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# Aeroelasticity

[air-oh-i-las-tik]
Aeroelasticity is the science which studies the interaction among inertial, elastic, and aerodynamic forces. It was defined by 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."

## Introduction

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.

Aeroelasticity can be divided in two fields of study: steady and dynamic 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

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).

### 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

Flutter is a self-starting 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, that the vibrational movement of the object increases an aerodynamic load which in turn drives the object to move further. If the energy during the period of aerodynamic excitation is larger than the natural damping of the system, the level of vibration will increase. The vibration levels can thus build up and are only limited when the aerodynamic or mechanical damping of the object match the energy input, this often results in large amplitudes and can lead to rapid failure. Because of this, structures exposed to aerodynamic forces - including wings, aerofoils, but also chimneys and bridges - are designed carefully within known parameters to avoid flutter. It is however not always a destructive force; recent progress has been made in small scale (table top) wind generators, for the third world designed specifically to take advantage of this effect , , .

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 the destruction of the aircraft. The following link shows a visual demonstration of flutter which destroys an RC aircraft.

Flutter can be prevented by using an automatic control system to limit structural vibration.

Flutter can also occur on structures other than aircraft. One famous example of flutter phenomena is the collapse of Galloping Gertie, the original Tacoma Narrows Bridge.

### 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

Buffeting is a high-frequency instability, caused by airflow separation or shock wave oscillations from one object striking another. It is a random forced vibration.

## 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.

## Media

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.

## Related books

• 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.