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National Advisory Committee for Aeronautics, Technical Notes - The Spacing of Orifices for the Measurement of Pressure Distributions

The following report has been prepared for publication
by the_National Advisory Committee for Aeronautics. Suitable
locations of orifices for the measurement of pressure distri—
butions have been discussed. Tables are given for quickly
laying out these locations and for quickly and easily comput—
ing the resultant air forces from the result of the measure—
ments.

For many aeronautical problems, the mechanical interac—
tion between adjacent particles of air or between particles
of'air and an adjacent rigid boundary can be assumed_exactlyi
enough to be a pressure. This means that the three shear
components become zero, and that the three remaining compres—
sion components become equal to a pressure, say p- The phys—
ical dimension of a pressure is Egiifi, and since with a
pure and genuine pressure the force is always directed at
right angle to its reference area, a pressure cannot be said
to have or to occupy any specific direction.

The pressure of moving air is generally different at dif-
ferent points. It has to be realized that the pressure now
under discussion really exists; it is the asiual force per
unit area between adjacent particles. There is, however, no
method known to measure an absolute pressure as such. Only
the difference between two pressures can be measured. It is,
therefore, necessary to establish a standard pressure before
entering into the discussion of any numerical relation. Such
standard or zero pressure is chosen differently in different
cases. When discussing heavier-than—air craft (neglecting any
buoyancy of the air) it is customary and most convenient to
consider as zero pressure the pressure of the atmosphere at
the same altitude, when at rest, that is,_in absence of the
airplane and of any wind. This standard pressure is not con—
stant under the ordinary assumptions of mechanics, but is dif-
ferent at different altitudes. On the other hand, when dis—
cussing the buoyancy of air as with airships, the Variation
_cf the pressure of resting air is of greatest importance and
the standard pressure has to be chosen otherwise.