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
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 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
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
-
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 and potentially distructive 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.
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 [[1]]
shows a visual demonstration of flutter which destroys an RC aircraft.
Flutter can also occur on structures other than aircraft. One famous example
of flutter phenomena is the collapse of the
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.
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