This work evaluates the long-term fatigue life and structural compatibility of integrated optical fiber sensors (OFS) within an H145 (or BK117 D-3) helicopter flexbeam. Utilizing fiber Bragg grating (FBG) arrays, the study compares different deployment techniques under a 100,000-cycle fatigue test: embedded, surface-integrated, and surface-applied. A validated three-dimensional (3D) finite element model (FEM) was developed to reconstruct cross-sectional loads and correlate experimental strain data. Validation against conventional electrical strain gauges (SG) confirms that embedded FBGs significantly outperform SGs in durability, maintaining functionality beyond the operational limit of traditional sensors. Furthermore, the methodology successfully tracks global stiffness evolution and degradation throughout the fatigue life. Micro-computed tomography (µCT) scans verify that the integrated fibers do not compromise structural integrity. These findings demonstrate the potential of embedded OFS for continuous, in-service load monitoring and condition-based maintenance (CBM) of flight-critical rotorcraft components.

Optical fibre strain and shape measurement sensors were deployed on two bearingless main rotor systems, Airbus Helicopters H135 and Airbus Helicopters H145 (or BK117 D-3) during ground runs with controlled pilot inputs and during a whirl tower test. The sensing capabilities of two optical fibre-based strain sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), and direct fibre optic shape sensing (DFOSS), a shape measurement based on the FSI approach, were benchmarked against conventional strain gauge measurements. Signal-to-noise ratios and modal properties were determined from the collected strain and displacement signatures using an improved operational modal analysis suitable for the removal of amplitude-modulated rotor harmonics and large rotor speed variations. It was shown not only that all fibre-optic based sensing techniques provide detailed understanding into the dynamic properties of the blade, but the measurements also offer insights into couplings from the airframe to the rotor and couplings from the drive train to the rotor. Results and discussions of the analysis of the measurements from the DFOSS system highlight its benefits over strain gauges or of the FBG strain sensing approach.

Optical fibre strain and shape measurement sensors were deployed on a 5-m long rotor blade during a full-speed (rotation rate 6.6 Hz) helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In Part 1 of a 2-part paper series, the strain sensing capabilities of the two optical fibre-based sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), are compared, while Part 2 (Kissinger et al 2022 Smart Mater. Struct. accepted) specifically investigates the blade shape measurement based on the FSI approach. In part 1, the rotor hub-mounted instrumentation is described, and data on the dynamics of the blade obtained from a sequence of controlled pilot inputs are analysed both in the time and spectral domains. It is shown that this can provide insights into the aeroelastic properties of the blade. Noise standard deviations of 0.2 n ϵ / Hz and 30 n ϵ / Hz for the FSI and FBG-based sensing approaches, respectively, were observed over a strain range of 3500 µϵ .

Optical fibre strain and shape measurement sensors were deployed on a rotor blade during a full-speed helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In part 2 of a 2-part paper series, two-dimensional direct fibre-optic shape sensing (DFOSS), using fibre segment interferometry-based interrogation is investigated. The concept of blade shape change visualisation over one rotation period using rotation displacement surfaces is introduced and the usefulness of DFOSS data to gain additional insights by determining operational modal frequencies independently for both horizontal and vertical vibration directions of the blade is demonstrated.

This paper compares two fibre optic sensing techniques for vibration characterisation: (a) optical fibre Bragg grating (FBG) strain gauges and (b) a novel direct fibre optic shape sensing (DFOSS) approach based on differential interferometric strain measurements between multiple fibres within the same fibre arrangement. Operational mode shapes and frequency measurements of an Airbus Helicopters H135 bearingless main rotor blade (5.1 m radius) were acquired during a series of ground vibration tests undertaken in a controlled laboratory environment. Data recorded by the fibre optic instrumentation systems were validated using commercially available accelerometers and compared against a baseline finite element model. Both fibre optic sensing systems proved capable of identifying the natural frequencies of the blade in the frequency range of interest (0-100 Hz). The data from the FBG sensors exhibited a dependency on their position relative to the neutral axes of the blade, which meant that full characterisation of the flapping and lagging modes required careful consideration of sensor location in the chordwise direction. The DFOSS system was able to identify all structural dynamics, despite being located on the neutral axis in the lagging direction, due to its sensitivity to angle changes, rather than strain, and its biaxial measurement capability. The DFOSS system also allowed the operational mode shapes of the blade to be determined directly, without the requirement for strain transfer from the blade to the sensor and without the requirement for a model of the underlying structure. The accuracy of obtained natural frequencies and operational mode shapes is assessed, demonstrating the potential of the use of both fibre optic sensing systems for determining blade structural dynamics. (c) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Two fibre-optic sensing techniques were evaluated and compared in a ground vibration test on a full-scale helicopter blade, with verification by measurements made using accelerometers. The first technique used an array of fibre Bragg gratings to monitor the amplitudes of the mode frequencies at specific locations along the blade. The second approach employed a novel direct fibre optic shape sensing (DFOSS) technique to directly measure the shape of the blade and characterise its modal frequencies. It is shown that modal frequencies can be detected with both techniques. However, only the DFOSS approach can detect all modal frequencies using a single sensor array. Crucially, the DFOSS approach provides ease of installation, as no strain transfer to the structure-under-test is required and thus its use can significantly speed up ground vibration test campaigns.

The use of optical fibre Bragg gratings (FBGs) and direct fibre optic shape sensing (DFOSS) in the ground vibration testing of a full size helicopter rotor blade has been evaluated, with the performance benchmarked against measurements made using accelerometers. An array of FBGs was used to monitor the amplitudes of the mode frequencies at specific locations along the blade and DFOSS was used to measure directly the shape of the blade and to characterise its modal frequencies. While it was possible to measure modal frequencies using both approaches, DFOSS proved capable of detecting modal frequencies using a single sensor array located along the longitudinal axis of the blade.

Results of a survey investigating commonly occurring minor rotor blade damage incidents are presented in this paper. Over 100 participants worldwide ranging from test pilots to commercial pilots and licensed engineers answered the survey. The focus of this work was to provide a user-oriented context that can inform the decision-making process for integrating state-of-the-art instrumentation systems for rotor blade health monitoring onboard operational helicopters. This paper highlights the dichotomy faced by designers who have a choice to follow either a reactive strategy based on operational experience or a preventative approach based on technological trends.

The use of a new aeroelastic computer framework called Flexit is described and the framework is used to analyse the dynamic aeroelastic behaviour of a four-bladed helicopter main rotor. Flexit implements a loose coupling between unsteady vortex lattice method (UVLM) and numerical solution of the inhomogeneous Euler-Bernoulli partial differential equation (PDE). The framework is fast because most of the intensive computational functionality is performed on GPU using NVIDIA CUDA technology, and this makes it suitable for use in the early design stages. The UVLM algorithm uses a free wake model, and solution of the Euler-Bernoulli PDE is approximated using a finite difference algorithm that includes a term to take account of centrifugal forces. The results of simulations are compared with analysis performed with CFD and FSI tools.

I have always loved finding solutions to all sorts of problems and I always knew that I wanted to work in a technical area. Yet, a lesson learnt during my apprenticeship, qualifying to become a Technical Draughtsperson in a small bus company, triggered my decision to becoming an engineer. I quickly realised how important it is not to come up with just “a” solution. A theoretical solution might result in a design that would never work in reality. The skill of combining practice and theory is the reason why I wanted to become an engineer, learning about the tools available to solve problems.

Helicopters operate in many different flight regimes. Often, they are required to perform extreme manoeuvres. Damage on rotor blades can have serious consequences. Due to the wide spectrum of operation, rotor blade designers envisaged a large design envelope. However, the missions flown by many users exercise only a small inner region of this design envelope and they rarely push the blade to its operational limits. The lack of knowledge about the actual loading environment and resulting blade deformation leads to severe limitations of progress in the area of helicopter maintenance, repair and servicing the rotor blades.