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National Advisory Committee for Aeronautics, Report - Some Effects of Frequency on the Contribution of a Vertical Tail to the Free Aerodynamic Damping of a Model Oscillating in Yaw

The directional damping and stability of a fuselage-vertical-
tail model oscillating freely in yaw were measured at a Mach
number of 0.1!, and compared with the damping and stability
obtained by consideration of the ejects of unsteady lift. The
contribution of the vertical tail to the damping in yaw of the
model agreed within the limits of experimental accuracy with
predictions of the approximate finite-aspect-ratio unsteady-lift
theories for the range of frequencies tested, and the two-
dimensional unsteady—lift theory predicted a much greater: loss
in damping with reduction in frequency than was shown by
experiment or by the approximate finite-aspectratio unsteady-lift
theories. For frequencies comparable to those of lateral airplane
motions, both the unsteady-lift theories and the experimental
resultsindicatedthatthecontribntionofthecerticaltailtothe
directional stability of the model was relatively independent of
frequency.

The advent of high-speed airplanes of high relative density
has focused attention on certain problems associated with
the dynamic stability of airplanes which, because of previous
unimportance, have heretofore been neglected. Among these
problems is the effect of the periodicity of the airplane motion
on the effective values of the various stability derivatives
Reference 1 and, more recently, references 2 and 3 have
indicated the possibility of sizeable effects from this problem.

An appreciable amount of theoretical work on these
unsteady-lift effects exists at present; however, only a small
amount of experimental substantiation of the results is
available for the low-frequency range of oscillation. As a
result, a program has been undertaken in the Langley sta-
bility tunnel to determine the effects of such variables as
frequency and amplitude of motion on the contribution of
the various airplane components to the stability derivatives
of present-day airplane configurations. The work reported,
herein covers that phase of the investigation which considers
frequency effects on the directional damping and stability
of a model undergoing a freely damped oscillatory yawing
motion. The effects of vertical—tail aspect ratio and com-
pressibility as predicted by the theoretical treatments are
discussed in relation to the experimental stability charac-
teristics obtained by the iree~oscillation and