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Space Vehicle Design Criteria - Flutter Buzz And Divergence

1.1 INTRODUCTION

Flutter is a self-excited oscillation of a vehicle surface or component caused
and maintained by the aerodynamic, inertia, and elastic forces in the system.
For a given structural system there are combinations of Mach number and
dynamic pressure which defines a flutter boundary. At a given Mach number,
flight at dynamic pressures below the flutter boundary wiii result in damped
oscillations. At the flutter boundary a transition occurs so that for higher
dynamic pressures, oscillations will be sustained at some Limiting amplitude or
wili diverge until a structural failure occurs. In addition to being dependent on
Mach number and dynamic pressure, the occurrence of flutter is dependent on
such factors as structural stiffness, mass and mass distribution, stiffness
changes due to steady and transient thermal inputs, control-surface actuation-
system dynamics, system tolerances, misalignments, and free piay.

Control-surface boss is a type of flutter involving only one degree of
freedom. it is usually a cure rotational oscillation of a control surface, but
may appear as a torsional “windup” oscillation if the surface is restrained near
one end. Buzz generally occurs in regions of transonic flow.

Divergence is a nonoscillatory instability which occurs when the restoring
moments within a system are exceeded by the external aerodynamic moments.
Divergence can be a critical design problem on some slender configurations.
It is usually not a consideration for sweptback surfaces.

Panel flutter will be considered in a separate section.

1.4.1 ANALYSIS

Flutter and divergence analyses should include all significant de-
grees of freedom such as symmetric and antisymmetric bending, torsion,
chord bending, rotation of lifting surfaces and control surfaces, and body
bending and torsion. The preferred formation of flutter analyses will utilize
vibration modes and frequencies although a formulation using aerodynamic and
structural influence coefficients is acceptable. Vibration modes can be either
coupled modes or uncoupled or assumed modes. If coupled or assumed modes
or an influence—coefficient approach is used the coupled vibration modes and
frequencies at zero airspeed should be calculated from the flutter equations for
correlation with measured modes and frequencies.