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 of lift, the bending moment, and the torque or pitching moment on the principle 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. Extensions 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, the data reduction time. The basic principles of strain-gage calibration procedures are illustrated by reference to the data for two aircraft structures of typical construction, one a straight and the other a swept horizontal stabilizer.