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. Aircraft are prone to aeroelastic effects because they need to be lightweight and withstand large aerodynamic loads. Aircraft are designed to avoid the following aeroelastic problems: divergence where the aerodynamic forces increase the angle of attack of a wing which further increases the force; control reversal where control activation produces an opposite aerodynamic moment that reduces, or in extreme cases, reverses the control effectiveness; and flutter which is the uncontained vibration that can lead to the destruction of an aircraft. 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 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. History 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 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 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, 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