# Aeroelasticity

 Contents

## Introduction

Aeroelasticity is the science related to the physical phenomena which involve interaction among inertial, elastic and aerodynamic forces.

No aircraft structure is completely rigid, so when it is subjected to aerodynamic forces it will normally deflect by a small amount. This effect can become very important at high speeds because any change in the shape of the body can cause the applied aerodynamic forces to change, leading in turn to further deflection and further changes in load. This vicious circle can rapidly develop into aeroelastic phenomena.

## Static Aeroelasticity

Static Aeroelasticity studies the interactions among aerodynamic and elastic forces. 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-excited oscilation that occurs when a lifting surface deflects under aerodynamic load so as to reduce the applied 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 aircrafts. One famous example of flutter phenomena is the Tacoma Narrows Bridge.

### Dynamic Response

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

### Buffetting

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

## 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 0486691896, 880 pgs;
• Dowell, E. H., A Modern Course on Aeroelasticity. ISBN 9028600574.

DLR Institute of Aeroelasticity (http://www.ae.go.dlr.de/)

The Aeroelasticity Group - Texas A&M University (http://aerounix.tamu.edu/aeroel/)

NACA Technical Reports - Langley Research Center (http://naca.larc.nasa.gov/)

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