Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial , elastic , and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity dealing with the static or steady state response of an elastic body to a fluid flow, and dynamic aeroelasticity dealing with the body's dynamic (typically vibrational ) response.
31-444: Aircraft are prone to aeroelastic effects because they need to be lightweight while enduring large aerodynamic loads. Aircraft are designed to avoid the following aeroelastic problems: Aeroelasticity problems can be prevented by adjusting the mass, stiffness or aerodynamics of structures which can be determined and verified through the use of calculations, ground vibration tests and flight flutter trials . Flutter of control surfaces
62-416: A self-oscillation and eventual failure. "Net damping" can be understood as the sum of the structure's natural positive damping and the negative damping of the aerodynamic force. Flutter can be classified into two types: hard flutter , in which the net damping decreases very suddenly, very close to the flutter point; and soft flutter , in which the net damping decreases gradually. In water the mass ratio of
93-433: A continuous stream of vortices known as a Kármán vortex street , which can induce structural oscillations. Strakes are typically wrapped around chimneys to stop the formation of these vortices. In complex structures where both the aerodynamics and the mechanical properties of the structure are not fully understood, flutter can be discounted only through detailed testing. Even changing the mass distribution of an aircraft or
124-527: 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 downstream of the wing. The methods for buffet detection are: In the period 1950–1970, AGARD developed the Manual on Aeroelasticity which details the processes used in solving and verifying aeroelastic problems along with standard examples that can be used to test numerical solutions. Aeroelasticity involves not just
155-400: A violent tail oscillation, which caused extreme distortion of the rear fuselage and the elevators to move asymmetrically. Although the aircraft landed safely, in the subsequent investigation F. W. Lanchester was consulted. One of his recommendations was that left and right elevators should be rigidly connected by a stiff shaft, which was to subsequently become a design requirement. In addition,
186-473: Is an idealized "radiating element" used as a reference ; an antenna that broadcasts power equally (calculated by the Poynting vector ) in all directions. The gain of an arbitrary antenna is usually reported in decibels relative to an isotropic antenna, and is expressed as dBi or dB(i). In cells (a.k.a. muscle fibers ), the term "isotropic" refers to the light bands ( I bands ) that contribute to
217-426: Is of the form where C is a coefficient, U is the free-stream fluid velocity, and α 0 is the initial angle of attack. This yields an ordinary differential equation of the form where The boundary conditions for a clamped-free beam (i.e., a cantilever wing) are which yields the solution As can be seen, for λL = π /2 + nπ , with arbitrary integer number n , tan( λL ) is infinite. n = 0 corresponds to
248-463: Is the loss (or reversal) of the expected response of a control surface, due to deformation of the main lifting surface. For simple models (e.g. single aileron on an Euler-Bernoulli beam), control reversal speeds can be derived analytically as for torsional divergence. Control reversal can be used to aerodynamic advantage, and forms part of the Kaman servo-flap rotor design. Dynamic aeroelasticity studies
279-433: Is uniformity in all orientations . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix a- or an- , hence anisotropy . Anisotropy is also used to describe situations where properties vary systematically, dependent on direction. Isotropic radiation has the same intensity regardless of the direction of measurement , and an isotropic field exerts
310-468: Is usually eliminated by the careful placement of mass balances . The synthesis of aeroelasticity with thermodynamics is known as aerothermoelasticity , and its synthesis with control theory is known as aeroservoelasticity . The second failure of Samuel Langley 's prototype plane on the Potomac was attributed to aeroelastic effects (specifically, torsional divergence). An early scientific work on
341-612: The National Physical Laboratory (NPL) was asked to investigate the phenomenon theoretically, which was subsequently carried out by Leonard Bairstow and Arthur Fage . In 1926, Hans Reissner published a theory of wing divergence, leading to much further theoretical research on the subject. The term aeroelasticity itself was coined by Harold Roxbee Cox and Alfred Pugsley at the Royal Aircraft Establishment (RAE), Farnborough in
SECTION 10
#1732780107264372-531: 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 the aircraft or lead to its destruction, as in Northwest Airlines Flight 2 in 1938, Braniff Flight 542 in 1959, or the prototypes for Finland's VL Myrsky fighter aircraft in
403-471: The early 1930s. In the development of aeronautical engineering at Caltech , Theodore von Kármán started a course "Elasticity applied to Aeronautics". After teaching the course for one term, Kármán passed it over to Ernest Edwin Sechler , who developed aeroelasticity in that course and in publication of textbooks on the subject. In 1947, Arthur Roderick Collar defined aeroelasticity as "the study of
434-506: The early 1940s. Famously, the original Tacoma Narrows Bridge was destroyed as a result of aeroelastic fluttering. In some cases, automatic control systems have been demonstrated to help prevent or limit flutter-related structural vibration. Propeller whirl flutter is a special case of flutter involving the aerodynamic and inertial effects of a rotating propeller and the stiffness of the supporting nacelle structure. Dynamic instability can occur involving pitch and yaw degrees of freedom of
465-452: 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
496-436: 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. Methods of predicting flutter in linear structures include the p-method , the k-method and the p-k method . For nonlinear systems , flutter is usually interpreted as a limit cycle oscillation (LCO), and methods from
527-440: The grain) and layered rocks such as slate . Isotropic materials are useful since they are easier to shape, and their behavior is easier to predict. Anisotropic materials can be tailored to the forces an object is expected to experience. For example, the fibers in carbon fiber materials and rebars in reinforced concrete are oriented to withstand tension. In industrial processes, such as etching steps, "isotropic" means that
558-481: The interactions among aerodynamic, elastic, and inertial forces. Examples of dynamic aeroelastic phenomena are: Flutter is a dynamic instability of an elastic structure in a fluid flow, caused by positive feedback between the body's deflection and the force exerted by the fluid flow. In a linear system , "flutter point" is the point at which the structure is undergoing simple harmonic motion —zero net damping —and so any further decrease in net damping will result in
589-406: 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 an aeroplane, two significant static aeroelastic effects may occur. Divergence is a phenomenon in which the elastic twist of the wing suddenly becomes theoretically infinite, typically causing
620-444: The pitch inertia of the foil to that of the circumscribing cylinder of fluid is generally too low for binary flutter to occur, as shown by explicit solution of the simplest pitch and heave flutter stability determinant. Structures exposed to aerodynamic forces—including wings and aerofoils, but also chimneys and bridges—are generally designed carefully within known parameters to avoid flutter. Blunt shapes, such as chimneys, can give off
651-453: The point of torsional divergence. For given structural parameters, this will correspond to a single value of free-stream velocity U . This is the torsional divergence speed. Note that for some special boundary conditions that may be implemented in a wind tunnel test of an airfoil (e.g., a torsional restraint positioned forward of the aerodynamic center) it is possible to eliminate the phenomenon of divergence altogether. Control surface reversal
SECTION 20
#1732780107264682-506: The process proceeds at the same rate, regardless of direction. Simple chemical reaction and removal of a substrate by an acid, a solvent or a reactive gas is often very close to isotropic. Conversely, "anisotropic" means that the attack rate of the substrate is higher in a certain direction. Anisotropic etch processes, where vertical etch-rate is high but lateral etch-rate is very small, are essential processes in microfabrication of integrated circuits and MEMS devices. An isotropic antenna
713-487: The propeller and the engine supports leading to an unstable precession of the propeller. Failure of the engine supports led to whirl flutter occurring on two Lockheed L-188 Electra aircraft, in 1959 on Braniff Flight 542 and again in 1960 on Northwest Orient Airlines Flight 710 . Flow is highly non-linear in the transonic regime, dominated by moving shock waves. Avoiding flutter is mission-critical for aircraft that fly through transonic Mach numbers. The role of shock waves
744-503: The same action regardless of how the test particle is oriented. Within mathematics , isotropy has a few different meanings: In the study of mechanical properties of materials , "isotropic" means having identical values of a property in all directions. This definition is also used in geology and mineralogy . Glass and metals are examples of isotropic materials. Common anisotropic materials include wood (because its material properties are different parallel to and perpendicular to
775-405: The structure further, which eventually brings the structure to the point of divergence. Unlike flutter, which is another aeroelastic problem, instead of irregular oscillations, divergence causes the lifting surface to move in the same direction and when it comes to point of divergence the structure deforms. Divergence can be understood as a simple property of the differential equation (s) governing
806-533: The study of dynamical systems can be used to determine the speed at which flutter will occur. 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
837-538: The subject was George Bryan 's Theory of the Stability of a Rigid Aeroplane published in 1906. Problems with torsional divergence plagued aircraft in the First World War and were solved largely by trial-and-error and ad hoc stiffening of the wing. The first recorded and documented case of flutter in an aircraft was that which occurred to a Handley Page O/400 bomber during a flight in 1916, when it suffered
868-608: The twist. Flight control surfaces Too Many Requests If you report this error to the Wikimedia System Administrators, please include the details below. Request from 172.68.168.133 via cp1102 cp1102, Varnish XID 571749177 Upstream caches: cp1102 int Error: 429, Too Many Requests at Thu, 28 Nov 2024 07:48:27 GMT Isotropic In physics and geometry , isotropy (from Ancient Greek ἴσος ( ísos ) 'equal' and τρόπος ( trópos ) 'turn, way')
899-409: The wing deflection . For example, modelling the airplane wing as an isotropic Euler–Bernoulli beam , the uncoupled torsional equation of motion is where y is the spanwise dimension, θ is the elastic twist of the beam, GJ is the torsional stiffness of the beam, L is the beam length, and M ’ is the aerodynamic moment per unit length. Under a simple lift forcing theory the aerodynamic moment
930-443: The wing to fail. Control reversal is a phenomenon occurring only in wings with ailerons or other control surfaces, in which these control surfaces reverse their usual functionality (e.g., the rolling direction associated with a given aileron moment is reversed). Divergence occurs when a lifting surface deflects under aerodynamic load in a direction which further increases lift in a positive feedback loop. The increased lift deflects
961-525: Was first analyzed by Holt Ashley . A phenomenon that impacts stability of aircraft known as "transonic dip", in which the flutter speed can get close to flight speed, was reported in May 1976 by Farmer and Hanson of the Langley Research Center . Buffeting is a high-frequency instability, caused by airflow separation or shock wave oscillations from one object striking another. It is caused by