Oleo-pneumatic Shock Absorber Research Articles

Preliminary Nose Landing Gear Digital Twin for Damage Detection

An increase in aircraft availability and readiness is one of the most desired characteristics of aircraft fleets. Unforeseen failures cause additional expenses and are particularly critical when thinking about combat jets and Unmanned Aerial Vehicles (UAVs). For instance, these systems are used under extreme conditions, and there can be situations where standard maintenance procedures are impractical or unfeasible. Thus, it is important to develop a Health and Usage Monitoring System (HUMS) that relies on diagnostic and prognostic algorithms to minimise maintenance downtime, improve safety and availability, and reduce maintenance costs. In particular, within the realm of aircraft structures, landing gear emerges as one of the most intricate systems, comprising several elements, such as actuators, shock absorbers, and structural components. Therefore, this work aims to develop a preliminary digital twin of a nose landing gear and implement diagnostic algorithms within the framework of the Health and Usage Monitoring System (HUMS). In this context, a digital twin can be used to build a database of signals acquired under healthy and faulty conditions on which damage detection algorithms can be implemented and tested. In particular, two algorithms have been implemented: the first is based on the Root-Mean-Square Error (RMSE), while the second relies on the Mahalanobis distance (MD). The algorithms were tested for three nose landing gear subsystems, namely, the steering system, the retraction/extraction system, and the oleo-pneumatic shock absorber. A comparison is made between the two algorithms using the ROC curve and accuracy, assuming equal weight for missed detections and false alarms. The algorithm that uses the Mahalanobis distance demonstrated superior performance, with a lower false alarm rate and higher accuracy compared to the other algorithm.

Study of the Two-Phase Flow Characteristics of a Damping Orifice in an Oleo-Pneumatic Shock Absorber

The oleo-pneumatic shock absorber involves a complex two-phase flow in the working process. In this paper, a simple oleo-pneumatic shock absorber model was established, and the volume-of-fluid (VOF) two-phase flow model was adopted to accurately simulate the distribution of the two-phase flow field in the shock absorber through the commercial software FLUENT 2020 R2. The accuracy of the simulation model was verified by the method of engineering damping force estimation, and the error of the numerical simulation results compared with the engineering estimation results was 7–8%. By numerical simulation, the influence of different orifice lengths and diameters on the maximum pressure, temperature, velocity and oil damping force inside the shock absorber was studied. The results showed that with the increase of the orifice length, the maximum pressure, flow rate and oil damping force in the shock absorber decreased. The temperature decreased first and then increased, but the overall effect was small. However, according to the oil volume fraction contour, the gas–liquid distribution in the shock absorber with an orifice larger than 15 mm was more chaotic. Increasing the diameter of the orifice had a great impact on the shock absorber. The maximum pressure, flow rate and damping force of the oil inside the shock absorber were sharply reduced, and the temperature continued to rise. These research results can provide reference for the optimization design of oleo-pneumatic shock absorbers.

Fuzzy control of a landing gear system with oleo-pneumatic shock absorber to reduce aircraft vibrations by landing impacts

PurposeThe purpose of this paper is to control a landing gear system with an oleo-pneumatic shock absorber with the fuzzy controller.Design/methodology/approachThe landing gear system with an oleo-pneumatic shock absorber is modeled mathematically. A fuzzy controller is designed for reducing aircraft vibrations. Stroke velocity and main mass velocity parameters were used to decide variable gas pressure with the fuzzy controller.FindingsThe fuzzy controller, designed according to stroke velocity and main mass velocity, reduces aircraft vibrations by the landing impacts. The controller can provide strong robustness because it shows similar good performance for different descent speeds.Research limitations/implicationsThis study was carried out through simulations in a computer environment and has not been experimentally tested in a real environment. In addition, signal and measurement delays are not taken into account. In future models, the effects of these signal delays can be added, and the controller can be tested on a real model.Originality/valueIn this study, to the best of the authors’ knowledge, for the first time, the gas pressure for the landing gear system using an oleo-pneumatic shock absorber was controlled by a fuzzy controller that adjusts the stroke velocity and the main mass velocity. Although the oleo-pneumatic shock absorber model contains high nonlinearities, the designed fuzzy controller gave successful results as robust.

Landing Gear Mechanical Network Synthesis for Improving Comfort at Landing Considering Aircraft Flexibility

The landing impact that occurs during aircraft touchdown results in the development of significant loads and accelerations within the airframe. Accurate knowledge of the landing loads is necessary not only for predicting the resulting stresses for design of the airframe, but also for designing strategies to mitigate the vibratory loads and improve the ride quality. Perceived passenger comfort is dependent both on the magnitude of the acceleration experienced by the passengers and on the frequency content of the vibrations. Using a flexible airframe model to better capture the loading regime and frequency response at landing impact, this study optimizes various single-port (two-terminal) passive mechanical networks that consist of an arrangement of springs, dampers, and inerters to minimize passenger discomfort and peak forces applied to the aircraft. The performance is compared with a baseline oleo-pneumatic shock absorber. First, the candidate layouts are optimized, and then the observations from this exercise are used to synthesize a mechanical network with a desired frequency response. All considered mechanical networks demonstrated the ability to control the frequency content of the input loading, thus resulting in a reduction in accelerations and an improvement in all comfort parameters used in this study over the oleo-pneumatic baseline.

Comprehensive design of an oleo-pneumatic nose landing gear strut

A comprehensive design cycle of a nose landing gear strut having an oleo-pneumatic shock absorber for a lightweight aircraft is proposed. Design and analysis of a retractable nose landing gear according to Airworthiness Standards FAA FAR Part 23 have been carried out. This research is focused on mathematical modeling of an oleo-pneumatic strut with an analytical solution of design variables at static and dynamic loading conditions. The variation of spring and damping characteristics of an oleo-pneumatic shock absorber with the stroke length is also presented. Feasibility of equivalent mechanical spring and damper along with comparison of pneumatic as air spring and oleo as hydraulic damper is studied. Numerical integration technique was used to solve the dynamic model of an oleo-pneumatic strut with forcing function of an impact force during touch down scenario. Energy conservation principle was used to determine height required for drop tests. Parametric study of anteversion angle within the constraints of ground clearance and volume in the fuselage determined an optimized angle of nose landing gear strut. Based on the maximum pressure and impact force encountered during landing, the hydraulic cylinder and piston design was finalized. In order to validate the proposed design cycle for preliminary phase, a structural integrity of cylinder and piston assembly was carried out using finite element analysis. Deformation, maximum stresses and factor of safety validated the proposed design cycle of a nose landing gear strut specific to a general aviation aircraft having all up mass of 1600 Kg.

Selection Methodology of an Electric Actuator for Nose Landing Gear of a Light Weight Aircraft

Landing gear system of an aircraft enables it to take off and land with safety and comfort. Because of the horizontal and vertical velocity of aircraft, upon landing, the complete aircraft undergoes different forcing functions in the form of the impact force that is absorbed by landing gears, shock absorbers, and actuators. In this research, a selection methodology has been proposed for an electrical actuator to be installed in the retraction mechanism of nose landing gear of an aircraft having 1600 kg gross takeoff weight. Nose landing gear and its associated components, like strut and shock absorbers, were modeled in CAD software. Analytical expressions were then developed in order to calculate the actuator stroke, translational velocity, force, and power for complete cycle of retraction, and some were subsequently compared with the computational results that were obtained using MSC ADAMS®. Air in the oleo-pneumatic shock absorber of nose landing gear was modeled as a nonlinear spring with equivalent spring constant, whereas hydraulic oil was modeled as a nonlinear damper with equivalent damping constant. The nose landing gear system was modeled as a mass-spring-damper system for which a solution for sinusoidal forcing functions is proposed. Finally, an electrical actuator has been selected, which can retract and extend nose landing gear, meeting all of the constraints of aircraft, like fuselage space, aircraft ground clearance, locking loads, power consumption, retraction and extension time, and dynamic response of aircraft. It was found that the selection of an electrical actuator is based upon the quantification of forces transmitted to electrical actuator during one point load at gross takeoff weight. The ability of retraction and extension time, as dictated by Federal Aviation Regulation, has also been given due consideration in the proposed methodology as significant criteria. The proposed system is now in the process of ground testing, followed by flight testing in the near future.

Multi-physics based Simulations of an Oleo-pneumatic Shock Absorber System for PHM

The paper presents multi-physics (Mechanical, Thermal, Hydraulic, and Pneumatic) based modelling and simulation of an Oleo-pneumatic shock absorber with fault capabilities. The fault simulated in this model is leakage due to eccentricity. The one-dimensional shock absorber system models to give loads at different sink velocities. These load values, used in the structural model to do static stress analysis. By using these loads directly from the system model eliminates the error in load computation from the load’s group, thereby eliminating the time and cost involved in this activity. The models and static stress analyses carried out with both 1-D and 3-D elements. The 3-D landing gear model meshed with using both auto and manual mesh generation options. The consequences of both 1-D and 3-D models mesh generation, discussed in this paper. The static stress analysis, compared with the experimental results and it is found that the results are within 5% deviation. Based on the static stress and fatigue analysis computed the life of a landing gear.

The Dynamic Characteristics of a non-linear main landing gear system of an aircraft during landing

The landing gear plays a very important role during landing by absorbing the high impact energy of the aircraft. The main landing gear absorbs the bulk of the load to reduce the load experienced by both the aircraft fuselage and the nose landing gear. In this paper, a mathematical approach is used to extract the dynamic characteristics of the system. A two-degree of freedom mathematical model of the main landing gear is developed. This model is used to derive the dynamic equations of the landing gear system and to study the behaviour of main landing gear during main gear and nose gear touchdown conditions. The non-linear stiffness and damping co-efficient in an Oleo-Pneumatic shock absorber are integrated into the system to achieve a more accurate response of the system. The response of this system is established by adopting a complex modal analysis approach to account for the non-classical damping exhibited by the system. The obtained spring force, damping force and responses are reported. This work provides an alternative approach using complex modal analysis to obtain results for complex systems exhibiting non-linear characteristics.

Optimal Inerter-Based Shock–Strut Configurations for Landing-Gear Touchdown Performance

This paper investigates the possibility of improving aircraft landing-gear touchdown performance by adding an inerter alongside a linear passive shock strut. The inerter is a novel mechanical element with the property that the applied force is proportional to the relative acceleration between its terminals. A simplified landing-gear model is presented, and the baseline performance of a conventional oleo-pneumatic shock absorber is established. Candidate layouts with linear mechanical components including inerters are considered using three objective functions: the strut efficiency, the maximum strut load, and the maximum stroke. It is demonstrated that improved touchdown performance can be achieved with a linear inerter-based configuration. However, it is also observed that the potential energy stored in the gear at the end of the first compression stroke exceeds that of the baseline nonlinear system. This suggests a poorer elongation stage might be observed. To address this, an additional constraint on energy dissipation is then considered. To achieve a reduced potential energy, a double-stage compression spring is introduced. With this, inerter-based configurations that provide improvements for the performance indices of interest are identified and presented.

Active control of a non-linear landing gear system having oleo pneumatic shock absorber using robust linear quadratic regulator approach

This paper deals with the active control of a non-linear active landing gear system equipped with oleo pneumatic shock absorber. Runway induced vibration can cause reduction of pilot’s capability of control the aircraft and results the safety problem before take-off and after landing. Moreover, passenger–crew comfort is adversely affected by vertical vibrations of the fuselage. The active landing gears equipped with oleo pneumatic shock absorber are highly non-linear systems. In this study, uncertain polytopic state space representation is developed by modelling the pneumatic shock absorber dynamics as a mechanical system with non-linear stiffness and damping properties. Then, linear matrix inequalities-based robust linear quadratic regulator controller having pole location constraints is designed, since the classical linear quadratic regulator control design is dealing with linearized state space models without considering the non-linearities and uncertainties. Thereafter, numerical simulation studies are carried out to analyse aircraft response during taxiing. Bump- and random-type runway irregularities are used with various runway class and wide range of longitudinal speed. Simulation results revealed that neglecting the non-linear dynamics associated with oleo pneumatic shock absorber results significant performance degradation. Consequently, it is demonstrated that proposed robust linear quadratic regulator controller has a superior performance in terms of passenger–crew comfort and operational safety when compared to classical linear quadratic regulator.

Optimization of an Oleo-Pneumatic Shock Absorber for Main Landing Gear of a Commercial Aircraft

In the current Aircraft landing system, efficiency of taxiing and landing is highly dependent on the profile of the runway. In a research experiment done by BOEING it was found that an isolated bump experienced during taxiing can cause as much discomfort as a harsh landing. These bumps appear when runway maintenance is due. The objective of the project was to design an oleo-pneumatic shock absorber for the main landing gear of a commercial aircraft, Boeing 737-200, taking into consideration passenger comfort during takeoff and landing. The design aims at reducing the dependence on runway maintenance, thus reducing repair and maintenance costs. It also aims at improving the overall efficiency of the system, making landing more comfortable for the passengers. This paper outlines the procedure followed in the optimization of the strut and also describes the final layout.

Multi-physics based Simulations of a Shock Absorber Sub-system for PHM

The multi-physics involve Mechanical, Thermal, Hydraulic, and Pneumatic based modeling and simulation of an oleo-pneumatic shock absorber with fault capabilities presented in this paper. The fault simulated in this model is leakage due to eccentricity. The one-dimensional shock absorber system models render loads at different sink velocities. These load values, used in the structural model to do static stress analysis. By using these loads directly from the system model eliminates the error in load computation from the loads group, thereby eliminating the time and cost involved in this activity. The models and static stress analyses are done with both 1-D and 3-D elements. The 3-D landing gear model meshed with using both auto and manual mesh generation options. The consequences of both 1-D, and 3-D models mesh generation are discussed in this paper. The static stress analysis, compared with the experimental results and it is found that the results are within 5% deviation. Based on the static stress analysis computed the life of a landing gear.

Design and Drop Test of Aircraft Landing Gear’s Shock Absorber Based on Magnetorheological Damper

The structure of a shock absorber based on magnetorheological (MR) damper with a metering pin is proposed, and structure of the magnetic circuit of MR damper is optimized. By drop tests, the damping characteristics of the shock absorber and damping effect are tested. The experimental results show that the shock absorber has the characteristic of a wide damping force adjustment range and lower energy dissipation. The maximum vertical load and shock absorber piston displacement can be changed at same drop height by changing the current. It has also been proved that shock absorber based on MR damper is better than conventional oleo-pneumatic shock absorber on adjustment of damping force.

Vibration Control of Aircraft Semi-Active Suspension System using PID-Bees Technique

The dynamic load and vibration caused by landing impact and the unevenness of runway will result in airframe fatigue, discomfort of crew/passengers and the reduction of the pilot’s ability to control the aircraft. The semi-active suspension system can provide good performance of both landing impact and taxi, and has the ability for adapting to various ground and operational conditions. This current paper designs Proportional Integral Derivative controller based on Bees Intelligent Algorithm as the optimization technique for nonlinear model of semi-active landing gear system that chooses damping performance of system at touchdown as object function. Optimal setting of controller parameters to achieve favorable time response using numerical software method based on Bees Algorithm is more simple and more effective than other traditional methods such as Ziegler-Nichols and experimental because it does not need high experience and complex calculations and leads to better results. This research develops nonlinear two-dimensional mathematical model to describe landing gear system with oleopneumatic shock absorber and linear tire. Based on this model, the dynamic equations are used to investigate the behavior of an aircraft semi-active landing gear system subject to runway disturbance excitation and the stability conditions of the landing system around static equilibrium position is studied. SIMULINK simulation software is applied to validate the theoretical analysis of system stability. Results of system numerical Simulation with optimized controller using Bees Algorithm in MATLAB software shows that the transmitted impact load to airframe, the vertical vibration of aircraft and time to return static equilibrium position at touchdown are significantly improved. General Terms Classical Technique, Optimization Algorithm

Application of Active Suspension System to Reduce Aircraft Vibration using PID Technique and Bees Algorithm

dynamic load and vibration caused by landing impact and the unevenness of runway will result in airframe fatigue, discomfort of crew/passengers and the reduction of the pilot's ability to control the aircraft. The aim of the current paper is to design Proportional Integral Derivative classical controller based on Bees Intelligent Algorithm as the optimization technique for nonlinear model of active landing gear system that chooses damping and stiffness performance of suspension system at touchdown as optimization object. Optimal setting of controller parameters to achieve desirable time response using numerical software method based on Bees Algorithm is easier and more effective than other traditional methods because it does not need high experience and complex calculations and leads to better results. This research develops nonlinear two-dimensional mathematical model to describe landing gear system with oleo-pneumatic shock absorber and linear tire. Based on this model, the dynamic equations derived are used to investigate the behavior of an aircraft active landing gear system subject to runway disturbance excitation and the stability conditions of the landing system around static equilibrium position is studied according to the Routh-Hurwitz criterion. Simulink control system simulation software is utilized to validate the theoretical analysis of system stability and results comparison and adaptation of this paper with research of Wang and Xing about investigation of active landing gear system. Results of system numerical Simulation with optimized controller using Bees Algorithm in MATLAB software shows that the transmitted impact load to airframe, the vertical vibration of aircraft and time to return static equilibrium position at touchdown are significantly improved compared with other control performances.

유공압 착륙장치 해석 및 시험평가

A landing gear absorbs the impact energy during the touchdown and it generally consists of an oleo-pneumatic shock absorber and structural components. It should be designed not only to satisfy the static and fatigue strength requirements but to have the sufficient shock absorbing efficiency. The design loads and shock absorbing performance are to be validated by tests, which is required by MIL Spec and FAR, etc. This paper presents the development procedures from the design requirements to the qualification tests and technical points to be considered, with examples of the helicopter landing gear development.

유공압 착륙장치 낙하시험

착륙장치는 완충장치를 이용하여 항공기 착륙 시의 충격을 흡수하는 역할을 한다. 다양한 종류의 완충장치가 존재하나, 완충효율 측면에서 가장 우수한 것은 유공압 방식이다. 착륙장치의 완충 성능은 반드시 낙하시험을 통해 입증하여야 하며, 이는 미 군사규격, 미연방 항공규정 등에서 공통적으로 요구하는 있는 사항이다. 이 논문에서는 낙하시험을 위한 설비 구성, 시험 절차 및 결과분석 방법을 실제 낙하시험 사례와 함께 제시한다. The main function of a landing gear is to absorb the impact energy during touchdown. Most landing gear use an oleo-pneumatic shock absorber which essentially consists of an oil damper and a gas spring. The performance of a shock absorber can be estimated by analysis but it should be verified by drop test, which is required by MIL Spec. and FAR. In the drop test, various data such as ground loads, shock absorber pressure, stroke and mass travel are analyzed to validate the shock absorbing efficiency and the mathematical model for analysis. This paper presents the introduction of drop test facility, the test procedure and data evaluation method with real drop test example.

Nonlinear Landing Gear Behavior at Touchdown

Landing gear dynamics for an aircraft has been analyzed with a heave-pitch model having telescopic main gear and articulated nose gear using oleopneumatic shock absorber. System equations have been presented incorporating the effects of linkage dynamics, frictional forces, and nonlinearities in the tyre, air spring, and oleo damping forces. Sensitivity of the system response to variations in some shock strut parameters has been investigated for the landing touchdown impact phase to bring about improvement in the performance.

Optimization of an oleo-pneumatic shock absorber of an aircraft during landing

Optimization of an oleo-pneumatic shock absorber of an aircraft during landing

Stresses in Shock Absorbers

This investigation is concerned with the stresses in a hollow circular cylinder of constant wall thickness, and closed by flexible plates, also of constant thickness. The cylinder is subjected to the combined action of an internal pressure and an axial force, both of which are statically applied. Although the particular case envisaged is that of a shock absorber, there are other engineering elements to which the analysis is applicable. Any closed circular cylinder of constant wall thickness subjected to an internal pressure is within the category. In a paper, Dr. D. M. A. Leggett investigated the case of an oleopneumatic shock absorber with rigid end plates, and what follows is an extension of his paper. As would be expected, the flexibility of the end plates causes a reduction in the flexural stress.