.L., Ashley, H. and Halfman, H., Aeroelasticity. Dover Science, 1996, ISBN 0-486-69189-6, 880 pgs;
- Dowell, E. H., A Modern Course on Aeroelasticity. ISBN 90-286-0057-4;
- Fung, Y.C., An Introduction to the Theory of Aeroelasticity. Dover, 1994, ISBN 978-0-486-67871-9;
- Hodges, D.H. and Pierce, A., Introduction to Structural Dynamics and Aeroelasticity, Cambridge, 2002, ISBN 978-0-521-80698-5;
- Wright, J.R. and Cooper, J.E., Introduction to Aircraft Aeroelasticity and Loads, Wiley 2007, ISBN 978-0-470-85840-0.
- Hoque, M. E., "Active Flutter Control", LAP Lambert Academic Publishing, Germany, 2010, ISBN 978-3-8383-6851-1.
- Collar, A. R., "The first fifty years of aeroelasticity," Aerospace, vol. 5, no. 2, pp. 12–20, 1978
- Garrick, I.E. and Reed W.H., "Historical development of aircraft flutter," Journal of Aircraft, vol. 18, pp. 897–912, Nov. 1981.