Aeroelastic phenomena occur when an object encounters a fluid at speed.
The speed involved in the encounter may be associated with the object and/or the flow.
Aeroelastic problems have “plagued” the development of aeronautics.
The 2nd failure of Samuel Langley’s prototype plane on the Potomac has been attributed to aeroelastic effects (specifically, torsional divergence).
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.
In 1926, Hans Reissner published a theory of wing divergence, leading to much further theoretical research on the subject
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 the wing to fail spectacularly.
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).
Control surface reversal is the loss (or reversal) of the expected response of a control surface, due to deformation of the main lifting surface.
Aeroelastic problems also affect bridges and chimneys.
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 a self-oscillation and eventual failure.
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 be discounted only 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.
One famous example of flutter phenomena is the collapse of the original Tacoma Narrows Bridge.
Flutter as a controlled aerodynamic instability phenomenon is used intentionally and positively in windmills for generating electricity and in other works like making musical tones on ground-mounted devices, as well as on musical kites.
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 downstream of the wing.
Flow is highly non-linear in the transonic regime, dominated by moving shock waves.
A phenenenon 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.
However, the joys of dynamic aeroelasticity have also been experienced by countless generations of stone throwers.
Stone skipping is a pastime which involves throwing a stone with a flattened surface across a lake or other body of water in such a way that it bounces off the surface of the water.
The object of the game is to see how many times a stone can be made to bounce before sinking.
Research undertaken by a team led by French physicist, Lydéric Bocquet, has discovered that an angle of about 20° between the stone and the water’s surface is optimal.
Work by Hewitt, Balmforth and McElwaine has shown that if the horizontal speed can be maintained skipping can continue indefinitely.
Earlier research reported by Bocquet calculated that the world record of 38 rebounds set by Coleman-McGhee, unchallenged for many years, required a speed of 12 m/s (25 mph), with a rotation of 14 revolutions per second.
The joys of dynamic aeroelasticity are also enjoyed by water skiers and barefoot skiers.
Barefoot skiing is water skiing behind a motorboat without the use of water skis, commonly referred to as “barefooting”.
Barefooting requires the skier to travel at higher speeds than conventional water skiing (30-45mph/50-70kmh).
The necessary speed required to keep the skier upright varies by the weight of the barefooter and can be approximated by the following formula: (W / 10) + 20, where W is the skier’s weight in pounds and the result is in miles per hour.
Fernando Reina Iglesias towed by a helicopter off Acapulco, Mexico.
He reached a speed of 153 mph, the fastest speed by a barefoot skier on record.
An understanding of dynamic aeroelasticity is very important in science…