Measurement of Profile Drag on an Airplane in Flight by the Momentum Method: Part 1 Page: 3 of 68
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N.A.C.A. Technical Memorandum ..N. 55?
rately converted only in cases of similarity of the flow diagrams.
Theoretically this takes place when the ratio of the inertia ef-
fect to the viscosity effect remains constant in the flow (Rey-
nolds law of similarity) or, in other .words, when the flow veloc-
ity multiplied by the comparative length and divided by the "kin-
ematic viscosity" (that is, by the Reynolds Number) gives a con-
stant quantity. This purpose is fulfilled in practice by the
product of the flow velocity and the comparative length alone
(Reynolds Number), since the kinematic viscosity is usually con-
stant enough.
For ordinary air-flow measurements, where the linear dimen-
sions of the model are roughly from 1/10 to 1/25 and the attain-
able velocities approximately 1/2 to fully those of an actual
airplane, Reynolds Number always differs so greatly from that of
the full-size airplane that the convertibility of the results is
impaired. The resulting uncertainty is, however, of no great im-
portance, owing to the moderate degree of accuracy of ordinary
airplane performance calculations. Test results must neverthe-
less be accurate, in order to keep pace with the gradual improve-
ment of airplane structures which (entirely apart from its scien-
tific importance) greatly affects the economical value and the
efficiency of very large airplanes.
A further consideration applies to wings. According to air-
flow measurements, their drag consists of induced drag, insepara-
bly connected with the production of lift, and of profile drag,2
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Schrenk, Martin. Measurement of Profile Drag on an Airplane in Flight by the Momentum Method: Part 1, report, March 1930; (https://digital.library.unt.edu/ark:/67531/metadc63672/m1/3/: accessed May 7, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.