Application of strain gauge pattern analysis

Strain gage pattern analysis, developed at the Royal Aircraft Establishment for measuring vibration modes of a rotating blade, is described. The analysis enables displacements in flap, lag and torsion to be derived from patterns of strain gage responses, which are related to displacements through a set of calibration modes. The latter can be obtained with the blade nonrotating, and conventional strain gage calibration is avoided. A theoretical study of the effects of measurement errors has shown that the technique is insensitive to random errors of a magnitude that might be expected in practice. Results obtained from a model rotor are given, and a blade-shrouding technique has been used to obtain modes with and without aerodynamic forces.