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National Advisory Committee for Aeronautics, Technical Notes - Damping in Flutter Models

The problem of determining values of structural damp—
ing for use in flutter calculations -iS discussed. The_
concept of equivalent viscous damping is reviewed and its
relation to the structural damping coefficient g intro»
duced in N.A.C.A. Technical Report No. 685 is shown. The
theory of-normal modes is reviewed and a number of methods
are described for separating the motions associated with',
different modes. Equations are developed for use in eval—
uating the damping parameters from experimental data.

Experimental results of measurements of damping in
several flutter models are presented.
One important step_in the study of the flutter prop—
erties of an airplane structure is the determination of
ths'struetural damping. -In an investigation of flutter
_carried out in the-N.A.Q.A. 8—foot high—speed tunnel, it
was-desirable to.determine the damping in the models test—
ed. The present report is a description of the methods
used, ‘together with a review and a critical discussion of
the Principles and the derivations pertaining to the- msas~
urement. of damping parameters“

The presence of damping in a structure can be i-n~
ferred from and its amount can be measured by a number of.
different effects.» The principal effects that depend upon
damping are: ' ' ‘ - '

(l) The rate of decay of free vibrations.

(2) The amplitudes produced by given applied forces
at a resonant frequency.
(3) The heat produced by vibrationr
(4) The elastic hysteresis loop.

A complete physical theory would have to account for
all the observed results in all the preceding effects. For
the purpose of flutter __calculations, however. a complete
physical theory is unn-ecessary. For eremple, the exact
shape of the hysteresis loop may be unaccounted for. It is
sufficient to express the damping in terms of certain efv
fective parameters that can be conveniently incorporated
into the flutter analysis. The present report reviews
some of the results of—other investigators on the physical
laws of damping but makes no attempt to find the true law
of damping in the flutter models.