Mega interesting video! 'Wing flutter'

[admin (Simon W)]

 

SW

[asquaddie]

Shame it didn't have audio to explain the flutter....

Edited January 11, 2017 by asquaddie

[admin (Simon W)]

Google says this  

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.[7] Methods of predicting flutter in linear structures include the p-method, the k-method and the p-k method.[6] In water the mass ratio of the pitch inertia of the foil vs 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.[8]

For nonlinear systems, flutter is usually interpreted as a limit cycle oscillation (LCO), and methods from the study of dynamical systems can be used to determine the speed at which flutter will occur.[9]

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,[10] as in Braniff Flight 542.

In some cases, automatic control systems have been demonstrated to help prevent or limit flutter-related structural vibration.[11]

Flutter can also occur on structures other than aircraft. One famous example of flutter phenomena is the collapse of the original Tacoma Narrows Bridge.[12]

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. Flutter is not always a destructive force; recent progress has been made in windmills for underserved communities in developing countries, designed specifically to take advantage of this effect. The oscillating motion allows variable-stroke waterpumping to match the variable power in the wind. Semirotary binary flutter can also have an upper critical airspeed at which it stops, affording automatic high wind protection[8] The resulting Wing'd Pump has been designed to mount on the well it pumps or float on the pond it draws from.[13] At its large scale the flutter is coupled by static gravity imbalance as well as dynamic imbalance. Further a gravity pendulum achieves large amplitude elasticity most practically.[14] The same annual output can be achieved with wing length equal to a multiblade rotary windpump's diameter, in half the windspeed regime.[15] P. Sharp and J. Hare showed a toy linear generator run by two flutter wings.[16] Recently, researchers have demonstrated the ability to harvest energy directly from beam's self-induced, self-sustaining limit cycle oscillations in airflow of a flexible, piezoelectric beam placed in a wind tunnel. While the general approach to harvesting energy from these "aeroelastic" vibrations is to attach the beam to a secondary vibrating structure, such as a wing section, the new design eliminates the need for the secondary vibrating structure because the beam is designed so that it produces self-induced and self-sustaining vibrations (LCO). As a result, the new system can be made very small, which increases its efficiency and makes it more practical for applications, such as self-powered sensors.[17][18]

The word flutter is typically linked to the form of aerodynamic instability discussed above. Though, a connection between dry friction and flutter instability in a simple mechanical system has been discovered,[19] watch the movie for more details.

SW