起落架是飞机上众多关键组件

1、起落架是飞机上众多关键组件之一，飞机起落架的基本功能是在地面上实现 的，包括滑跑、起飞和着陆。在上述中、最重要的一部分就是着陆部分，因为它 包括许多能量的转换，并且在一些情况下系统一定要足够稳定。一些约束条件和 需求对于起落架的性能来说，包括耐久性、平顺性、重量、高度、冲程、起落架 收上和转向。以上所述的性能在地面操作上实现合理的性能必须做出恰当的优 化。虽然有很多不同的起落架模型，但是传统的模型还是包括轮胎部分、缓冲器 部分、支承结构部分。轮胎部分包括一个主要车轮装配连接机身和一个前轮装配 连接到机头。有三种常见的起落架布局：后三点式，前三点式、串联式起落架。 本论文主要研究前三点式起落架。

2、在着陆期间，主轮也就是后排机轮首先与地面 接触，然后机头部分的轮胎也就是前轮与地面接触。一般有三种为前起落架所采 用的几何图形布局，包括套筒式、铰接式、和半铰接式。如图1.1。图1.1起落架车匕胎布局套筒式轮胎布局设计在运动学上来说很简单，但在几何学上来讲得不到大的 支撑力量。自从支撑在表达冲击设计两端铰接，它仅产生的轴向力，并减少轴颈摩 擦力,却使得运动变得复杂。冲击支撑在半铰接设计中只是一端铰接，但性能可以 通过选择适当的齿轮参数进行优化。两个主要的起落架性能：（a）降落过程中的性能。（b）飞机滑跑、着陆和随 后的缓冲中轨道粗糙度对励磁性能的影响。高水平的瞬变激发在着陆期间必须控 制的平滑

3、，为了实现稳态在合理的时间内。1.2起落架领域的发展飞机起落架的设计和发展有着大量的科研成果，大部分的科研成果都是在某 些特定的情况下做出来的。在起落架设计的各个环节中，Currey通过实验成果 与经验总结，很好的描述了起落架的性能需求和缓冲器的设计。来自美国国家航 空宇宙航行局的Jocelyn也细节的解释了起落架动态特性，尤其是在摆动和制动 过程的振动，并且很好的总结了工作文件在过去的10年里，突显了解决振荡问 题上的最新成果。依照他们的工作，起落架振荡包括由自身引起的振动称为摆动 和制动过程的振动。可能性的原因对于摆动是低扭转刚度，在装置中有过多的空 转，车轮的不平衡或磨损的部件。制动过程

4、包括装置行驶，尖叫和振荡等由摩擦 特性导致的，介于刹车系统旋转和非旋转部分。这篇文章也解释了 Moreland在 飞机起落架动态理论中所做的工作。Moreland发现为了更加准确的描述系统和 振动现象，数学模型的建立需要5个自由变量：轮胎偏差、旋转角度、支撑偏差、 阻尼器的联系应变和机体运动。尽管这些自由变量数据看起来比较难以处理，但 是他们对于充分理解实质性的工作条件发挥着相当重要的作用。W. Kruger et al在宽广的理念中备份了飞机起落架动态特性，涵盖了在 控制方面的着陆和地面操纵。有两个控制观念对自动控制系统很有益：（a）在一个完全活动的悬浮执行机构提供了动力，它被直接称为一个控

5、制 法则。（b）在一个半主动悬浮中，只有阻尼力在瞬间减震器取代方向上被调整去 控制阻尼系数。他们的工作分析了设计要求的问题并对所有变量的系统进行仿 真。由于缺少高性能计算设备，为了减少结果方程的复杂程度，大多数早期的模 型都是非常简单的，D.Yadav和R.P.Ramamoorthy在Yadav和Kapadia所研究的 举模型的基础上进行了更加深入的分析。这个飞机模型通过两轮减震器弹簧被理 想化的作为刚性梁。大多数的轮子和其中的联系是集中在各自近似为轴轮和簧载 质量，被认为铰链和纵摇的自由度。举模型忽略了纵摆自由度及其耦合模型，而举投模型合并这些效应分析数 据。在着陆冲击时，举模型运用了两轮铰

6、链式前起落架和套管式伸缩的主轮几何 模型。他们也第一次学习了联动动力学对于变形的轮胎减震支柱的影响。模型如 图1.2所示。本文中，铰接前起落架有三个缓冲参数一锐孔流量系数（Cd），初始空气压 力（Po）和气动范围（Aa）被选做变量。通过不同的锐孔流量系数作用于前起落 架，系统行为表明着陆应该产生更小的振幅。这不会有主要的影响在最大振幅的反弹质量。他们也概括了一个着陆更小的初始空气压力，这可以为冲击增加时间， 能很好地应用于减振器的主动制。图1.2举模型1.1Overview of Landing GearsOne of the many critical components that mak

7、e an aircraft function is the landing gear. The basic function of a landing gear on an aircraft is to maneuver it during its gr ound operations which include taxi, takeoff and landing. Of these, the most critical ph ase is the landing because it involves a massive amount of energy transfer and the s

8、ystemhas to best able enough to operate under these conditions. Some of the constrai nts and requirements for the performance of landing gears are crash survivability, riding performance, weight, height, stroke length, retraction, and steering. All of the above would have to be optimized for the rea

9、sonable performance of an aircraft on its ground operations.Although there are so many different variants of the landing gear models, the conventional one has a tired wheel unit, a shock absorbing unit and a supporting structure 1. The wheel unit has a main wheel assembly attached to the fuselage an

10、d a nose wheel assembly attached to the nose of the aircraft. There are three common types of landing gear: conventonal, tricycle, and tandem. This work has been confined to the tricycle type of landing gear. During landing, the main wheels come in contact with the ground first which is called the t

11、ouchdown and then the nose wheel makes contact with the ground.套筒式铉接式半钗接式Fig. 1 . 1 Landing gear typesThe telescopic design is kinematically simpler but the geometry may not account for the large strut forces. Since the shock strut in the articulated design has both end hinged, it produces only axia

12、l forces and so doe minimize the journal frictional forces, but at the cost of complex kinematics. The shock strut in the semi articulated gear design has only one end hinged, but the performance can be optimized by proper selection of gear parameters.The two major concerns for landing gear performa

13、nce are:behavior during touch down impact.and performance excitation induced by track roughness during taxi, takeoff and the later part of landing runs. The high level of transients induced during touchd own have to be controlled smoothly in order to achieve the stady state in a reasonable amount of

14、 time.1.2.Developments in the Field of Landing GearsThere has been considerable research in the field of landing gear design and devel opment and most of these works are specific to a particular case under consideration. The various steps in .landing gear design, its performance requirements and sho

15、ck strut design are well described by Currey , based on experience and experiments. Jocelyn of NASA has explained in detail the landing gear dynamics, especially shimmy and brake-induced vibration and has well summarized the work documented from the last ten years to highlight the latest efforts in

16、solving these vibration problems. According to their work, the landing gear vibration includes self-induced oscillations called shim my and brakeinduced vibration. Possible causes for shimmy are low torsional stiffness excessive freeplay in the gear, wheel imbalance, or worn parts. Brake-induced vib

17、rati on includes gear walk, squeal and chatter, which are caused by the frictional character istics between the brake rotating and nonrotating parts. This paper also explains the work done by Moreland 5 in landing gear dynamics and the theory of shimmy. More land found that to precisely describe the

18、 system and the shimmy phenomena, the math ematical model required 5 degrees of freedom: tire deflection, swivel angle, strutdefle ction, damperlinkage strain, and airframe motion. Even though it may seem cumberso me to include these degrees of freedom to the specific model of study, they play an im

19、 portant role if the model has to be comprehensive enough to consider all the practical working conditions.W. Kriiger et al. 6 documented the aircraft landing gear dynamics in a wider se nse, considering both landing and ground maneuvering with control aspects. There are two control concepts which a

20、re of interest to automotive systems: (a) in a fully-active suspension an actuator provides force which is directly defined by a control law, (b) in a semi-active suspension only the damping force in the direction of the momentary damper displacement is modulated to control the damping coefficient.

21、Their work analyzed the problem from design requirements to the simulation of the system with all probable variables.Most of the earlier models were simplified to reduce the complexity of the resulting equations due to lack of computational facilities and they assumed the gear geometry to be telesco

22、pic. D. Yadav and R. P. Ramamoorthy 2 analyzed an extended version of the heave model created by Yadav and Kapadia (1990). This model idealized the air craft as a rigid beam supported by the shock absorbers over two wheel springs. The masses of the wheels and the linkages were approximated as lumped

23、 at the respective wheel axes and the sprung masses were assumed to have heave and pitch degrees of freedom.The heave model neglected the pitch degrees of freedom and its coupling with the model, whereas the heave pitch model incorporated these effects into the analysis.The heave pitch model used a two-wheeled articulated nose gear and telescopic main gear geometries during landing impact.