Procedure for determining speed and climbing performance of airships Page: 4 of 40
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N.A.0,A. Technical Note No. . 564'
AIR-- SPEED IEASUREME NTS
Instrument Method
Effect of velo.ci _field.- The velocity of the air
relative to an airship in flight is influenced over a wide
field by the presence of the airship.'and control car or
other protuberances. The local velocity at any point is
dependent on the shape of the airship and protuberances,
on the location of this point relative to the body or bod-
ies causing the disturbance, and on the direction of the
relative wind. In addition to this general velocity field,
which extends to a great distance iri all directions, there
is the so-called "boundary layer" of air close to the body
in which the velocity is retarded by friction.. Although
this boundary layer increases in. thickness from the bow to
the stern, it is relatively thin and easily avoided in
making measurements of air speed.
The general nature of the velocity field close to the
hull is indicated by the distribution of normal pressure
on the hull. At the.bow and stern the normal pressures
are higher than true static, and the velocities in these
regions are correspondingly lower than the true air speed
by as much as 100 percent. Amidships the normal pressures
are less than.true static and the velocities are corre-
spondingly higher than the true air speed by as much as
about 10 percent. Between these regions of low and high
velocity there ar.e marginal regions in which the true air
speed prevails, but they are of small practical signifi-
cance as regards air-speed measurements. The location.of
these marginal regions is dependent on the trim of the
airship and local irregularities of contour and, further-
more, tests would bo required to establish the location
for any given trim condition.
In order to avoid the effect of the velocity field,
it is necessary to place the air-speed head used in speed
trials at a considerable distance from the airship. An
indication of the distance that is required is shown in
figure 3. The curves shown in this figure apply to cal-
culated values for the U.S.S. Akron hull at zero pitch
and, although they do not apply exactly to other airships,
they can be regarded as approximately representative of.
the general case. As shown in the figure, beyond the dis-
tance of 1-1/2 diameters from the midship section the er-
ror in local velocity becomes very small. Thus with an3
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Thompson, F. L. Procedure for determining speed and climbing performance of airships, report, April 1936; (https://digital.library.unt.edu/ark:/67531/metadc54166/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.