String potentiometer blade motion measurement system applied to fully articulated inter-blade rotor

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Dornetti, M.
Trezzini, A.A.
Fosco, E.
Colombo, A.
Facchini, G.
Piccirillo, G.
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Innovations in design represent an essential ingredient to continuously improve performances and reliability of flying products. In particular, the competitiveness of a helicopter is directly related to the efficiency of its rotors, which determine performances, loads and comfort for the entire aircraft and its occupants. The architecture of the rotor and its behavior in flight are important ingredients to be investigated in order to obtain the best compromise in terms of market attractiveness and engineering feasibility; on this subject, one of the key factors in the development and evaluation of a rotor system is the knowledge of in-flight blade motions, which is a task that historically required significant engineering effort in order to get accurate and reliable measurements. Present paper describes the application of a new concept of blade motion measurement system applied to a recently developed main rotor inter-blade architecture, which employs rotational dampers fitted in the tension links of the corresponding blades and connected also to the following blades by means of metallic links. The requirements at the basis of the development of a new measurement system are the need of very compact equipment, compatible with the limited space available on the new hub architecture, together with reliability improvements with respect to existing devices. In general, the importance of such a system is to allow the reconstruction of the rotor motion providing pitch, flap and lag angles time histories of the blade; these data are then used basically to correlate with flight mechanics models, compare with rotor loads scenario and provide on-field measurements helping in the life definition of elastomeric components. In this paper we deal with all the system stages, starting from the early hardware design and development activity, through dedicated software implementation, up to the final phases of system calibration, flight data processing and correlation. As far as hardware design, measurements on new inter-blade rotor architecture cannot be managed as for hub-to-blade solution, which used damper motion as input for the kinematics computation; in fact, in the inter-blade architecture, being the damper connected to two consequent blades, it doesn’t provide information about absolute position of the blade. Moreover, the absence of the damper attachment point on the hub makes difficult to fit an analogous mechanical system, consisting of angular potentiometers fitted on a crank connected-rod mechanism. For these reasons, a more compacted measurement technology based on string potentiometers sensors has been adopted, helping both in terms of ease of installation and system flexibility. In fact, string potentiometers can be easily installed on the rotor and cable path can be simply managed in terms of optimal position in order to guarantee both measurement effectiveness and clearance with all the moving parts of the rotor. The adopted measurement system configuration is made of three sensors for each blade: potentiometer bodies are installed on the tension link through a removable plate, whereas cables are linked to the hub by means of a dedicated support, providing blade positioning with respect to the hub itself. The final location of the attachment points, both on the blade and hub sides, is the result of a detailed evaluation assessment, carried out with the aim of finding the optimum between measurements accuracy and motions decoupling (in terms of flap, lag and pitch degrees of freedom), within mechanical clearance constraints. This work has been performed starting from CATIA environment and developing a multi-body model to analyze in depth kinematics and support the design phase; in parallel, an in-house Matlab code has been developed and validated with previous models, in order to manage design sensitivity analyses and provide a valid tool to process flight data on the prototypes. Starting from the six string potentiometers signal, mentioned code allows basically the reconstruction of the time histories related to the angles of two consecutive blades; using these results and modeling the kinematics of the inter-blade rotor architecture as rigid, computation of damper angle and full set of damper link rod-end rotations have been provided. Once calibrated, this system flew for over 100 flight hours without technical issues and providing high accuracy of its measures. Moreover, the good reconstruction of damper motions starting from measurements on two consecutive blades can be considered a significant achievement, validating the entire process and providing a solid basis to infer rod-end rotations from the reconstructed kinematics. The new system can be easily tailored on other rotors and different architectures, with minimum design impact and optimization effort.