Stability Analysis of Nonlinear Models of Nose Landing Gear Shimmy

Shimmy can reduce the service life of the nose landing gear, affect ride comfort, and even cause fuselage damage leading to aircraft crashes. Taking a light aircraft as the research object, the torsional freedom of landing gear around strut axis and lateral deformation of tire are considered. Since the landing gear shimmy is a nonlinear system, a nonlinear mechanical model of the front landing gear shimmy is established. Sobol index method is proposed to analyze the influence of structural parameters on the stability region of the nose landing gear, and Routh-Huritz criterion is used to verify the reliability of the analysis results of Sobol index method. We analyse the effect of torsional stiffness of strut, caster length, rated initial tire inflation pressure, rake angle, and vertical force on the stability region of theront landing gear. And the research shows that the optimization of the torsional stiffness of the strut and the caster length of the nose landing gear should be emphasized, and the influence of vertical force on the stability region of the nose landing gear should be paid attention to.

Design and Dynamics Analysis of Anti-skid Braking System for Aircraft with Four-wheel Bogie Landing Gears

In asymmetric conditions,the movement and loads of left/right wheels or front/back wheels of the aircraft with multi-wheel or four-wheel bogie landing gears are inconsistent.There are few open literatures related to anti-skid braking system for multi-wheels due to technology blockade.In China,the research on multi-channel control and non-equilibrium regulation has just started,and the design of multi-channel control system for anti-skid braking,the simulation of asymmetry taxiing under braking are not studied.In this paper,a dynamics model of ground movement for aircraft with four-wheel bogie landing gears is established for braking simulation, considering the six-degree-of-freedom aircraft body and the movement of bogies and wheels.A multi-channel anti-skid braking system is designed for the wheels of the main landing gears with four-wheel bogies.The eight wheels on left and right landing gears are divided into four groups,and each group is controlled via one channel.The cross protection and self-locked protection modules are added between different channels.A multi-channel anti-skid braking system with slip-ratio control or with slip-velocity control is established separately.Based on the aircraft dynamics model,aircraft braking to stop with anti-skid control on dry runway and on wet runway are simulated.The simulation results demonstrate that in asymmetric conditions,added with cross protection and self-locked protection modules,the slip-ratio-controlled braking system can automatically regulate brake torque to avoid deep slipping and correct aircraft course.The proposed research has reference value for improving brake control effect on wet runway.

TEST AND CONTROL SYSTEM DEVELOPMENT AND APPLICATION OF LANDING GEAR DROP TEST RIG

A control and test system of a landing gear drop test rig is developed considering the drop test specifica- tions for the ＂Seagull 300＂ multi-functional amphibious airplane. In order to realize the automation of drop test process, a servo system is proposed and programmable logic controller（PLC） technology is used. Several key technologies for measuring the horizontal load, the vertical load and the transient rotational speed are studied. According to the requirements of CCAR-23-R3, the drop test of landing gears of the ＂Seagull 300＂ airplane is accomplished. Test results show that the system has a high accuracy of data collection. The system is stable and reliable. The drop test satisfies the requirements of the drop test specifications and the results can be used as the certification of airworthiness for this kind of airplane.

Application of High-speed Solenoid Valve to the Semi-active Control of Landing Gear

To select or develop an appropriate actuator is one of the key and difficult issues in the study of semi-active controlled landing gear. Performance of the actuator may directly affect the effectiveness of semi-active control. In this article, parallel high-speed solenoid valves are chosen to be the actuators for the semi-active controlled landing gear and being studied. A nonlinear high-speed solenoid valve model is developed with the consideration of magnetic saturation characteristics and verified by test. According to the design rule of keeping the peak load as small as possible while absorbing the specified shock energy, a fuzzy PD control rule is designed. By the rule controller parameters can be self-regulated. The simulation results indicate that the semi-active control based on high-speed solenoid valve can effectively improve the control performance and reduce impact load during landing.

DESIGN OF NEW PASSIVE ADAPTIVE SHOCK ABSORBER OF LANDING GEAR AND STUDY OF ITS LANDING PERFORMANCE

A new passive adaptive shock absorber of the landing gear with double-cavity and dual-damping is studied. Its mathematical model and the virtual prototype are established based on the dynamics simulation software ADAMS. The landing dynamic characteristics and the effect of the parameters on the proposed adaptive shock absorber are analyzed. The results show that the proposed adaptive shock absorber has the slightly better landing performance at the normal load case and much less overload at the crudely landing case than the shock absorber with single-cavity and variable orifice. It also can be concluded that the overload of the proposed adaptive shock absorber can be reduced through increasing the volumes of both cavities or decreasing the pressure of the high pressure cavity or increasing the pressure of the low pressure cavity.

Hierarchical Optimization of Landing Performance for Lander with Adaptive Landing Gear

A parameterized dynamics analysis model of legged lander with adaptive landing gear was established. Based on the analysis model, the landing performances under various landing conditions were analyzed by the optimized Latin hypercube experimental design method. In order to improve the landing performances, a hierarchical optimization method was proposed considering the uncertainty of landing conditions. The optimization problem was divided into a higher level(hereafter the "leader") and several lower levels(hereafter the "follower"). The followers took condition?ing factors as design variables to find out the worst landing conditions, while the leader took bu er parameters as design variables to better the landing performance under worst conditions. First of all, sensitivity analysis of landing conditioning factors was carried out according to the results of experimental design. After the sensitive factors were screened out, the response surface models were established to reflect the complicated relationships between sensi?tive conditioning factors, bu er parameters and landing performance indexes. Finally, the response surface model was used for hierarchical optimization iteration to improve the computational e ciency. After selecting the optimum bu er parameters from the solution set, the dynamic model with the optimum parameters was simulated again under the same landing conditions as the simulation before. After optimization, nozzle performance against damage is improved by 5.24%, the acceleration overload is reduced by 5.74%, and the primary strut improves its performance by 21.10%.

Aircraft Landing Gear Control with Multi-Objective Optimization Using Generalized Cell Mapping

This paper presents a numerical algorithm tuning aircraft landing gear control system with three objectives,including reducing relative vibration, reducing hydraulic strut force and controlling energy consumption. Sliding mode control is applied to the vibration control of a simplified landing gear model with uncertainty. A two-stage generalized cell mapping algorithm is applied to search the Pareto set with gradient-free scheme. Drop test simulations over uneven runway show that the vibration and force interaction can be considerably reduced, and the Pareto optimum form a tight range in time domain.

GA-Based Model Predictive Control of Semi-Active Landing Gear

Semi-active landing gear can provide good performance of both landing impact and taxi situation, and has the ability for adapting to various ground conditions and operational conditions. A kind of Nonlinear Model Predictive Control algorithm （NMPC） for semi-active landing gears is developed in this paper. The NMPC algorithm uses Genetic Algorithm （GA） as the optimization technique and chooses damping performance of landing gear at touch down to be the optimization object. The valve＇s rate and magnitude limitations are also considered in the controller＇s design. A simulation model is built for the semi-active landing gear＇s damping process at touchdown. Drop tests are carried out on an experimental passive landing gear systerm to validate the parameters of the simulation model. The result of numerical simulation shows that the isolation of impact load at touchdown can be significantly improved compared to other control algorithms. The strongly nonlinear dynamics of semi-active landing gear coupled with control valve＇s rate and magnitude limitations are handled well with the proposed controller.

Landing gear noise control using perforated fairings

Landing gears of commercial aircraft make an important contribution to total aircraft noise in the approach configuration. Using fairings to shield components from high speed impingement reduces noise. Furthermore, perforating these fairings has been confirmed by flight tests to further enable noise reduction. Following an earlier fundamental study of the application of perforated fairings, a study has been performed to investigate and optimize the benefits of bleeding air through landing gear fairings. By means of wind tunnel tests, an aerodynamic and acoustic survey has been performed on a simplified generic main landing gear to explore the influence of （perforated） fairings on the lower part of the gear. The results show that for this specific case, the application of impermeable fairings reduces noise in the mid- and high frequency range by shielding sharp edged components from high velocity impingement. However, below 1 kHz the noise is shown to increase significantly. Application of the perforations is shown to diminish this low frequency increase whilst maintaining the reduction in the mid- and high frequency range. The aerodynamic and acoustic measurements point in the direction of the separated flow of the fairings interacting with the downstream gear components responsible for the low frequency noise increase. Bleeding of the air through the fairings reduces the large scale turbulence in the proximity of these components and hence diminishes the low frequency noise increase.

Performance Analysis on Retractable Landing Gear and Design of Ground Test System

Landing gears are one of the key components for large or middle unmanned aerial vehicles,and their working performances directly affect flying security and aircraft taking-off and landing performance.Thus,it is meaningful to study the retraction system.Based on CATIA and ADAMS software platforms,a virtual model of landing gear retraction system is built for performance test,and then dynamic simulation is carried out.Afterwards,a test system for landing gear retraction is established,and the test data are compared with the results acquired from dynamics simulation.The main factors which affect the dynamic performance of retractable landing gear are analyzed emphatically.The simulation results show that aerodynamic load has an impact on retraction time,the mass force affects extension process,and the oil hole size of hydraulic actuator has an effect on both retraction time and extension time.

Deployment Reliability Test and Assessment for Landing Gear of Chang' E-3 Probe

Chang'E-3 probe incorporates four landing gears to assure the soft-landing,which are stowed while launching and deployed after separated from rocket. Deployment reliability is quite crucial for this mission. The deployment reliability test( DRT) method and assessment method were developed in this paper. Then DRT was conducted and the deployment reliability estimate of Chang'E-3probe was used to verify the proposed methods.

Uncertain Modal Analysis of Unmanned Aircraft Composite Landing Gear

Through taking uncertain mechanical parameters of composites into consideration,this paper carries out uncertain modal analysis for an unmanned aircraft landing gear.By describing correlated multi-dimensional mechanical parameters as a convex polyhedral model,the modal analysis problem of a composite landing gear is transferred into a linear fractional programming(LFR)eigenvalue solution problem.As a consequent,the extreme-point algorithm is proposed to estimate lower and upper bounds of eigenvalues,namely the exact results of eigenvalues can be easily obtained at the extreme-point locations of the convex polyhedral model.The simulation results show that the proposed model and algorithm can play an important role in the eigenvalue solution problem and possess valuable engineering significance.It will be a powerful and effective tool for further vibration analysis for the landing gear.

IDENTIFICATION OF NONLINEAR PROPERTIES OF AN AIRCRAFT LANDING GEAR SHOCK STRUT

A simple but realistic method for identifying nonlinear stiffness and damp-ing of an air-oil shock strut widely used in aircraft is developed.In the method a powerseries expansion is used to niodel the nonlinear dynamic properties of the strut. and after introducing new variables lhe nonlinear identitication problem can be reduced to alinear one with unknown linear paranieters. An unbiased, efficient and consistentestimator for the vector of the linear parameters is obtained under conditions of mini-mizing the sum of squared residuals which is assumed to be stationary and uncorrelatedwith the observed data.The order and the most effective independent variables in themodel are detennined by the criterion of residual series correlation infonnation entropyand the procedure of best subset regression, respectively. Experiinent demonstrates thatthe results are quite satisfactory, and the method developed is realistic, which can beused to study the dynamic properties of a strut in full detail.

Reliability Design for the Landing Gear of Chang'E-3 Probe

Landing gear is one of the important components of Chang'E-3 Probe.The device was used to absorb the impact energy of Chang'E-3 Probe during the landing process.After landing on the moon,it can support the lander steadily for a long time.In order to assure the reliability of the landing gear and deal with the extremely severe landing conditions,many reliability measures were adopted in the design procedure,such as selection of cushion materials,optimization of mechanism configuration,design of deployment mode,allocation of buffer force and control of its variation.The successful landing on the moon of Chang'E-3 Probe has completely verified the high reliability of the landing gear.

PARAMETRIC MODELING OF COMPONENT LIBRARY FOR LANDING GEAR BASED ON CATIA/CAA

The design of landing gear is complicated due to the numerous considered elements.And the initial elements related to each other can also be influenced by different factors.Landing gear design often involves a very large variety of configurations,especially in the conceptual design phase.However,traditional method costs more time to complete the whole procedure for suitable configurations of landing gears.Therefore,the parametric modeling of component library for landing gear based on computer aided three-dimensional interface application/component application architecutre(CATIA/CAA)is proposed.According to the analysis of the characteristics of landing gear components,a method is presented to extract the primary parameters of landing gear components so that a systematic classification can be established.Further,the related theories and methods,including receiving geometrical parameters of the components and updating the parametrical model,displaying the component parts,are also illustrated.Finally,the development technology for component library is explained.The proposed modeling method can improve the efficiency of the whole design cycle for landing gear.

Cable-driven legged landing gear for unmanned helicopter:Prototype design,optimization and performance assessment

Unmanned helicopters equipped with adaptive landing gear will dramatically extend their application especially in dealing with challenging terrains.This study presents a novel cable-driven legged landing gear(CLG)with differential transmission for unmanned helicopters in complex landing environments.To obtain the preferred configuration of the legged mechanism,a multi-objective optimization framework for the CLG is constructed by concurrently considering terrain adaptability,landing stability and reasonable linkage of internal forces.The non-dominated sorting genetic algorithmⅡis employed to numerically acquire the optimal scale parameters that guide the mechanical design of the CLG.An unmanned helicopter prototype equipped with the devised CLG is developed with key performance assessment.Experimental results show that the devised CLG can provide energy-efficient support over uneven terrains(totally driven torque demand less than 0.1 N m)in quasi-static landing tests,and favorable terrain adaptability(posture fluctuation of the fuselage less than±1°)in unknown slope landing tests.These exhibited merits give the proposed CLG the potential to enhance the landing performance of future aircraft in extreme environments.

DYNAMIC RESPONSE ANALYSIS OF CARRIER-BASED AIRCRAFT DURING LANDING

In view of the complexity of landing on the deck of aircraft carrier,a systematic model,composed of sixdegree-of-freedom mathematic model of carrier-based aircraft,four-degree-of-freedom model of landing gears and six-degree-of-freedom mathematic model of carrier,is established in the Matlab-Simulink environment,with damping function of landing gears and dynamic characteristics of tires being considered.The model,where the carrier movement is introduced,is applicable for any abnormal landing condition.Moreover,the equations of motion and relevant parameter are also derived.The dynamic response of aircraft is calculated via the variable step-size RungeKuta algorithm.The effect of attitude angles of aircraft and carrier movement during the process of landing is illustrated in details.The analytical results can provide some reference for carrier-based aircraft design and maintenance.

SIMULATION OF LARGE CIVIL AIRCRAFT LANDING RESPONSE UNDER CONSIDERATIONS OF FLEXIBLE AIRFRAME

To evaluate the landing response of the large civil aircraft in the conceptual design phase , a method for simulating aircraft landing is given.The model for the shock absorber is investigated.The flexible airframe model is established using finite element model ( FEM ) to analyze its modes.Then , the whole aircraft model with flexible airframe is made for the multibody simulation.Tail-down , two-point , three-point and sideslip landing scenarios are studied.The influence on the landing performance considering mode superposition of the flexible airframe is analyzed.Both longitudinal and spanwise positions of the main landing gear are changed to research the influence on the landing performance.Results show that the method is feasible.The shock absorber axial force of the main landing gear with the flexible airframe is smaller than that of rigid airframe.The number of mode superposition and the position of main landing gear can influence the landing response.

大型飞机多支柱交联式起落架一体化载荷校准及建模研究

针对某大型飞机多支柱交联式起落架结构受力及传力特点,首次采用一体校准、独立建模的载荷试验方法,三支柱起落架侧挂正向一体安装、一体加载,解决了多支柱之间的载荷影响问题。根据交联式多支柱结构受力及传力特点,进行应变电桥及载荷工况设计,载荷工况既包括常规的单支柱加载,还包括满足多支柱影响的两支柱及三支柱组合加载工况。提出了多支柱广义响应系数概念,并通过广义响应系数对支柱之间的影响效果进行分析,显示支柱载荷向前单向传递,后支柱影响紧邻前支柱,前支柱不影响后支柱。据此提出了支柱独立建模、载荷逐次迭代、基于广义响应系数的支柱消扰载荷建模方法,获得了满足精度要求的载荷模型。

不平整跑道激励下飞机起落架随机响应及可靠性分析

采用非线性二自由度系统构建了飞机起落架模型,通过时域噪声刻画了不平整跑道对飞机起落架系统的随机激励,并引入路面不平度系数描述了跑道的不平整程度;基于飞机起落架模型响应的概率密度函数及其统计量,开展了不平整跑道对飞机起落架系统影响规律的研究。应用随机可靠性分析方法以及人体振动舒适性理论,通过建立安全域和舒适域与系统响应之间的关系,分析了起落架模型在不同路面不平度系数下的可靠性,并对乘客舒适性进行了评价。研究结果表明:路面不平度系数越大,系统状态变量的波动越剧烈,系统的可靠性和舒适性与路面不平度系数呈一定的负相关性;在路面不平度系数较小的情况下,系统平均首次穿越时间以及舒适性受随机扰动的影响更加显著。