Dynamic aeroelasticity studies the interactions among aerodynamic, elastic, and inertial forces. Examples of dynamic aeroelastic phenomena are:
Flutter 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. "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 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 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 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 the early 1940s. Famously, the original
Tacoma Narrows Bridge was destroyed as a result of aeroelastic fluttering.
Aeroservoelasticity In some cases, automatic control systems have been demonstrated to help prevent or limit flutter-related structural vibration.
Propeller whirl flutter 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 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.
Transonic aeroelasticity 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 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 F/A-18 wing
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. The methods for buffet detection are: • Pressure coefficient diagram • Pressure divergence at trailing edge • Computing separation from
trailing edge based on
Mach number • Normal force fluctuating divergence == Prediction and cure ==