<p>Stability Control Augmentation Systems (SCAS) are widely adopted in rotary wing flight controls to enhance the vehicle stability in difficult aerodynamic conditions, such as low altitude missions, stationary flight nearby vertical walls or in presence of heavy gusts. SCAS systems are based upon small electro-hydraulic position-controlled servosystems on the command linkage mechanism of the flight control actuator. Due to their short stroke and responding to safety concerns, SCAS actuators have a limited authority over the movement of the main actuator. Being critical for the operability of the helicopter, the definition of a Prognostics and Health Management framework would provide significant advantages to the users, such as better risk mitigation, improved availability and a reduction in the occurrences of unpredicted failures which still represent one of the most known downsides of helicopters. A preliminary analysis on the effects of the inception and progression of several degradation types is the first step towards assessing if such PHM system is feasible, and which failure modes are more likely to be observed. Authors already investigated the effects of several faults mode on electro-hydraulic servo actuators (EHSA) for fixed wing aircrafts in previous works, addressing the feasibility of the definition of a dedicated PHM system without resorting to additional sensors. Results of these research activities are not however directly applicable to the case of the SCAS actuators; these devices are in fact different in design and type of usage, while also lacking some signals instrumental to the observation of faults within EHSAs for other applications. SCAS actuators are not directly subjected to the aerodynamic load, and the only force disturbance acting on their control loop is due to the combined effect of a recentring spring., which is fundamental for the system safety, and friction. To pursue this analysis, a detailed model of the flight control actuator has been prepared and a number of significant failure modes investigated. For the servovalve, issues in the feedback spring (backlash and crack inception), in the torque motor (windings short and degradation of the permanent magnets) and in the jet-pipe (occlusion and deformation of the channel) were considered. For the actuator, wear of the dynamic seals, crack inception in the recentring spring, increase in friction and backlash at the spherical joints were considered. Then, an in-depth simulation campaign was pursued with the aim of studying the interactions between different failure modes, the effects that the propagating degradations have on the system performances and which signals can be used to define a robust set of features. The paper introduces the case-study under analysis, presenting the most prominent peculiarities of the system and the effect of such peculiarities on the definition of health monitoring schemes. Hence, the model used to describe the behaviour of the system under nominal and degraded conditions is introduced and the selected fault modes described with details. Hence, results of a wide-ranging simulation campaign are presented, where the time-domain response of the system is used to guide in the definition of a proper set of features able to characterize the selected fault cases. The possible features are hence ranked according to significant metrics such as correlation with the simulated degradations, signal-to-noise ratio and accuracy. Finally, preliminary conclusions are drawn on which fault modes are detectable and which features are most promising for future PHM systems implementation.&nbsp;</p>

A Simulation Survey on the Effects of Progressing Faults Within the SCAS of a Flight Control Actuator for Helicopters

Paper Proceedings: https://asmedigitalcollection.asme.org/FPST/FPMC2021/volume/85239

2021 ASME/BATH Symposium on Fluid Power and Motion Control (FPMC2021)

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