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National Advisory Committee for Aeronautics, Technical Notes - Some Measurements of Atmospheric Turbulence Obtained from Flow Direction Vanes Mounted on an Airplane

The power spectrum of relativeLy short wavelength turbulence in
the atmosphere was calculated from measurements made in flight. The
range of wavelengths covered by these measurements was from 10 feet to
200 feet. The power spectral density varies with the Square of the
wavelength of the turbulence. This variation is in agreement with the
high—frequency asymptote of the spectrum form generally assumed for
isotropic turbulence. Flow—direction vanes Were used to measure the
vertical and horizontal components of gust velocity normal to the
flight direction. The powar spectral densities of the two components
are, for practical purposes, equal. The use of vanes is shown to
afford a simple, direct method of obtaining the power spectral density
of atmospheric turbulence in the relatively high frequency range of
airplane response.

The introduction of the methods of generalized harmonic analysis
to measurements of atmospheric turbulence has provided a form of sta-
tistical information which makes it possible to analyze problems con-
cerning the motion or the flight path of an airplane which is flying
in rough air. Because of the random nature of turbulence, previous
methods available to airplane designers for describing rough air pro—
vided only statistical estimates of peak values of gust velocity,
velocity gradients, and the frequency of their occurrence. Although
this information is useful for calculating maximum values of airplane
gust response, it does not describe the gust disturbance in sufficient
detail for calculating the average effect of the continuous gust dis—
turbance on the response of an airplane which is encountering atmospheric
turbulence. The airplane response to gusts is determined by the dynamic
characteristics of the airplane. The response is, therefore, dependent
upon the gust wavelength. Calculating the response of an airplane in
turbulent air requires information pertaining to the gust disturbance
which distinguishes between gusts of different wavelengths. The appli-
cation of the methods of generalized harmonic analysis to gust data
accomplishes this purpose.