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We have developed an optical fiber distributed sensing system based on optical frequency domain reflectometry (OFDR) that uses long-length fiber Bragg gratings (FBGs). This technique obtains strain data not as a point data from an FBG but as a distributed profile within the FBG. This system can measure the strain distribution profile with an adjustable high spatial resolution of the mm or sub-mm order in real-time. In this study, we applied this OFDR-FBG technique to a flying test bed that is a mid-sized jet passenger aircraft. We conducted flight tests and monitored the structural responses of a fuselage stringer and the bulkhead of the flying test bed during flights. The strain distribution variations were successfully monitored for various events including taxiing, takeoff, landing and several other maneuvers. The monitoring was effective not only for measuring the strain amplitude applied to the individual structural parts but also for understanding the characteristics of the structural responses in accordance with the flight maneuvers. We studied the correlations between various maneuvers and strains to explore the relationship between the operation and condition of aircraft.

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We conduct flight demonstrations of aircraft monitoring by using optical fiber distributed sensing technique. We monitor a main wing of a flying test bed that is a middle-sized passenger jet. We use optical frequency domain reflectometry and long-length fiber Bragg gratings (FBGs). The sensing system measures the strain distribution profile within the FBGs with a mm order spatial resolution in real-time. Thanks to the high spatial resolution, we could observe local strain distributions due to the ribs of the wing. We also monitored strain distribution variations corresponding to various maneuvers during flight. We discussed the correlation with flight maneuvers and the interpretation of the monitoring data.

Optical fiber sensors have many advantages for structural health monitoring and are often used to monitor the strain distribution of structures. However, using Fiber Bragg Grating (FBG) sensors with a general interrogation system, e.g. Wavelength Division Multiplexing (WDM), permits only the measurement of average strain within the gauge length and moreover, a large number of FBG sensors may be necessary to monitor overall stress concentrations. A distributed strain sensor with a higher spatial resolution and longer sensing length is therefore desired for the accurate and effective monitoring of stress concentration. To meet this need, we present a new strain measurement system with a long gauge FBG sensor based on the principle of Optical Frequency Domain Reflectometry (OFDR) which enables us to measure fully distributed strain at high spatial resolution. In this paper, we describe the principle and optical simulation model of our proposed measurement system and show the results of numerical calculations.