Flutter is the effect of control surfaces start oscillating. As air travels over the surface it may start the surface to oscillate which can make the surface go up and down in a way it might get damaged or totally ripped of. It may start as a dull buzz but can increase in a matter of seconds in a potential danger (figure 1.1). Flutter is a self-excited oscillation of the aircraft structure, where energy is absorbed from the airstream. When the elastic structure of the aircraft is disturbed at speeds below the flutter speed, the resulting oscillatory motions decay. However, when the structure is disturbed at speeds above the flutter speed, the oscillatory motions will abruptly increase in amplitude and can ultimately lead to catastrophic failure of the structure. In some instances, flutter oscillations are limited to just a single airplane component such as the wing, whereas in other instances the oscillations may be considerably more complex and involve coupling of wing, fuselage, and empennage vibrations.

The opportunity for flutter to occur may be created by an overpowered aircraft. This can lead to flutter as the construction of the wing and tail are insufficient to keep the surface centered due to high speed air flow. In this case flutter can be prevented by decelerating the aircraft to a speed recommended by the manufacturer.

Figure 1.1 Model wind-tunnel tests

Determining whether an aircraft or part of the aircraft is likely to experience flutter during normal operation requires carefully planned analytical and experimental studies of the complex aeroelastic interactions that the aircraft will experience throughout its flight envelope with sufficient margins beyond the expected flight conditions. In case the aircraft experiences any flutter tendencies, the design of the aircraft should be revised and adjusted. A common modification is to increase the most critical structural stiffness and could result in an increase of the structural aircraft weight.

When testing an aircraft on how it performs under certain circumstances and flight conditions a dynamically scaled model of the actual aircraft is used and placed in subsonic wind tunnels like illustrated above.

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