Theoretical Investigation of Methods for Computing Drag From Wake Surveys at High Subsonic Speeds Page: 4 of 30
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ECA ARR No. 5021
coefficiont obtained by means of the momentum method are
reliable u to speeds near the velocity of sound and that
the presence of shook waves of limited extent does not in-
validate the result. Theae conclusions must not be con-
sidered as a complete negation of Taylor's results, since
his investigation of the pressure distributions encountered
in practice, as given in reference 1, showed that the error
due to mixing was in the worst case much smaller than was
theoretically possible.
The a sressions for drag coefficients, for either the
compressible or incompressible case, appear as integrals
which are to be evaluated in the measurement plane. Ac-
tual values of head lose and static-pressure variation cann
be determined. by means of a pitot-traverse method and, as
a consequence, the given integrals can be evaluated by
numerical1 integration. In order to avoid the lengthy com-
putation involved, an alternate approach has been developed
by Bilverstein and Katuoff (reference 5) in which it is
assumed thbr.t drag is proportional to the integral of the
total-head loss across the wake. The integral of head loss
is obtrAinod from an averaging rake, an integrating manometer,
or by an integration of measurements made by a traversing
tube. The proportionality factor, in turn, is evaluated
by assuming a definite type of wake shape and, in this re-
port, is tabulated as a function of maximum total-head loss,
static-pressure decrement, and lach number of the free
stream. The results given herein deviate somewhat from
those given in reference 5 for reasons that will be dis-
cussed later.
In reference 6 a comparison was made between values
of drag coefficient computed by means of point-by-point
integration and those determined by the integrating method
and proportionality constants of reference 5. It was
found that the latter procedure gave results in excellent
agreement with those obtained by the former, at least up
to values of the 3ach number of the free stream in the
neighborhood of 0.6. A wide range of shapes was consid-
ered and, remarkably enough, the agreement remained uni-
formly good for the Mach numbers considered.
Since in high-speed wind tunnels it is necessary to
evaluate draG at Iach numbers above those considered in
reference 6, and since the recalculation of the nropor-
tionality constants resulted in a change in their previ-
ously determined values, the present paper has undertaken
the comparison of theoretical drag coefficients obtained,_
3
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Heaslet, Max A. Theoretical Investigation of Methods for Computing Drag From Wake Surveys at High Subsonic Speeds, report, June 1945; (https://digital.library.unt.edu/ark:/67531/metadc61268/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.