The structural origins of flutter.

By Barnaby Wainfan

Balancing control surfaces requires care and, often, special tools to properly support the surface and yet allow it to move with little friction. Here, the expected weight of the paint to come is temporarily applied to help get the basic balance right. Final balancing must be done after the paint is applied.

Last month we started a look at aeroelasticity. As we saw, airplanes are not perfectly rigid. No matter how stiff the structure is, it does distort under the influence of aerodynamic forces. And structures do not just deflect; they can also vibrate or oscillate.

Flutter is a sustained or increasing oscillation of the airplane’s structure that is driven by aerodynamic forces. Sometimes flutter appears as a sustained limit cycle oscillation that maintains a steady amplitude, but does not damp out. Such a flutter is often referred to as “buzz.” It is undesirable because of the cyclic loads it puts on the airframe, which can cause failures due to fatigue or working fasteners loose. In general, having a sustained aerodynamically driven oscillation in the airframe structure is considered unacceptable for safe operation of the airplane. …

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