naca-report-270
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National Advisory Committee for Aeronautics, Report - The Measurement of Pressure Through Tubes in Pressure Distribution Tests
The tests described in this report were made by the National Advisory Committee for
Aeronautics to determine the error caused by using small tubes to connect orifices on the surface
of aircraft to central pressure capsules in making pressure distribution tests.
Aluminum tubes of Tag-inch inside diameter were used to determine this error. Lengths
from 20 feet to 226 feet and pressures whose maxima varied from 2 inches to 140 inches of
water were used. Single-pressure impulses for which the time of rise of pressure from zero
to a maximum varied from 0.25 second to 3 seconds were investigated.
The results show that the pressure recorded at the capsule on the far end of the tube lags
behind the pressure at the orifice end and experiences also a change in magnitude. For the
values used in these tests the time lag and pressure change very principally with the time of rise
of pressure from zero to a maximum and the tube length. Curves are constructed showing the
time lag and pressure change. Empirical formulas are also given for computing the time lag.
Analysis of pressure-distribution tests made on airplanes in flight shows that. the recorded
pressures are slightly higher than the pressures at the orifice and that the time lag is negligible.
The apparent increase in pressure is usually within the experimental error, but in the case of the
modern pursuit type of airplane the pressure increase may be 5 per cent. For pressure-distribu-
tion tests on airships the analysis shows that the time lag and pressure change may be neglected.
The air pressure acting on an aircraft in flight is usually measured by providing an orifice
at the point to be investigated and connecting this orifice to a manometer by means of a tube
of comparatively small diameter. During steady rectilinear mot-ions of the aircraft the pressure
in the tube at the manometer end differs from the pressure at the orifice end only by the amount
due to the differences in level of the two ends of the tube. This difierence of pressure is usually
quite small but may easily be computed.
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