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# Aeroelasticity

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

Modern airplane structures are not completely rigid, and aeroelastic phenomena arise when structural deformations induce changes on aerodynamic forces. The additional aerodynamic forces cause increasing of the structural deformations, which leads to greater aerodynamic forces. 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: static and dynamic aeroelasticity.

## Static aeroelasticity

Static 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

Main article: Control 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 vibration that occurs when a lifting surface bends under aerodynamic load. Once the load reduces, the deflection also reduces, restoring the original shape, which restores the original load and starts the cycle again. In extreme cases the elasticity of the structure means that when the load is reduced the structure springs back so far that it overshoots and causes a new aerodynamic load in the opposite direction to the original. 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.

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

### Dynamic response

Dynamic response or forced response is the response of an aircraft to gusts and other external atmospheric disturbances.

### Buffeting

Buffeting is a high-frequency instability, caused by airflow disconnection from the airfoil or shock wave oscillations. 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.

AAW time lapse test

Time lapsed film of Active Aeroelastic Wing (AAW) Wing loads test, December, 2002.

AAW flight test (with audio)

F/A-18A (now X-53) Active Aeroelastic Wing (AAW) flight test, December, 2002.

• Problems seeing the videos? See media help.

## 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.