Development and application of optical fibre strain and pressure sensors for in-flight measurements

Abstract

Fibre optic based sensors are becoming increasingly viable as replacements for traditional flight test sensors. Here we present laboratory, wind tunnel and flight test results of fibre Bragg gratings (FBG) used to measure surface strain and an extrinsic fibre Fabry-Perot interferometric (EFFPI) sensor used to measure unsteady pressure. The calibrated full scale resolution and bandwidth of the FBG and EFFPI sensors were shown to be 0.29% at 2.5 kHz up to 600 με and 0.15% at up to 10 kHz respectively up to 400 Pa. The wind tunnel tests, completed on a 30% scale model, allowed the EFFPI sensor to be developed before incorporation with the FBG system into a Bulldog aerobatic light aircraft. The aircraft was modified and certified based on Certification Standards 23 (CS-23) and flight tested with steady and dynamic manoeuvres. Aerobatic dynamic manoeuvres were performed in flight including a spin over a g-range −1g to +4g and demonstrated both the FBG and the EFFPI instruments to have sufficient resolution to analyse the wing strain and fuselage unsteady pressure characteristics. The steady manoeuvres from the EFFPI sensor matched the wind tunnel data to within experimental error while comparisons of the flight test and wind tunnel EFFPI results with a Kulite pressure sensor showed significant discrepancies between the two sets of data, greater than experimental error. This issue is discussed further in the paper.

On the development of flight-test equipment in relation to the aircraft spin

“…For example, the miniature 7-hole probe air data system, used for spin testing by Li et al, is just 4mm in diameter and weighs only 150g but is still capable of measuring air flow angles to an accuracy of 1% [60]. Similarly, an SBG Systems IG-500A Attitude and Heading Reference System (AHRS), utilised in studies regarding a Scottish Aviation Bulldog [61,42], weighs just 39g, but can provide 3-dimensional attitude and acceleration data in a dynamic manoeuvre accurate to within 2 • [62]. Advances in technology such as these allows representative data to be gathered with minimum impact of the added equipment on the aircraft's centre of gravity, total weight, and general aerodynamics.…”

“…Firstly, Lawson et al [61,63] describes the development of the fibre optic sensors for flight test measurements of surface strain and unsteady pressure, using a Scottish Aviation Bulldog. Fibre Bragg gratings (FBGs), distributed along the port side wing were used to record wing strain data.…”

“…The performance of the fibre optic sensors was verified by Lawson et al [61,63] before the same instrumentation was used in a further study regarding the effect of mass distribution on the characteristics of a spin [42]. The tests involved altering the fuel load on board to identify the changes in moments of inertia during a spin.…”

Development of Fibre Bragg grating (FBG) dynamic pressure transducer with diminutive voltage inconsistency

“…This formula is established by Nobel Laureate Sir William Lawrence Bragg in 1915 about diffraction of X-Ray from crystals and he expressed it into a simple mathematical formula. The Bragg wavelength is sensitive to physical changes in the grating due to strain and temperature [30]. Thermal expansion in the FBG causes the effective refractive index and the spacing of the gratings to change simultaneously and produced a wavelength shift.…”

Photonic crystal fiber micro pressure sensor based on an extrinsic Fabry-Perot interferometer fabricated by femtosecond laser

“…Fiber-optic pressure sensors have been applied in down-hole pressure monitoring [1], engines [2], aerodynamics [3], biomedical field [4], etc. In recent years, fiber-optic extrinsic FPI pressure sensors based on diaphragm deformation have been studied.…”