One of the most dangerous events that can occur in flight is a phenomena called "flutter". Flutter is an aerodynamically induced vibration of a wing, tail, or control surface that can result in total structural failure in a matter of seconds. The prediction of flutter is not a precise science and requires flight verification that flutter will not occur within the normal flight envelope.
The aerodynamic surfaces of an airplane are constructed so that they can carry the loads that are produced in flight. For example the wing must be capable of supporting the weight of the airplane as well as the additional lift produced during turning flight. The resulting wing structure can be viewed as a blade or spring extending from the fuselage. If we "tap" the spring with a hammer, it will vibrate at a frequency which relates to the stiffness of the spring. A stiff spring will vibrate at a higher frequency than a more limber spring. This frequency is known as the "natural frequency" of the spring.
Flutter will usually occur at or near the natural frequency of the structure, that is, some small aerodynamic force will cause the structure to vibrate at its natural frequency. If this small force persists at the same frequency as the natural frequency of the structure, a condition called "resonance" occurs. Under a resonant condition, the amplitude of the vibration will increase dramatically in a very short time and can cause catastrophic failure in the structure.
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