Browse Topic: Reliability
Gearbox casing cracks in helicopters would be critical impacting the aircraft's reliability and operation safety directly. The Defense Science and Technology Group (DSTG) HUMS2025 gearbox casing failure data set was the unexpected result of a test stand operation. The gearbox undergoes high cycle (> 400 acquisitions) under high torque (100% and 125% nominal torque) conditions. We hypothesized that the any cracking would be due to the planet/ring gear interaction. A condition indicator (CI) would be sensitive to a crack feature and this would be sensitive to change in gearbox torque. This paper explores the development of both a cyclo-stationary based CI (frequency-domain) and a time synchronous average CI (time-domain). The trend shows that proposed methods can help to detect localized defects in gearbox casing at an early stage and trend as the crack propagates before catastrophic failure occurs.
In this work, a vision-based solution is developed to address the challenge of landing on a ship deck with precision and accuracy. For an autonomous landing, it is important to have a fast and accurate pose estimation system along with a reliable control strategy. This research uses fractal ArUCo markers instead of multiple separate markers to allow smooth pose estimation at different heights. Pose estimates are further improved using an Extended Kalman Filter, and a tracking algorithm then uses these estimates to guide the landing. A four degree-of-freedom (roll, pitch, heave and sway) simulator platform was built and used to validate the algorithm. The accuracy of the vision system is compared against that of a motion capture system. Real-world experiments were performed on different quadrotors to demonstrate tracking and landing on the platform with sway, roll, and pitch motions. The results show that the system is efficient and reliable in achieving safe and successful landings. The proposed landing system is concluded to be applicable for landings on the deck of the ship under sea-state 4.
Rotorcraft dynamic component fatigue lives and corresponding reliability have long been derived from three major contributors: material strength, loads, and usage. This paper provides a historical perspective of the contribution of aircraft usage to overall U.S. Army rotorcraft dynamic component reliability. A quick background of how we got to a six-nines reliability requirement is first provided. Different types of usage spectra and the nuances and trade-offs of two specific usage gathering methods, pilot surveys and usage monitoring, are discussed. Finally, I describe where usage spectrum fits into fatigue life calculations and the existing reliability policy and requirements. Each OEM (e.g., Bell Helicopter, Boeing, Sikorsky) has been free to develop their own fatigue methods over the years. These differences in method can lead to vastly different results, even with the same input parameters as evidenced by a now well-known round robin problem. There is notable variability between OEM methodologies, each with viable solutions to this trivariate problem. In the interest of normalizing independent U.S. Government (USG) assessments across multiple OEM paradigms, the Army is investigating a USG method to assess the reliability contribution from usage. No new methods are presented herein, only findings of previous work. Uncited opinions herein are those of the author based on literature review, peer discussions, and experience with U.S. Army and U.S. Air Force (USAF) airworthiness processes. Reliability values in this paper are approximate, as there are elements of statistical distribution and non-statistical estimation that contribute.
Wear debris monitoring and analysis is a common practice for the condition assessment of engine and transmission health. Oil debris monitoring (ODM) and electronic chip detectors (ECD) are two common methods deployed for continuous monitoring of oil wetted component health in-flight. This study evaluates the diagnostic performance of the two sensing technologies within controlled rolling element bearing (REB) fault experiments. Progressive visual inspection of the REB spall progression through failure provided a ground truth against which both systems could be compared. Quantifiable metrics of reliability, diagnostic accuracy, provided maintenance interval were defined to create a framework for condition-based maintenance (CBM) program decision making. In summary, it was found that the ODM sensor system provided earlier fault notice, but more so, vastly outperformed the ECD in reliability and avoidance of false positives.
A framework for statistical comparison between analytical and experimental structural loads has been developed and applied to approximately 100 counters within the UH-60A Airloads test program. This framework relies on established structural load variability methods with novel applications to analytical structural load development maneuver time transient analysis. The analytical results are from Rotorcraft Comprehensive Analysis System (RCAS) spanwise structural loads developed with hub load and spanwise aerodynamic loads prescribed. RCAS consistently under predicted the Coefficient of Variation (COV) associated with spanwise Normal bending when compared to flight data. This resulted in significant scale factors required to achieve a μ+2σ reliability for structural load development. RCAS results for Edgewise bending scale factors proved slightly better than Normal bending in addition to more even over / under prediction of COV when compared to flight data.
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The paper discusses the application of the Array Controlled Turn-less Structures (ACTS) motor for VTOL application. The motor enhances the three main competing characteristics of electric motors; namely specific power, efficiency and reliability. The motor arrays an ensemble of elemental turn-less motors which include turn-less elements each with their dedicated inverters which are operated in synchronism. The resulting small pole size enhances the power density, the enhanced conductor packing enhances the efficiency, and the massive parallelism enhance the reliability. Vertical takeoff requires much higher thrust compared to wing assisted takeoff. With limited on-board power, this higher thrust is presently provided by in ordinary larger propulsion disk area which reduces the craft aerodynamics, and the cruising Lift-to-Drag (L/D) ratio and accordingly the flight efficiency and range. The high specific power of the ACTS motor allows for a different scenario and thus craft architecture. By substantially increasing the takeoff power which is now possible with the higher specific power of the ACTS motor, the propulsion disk area can be substantially reduced, with the resulting greater streamlined, high L/D craft, and thus longer range. The paper discusses key aspect of the motor and inverter architecture and technology. Furthermore, it discusses its application to a high L/D VTOL and prospective performance.
An alternative probabilistic approach is proposed to assess the reliability of rotorcraft structures. According to the approach, safe boundaries of reliability predictions (i.e., conservative ones with additional safety margins) are calculated instead of exact values of the reliability as usually estimated in conventional analysis. Due to additional safety margins, these boundaries are suggested for practical engineering applications. The proposed approach is based on two main ideas, namely a) prediction of a relatively small population of independent coarse estimations of reliability and b) application of sampling methods to predict reliability for each individual coarse estimation. Robustness and convenience of the developed approach and its computational implementation is demonstrated for four scenarios considering two problems (simplified analytical and realistic FEA-based ones) under two sets of input data providing probability of failure (POF) close to 1e-6 and 1e-9, respectively. High accuracy of predictions according to the developed approach is independently verified by comparison with known “exact” references: closed-form solutions for the analytical problem and by direct MCS for the numerical one. The proposed approach can be recommended as an efficient practical solution for a broad range of reliability assessments of rotorcraft structures requiring both i) low POF (e.g., below 1e-6) and ii) relatively complex time-consuming structural definitions (e.g., compute times in hours for each quasi-deterministic simulation).
ABSTRACT Classical reliability modeling methods such as reliability block diagrams and fault trees express system reliability in terms of the reliability of the constituent subsystems and the architecture of that system. In recent years, prognostics and health management (PHM) has emerged as a promising method to combine sensing and algorithms to estimate important measures of reliability such as the probability that a subsystem possesses sufficient remaining useful life to conduct a mission without failure. This is especially important for mission critical systems. However, methods from classical reliability do not explicitly consider PHM. To overcome this limitation, this paper develops a modeling approach to consider reliability outcomes as well as PHM decisions, which should exhibit strong correlation in order to correctly classify the true state of the subsystem or component as healthy or unhealthy. We draw upon more general reliability modeling methods to characterize the correlation between the state of a subsystem’s reliability and PHM decision. We subsequently propose an approach to obtain analytical expressions to assess system availability and cost in terms of these pairs of subsystem reliabilities and PHM decisions. Models that combine concepts from reliability and PHM will complement existing reliability, availability, and cost models, enabling sensitivity analysis within trade studies that can identify how improvements to subsystem-specific PHM techniques will impact system and fleet-level measures.
ABSTRACT The pericyclic transmission provides the opportunity to vastly impact transmission design in rotorcraft due to its ability to provide exceedingly high reduction ratios in a single stage that would normally require multiple gear stages. This could lead to lighter transmissions with fewer components, increased reliability, efficiency, speed and decreased cost to maintain. While many previous studies have focused upon the gearing within the pericyclic transmission, this work focused on what influences pericyclic geometry, and how changes in geometry impact bearing loads. Specifically, the loading of bearings that must deliver power from the input shaft to the nutating and rotating gears of the system were of primary concern. A comprehensive look at dynamic loads generated by nutating bodies was performed. Methods to address these dynamic loads via application of counterbalances, and deviation from conventional pericyclic transmission designs were utilized to negate the dynamic moment of concern. Counterbalances negating the dynamic moment were shown to weigh between 30-50% of the pericyclic motion converter gears in a 40:1 reduction ratio pericyclic design at 12,000 rpm input speed and reduced applied moments by three orders of magnitude. Finally, a static solver was used to determine the bearing loads with updated component geometries and mass moment of inertias that included the required counterbalances.
ABSTRACT Torque indicating issues continue to plague Power Turbine Modules (PTMs) as a top reason for return. These issues can be linked to failed torque and overspeed sensors and PTM torque reference shaft issues including; sticking shafts due to torque stiction and sheared pins connecting PTM drive shafts to torque reference shafts. At the end of the PTM shaft is the pinning hardware for the torque reference shaft. In shipping and during handling of the PTM damage can occur causing the pin to shear and fail. They can fail due to inadvertent contact with the PTM or torque shaft, during shipping and handling, or after prolonged usage. Defining a reliable inspection procedure to make the determination that the PTM shaft is faulty prior to ‘on wing’ installation can save time and money for the operator. In addition, the repair procedure for sheared pin is to replace pinning hardware at Corpus Christi Army Depot (CCAD) machine shop. During the repining of the hardware the holes must be located a minimum distance from each other to preserve the integrity of PT shaft and aft insert parent metal according to the Depot Maintenance Work Requirement manual. In the presence of machine shop error, failing to maintain the minimum distance between pin holes prevents viable PT shafts from use in production. Identifying a process improvement for repair procedures of pin hardware at the machine shop can save time and money invested in overhauled PT shafts for the depot.
ABSTRACT Usage credits may be used to extend retirement lives for structural components. However, any credit substantiation must account for the contribution of conservative usage assumptions to the current level of safety. Structural reliability methods have been proposed as a means to achieve this end. Herein a new, relative method to determine a practically equivalent reliability (and safety) for aircraft fleets is developed using system reliability theory. Simple mathematical examples are used to illustrate the basic principles. A more realistic example based on the AHS Fatigue and Damage Tolerance subcommittee Round Robin problem is presented. These examples show that, even if only a few aircraft in a fleet operate in a severe manner, these aircraft drive the overall fleet reliability. This means that many aircraft may be able to receive credit without having any appreciable change on fleet reliability. A generalized procedure to apply the method to real world problems is developed. Use of the method as part of a certification methodology is presented along with a justification of what change in reliability would be practically equivalent. Application of this method may allow for safe extension of component lives based on usage.
ABSTRACT A Health and Usage Monitoring System (HUMS) records a large number of flight parameters that can be utilized for regime recognition and tracking of individual rotorcraft usage. The availability of this information from a fleet offers the additional opportunity to work with these large datasets within an Advanced Structural Integrity Framework (ASIF) developed to perform reliability-based design and maintenance, considering durability and damage tolerance. Probabilistic analytical techniques were explored using a sample of UH-60 Integrated Vehicle Health Monitoring System (IVHMS) fleet data that was analyzed for the horizontal stabilator in a case study to estimate the reliability associated with a specified component replacement time and inspection interval. Probabilistic sensitivity studies showed that the deterministic design and substantiation practices used for the horizontal stabilator very closely approximated the industry accepted reliability levels for fatigue and crack growth. The probabilistic methodology also effectively utilized the available IVHMS regime recognition output, but the reliability results depended greatly on assumptions used to describe statistical distributions of key random input parameters, especially loads, which were not available from the fleet data for this component. The ASIF was shown to be a powerful tool for reliability-based management of rotorcraft structural integrity. Further work is needed to confidently configure and fully exploit such methods.
ABSTRACT Operations and support cost constitutes nearly 70% of rotorcraft lifecycle cost. When considering new rotorcraft concepts and technology infusion for current concepts, quantitative performance evaluation is undertaken during conceptual design. However, the effect of design decisions on operations and support metrics are typically evaluated qualitatively. Since operation and support costs constitute an overwhelming majority of rotorcraft lifecycle cost, quantitative evaluation of these metrics is required to fully capture the design trade space. To this end, an integrated discrete-event simulation environment is developed to quantify the impact of architectural decisions and subsystem technology infusion on key metrics including the operational availability, system mean time between failures, Maintenance Free Operating Period, repair cost, and maintenance man-hours needed for a given period of operation. Since data needs are immense, it is appropriate to use data from existing platforms to populate unknown fields. An example is presented in this paper for a notional helicopter to demonstrate the use of the discrete-event simulation environment as a tradeoff environment for operations and supportability metrics.
ABSTRACT Reliability analysis of a rotorcraft transmission bolted connection using actual service records data is the focus of this paper. The analysis is conducted for two different bolt materials used in the gearbox to evaluate the difference in reliability. As part of a continued operational safety bolt preload assurance check, the bolted connection between the spiral bevel ring gear and first stage sun gear undergoes a periodic field inspection: the inspection is said to have failed if the application of a prescribed torque to the nut causes the entire bolt assembly to spin. Reliability analysis employs a two-parameter Weibull probability density function to model the life of the bolted connection, with parameters derived using the Maximum Likelihood Estimator method. Results for each bolt type are discussed and an observation is made as one possible explanation for disparity in reliability levels for the two different bolt materials which is supported by direct measurement of bolt tension at initial assembly. As a result of the findings in this paper, recommendations for future work, including modification of the bolted connection assembly procedure, is discussed.
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