Fiber-optic balanced double-polarization Michelson interferometers with fringe-counting read-out using passive-quadrature demodulation are employed for remote sensing of the surface strain of plates made from carbon-fiber composites, which are screwed to the main wing spar of a Cessna C207A. Two series of flight tests have been performed with three different sensors. During parabolic flight maneuvers, quasistatic strain variations of up to 500 mu in are measured along with superimposed engine- and aerodynamically induced vibrations with strain amplitudes of 5 mu in at (sub-) harmonics of the engine speed (40 Hz), and compared with the read-out of a conventional resistive strain gauge.
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