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National Advisory Committee for Aeronautics, Report - Calibration of Strain Gage Installations in Aircraft Structures for the Measurement of Flight Loads

A general method has been developed for calibrating strain-
gage installations in aircraft structures, which permits the
measurement in flight of the shear or lift, the bending moment,
and the torque or pitching moment on the principal lifting or
control surfaces. Although the stress in structural members
may not be a simple function of the three loads of interest, a
straightforward procedure is given for numerically combining
the outputs of several bridges in such a way that the loads may
be obtained. Ertensions of the basic procedure by means of
electrical combination of the strain-gage bridges are described
which permit compromises between strain-gage installation
time, availability of recording instruments, and data reduction
time. The basic principles of strain-gage calibration pro-
cedures are illustrated by reference to the data for two aircraft
structures of typical construction, one a straight and the other
a swept horizontal stabilizer.

The measurement of loads on aircraft in flight is required
for a variety of purposes such as in research investigations,
structural integrity demonstrations, and developmental
flight testing. Although pressure-distribution methods per-
mit the determination of aerodynamic loads without correc—
tions for inertia effects, pressure installations must be very
complete in order that accurate load data may be obtained.
Since the time of installation and data reduction may be
lengthy, the general use of pressure-distribution methods in
the measurement of loads on aircraft in flight is avoided
except when specific detailed load—distribution data are-
desired.

A more useful tool for the measurement of the overall
loads on aircraft structures appears to be the wire resistance
strain gage. Properly installed and calibrated, such gages
may be used to determine the structural loads on control
surfaces, landing-gear structures, and relatively complex
built-up wing and empennage assemblies. The measured
structural loads can, in turn, be converted to aerodynamic
loads provided the structural weight distribution is lmown
and the acceleration distribution has been measured.