Aeroelasticity
Encyclopedia
Aeroelasticity is the science which studies the interactions among inertial, elastic
, and aerodynamic forces. It was defined by Arthur 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.
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.
Aeroelasticity can be divided in two fields of study: steady (static) and dynamic
aeroelasticity.
properties are not significant in the calculations of this type of phenomena.
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 can also occur on structures other than aircraft. One famous example of flutter phenomena is the collapse of the original Tacoma Narrows Bridge
. Another is a particular playground swing in Firmat
which "swings itself" with such force that it leads many to believe it is haunted.
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 distinguish between flutter wings, flip wings, and oscillating tensionally-held sweeping membrane wings for wind milling.
Generally it affects the tail unit of the aircraft structure due to air flow down stream of the wing.
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.
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.
-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 aileron
s and leading-edge flaps used to induce the twist.
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
, and aerodynamic forces. It was defined by Arthur 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
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
medium.
Introduction
AirplaneFixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
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
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
process. These interactions may become smaller until a condition of equilibrium is reached, or may diverge catastrophically if resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
occurs.
Aeroelasticity can be divided in two fields of study: steady (static) and dynamic
Dynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...
aeroelasticity.
Steady aeroelasticity
Steady aeroelasticity studies the interaction between aerodynamic and elastic forces on an elastic structure. MassMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
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-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibrationVibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
to produce rapid periodic motion. Flutter can occur in any object within a strong fluid flow, under the conditions that a positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
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
Stiffness
Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
of one component can induce flutter in an apparently unrelated aerodynamic component. At its mildest this can appear as a "buzz
Buzz
-People:*Buzz Aldrin , American pilot and astronaut, second person to set foot on the Moon*George Beurling , Canadian World War II fighter pilot*Buzz Hargrove , National President of the Canadian Auto Workers trade union...
" 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
Braniff Flight 542
Braniff Airways Flight 542, a Lockheed L-188 Electra, registration N9705C, was a scheduled domestic flight from Houston, Texas, bound for New York with scheduled stops in Dallas and Washington, D.C....
.
In some cases, automatic control systems have been demonstrated to help prevent or limit flutter-related structural vibration.
Flutter can also occur on structures other than aircraft. One famous example of flutter phenomena is the collapse of the original Tacoma Narrows Bridge
Tacoma Narrows Bridge (1940)
The 1940 Tacoma Narrows Bridge was the first incarnation of the Tacoma Narrows Bridge, a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1, 1940, and dramatically collapsed...
. Another is a particular playground swing in Firmat
Firmat
Firmat is a city located in the south of Santa Fe Province, Argentina in the Departamento General López. The main economic resources are agro-industry and agriculture. The city covers an area of about 226 km² . The population of Firmat stood at 18,294 per the 2001 national census...
which "swings itself" with such force that it leads many to believe it is haunted.
Flutter as a controlled aerodynamic instability phenomenon
Controlled Aerodynamic Instability Phenomena
The term controlled aerodynamic instability phenomena was first used by Cristiano Augusto Trein in the Nineteenth KKCNN Symposium on Civil Engineering held in Kyoto – Japan in 2006...
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
Kite types
Kites are tethered flying objects which fly by using aerodynamic lift, requiring wind, , for generation of airflow over the lifting surfaces.-Kite types:...
. 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 distinguish 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
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.
Other fields of study
Other fields of physics may have an influence on aeroelastic phenomena. For example, in aerospace vehicles, stressStress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
induced by high temperatures is important. This leads to the study of aerothermoelasticity. Or, in other situations, the dynamics of the control system
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
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, dampingDamping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
and mass characteristics of the aircraft. Prediction involves making a mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
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 NASANASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
-Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
flight research program to investigate the potential of aerodynamically twisting flexible wings to improve maneuverability of high-performance aircraft at transonic and supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds, with traditional control surfaces such as aileron
Aileron
Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
s 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 (watch a movie for more details ).See also
- Adaptive Compliant WingAdaptive Compliant WingThe Adaptive Compliant Wing is a wing design developed by FlexSys, Inc., which is flexible so that aspects of its shape can be changed in flight....
- Aerospace engineeringAerospace engineeringAerospace engineering is the primary branch of engineering concerned with the design, construction and science of aircraft and spacecraft. It is divided into two major and overlapping branches: aeronautical engineering and astronautical engineering...
- Flutter (electronics and communication)
- Kármán vortex street
- Mathematical modeling
- OscillationOscillationOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
- Parker Variable WingParker Variable WingThe Parker Variable Wing is a wing configuration in biplane or triplane aircraft designed by H.F. Parker in 1920. His design allows a supplement in lift while landing or taking-off....
- Tacoma Narrows Bridge (1940)Tacoma Narrows Bridge (1940)The 1940 Tacoma Narrows Bridge was the first incarnation of the Tacoma Narrows Bridge, a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1, 1940, and dramatically collapsed...
- TWA Flight 599TWA Flight 599Transcontinental and Western Air Flight 599 was a Fokker F.10 Trimotor en route from Kansas City, Missouri, to Los Angeles, California, on March 31, 1931. It crashed a few miles north west of Bazaar, Kansas; all eight on board died...
- Vortex-induced vibration
- Vortex sheddingVortex sheddingVortex shedding is an unsteady flow that takes place in special flow velocities . In this flow, vortices are created at the back of the body and detach periodically from either side of the body. See Von Kármán vortex street.Vortex shedding is caused when a fluid flows past a blunt object...
- X-53 Active Aeroelastic Wing
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 978-0486678719;
- Hodges, D.H. and Pierce, A., Introduction to Structural Dynamics and Aeroelasticity, Cambridge, 2002, ISBN 978-0521806985;
- 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.