Aircraft Hydraulic Systems Research Articles

Frequency-domain analysis of fluid-structure interaction in aircraft hydraulic pipeline systems: numerical and experimental studies

Frequency-domain analysis of fluid-structure interaction in aircraft hydraulic pipeline systems: numerical and experimental studies

Effect of Structural Parameters on Output Characteristics of a Novel Self-Supplied Aviation Intelligent Pump

The aviation intelligent pump system is an effective solution to aircraft hydraulic systems’ inefficient power consumption and temperature increase. A self-supplied aviation intelligent pump (SAIP) has a high power-to-weight ratio and compact structure, making it the optimal choice for an intelligent pump. To analyze the output characteristics of a novel aviation intelligent pump, it is crucial to establish an accurate mathematical model that describes its dynamic characteristics. This can be achieved by analyzing the working principle and exploring the influence of critical parameters. The paper introduces the composition and working principle of a self-supplied electro-hydraulic servo variable displacement pump. It then establishes a mathematical model of the whole pump, with a detailed analysis and modeling of the critical variable mechanism and the swash plate assembly’s load moment. A simulation model was created to examine the impact of crucial structural parameters, such as the offset spring’s stiffness and control piston’s diameter, on the output characteristics of the intelligent pump. An experimental platform was also constructed, and the experimental results confirm the accuracy of the SAIP model presented in this paper. The investigation of the output characteristics fully reveals the dynamic performance of the SAIP. This provides the basis for the subsequent design of high-performance flow and pressure control strategies and aids in researching intelligent aircraft hydraulic systems.

Effect of Temperature on S32750 Duplex Steel Welded Joint Impact Toughness.

The search for alternative materials that can be used for parts of aircraft hydraulic systems has led to the idea of applying S32750 duplex steel for this purpose. This steel is mainly used in the oil and gas, chemical, and food industries. The reasons for this lie in this material's exceptional welding, mechanical, and corrosion resistance properties. In order to verify this material's suitability for aircraft engineering applications, it is necessary to investigate its behaviour at various temperatures since aircrafts operate at a wide range of temperatures. For this reason, the effect of temperatures in the range from +20 °C to -80 °C on impact toughness was investigated in the case of S32750 duplex steel and its welded joints. Testing was performed using an instrumented pendulum to obtain force-time and energy-time diagrams, which allowed for more detailed assessment of the effect of testing temperature on total impact energy and its components of crack initiation energy and crack propagation energy. Testing was performed on standard Charpy specimens extracted from base metal (BM), welded metal (WM), and the heat-affected zone (HAZ). The results of these tests indicated high values of both crack initiation and propagation energies at room temperature for all the zones (BM, WM, and HAZ) and sufficient levels of crack propagation and total impact energies above -50 °C. In addition, fractography was conducted through optical microscopy (OM) and scanning electron microscopy (SEM), indicating ductile vs. cleavage fracture surface areas, which corresponded well with the impact toughness values. The results of this research confirm that the use of S32750 duplex steel in the manufacturing of aircraft hydraulic systems has considerable potential, and future work should confirm this.

Drive Systems for Landing Gears Numerically Simulated in AMESim Software

This work presents two versions of landing gear drives numerically simulated. We follow to obtain the right driving sequence of the legs and doors and the estimation of the landing gear deploy and retract time. Although the studies in this paper do not exhaust all the aspects concerning landing gear hydraulic system functioning, these are useful in the design phase of the aircraft hydraulic systems. Regarding the landing gear hydraulic system, we highlight one version of the drive stall phenomenon. When the hydraulic feeding pressure decreases, it is possible that the forces developed by the hydraulic cylinders are enough to retract completely the legs. The landing gear remains in an intermediate position, and the aircraft cannot accomplish its mission safely. For a second version, logic command functions are synthesized using Karnaugh diagrams, and after that, the numerical simulation is accomplished to validate the solution. We used the AMESim software to perform numerical simulations.

A digital twin framework for aircraft hydraulic systems failure detection using machine learning techniques

Since the last decade, aircraft systems, such as flight control and landing gear, have been requiring increasing power, and consequently, the complexity of hydraulic aircraft systems has escalated. Inevitably, this complexity has resulted in the need for the troubleshooting of hydraulic aircraft systems that are dispersed around an aircraft and supply power to critical flight systems. This study proposes a novel digital twin-based health monitoring system for aircraft hydraulic systems to enable diagnostics of system failures early in the design cycle using machine learning (ML) methods. The scope of the systems is limited to hydraulic systems at the aircraft level using 20 failure scenarios. The support vector machine and several ensemble learning algorithms of ML methods were used to identify these failures. A comparison of the ML methods revealed that the random forest algorithm performed superior to the other ML algorithms. The developed digital twin framework for hydraulic system of aerial vehicle platforms, can help researchers and engineers to evaluate diagnostics systems early in the design phase.

Challenges and Solutions for High-Speed Aviation Piston Pumps: A Review

As a core power component, aviation piston pumps are widely used in aircraft hydraulic systems. The piston pump’s power-to-weight ratio is extremely crucial in the aviation industry, and the “ceiling effect” of the PV value (product of compressive stress and linear velocity) limits the piston pump’s ability to increase working pressure. Therefore, increasing the piston pump’s speed has been a real breakthrough in terms of further enhancing the power-to-weight ratio. However, the piston pump’s design faces several challenges under the extreme operating conditions at high speeds. This study reviews several problems aviation axial piston pumps face under high-speed operating conditions, including friction loss, cavitation, cylinder overturning, flow pressure pulsation, and noise. It provides a detailed description of the research state of the art of these problems and potential solutions. The axial piston pump’s inherent sliding friction pair, according to the report, considerably restricts further increasing of its speed and power-to-weight ratio. With its mature technology and deep research base, the axial piston pump will continue to dominate the aviation pumps. Furthermore, breaking the limitation of the sliding friction pair on speed and power density, thus innovating a novel structure of the piston pump, is also crucial. Therefore, this study also elaborates on the working principle and development process of the two-dimensional (2D) piston pump, which is a representative of current high-speed pump structure innovation.

Propagation Characteristics of Pressure Pulsation in Hydraulic Hose

Most modern aircraft hydraulic systems use variable plunger pumps. The pulsating flow output is its inherent characteristic. The resulting pressure pulsation often causes serious damage to the energy pipeline system and endangers the lives of the occupants. This paper studies the propagation law of hydraulic oil in hydraulic hoses, fully considers the coupling vibration of hydraulic oil high-pressure fluid and flexible solids of the hose, establishes a fluid-solid coupling vibration model of hydraulic hoses, and uses ANSYS for numerical simulation to study different frequencies and pipe bending. Using ANSYS for numerical simulation to study the influence of different frequencies and pipe bending radii on the amplitude of fluid pressure pulsation, after comparative analysis, the law of fluid pressure pulsation propagation in hydraulic hoses is obtained.

Investigation on the Characteristics of a New Structure Brake Distribution Valve

Abstract A brake distribution valve is an important part of aircraft hydraulic systems. To realize the integration and lightweight design of traditional structure valves, boost the response speed of valves, and reduce the influence of nonwork zone and damping on valve performance, this paper presents a new structure brake distribution valve which consists of pressure reducing unit, overflow unit, and embedded shuttle valve instead of the traditional valve structure. Numerical simulations and model verification experiments of the entire valve demonstrate that the new structure valve's dynamic performance has been significantly improved on the premise of ensuring linear characteristics. The simulation results show that the response time of the new structure valve is shorter than that of the traditional structure valve, with a rate of 30.77%, in case of step response simulations, and the tracking error of the valve reduces by nearly 5% in sinusoidal response simulations.

Review of cylinder block/valve plate interface in axial piston pumps: Theoretical models, experimental investigations, and optimal design

Axial piston pumps have been widely used in aircraft hydraulic systems to supply the system with pressurized fluid. The continuous improvement of the aircraft performance has put forward the demand on aviation piston pumps for high power density, safety, and reliability. The lubricating interfaces in axial piston machines are the key design issue that greatly determines the pump performance and service life. The cylinder block/valve plate interface is one of these critical lubricating interfaces and has received considerable attention from many researchers in the last half century. This study aims to review the state‐of‐the‐art literature on the cylinder block/valve plate interface comprehensively and systematically. First, we introduce various theoretical models developed to investigate the lubrication behaviors of the interface and compare them in terms of their assumptions and limitations. Second, the experimental studies on the cylinder block/valve plate interface are presented comprehensively, where the involved test rigs are divided into three types according to their fidelity levels and measurement functionality. Third, we summarize some typical approaches of structure optimization, surface shaping, and surface strengthening, which help improve the load-carrying and anti-wear capacities of the interface under severe operating conditions. Finally, the challenges and future trends of the cylinder block/valve plate interface research are discussed briefly.

Use of phase portraits of hydro-mechanical units for diagnosing aircraft hydraulic systems

The article deals with diagnostics of hydraulic systems using phase portraits. A brief review of the existing methods for diagnosing hydraulic units identifying their advantages and disadvantages is given. An approach based on the analysis of dynamic characteristics of a hydraulic system and phase portraits of hydro-mechanical units in their operational and faulty conditions is proposed. As an example, we consider a dynamic model of a simplified hydraulic system consisting of standard components. By adjusting the model parameters characteristic faults typically occurring in operation, such as internal leaks in the pump, contamination of the hydraulic fluid with mechanical impurities, sticking of the valve, etc. were artificially introduced in hydro-mechanical units. A family of phase portraits of a hydraulic system for the operational condition and various faulty ones was constructed. A quantitative estimate of their changes, based on calculating the difference in the areas of the figures restricted by their graphs, is proposed. As a result, it was established that failures and malfunctions introduce changes in the phase portraits of hydro-mechanical units, which makes it possible to apply the proposed approach as a basis for diagnosing the technical condition of hydraulic systems.

Influence of Friction Stir Processing Parameters on Tensile properties and Microstructure of Dissimilar AA 8011-H24 and AA 6061-T6 aluminum alloy joints in Nugget Zone

The recently developed innovative joining process, Friction stir welding (FSW) is connected to make the best quality of strength and hardness of the welded joints. The lightweight nonferrous materials are focused on aircraft hydraulic systems, fuel tank and transportation ventures due to its high strength to weight ratio and corrosion resistance. The joining of aluminum alloys is extremely troublesome in the conventional fusion welding process because of its cementing shrinkage, hot breaking, scatter, fumes and residual stresses. The friction stir processing is connected to enhance the mechanical properties of the friction stir welded joints by the addition of Nanoparticles viz., SiC and AkO3. The investigation was made on the microstructural analysis of the similar and dissimilar friction stir welded and processed AA 8011-H24 and AA 6061-T6 aluminum alloy joints by Scanning Electron Microscope and the presence of alloying elements in the welded region was found by Electron Dispersive Spectroscopy.

Intelligent Diagnostics for Aircraft Hydraulic Equipment

In aviation industry, unscheduled maintenance costs may vary in a large range depending on several factors, such as specific aircraft system, operational environment and aircraft usage and maintenance policy. These costs will become more noteworthy in the next decade, due to the positive growing of worldwide fleet and the introduction of more technologically advanced aircraft. The new implemented technologies will bring new challenges in the Maintenance, Repair and Overhaul (MRO) companies, both because of the rising number of new technologies and high volume of well-established devices, such as Electro-Hydraulic Servo Actuators for primary flight control. Failures in aircraft hydraulic systems deeply influence the overall failure rate and so the relative maintenance costs. For this reason, overhaul procedures for these components still represents a profitable market share for all MRO stakeholders. Innovative solutions able to facilitate maintenance operations can lead to large cost savings. This paper proposes new methodologies and features of the Intelligent Diagnostic system which is being developed in partnership with Lufthansa Technik (LHT). The implementation of this innovative procedure is built on a set of failure detection algorithms, based on Machine Learning techniques. This development requires first to bring together the results from different parallel research activities: Identification of critical components from historical data; Designing and testing automatic and adaptable procedure for first faults detection; High-fidelity mathematical modeling of considered test units, for deeper physics analysis of possible failures; Implementation of Machine Learning reasoner, able to process experimental and simulated data.

Investigation into the Effects of the Variable Displacement Mechanism on Swash Plate Oscillation in High-Speed Piston Pumps

High-speed, pressure-compensated variable displacement piston pumps are widely used in aircraft hydraulic systems for their high power density. The swash plate is controlled by the pressure-compensated valve, which uses pressure feedback so that the instantaneous output flow of the pump is exactly enough to maintain a presetting pressure. The oscillation of the swash plate is one of the major excitation sources in the high-speed piston pump, which may cause lower efficiency, shorter service life, and even serious damage. This paper presents an improved model to investigate the influence of the variable displacement mechanism on the swash plate oscillation and introduces some feasible ways to reduce oscillation of the swash plate. Most of the variable structural parameters of the variable displacement mechanism are taken into consideration, and their influences on swash plate oscillation are discussed in detail. The influence of the load pipe on the oscillation of the swash plate is considered in the improved model. A test rig is built and similarities between the experiments and simulated results prove that the simulation model can effectively predict the variable displacement mechanism state. The simulation results show that increasing the volume of the outlet chamber, the spring stiffness of the control valve, the action area of the actuator piston, and offset distance of the actuator piston can significantly reduce the oscillation amplitude of the swash plate. Furthermore, reducing the diameter of the control valve spool and the dead volume of the actuator piston chamber can also have a positive effect on oscillation amplitude reduction.

Electrochemical Detection of Tricresyl Phosphates in Gas

Tricresyl phosphate (TCP) is a class of neurotoxic organophosphate cholinesterase inhibitors, exposure to which may induce nausea, vomiting, diarrhea and abdominal pain. Despite of its acute toxicity, TCP is widely used in industrial applications, including flame retardants, and plasticizers. Many military and commercial aircrafts have employed hydraulic fluids containing TCP organophosphates additive since the 1970s. Potentially TCP could be released into the cabin air environment during oil leakage on aircraft hydraulic systems. Exposure to passengers and crew occurs mainly through ingestion, inhalation of aerosols, and dermal sorption. In this study, an automatic electrochemical detection system was developed to measure the TCP in gas. TCP was absorbed and hydrolyzed by an alkaline alumina column, to obtain hydrolysates that are electrochemically active, i.e. cresol, and then washed out by buffer for direct electrochemical detection. After each detection, NaOH solution was used to regenerate the column. The effects of different experiment conditions were investigated such as elution time, alumina type, the composition of regeneration solution, flow rate, temperature, etc. Through the optimization, an automatic TCP detection device was obtained with features of high sensitivity, regeneration capability and portability, which could be utilized in site test of cabin.

Aliasing Signal Separation of Superimposed Abrasive Debris Based on Degenerate Unmixing Estimation Technique.

Leakage is the most important failure mode in aircraft hydraulic systems caused by wear and tear between friction pairs of components. The accurate detection of abrasive debris can reveal the wear condition and predict a system’s lifespan. The radial magnetic field (RMF)-based debris detection method provides an online solution for monitoring the wear condition intuitively, which potentially enables a more accurate diagnosis and prognosis on the aviation hydraulic system’s ongoing failures. To address the serious mixing of pipe abrasive debris, this paper focuses on the superimposed abrasive debris separation of an RMF abrasive sensor based on the degenerate unmixing estimation technique. Through accurately separating and calculating the morphology and amount of the abrasive debris, the RMF-based abrasive sensor can provide the system with wear trend and sizes estimation of the wear particles. A well-designed experiment was conducted and the result shows that the proposed method can effectively separate the mixed debris and give an accurate count of the debris based on RMF abrasive sensor detection.

Fault diagnosis of an intelligent hydraulic pump based on a nonlinear unknown input observer

Hydraulic piston pumps are commonly used in aircraft. In order to improve the viability of aircraft and energy efficiency, intelligent variable pressure pump systems have been used in aircraft hydraulic systems more and more widely. Efficient fault diagnosis plays an important role in improving the reliability and performance of hydraulic systems. In this paper, a fault diagnosis method of an intelligent hydraulic pump system (IHPS) based on a nonlinear unknown input observer (NUIO) is proposed. Different from factors of a full-order Luenberger-type unknown input observer, nonlinear factors of the IHPS are considered in the NUIO. Firstly, a new type of intelligent pump is presented, the mathematical model of which is established to describe the IHPS. Taking into account the real-time requirements of the IHPS and the special structure of the pump, the mechanism of the intelligent pump and failure modes are analyzed and two typical failure modes are obtained. Furthermore, a NUIO of the IHPS is performed based on the output pressure and swashplate angle signals. With the residual error signals produced by the NUIO, online intelligent pump failure occurring in real-time can be detected. Lastly, through analysis and simulation, it is confirmed that this diagnostic method could accurately diagnose and isolate those typical failure modes of the nonlinear IHPS. The method proposed in this paper is of great significance in improving the reliability of the IHPS.

Design and demonstration of a flexible matrix composite morphing control surface for air gap control in a Fowler flap

A full-scale commercial aircraft morphing control surface using flexible matrix composite actuators was designed and demonstrated in this research. The muscle-like flexible matrix composite actuator is ideal for morphing structures as it deforms while actuating with the structure, and the hydraulically driven actuator can operate at pressures similar to existing aircraft hydraulic systems. Through a series of parameter studies performed with finite element models, an active spoiler was designed to control the gap between a spoiler and deployed Fowler flap. A full-scale prototype was then fabricated and tested under pseudo-aerodynamic loads. The prototype demonstrated that it can achieve the necessary deflections under the given loading condition. A closed-loop control system that allows the tip deflection of the spoiler to be precisely controlled under loading was designed.

Коливання тиску в перехідних процесах гідрав- лічних систем з насосами змінної подачі

У авіаційному гідроприводі великої потужності в якості джерел живлення отримали розповсюдження аксіально-поршневі насоси регульованої подачі. Режими роботи насоса, за яких відбувається виділення повітря і кавітація, супроводжуються підвищеним шумом, зниженням подачі, інтенсивними коливаннями тиску. Негативні наслідки цих явищ – ерозія деталей, втомне руйнування трубопроводів, зниження в'язкості робочої рідини, її забруднення продуктами зносу. У статті розглянуто механізм виникнення кавітації в аксіально-поршневих насосах, визначені фактори, що впливають на виникнення кавітації та аерації робочої рідини в цих насосах. Також розглянуто особливості перехідних процесів в гідросистемі літака. Показано, що при зміні подачі насоса з мінімальної до максимальної можливі значні коливання тиску в напірній магістралі гідросистеми літака, які негативно впливають на ресурс насоса. Даються рекомендації щодо зменшення таких коливань тиску.

Investigation of the seizing mechanism of aviation hydraulic drive spool–and–sleeves

The paper is devoted to the study of the process of seizure of spool-and-sleeve pairs of aviation hydraulic drives under the influence of contamination particles contained in working fluids of aircraft hydraulic systems. The mechanism of the particles action on the hydraulic drive is discussed. The process of seizure development is analysed in detail. Two techniques based on the analysis of the results are proposed that make it possible to estimate the state of the spool-and-sleeve. The first one is based on direct measurement of the gap in the spool-and-sleeve of the hydraulic drive during repair. To implement the technique it is necessary to analytically determine the minimal allowed size of the gap that would provide stable functioning of the unit over the whole pressure range of the hydraulic system. The second technique is less labour-intensive (but also less precise) in comparison to the first one and is based on the measurement of the hydraulic drive macroparameter the variation of which depends on the change of the gap size in the spool-and-sleeve during maintenance. It is necessary to determine the dependence of the macroparameter chosen on the size of the spool-and-sleeve gap. Implementation of the described techniques will improve economic efficiency of aircraft usage by reducing the costs and the idle time in repair.

Aircraft Hydraulic System Leakage Detection and Servicing Recommendations Method


 
 
 Aircraft hydraulic systems are composed of several components connected and distributed along the aircraft. Monitoring leakage of these components are time-consuming tasks, and often cover only some parts of the system. The objective of this work is to present a method to estimate hydraulic leakage and recommend maintenance and servicing tasks using aircraft standard sensors such as fluid temperature and reservoir level.The proposed method was tested using several aircraft operating data with different levels of degradation (external leakages) and the results were analyzed in order to evaluate its precision on estimating leakage. Results showed the capability to detect leakage although uncertainties must be considered when evaluating maintenance interventions.