On floats and float tests Page: 2 of 31
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
TECHNICAL MEMORANDUM NO. 639
ON FLOATS AND FLOAT TESTS*
By Friedrich Seewald
The principal source of information on float resist-
ance is the model test. In view of the insuperable dif-
ficulties opposing any attempt at theoretical treatment of
the resistance problem, particularly at attitudes which
tend toward satisfactory take-off, such as the transitory
stage to planing, the towing test is and will remain the
primary method for some time to come. Consequently, the
importance of the model test from the vioewoint of reduc-
tion of model test data to full scale cannot be overesti-
When a model test is not in close agreement with ac-
tual full scale experience as, in fact, one phase of the
float tests postulates, one is apt to become skeptical as
to the validity of the law. of similitude, and for that
reason the underlying principles of these questions are
When, as is accepted practice in model float expori-
ments, the forces impressed by a fluid flow on a body are
measured, the total force set up by the fluid can, strict-
ly speaking, be similated to another geometrically similar
body in its entirety only when it is known that the indi-
vidual comnonents of this force which are contingent upon
different physical properties change in the same ratio
while being reduced to other body dimensions. Foremost
among these components is the force produced by skin fric-
tion, that is, by the viscosity of the fluid. Next in im-
portance is the force due to the mass inertia of the water
articles owing to their deflection from their direction
of motion. Third, is the force due to gravity in so far
as the pressure. varies with the depth of the water, and
in the change in pressure distribution attributable to the
variation in water level by the wavos. (other possible
* "Uber Schwimmer und Schwilmerversucho." -From Zeitschrift
fur Flugtechnik und Motorluftschiffahrt, May 15, 1931, pp.
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Seewald, Friedrich. On floats and float tests, report, September 1, 1931; (digital.library.unt.edu/ark:/67531/metadc277527/m1/2/: accessed May 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.