1336. The “”Light Weight”” System – A Novel Concept for On-Board Weight and Balance Measurement Using Fibre Optics


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A C MacDougall, R M H Dr. Cheng: 1336. The ”Light Weight” System – A Novel Concept for On-Board Weight and Balance Measurement Using Fibre Optics. 1980.



This paper describes recent theoretical and experimental work carried out by DSL Dynamic Sciences Limited, in an effort to develop an onboard weight and balance system (OBWB) suitable for small transport aircraft such as the Canadair ‘Challenger’ and the De Havilland ‘Dash 7’.
The work was instigated and funded by Transport Canada Research and Development Centre, in response to Canadair’s failure to locate a supplier of an OBWB for the Challenger – available systems either experienced problems of accuracy and repeatability, or were unsuitable for small aircraft.
Dynamic Sciences developed a concept for solving these problems by using optical transducers to detect deflections inside the landing gear axles. Such transducers would be electrically passive, highly sensitive to deflections, and lightweight.
The optical technique chosen was the ‘fiber optic strain gauge’, which operates on the principle that light takes a longer or shorter time to pass through a glass fiber which is stretched or compressed. An interferometer incorporating this technique was constructed, and it was demonstrated in the laboratory that such a system is capable of measuring deflections of axles on small transport aircraft to the required accuracy.
A preliminary design for an aircraft weight sensor based on this fiber optic technology was produced and presented to the Canadian aircraft manufacturers, who concluded that it was practicable.
The sensor consists of a hollow tube to which glass fibers are bonded. The fibers are bonded in such a way that the sensor is sensitive only to vertical shear due to vertical load. Two such tubes are mounted inside each axle. Light from a laser inside the aircraft is fed by glass fibers to each sensor tube. An interference pattern, produced by the fibers bonded to the tube, is transmitted by a fiber bundle to a fringe counter located inside the aircraft. The fringe counters provide inputs to a digital computer which computes and displays the aircraft’s gross weight and centre of gravity location.
The next stage in the development of the ‘Light-Weight’ OBWB system will be the production of an experimental optical transducer system and its calibration under controlled laboratory conditions.


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