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The Effect of the Nature of Surfaces on Resistance, as Tested on Struts

It has been shown by measurements previously taken of
model balloons,** that comparatively slight alterations in
surface may have considerable effect upon the resistance of
a body. Reference need only be made to the very striking
results obtained by those tests as regards the resistance of
a model balloon covered with fabric. The raised threads of
the fabric having been scorched with flame, the resistance
of the balloon more than doubled in value, contrary to any
procedure hitherto observed, when the REYNOLDS symbol in—
creased from about 100000 to 350000.

In testing struts, similar observations have been made
and are briefly reported as follows. The struts in queg-
Notes communicated by the aerodynamic model testing labor—
atory at thtingen.

Compare 0. WIESELSBERGER’S “Similitude Tests of Model Bal-
loons and onuthe Effect of the Nature of Surfaces," Bul.
80, of the Gottingen Laboratory. Published in the “Zeit—
schrift fur Flugtechnik und Motorluftschiffahrt,“ 1915,

The first series of measurements was made during the war on
behalf of the Deutschen Flugzeugwerke, Leipzic. The stri—
ing nature of the results obtained led to further measure-
ments being taken in a second series of tests carried out
at the Laboratory.

The wind relocity heing raised to 45 m/s, characteristic
values of E were attained up to about 8000 m/sec
nm., that is, the product of the velocity of the air and the
thickness of the struts. The length of the struts was 2.5 m.
They were measured at that length for resistance, and as the
diameter of the airflow was only 2.23 m., the struts pro—
jected out of it, at each end, into the still air. ‘To What
extent such airflow conditions resemble those corresponding
to unlimited length of strut, that is, to uniform airflow,
was established by means of special tests which are to be
reported later on. .

As as are at present chiefly concerned with the varia—
tions of resistance brought about by the nature of the sur—
face, the measurement of such resistance will not be sub—
jected to any particular regulation. It need only he mans
tioned that the method of testing employed evidently produces
somewhat higher resistance than when the airflow is perfectly
uniform. The direction of the airflow was parallel,
in all cases, to the line of symmetry of the profile.